WO2010146524A1 - Conformable electronic devices and methods for their manufacture - Google Patents

Conformable electronic devices and methods for their manufacture Download PDF

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Publication number
WO2010146524A1
WO2010146524A1 PCT/IB2010/052644 IB2010052644W WO2010146524A1 WO 2010146524 A1 WO2010146524 A1 WO 2010146524A1 IB 2010052644 W IB2010052644 W IB 2010052644W WO 2010146524 A1 WO2010146524 A1 WO 2010146524A1
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WO
WIPO (PCT)
Prior art keywords
substrate
contact pads
circuit element
circuit elements
layer
Prior art date
Application number
PCT/IB2010/052644
Other languages
French (fr)
Inventor
Frank Anton Van Abeelen
Peter Douglas Fairley
Steffen Reymann
Original Assignee
Koninklijke Philips Electronics N.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2010146524A1 publication Critical patent/WO2010146524A1/en

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    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/303—Surface mounted components, e.g. affixing before soldering, aligning means, spacing means
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/04—Automatically aligning, aiming or focusing the laser beam, e.g. using the back-scattered light
    • B23K26/042—Automatically aligning the laser beam
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/08—Devices involving relative movement between laser beam and workpiece
    • B23K26/082—Scanning systems, i.e. devices involving movement of the laser beam relative to the laser head
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/361—Removing material for deburring or mechanical trimming
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/36—Removing material
    • B23K26/40—Removing material taking account of the properties of the material involved
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
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    • H01L23/498—Leads, i.e. metallisations or lead-frames on insulating substrates, e.g. chip carriers
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    • H01L25/00—Assemblies consisting of a plurality of individual semiconductor or other solid state devices ; Multistep manufacturing processes thereof
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    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
    • H05K3/321—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by conductive adhesives
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2103/00—Materials to be soldered, welded or cut
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    • B23K2103/30—Organic material
    • B23K2103/42—Plastics
    • B—PERFORMING OPERATIONS; TRANSPORTING
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    • B23K2103/50—Inorganic material, e.g. metals, not provided for in B23K2103/02 – B23K2103/26
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    • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
    • H05K2201/09—Shape and layout
    • H05K2201/09209—Shape and layout details of conductors
    • H05K2201/09218—Conductive traces
    • H05K2201/09263—Meander
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
    • H05K2201/10—Details of components or other objects attached to or integrated in a printed circuit board
    • H05K2201/10613—Details of electrical connections of non-printed components, e.g. special leads
    • H05K2201/10954—Other details of electrical connections
    • H05K2201/10984—Component carrying a connection agent, e.g. solder, adhesive
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/01—Tools for processing; Objects used during processing
    • H05K2203/0104—Tools for processing; Objects used during processing for patterning or coating
    • H05K2203/0139—Blade or squeegee, e.g. for screen printing or filling of holes
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/01—Tools for processing; Objects used during processing
    • H05K2203/0147—Carriers and holders
    • H05K2203/0156—Temporary polymeric carrier or foil, e.g. for processing or transferring
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/01—Tools for processing; Objects used during processing
    • H05K2203/0191—Using tape or non-metallic foil in a process, e.g. during filling of a hole with conductive paste
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/10—Using electric, magnetic and electromagnetic fields; Using laser light
    • H05K2203/107—Using laser light
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/15—Position of the PCB during processing
    • H05K2203/1581—Treating the backside of the PCB, e.g. for heating during soldering or providing a liquid coating on the backside
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/40—Forming printed elements for providing electric connections to or between printed circuits
    • H05K3/4038—Through-connections; Vertical interconnect access [VIA] connections
    • H05K3/4053—Through-connections; Vertical interconnect access [VIA] connections by thick-film techniques
    • H05K3/4069—Through-connections; Vertical interconnect access [VIA] connections by thick-film techniques for via connections in organic insulating substrates
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • This invention relates to conformable electronic devices, that is to say electronic devices which can be physically deformed to conform to different surfaces without experiencing short-term physical or functional degradation.
  • the invention relates to such devices which are formed as sheets and are capable of both bending and stretching, such that the devices can be made to conform to surfaces having curvature in at least two different planes.
  • the invention also relates to a method of manufacturing conformable electronic devices which are capable of bending and stretching.
  • wearable electronic devices include devices which are intended to stimulate the healing of wounds by exposure to electric fields or light. More diverse applications for conformable electronic devices in general relate to display devices and sensors which can be manufactured as flat sheets and then applied to curved surfaces.
  • the simplest form of conformability for sheet-like electronic devices is bendability. It is already known to provide electronic devices which are capable of bending. Such devices are typically realized by preparing a thin electronically active layer on a flexible substrate such that the active layer coincides with the neutral mechanical plane of the device. In this case, the mechanical strain experienced by the relatively brittle active layer can be maintained below a failure limit, which is typically of the order of 1 %. Bendability enables sheet-like electronic devices to conform to surfaces having simple curvature in one plane, such as cylindrical surfaces. However, to enable such devices to conform to surfaces having curvature in multiple different planes, such as spherical surfaces or the surface of the skin of the human or animal body, a degree of stretchability is also required. To be of any practical value in this regard, the stretchability of a device must be significantly greater than that which is available from the known bendable devices described above (which can provide approximately 1 % stretch).
  • this device is more stretchable than the bendable devices described above, stretchability is still limited by the need to minimize strain in the active layer as it buckles (bends) under the compressive influence of the elastic substrate.
  • the available stretch is limited to around 10 %, which restricts the ability of the device to conform to deeply curved surfaces.
  • Another problem associated with the device is that the wrinkled surface of the active layer may be inappropriate for some applications.
  • Figs. Ia and Ib show the stretching of this device schematically in Figs. Ia and Ib.
  • Fig. Ia shows the arrangement of the islands and the conductive tracks of the device in the flat, undeformed state.
  • Fig. Ib shows the arrangement of the islands and the conductive tracks when the device is applied to a spherical surface. It will be noted that the stretching of the device is provided by extension of the areas of the flexible substrate between the rigid islands.
  • the conductive tracks are shown in greater detail in Fig. 2.
  • the tracks are typically formed of plated copper and follow a meandering path between the islands.
  • the device shown in the Figs is capable of greater stretchability, for example up to 50 %, the known method for its manufacture is inefficient and therefore costly.
  • the device is manufactured by electrically and mechanically attaching the circuit elements, in turn (sequentially), to the conductive tracks of the flexible substrate, typically by soldering.
  • a method for manufacturing a sheet-like electronic device which is bendable and stretchable comprising: (a) providing a first support substrate having arranged thereon a plurality of spaced apart electronic circuit elements, each circuit element having at least one conductive contact pad facing away from the substrate; (b) providing a second substrate having arranged thereon a plurality of conductive tracks, ends of the conductive tracks having conductive contact pads; (c) bringing the first and second substrates together such that the contact pads of the circuit elements face the contact pads of the conductive tracks and attaching the circuit elements to the second substrate such that the contact pads of the circuit elements are electrically connected to the contact pads of the conductive tracks; and (d) releasing the circuit elements from the first support substrate, wherein the second substrate of the device, to which the circuit elements are attached, is bendable and stretchable.
  • the invention thus provides a method by which highly conformable electronic devices may be efficiently manufactured.
  • the costs associated with the prior art technique of individually manipulating and attaching the circuit elements one at a time may be avoided.
  • Various techniques may be used for releasing the circuit elements from the first support substrate, including so-called laser release processes.
  • a bendable substrate may be one capable of supporting a radius of curvature of 500 mm or less, preferably 300 mm or less, and most preferably 100mm or less.
  • a stretchable substrate may be one capable of 5 % stretch or more, preferably 10 % stretch or more, and most preferably 20 % stretch or more.
  • the circuit elements may comprise one or more components of an electronic circuit.
  • the circuit elements may, for example, comprise sensors, display devices, and other types of semiconductor devices. Individual circuit elements may constitute complete circuits, portions of complete circuits or individual components of complete circuits.
  • the conductive tracks of the second substrate serve as interconnects between the circuit elements.
  • the first substrate may be a rigid substrate, such as glass. Rigid substrates can in general be processed more easily and with fewer modifications to standard manufacturing equipment.
  • the second substrate is itself bendable and stretchable, but may be attached to a further rigid support substrate for processing efficacy. In the latter case, the second substrate is released from the further rigid support substrate after the circuit elements have been attached to the second substrate, typically at the end of the manufacturing method.
  • the electronic circuit elements may themselves be flexible.
  • the circuit elements may be bendable to some degree, but are not generally stretchable.
  • the step of providing the first support substrate having the circuit elements may comprise: (a) applying a polymer coating, such as polyimide, over the first support substrate to define a circuit element substrate; (b) forming thin film electronic circuitry and conductive contact pads over the circuit element substrate, the thin film circuitry and the contact pads defining a plurality of spaced apart areas; and (c) removing portions of the circuit element substrate from between the spaced apart areas to thereby leave the circuit elements having the contact pads.
  • a polymer coating such as polyimide
  • the spaced apart areas may be defined by the edges of the circuitry and/or the contact pads, in which case the subsequently removed portions of the circuit element substrate will generally remain exposed until they are removed.
  • the spaced apart areas may be arbitrarily defined, in which case the subsequently removed portions of the circuit element substrate may be covered with one or more layers which remain from the forming of the circuitry and/or the contact pads.
  • the portions of the circuit element substrate which are removed may be cut out by dicing or laser cutting and may be released from the first support substrate by a laser release process. In embodiments where these portions of the circuit element substrate are covered with other layers, the circuit element substrate and these layers may be released together,
  • a laser release process may also be used to release the circuit elements from the first support substrate at the end of the manufacturing method.
  • a suitable laser release process comprises directing ultraviolet laser light at the circuit element substrate through the first support substrate.
  • a specific process of this type is the electronics on plastic by laser release (EPLaR) process described in WO 2005/050754, the entire disclosure of which is incorporated herein by reference.
  • the contact pads of the circuit elements may be electrically connected to the contact pads of the conductive tracks through holes formed in a structural layer.
  • the step of providing the first support substrate having the circuit elements may further comprise, before removing the portions of the circuit element substrate: (a) applying a structural film over the thin film circuitry, contact pads and the circuit element substrate, the structural film comprising a structural layer and a releasable layer arranged over the structural layer; (b) forming holes in the structural film to expose the contact pads and filling the holes with a curable conductive paste; and (c) removing the releasable layer from the structural layer of the structural film.
  • Portions of the structural film overlying the circuit element substrate are then removed at the same time that portions of the circuit element substrate are removed to leave the spaced apart circuit elements having the contact pads.
  • the structural layer of the structural film may comprise a part-cured thermosetting resin and the releasable layer of the structural film may comprise a silicone- coated polymer such as polyethylene terephthalate (PET).
  • PET polyethylene terephthalate
  • the part-cured resin should be capable of retaining its shape, but at the same time be moldable and sticky enough to adhere to other surfaces.
  • Attaching the circuit elements to the second substrate may comprise applying heat and/or pressure to the curable conductive paste and the part-cured thermosetting resin to thereby cure the paste and the resin.
  • the cured conductive paste electrically connects the contact pads of the circuit elements to the contact pads of the conductive tracks.
  • the above-described method is suitable for the manufacture of devices in which it is acceptable for the circuit element substrate to face away from the second (stretchable) substrate.
  • it may be more appropriate for the circuit element substrate to face towards the second substrate.
  • the step of providing the first support substrate having the circuit elements needs to be modified and may comprise: (a) applying a polymer coating, such as polyimide, over an auxiliary support substrate to define a circuit element substrate; (b) forming thin film electronic circuitry and conductive contact pads over the circuit element substrate, the thin film circuitry and the contact pads defining a plurality of spaced apart areas; (c) forming a transparent cover layer over the thin film circuitry, contact pads and exposed areas of the circuit element substrate; (d) attaching the first support substrate to the transparent cover layer with an adhesive layer; (e) releasing the circuit element substrate from the auxiliary support substrate; and (f) removing portions of the circuit element substrate and the transparent cover layer from between the spaced apart areas to thereby leave the circuit elements having the contact pads.
  • a polymer coating such as polyimide
  • the spaced apart areas may be defined by the edges of the circuitry and/or the contact pads, in which case the subsequently removed portions of the circuit element substrate will generally remain exposed until they are removed.
  • the spaced apart areas may be arbitrarily defined, in which case the subsequently removed portions of the circuit element substrate may be covered with one or more layers which remain from the forming of the circuitry and/or the contact pads.
  • the thin film electronic circuitry and conductive contact pads may be formed such that the contact pads face towards, and are in contact with, the circuit element substrate.
  • the circuit element substrate may be released from the auxiliary support substrate by directing ultraviolet laser light through the auxiliary support substrate at the circuit element substrate.
  • the adhesive layer comprises a non-permanent adhesive, and the circuit elements and/or the portions of the circuit element substrate and the transparent cover layer (and any other layers formed therebetween) are released from the first support substrate by releasing the adhesive bond.
  • the step of providing the first support substrate having the circuit elements may further comprise, before removing the portions of the circuit element substrate and the transparent cover layer: (a) applying a structural film over the circuit element substrate, the structural film comprising a structural layer and a releasable layer arranged over the structural layer; (b) forming holes in the structural film and the circuit element substrate to expose the contact pads and filling the holes with a curable conductive paste; and (c) removing the releasable layer from the structural layer of the structural film, and wherein portions of the structural layer overlying the circuit element substrate are removed at the same time as the portions of the circuit element substrate and the transparent cover layer are removed to leave the circuit elements having the contact pads.
  • the second substrate may be substantially elastic, in which case it may comprise a polyurethane film having a low hardness (for example, Shore A 85 or less).
  • the step of providing the second substrate having the conductive tracks comprises: (a) forming metallic tracks on a surface of a first film layer, ends of the conductive tracks having the conductive contact pads; (b) applying a second film layer over the metallic tracks, the contact pads and exposed areas of the surface of the first film layer; and (c) forming holes in at least one of the first and second film layers to expose the contact pads.
  • the metallic tracks may comprise copper.
  • Each of the tracks may take a meandering path between the contact pads at its ends.
  • the tracks may define alternating horseshoe shapes, since tracks having this shape have been found to be highly extensible.
  • the invention also provides an electronic device obtained using the method described above.
  • a sheet-like electronic device which is bendable and stretchable, the device comprising: (a) a bendable and stretchable substrate having arranged thereon a plurality of conductive tracks, ends of the conductive tracks having conductive contact pads; and (b) a plurality of spaced apart electronic circuit elements attached to the substrate, each circuit element having at least one conductive contact pad, (c) wherein the circuit elements are separated from the substrate by a structural layer, and wherein the contact pads of the conductive tracks are electrically connected to the contact pads of the circuit elements through holes formed in the structural layer.
  • the device may be one capable of supporting a radius of curvature of 500 mm or less, preferably 300 mm or less, and most preferably 100 mm or less.
  • the device may be capable of 5 % stretch or more, preferably 10 % stretch or more, and most preferably 20 % stretch or more.
  • the electronic circuit elements may be spaced apart in two orthogonal directions.
  • a ratio of the width of the spaces to the width of the circuit elements may have any positive value. In general, a higher ratio may provide a device having a greater degree of stretchability.
  • the ratio may, for example, be at least 1.0 or at least 2.0.
  • Figs. Ia and Ib are schematic drawings showing a known stretchable electronic device
  • Fig. 2 shows the conductive tracks of the device shown in Figs. Ia and Ib;
  • Fig. 3 is a flow chart showing a first general method of manufacturing an electronic device according to the invention.
  • Fig. 4 shows a first substrate having electronic circuit elements for use in the first general method of manufacturing an electronic device according to the invention
  • Figs. 5a to 5d show processing steps for the first substrate shown in Fig. 4;
  • Fig. 6 shows a second substrate having conductive tracks for use in the first general method according to the invention
  • Fig. 7 shows the combining of the first and second substrates according to the first general method according to the invention
  • Figs. 8a and 8b show the electronic device manufactured by the first general method according to the invention
  • Figs. 9a to 9f show the processing steps of a first specific method according to the invention.
  • Figs. 10a to 10c illustrate a variation of the first specific method according to the invention.
  • Figs. 1 Ia to 1 Ig show the processing steps of a second specific method according to the invention.
  • Fig. 12 illustrates a variation of the second specific method according to the invention.
  • the invention provides a method for manufacturing a sheet-like electronic device which is bendable and stretchable and can therefore be made to conform to surfaces having curvature in at least two different planes, such as a spherical surface.
  • the method in a general sense, is represented as a flow chart in Fig. 3.
  • a first support substrate is provided.
  • the first substrate has arranged thereon a plurality of spaced apart electronic circuit elements, each circuit element having at least one conductive contact pad facing away from the substrate.
  • the circuit elements each constitute a part of a complete electronic circuit.
  • a second substrate which is bendable and stretchable is provided.
  • the second substrate has arranged thereon a plurality of conductive tracks for interconnecting the circuit elements. Ends of the conductive tracks are provided with conductive contact pads.
  • a third step 5 of the method the first and second substrates are brought together such that the contact pads of the circuit elements face the contact pads of the conductive tracks.
  • the circuit elements are then attached to the second substrate such that the contact pads of the circuit elements are electrically connected to the contact pads of the conductive tracks.
  • a fourth step 7 of the method the circuit elements are released from the first support substrate, leaving them attached to the second substrate. Since the second substrate is bendable and stretchable, the resulting device can be made to conform to different curved surfaces.
  • Fig. 4 shows the first substrate 11 having the electronic circuit elements in a part-finished state.
  • Figs. 5a to 5d illustrate subsequent processing steps for the first substrate 11.
  • Fig. 6 shows the second substrate 51 having the conductive tracks 53.
  • Fig. 7 shows the combining of the first and second substrates 11, 51.
  • Figs. 8a and 8b show the finished electronic device 101 manufactured by the method.
  • the first substrate 11 having the electronic circuit elements is manufactured using a variation on the so-called EPLaR (electronics on plastic by laser release) process.
  • EPLaR electronics on plastic by laser release
  • the EPLaR process is described in greater detail in WO 2005/050754, but a brief description of the process will be provided herein for completeness.
  • the EPLaR process is a technique whereby a substrate arrangement is manufactured comprising a rigid support substrate and a flexible plastic substrate over the rigid support substrate. According to the normal EPLaR process, the flexible plastic substrate is released from the rigid support substrate after thin film circuitry has been formed over the plastic substrate. This enables substantially conventional substrate handling and processing to be employed, while at the same time providing a sheet-like electronic device which has some bendability (but very limited stretchability).
  • a polymer layer in the form of polyimide is spin coated over the surface of a first support substrate 11 to define a substrate 13 for the circuit elements.
  • the first support substrate 11 is formed of glass to be rigid.
  • the polyimide is typically applied in a thickness of 3 to 30 ⁇ m and has a low coefficient of thermal expansion.
  • Thin film circuitry 15 is formed over the circuit element substrate 13 using conventional techniques.
  • the thin film circuitry 15 may comprise a TFT display array or other semiconductor device.
  • Contact pads 17 formed of copper are also deposited over the circuit element substrate 13 to face away from the first support substrate 11. The contact pads 17 are patterned using known techniques and are electrically connected to the thin film circuitry 15.
  • the thin film circuitry 15 and contact pads 17 are formed over the substrate 13 in a plurality of spaced apart areas.
  • the areas are typically 1 to 10 mm wide, although other sizes are possible.
  • the spaced apart areas define a regular array of orthogonal rows and columns, although it should be noted that other arrangements are possible.
  • the spacings between the areas may be such that a ratio between the width of the spaces and the width of the circuit elements may be at least 1.0 and preferably at least 2.0.
  • circuit element substrate 13 thin film circuitry 15 and contact pads 17 are represented as a single layer referred to hereinafter as the circuit element layer 19.
  • the arrangement shown in Fig. 5a corresponds to that shown in Fig. 4 and represents the starting point for the further processes shown in Figs. 5a to 5d.
  • Figs. 5b and 5c which are sectional and plan views of the substrate arrangement, respectively, the spaced apart areas 21 in which the thin film circuitry 15 and contact pads 17 have been formed are cut out by dicing or a laser, for example a CO 2 laser.
  • the cuts 23 may follow the circumference of the spaced apart areas 21 only, or for practical reasons the cuts 23 may be arranged to run the entire length or width of the substrate arrangement, as shown in the Figures. In either case, the cuts 23 delimit the spaced apart areas 21 from portions of the circuit element layer 25 which are arranged between the spaced apart areas 21.
  • the portions of the circuit element layer 25 which are arranged between the spaced apart areas 21 are removed from the first support substrate 11.
  • the rear surface of the polyimide circuit element substrate 13 is exposed to a laser that can pass through the glass of the first support substrate 11, but which is strongly absorbed in polyimide.
  • the laser therefore typically emits with a wavelength in the range 300 to 410nm.
  • the laser is strongly absorbed within a very thin layer of the polyimide circuit element substrate 13, which is ablated. This leaves a very thin layer of polyimide on the first support substrate 11 and releases most of the circuit element layer 19.
  • the spaced apart areas 21 of the circuit element layer 19 are not exposed to the laser, for example by being masked, and therefore remain attached to the first support substrate 11.
  • the first support substrate 11 shown in Fig. 5d is suitable for use in the first step 1 of the method of the invention shown in Fig. 3.
  • the spaced apart areas 21 are the circuit elements.
  • Fig. 6 shows the second substrate 51 having the conductive tracks 53, for use in the second step 3 of the method shown in Fig. 3.
  • the second substrate 51 is prepared separately from the first support substrate 11 described above.
  • the second substrate 51 is a flexible (bendable and substantially elastically stretchable) substrate formed of a polyurethane (PU) material a low hardness (Shore hardness of A85 or less). As shown in Fig. 6, a thin layer of copper is formed over a stretchable layer of the polyurethane material which defines the second substrate 51. The copper layer is patterned to form the conductive tracks 53, ends of which terminate in enlarged contact pads 55. The copper layer may be patterned using conventional techniques known to those skilled in the art. The second substrate typically has a thickness of lOO ⁇ m.
  • the conductive tracks 53 are arranged to take a meandering path between the contact pads 55 and define alternating horseshoe shapes, similar to those illustrated in Fig. 2.
  • the conductive tracks 53 and contact pads 55 are also arranged such that, when first and second substrates 11, 51 are brought together and the facing contact pads 17, 55 are electrically connected, the conductive tracks 53 serve to interconnect the circuit elements 21 of the first substrate 11 to thereby make a spatially distributed electrical circuit.
  • An additional thin layer 57 of the stretchable polyurethane material is formed over the conductive tracks 53, the contact pads 55 and exposed areas of the second substrate 51 to provide vertical electrical isolation and to prevent corrosion.
  • the additional polyurethane layer 57 is typically formed to a thickness of around 25 ⁇ m.
  • Holes 59 are formed in the additional polyurethane layer 57 at the positions of the contact pads 55 to enable electrical connections to be made.
  • the holes 59 may be formed using a CO 2 laser, which is able to form the holes 59 without penetrating the metallic contact pads 55.
  • first and second substrates 11, 51 are brought together in a facing relationship in the third step 5 of the method shown in Fig. 3. This step is illustrated in more detail in Fig. 7. As shown in Fig. 7, the substrates 11, 51 are brought together so that the circuit elements 21 attached to the first support substrate 11 face the exposed contact pads 55 of the second (flexible) substrate 51. The substrates 11, 51 are also spatially aligned such that the contact pads 17 of the circuit elements 21 are in registration with the contact pads 55 of the second substrate 51.
  • the spaced apart circuit elements 21 are then attached to the second substrate 51, for example by adhesion, and the facing contact pads 17, 55 are electrically connected such that the circuit elements 21 are interconnected by the conductive tracks 53 of the second substrate 51 to form a spatially distributed electrical circuit.
  • the spaced apart circuit elements 21 are released from the first support substrate 11 to leave a sheet-like electronic device which is bendable and stretchable.
  • the circuit elements 21 are released by directing a laser at the polyimide of the circuit element substrate 13, using essentially the same technique which is described above for releasing the portions of the circuit element substrate 13 arranged between the circuit elements 21.
  • the polyurethane material of the second substrate 51 may be selected to be transparent to the wavelengths output by the laser.
  • Figs. 8a and 8b are sectional and plan views, respectively.
  • the circuit elements 21 of the device 101 are bendable to some degree because of use of the modified EPLaR process for their manufacture. However, the circuit elements 21 themselves are not capable of being stretched to any useful degree. Instead, stretchability is provided by the second substrate 51 to which the circuit elements 21 are bonded.
  • the substantially elastic nature of the second substrate 51 enables the flat, sheet-like device 101 to confirm to surfaces which have curvature in at least two different planes, such as spherical surfaces.
  • the meandering conductive tracks 55 formed on the second substrate 51 are capable of being stretched (and contracted) with the second substrate 51 without suffering significant degradation.
  • hpi and ⁇ pi are the thickness of the circuit element substrate 13 (formed of polyimide) and the stress across the circuit element substrate 13, respectively
  • hpu and ⁇ pu are the total thickness of the second substrate 51 (formed of polyurethane) and the stress across the second substrate 51, respectively.
  • the mechanical effects on the circuit element 21 of the thin film circuitry 15 and the attached portion of the second substrate 51 are relatively small and have therefore been neglected in equation (1).
  • ⁇ pi is the strain across the circuit element substrate 13 (formed of polyimide) and Epj is the Young's modulus of the material of the circuit elements substrate 13.
  • Equations (1) and (2) can be combined into the following equation:
  • the polyurethane material of the second substrate has a Shore hardness of A85 or less. Provided this material is stretched by no more than 50 %, it can be demonstrated that stress in the material remains below 10 MPa. The Young's modulus for this material is 8.5 GPa. Substituting these values, together with the above-mentioned failure limit of 1 %, into equation (3) gives the following requirement:
  • the circuit element substrate 13 of each circuit element 21 must have a thickness of at least 12 ⁇ m, which is technically feasible.
  • Figs. 9a to 9f show the processing steps of a first specific method according to the invention.
  • the first specific method is similar to the method described above, except that the first support substrate 11 is processed further before it is combined with the second (flexible) substrate 51. Only the additional processing steps will be described in detail herein.
  • Like reference numerals are used to refer to like elements in the drawings.
  • a structural film 27 is applied to the first support substrate 11 described above with reference to Fig. 4, prior to carrying out the subsequent processing steps illustrated in 5 a to 5 d.
  • the structural film 27 comprises a structural layer 29 of semi-cured epoxy resin in contact with the circuit elements 21 and exposed areas of the circuit element substrate 13 and a releasable layer 31 of silicone coated PET film arranged in contact with the structural layer 29.
  • the semi-cured resin of the structural layer 29 is sufficiently cured to retain its own shape while at the same time being mouldable under pressure and sticky enough to ensure reliable adhesion to adjacent layers (other than the releasable layer 31).
  • holes 33 are formed through the structural film 27 to expose the contact pads 17 of the circuit elements 21.
  • the holes 33 may be formed using a CO 2 laser, which is able to penetrate the structural film 27 but does not penetrate the metallic contact pads 17.
  • the holes 33 are then filled with a curable conductive paste 35 of the type conventionally used for via filling (or similar).
  • the holes 33 are filled with the paste 35 using a squeegee 37, as shown in Fig. 9c.
  • the first support substrate 11 having the filled holes 33 is then processed to cut out and remove portions of the circuit element substrate 13 arranged between the circuit elements 21, as described above with reference to Figs. 5a to 5d.
  • the laser release process not only removes portions of the circuit element substrate 13 but also removes overlying portions of the structural film 27.
  • the releasable layer 31 is removed from the structural layer 29 overlying the circuit elements 21, as shown in Fig. 9d.
  • the portions of conductive paste 35 stand proud in the resulting arrangement.
  • Fig. 9d also shows the step of bringing the first support substrate 11 and the second substrate 51 together in preparation for attaching the circuit elements 21 to the second substrate 51 and electrically connecting the contact pads 17, 55 of the substrates 11, 51.
  • the substrates 11, 51 are aligned such that the portions of conductive paste 35 are in registration with the holes 59 in the second substrate 51 through which the contact pads 55 are exposed.
  • the circuit elements 21 are attached to the second substrate 51 by pressing the first and second substrates 11, 51 together and heating. As shown in Fig. 9e, the conductive paste 35 is compressed between the substrates 11, 51 and displaces the moldable structural layer 29 formed of part-cured resin in a lateral direction. The structural layer 29 cures to thereby bond the circuit elements 21 to the second substrate 51. The conductive paste 35 cures to provide a reliable electrical connection between the contact pads 17, 55 of the substrates 11, 51. The curing temperature is below the melting temperature of the polyurethane material of the second substrate 51.
  • the circuit elements 21 are laser released from the first support substrate 11 to leave the finished sheet-like electronic device 101, which is bendable and stretchable.
  • the lateral displacement of the structural layer 29 causes some of the structural layer 29 to spread beyond the edge of the circuit element 21 and contact the first support substrate 11.
  • the cured structural layer 29 may then become adhered to the first support substrate 11, making release of the circuit elements 21 from the first support substrate 11 difficult.
  • This problem may be avoided by selecting a resin material for the structural layer 29 which can be laser released at the same time as the circuit element substrate 13 formed of polyimide.
  • the method may be modified as illustrated in Figs. 10a to 10c to prevent the structural layer 29 contacting the first support substrate 11.
  • the method is modified so that the structural layer 29 is removed from around the circumference 39 of the circuit elements 21.
  • the structural layer 29 is laterally displaced by the conductive paste 35, it does not spread beyond the edges of the circuit elements 21.
  • Fig. 10b shows the position of the structural layer 29 after it has cured.
  • the structural layer 29 may be removed from around the circumference 39 of the circuit elements 21 using a CO 2 laser, for example at the same time that portions of the circuit element substrate 13 are cut out from between the circuit elements 21.
  • a metallic band 41 may be formed around the circumference of each circuit element 21, for example at the same time as the contact pads 17 are formed, to prevent the CO 2 laser penetrating the circuit element substrate 13 of the circuit elements 21 when the structural layer 29 is removed.
  • a suitable metallic band 41 is shown in Fig. 10c, which is a plan view of the first support substrate 11 having the circuit elements 21.
  • Figs. 1 Ia to 1 Ig show the processing steps of a second specific method according to the invention.
  • the second specific method is similar to the first specific method described above, except that the first support substrate 11 is processed differently so that the circuit element substrate 13 does not face outwardly in the finished sheet-like electronic device 101.
  • This may be advantageous for devices which emit or sense light, such as display devices, since the polyimide material of the circuit element substrate 13 is typically yellow in color and not fully transparent. Only the modified processing steps will be described in detail herein.
  • Like reference numerals are used to refer to like elements in the drawings.
  • Fig. 11a shows a support substrate 43 which is substantially the same as the first support substrate 11 described above with reference to Fig. 4, prior to carrying out the subsequent processing steps illustrated in 5a to 5d.
  • the support substrate 43 shown in Fig. 11a differs from the substrate 11 shown in Fig. 4 only in that the contact pads 17 are formed underneath the thin film circuitry 15 so that they face towards the substrate 43 and are adjacent to the circuit element substrate 13.
  • a transparent cover layer 45 is also formed over the thin film circuitry 15 and the exposed portions of the circuit element substrate 13.
  • the support substrate 43 shown in Fig. 1 Ia is an auxiliary (temporary) support substrate which, like the first support substrate 11 described above, is rigid and formed of glass. Substantially conventional substrate handling and processing techniques may therefore be employed.
  • the first support substrate 11 is attached to the transparent cover layer 45 with a (non-permanent) adhesive film 47, as shown in Fig. 1 Ib.
  • the whole of the substrate arrangement is then released from the auxiliary support substrate 43 using the EPLaR laser release process described above and the substrate arrangement is inverted, as shown in Fig. l ie.
  • Fig. l ie also shows the application of a structural film 27 to the circuit element substrate 13.
  • the structural film 27 is the same as that described above with reference to Fig. 9a and comprises a structural layer 29 of semi-cured epoxy resin in contact with the circuit element substrate 13 now facing outwards and a releasable layer 31 of silicone coated PET film arranged in contact with the structural layer 29.
  • the semi-cured resin of the structural layer 29 is sufficiently cured to retain its own shape while at the same time being moldable under pressure and sticky enough to ensure reliable adhesion to adjacent layers (other than the releasable layer 31).
  • holes 33 are formed through the structural film 27 and the circuit element substrate 13 to expose the contact pads 17 of the circuit elements 21.
  • the holes 33 may be formed using a CO 2 laser, which is able to penetrate the structural film 27 and the circuit element substrate 13 but does not penetrate the metallic contact pads 17.
  • the holes 33 are then filled with a curable conductive paste 35 of the type conventionally used for via filling (or similar).
  • the holes 33 are filled with the paste 35 using a squeegee 37, as shown in Fig. l ie.
  • the first support substrate 11 having the filled holes 33 is then processed to cut out and remove portions of the circuit element substrate 13 arranged between the circuit elements 21, as described above with reference to Figs. 5a to 5d.
  • the laser release process not only removes portions of the circuit element substrate 13 but also removes the other layers overlying the first support substrate 11.
  • the releasable layer 31 is removed from the structural layer 29 overlying the circuit elements 21, as shown in Fig. 1 If.
  • the portions of conductive paste 35 stand proud in the resulting arrangement.
  • Fig. 1 If also shows the step of bringing the first support substrate 11 and the second substrate 51 together in preparation for attaching the circuit elements 21 to the second substrate 51 and electrically connecting the contact pads 17, 55 of the substrates 11, 51.
  • the substrates 11, 51 are aligned such that the portions of conductive paste 35 are in registration with the holes 59 in the second substrate 51 through which the contact pads 55 are exposed.
  • the circuit elements 21 are attached to the second substrate 51 by pressing the first and second substrates 11, 51 together and heating.
  • the conductive paste 35 is compressed between the substrates 11, 51 and displaces the mouldable structural layer 29 formed of part-cured resin in a lateral direction.
  • the structural layer 29 cures to thereby bond the circuit elements 21 to the second substrate 51.
  • the conductive paste 35 cures to provide a reliable electrical connection between the contact pads 17, 55 of the substrates 11, 51.
  • the curing temperature is below the melting temperature of the polyurethane material of the second substrate 51.
  • the circuit elements 21 are released from the first support substrate 11 by releasing the adhesive layer 47 to leave the finished sheet-like electronic device 101, which is bendable and stretchable.
  • the lateral displacement of the structural layer 29 causes some of the structural layer 29 to spread, which can make release of the circuit elements 21 from the first support substrate 11 difficult.
  • this problem may be avoided by selecting a resin material for the structural layer 29 which can be laser released at the same time as the circuit element substrate 13 formed of polyimide.
  • the method may be modified as illustrated in Fig. 12 to prevent the structural layer 29 contacting the first support substrate 11.
  • the method is modified so that the structural layer 29 and the circuit element substrate 13 are removed from around the circumference 39 of the circuit elements 21. In this way, when the structural layer 29 is laterally displaced by the conductive paste 35, it does not spread beyond the edges of the circuit elements 21.
  • the structural layer 29 may be removed from around the circumference 39 of the circuit elements 21 using a CO 2 laser, for example at the same time that portions of the circuit element substrate 13 are cut out from between the circuit elements 21.
  • a metallic band 41 may be formed around the circumference of each circuit element 21, for example at the same time as the contact pads 17 are formed, to prevent the CO 2 laser penetrating beyond the circuit element substrate 13 of the circuit elements 21 when the structural layer 29 is removed.
  • circuit elements described above are prepared using the EPLaR (electronics on plastic by laser release) process, but circuit elements prepared by other processes are suitable. Wholly rigid circuit elements may be used.
  • the second substrate may be formed of materials other than polyurethane. For example, synthetic rubber materials may be used.

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Abstract

The invention relates to a method for manufacturing a sheet-like electronic device which is bendable and stretchable. A first support substrate (11) is provided having arranged thereon a plurality of spaced apart (17) electronic circuit elements, each circuit element having at least one conductive contact pad facing away from the substrate (11). A second substrate (51) is also provided having arranged thereon a plurality of conductive tracks (53), ends of the conductive tracks having conductive contact pads (55). The first and second substrates (11, 51) are brought together such that the contact pads of the circuit elements face the contact pads (17) of the conductive tracks. The circuit elements (21) are attached to the second substrate such that the contact pads (55) of the circuit elements are electrically connected to the contact pads of the conductive tracks (53). The circuit elements (21) are then released from the first support substrate (11). The second substrate (51) of the device, to which the circuit elements (21) are attached, is bendable and stretchable, and this allows the device to conform to different curved surfaces.

Description

Conformable electronic devices and methods for their manufacture
FIELD OF THE INVENTION
This invention relates to conformable electronic devices, that is to say electronic devices which can be physically deformed to conform to different surfaces without experiencing short-term physical or functional degradation. In particular, the invention relates to such devices which are formed as sheets and are capable of both bending and stretching, such that the devices can be made to conform to surfaces having curvature in at least two different planes.
The invention also relates to a method of manufacturing conformable electronic devices which are capable of bending and stretching.
BACKGROUND OF THE INVENTION
There is an increasing market demand for sheet-like electronic devices which can be made to conform to different surfaces. One application for such technology is electronic devices which are worn on the human or animal body. For example, it is advantageous that electronic devices which sense activity or monitor the human body during physical exercise are held as close as possible to the body by conforming to the surface of the wearer's skin. Maintaining conformity in this application is complicated by the fact that the curvature of the skin's surface tends to vary during exercise.
Other applications for wearable electronic devices include devices which are intended to stimulate the healing of wounds by exposure to electric fields or light. More diverse applications for conformable electronic devices in general relate to display devices and sensors which can be manufactured as flat sheets and then applied to curved surfaces.
The simplest form of conformability for sheet-like electronic devices is bendability. It is already known to provide electronic devices which are capable of bending. Such devices are typically realized by preparing a thin electronically active layer on a flexible substrate such that the active layer coincides with the neutral mechanical plane of the device. In this case, the mechanical strain experienced by the relatively brittle active layer can be maintained below a failure limit, which is typically of the order of 1 %. Bendability enables sheet-like electronic devices to conform to surfaces having simple curvature in one plane, such as cylindrical surfaces. However, to enable such devices to conform to surfaces having curvature in multiple different planes, such as spherical surfaces or the surface of the skin of the human or animal body, a degree of stretchability is also required. To be of any practical value in this regard, the stretchability of a device must be significantly greater than that which is available from the known bendable devices described above (which can provide approximately 1 % stretch).
It has been proposed to provide a stretchable sheet-like electronic device by bonding a bendable electronically active layer to a pre-stretched flexible elastic substrate. When the device is allowed to relax, it partially contracts and a series of wrinkles are formed in the active layer as it buckles under the compressive influence of the elastic substrate. Subsequent stretching of the device is accommodated by allowing the wrinkles of the active layer to straighten out.
Although this device is more stretchable than the bendable devices described above, stretchability is still limited by the need to minimize strain in the active layer as it buckles (bends) under the compressive influence of the elastic substrate. In practice, it has been found that the available stretch is limited to around 10 %, which restricts the ability of the device to conform to deeply curved surfaces. Another problem associated with the device is that the wrinkled surface of the active layer may be inappropriate for some applications.
It has also been proposed to provide a stretchable sheet-like electronic device by attaching a plurality of rigid electronic circuit elements, such as integrated circuits, to a flexible elastic substrate having extensible conductive tracks for connecting the circuit elements together. The circuit elements are spaced apart on the elastic substrate and effectively define discrete "islands" which are interconnected by the conductive tracks.
The stretching of this device is shown schematically in Figs. Ia and Ib. Fig. Ia shows the arrangement of the islands and the conductive tracks of the device in the flat, undeformed state. Fig. Ib shows the arrangement of the islands and the conductive tracks when the device is applied to a spherical surface. It will be noted that the stretching of the device is provided by extension of the areas of the flexible substrate between the rigid islands. The conductive tracks are shown in greater detail in Fig. 2. The tracks are typically formed of plated copper and follow a meandering path between the islands.
Although the device shown in the Figs, is capable of greater stretchability, for example up to 50 %, the known method for its manufacture is inefficient and therefore costly. In particular, the device is manufactured by electrically and mechanically attaching the circuit elements, in turn (sequentially), to the conductive tracks of the flexible substrate, typically by soldering.
SUMMARY OF THE INVENTION
According to an aspect of the invention, there is provided a method for manufacturing a sheet-like electronic device which is bendable and stretchable, the method comprising: (a) providing a first support substrate having arranged thereon a plurality of spaced apart electronic circuit elements, each circuit element having at least one conductive contact pad facing away from the substrate; (b) providing a second substrate having arranged thereon a plurality of conductive tracks, ends of the conductive tracks having conductive contact pads; (c) bringing the first and second substrates together such that the contact pads of the circuit elements face the contact pads of the conductive tracks and attaching the circuit elements to the second substrate such that the contact pads of the circuit elements are electrically connected to the contact pads of the conductive tracks; and (d) releasing the circuit elements from the first support substrate, wherein the second substrate of the device, to which the circuit elements are attached, is bendable and stretchable.
The invention thus provides a method by which highly conformable electronic devices may be efficiently manufactured. In particular, by providing a plurality of electronic circuit elements on a first support substrate, and then attaching the circuit elements to the second substrate before they are released from the first substrate, the costs associated with the prior art technique of individually manipulating and attaching the circuit elements one at a time may be avoided. Various techniques may be used for releasing the circuit elements from the first support substrate, including so-called laser release processes.
The terms "bendable" and stretchable" have well defined meanings which will be known to those skilled in the art. However, a bendable substrate may be one capable of supporting a radius of curvature of 500 mm or less, preferably 300 mm or less, and most preferably 100mm or less. A stretchable substrate may be one capable of 5 % stretch or more, preferably 10 % stretch or more, and most preferably 20 % stretch or more.
The circuit elements may comprise one or more components of an electronic circuit. The circuit elements may, for example, comprise sensors, display devices, and other types of semiconductor devices. Individual circuit elements may constitute complete circuits, portions of complete circuits or individual components of complete circuits. The conductive tracks of the second substrate serve as interconnects between the circuit elements. The first substrate may be a rigid substrate, such as glass. Rigid substrates can in general be processed more easily and with fewer modifications to standard manufacturing equipment. The second substrate is itself bendable and stretchable, but may be attached to a further rigid support substrate for processing efficacy. In the latter case, the second substrate is released from the further rigid support substrate after the circuit elements have been attached to the second substrate, typically at the end of the manufacturing method.
The electronic circuit elements may themselves be flexible. For example, the circuit elements may be bendable to some degree, but are not generally stretchable. In this case, the step of providing the first support substrate having the circuit elements may comprise: (a) applying a polymer coating, such as polyimide, over the first support substrate to define a circuit element substrate; (b) forming thin film electronic circuitry and conductive contact pads over the circuit element substrate, the thin film circuitry and the contact pads defining a plurality of spaced apart areas; and (c) removing portions of the circuit element substrate from between the spaced apart areas to thereby leave the circuit elements having the contact pads.
During the step of forming the thin film electronic circuitry and the conductive contact pads, the spaced apart areas may be defined by the edges of the circuitry and/or the contact pads, in which case the subsequently removed portions of the circuit element substrate will generally remain exposed until they are removed. However, it is also intended that the spaced apart areas may be arbitrarily defined, in which case the subsequently removed portions of the circuit element substrate may be covered with one or more layers which remain from the forming of the circuitry and/or the contact pads.
The portions of the circuit element substrate which are removed may be cut out by dicing or laser cutting and may be released from the first support substrate by a laser release process. In embodiments where these portions of the circuit element substrate are covered with other layers, the circuit element substrate and these layers may be released together,
A laser release process may also be used to release the circuit elements from the first support substrate at the end of the manufacturing method.
Where the circuit element substrate is formed of polyimide, a suitable laser release process comprises directing ultraviolet laser light at the circuit element substrate through the first support substrate. A specific process of this type is the electronics on plastic by laser release (EPLaR) process described in WO 2005/050754, the entire disclosure of which is incorporated herein by reference. In the electronic device, the contact pads of the circuit elements may be electrically connected to the contact pads of the conductive tracks through holes formed in a structural layer. In this case, the step of providing the first support substrate having the circuit elements may further comprise, before removing the portions of the circuit element substrate: (a) applying a structural film over the thin film circuitry, contact pads and the circuit element substrate, the structural film comprising a structural layer and a releasable layer arranged over the structural layer; (b) forming holes in the structural film to expose the contact pads and filling the holes with a curable conductive paste; and (c) removing the releasable layer from the structural layer of the structural film.
Portions of the structural film overlying the circuit element substrate are then removed at the same time that portions of the circuit element substrate are removed to leave the spaced apart circuit elements having the contact pads.
The structural layer of the structural film may comprise a part-cured thermosetting resin and the releasable layer of the structural film may comprise a silicone- coated polymer such as polyethylene terephthalate (PET). The part-cured resin should be capable of retaining its shape, but at the same time be moldable and sticky enough to adhere to other surfaces.
Attaching the circuit elements to the second substrate may comprise applying heat and/or pressure to the curable conductive paste and the part-cured thermosetting resin to thereby cure the paste and the resin. The cured conductive paste electrically connects the contact pads of the circuit elements to the contact pads of the conductive tracks.
The above-described method is suitable for the manufacture of devices in which it is acceptable for the circuit element substrate to face away from the second (stretchable) substrate. In certain types of device, such as devices having displays, it may be more appropriate for the circuit element substrate to face towards the second substrate. In this case, the step of providing the first support substrate having the circuit elements needs to be modified and may comprise: (a) applying a polymer coating, such as polyimide, over an auxiliary support substrate to define a circuit element substrate; (b) forming thin film electronic circuitry and conductive contact pads over the circuit element substrate, the thin film circuitry and the contact pads defining a plurality of spaced apart areas; (c) forming a transparent cover layer over the thin film circuitry, contact pads and exposed areas of the circuit element substrate; (d) attaching the first support substrate to the transparent cover layer with an adhesive layer; (e) releasing the circuit element substrate from the auxiliary support substrate; and (f) removing portions of the circuit element substrate and the transparent cover layer from between the spaced apart areas to thereby leave the circuit elements having the contact pads.
During the step of forming the thin film electronic circuitry and the conductive contact pads, the spaced apart areas may be defined by the edges of the circuitry and/or the contact pads, in which case the subsequently removed portions of the circuit element substrate will generally remain exposed until they are removed. However, it is also intended that the spaced apart areas may be arbitrarily defined, in which case the subsequently removed portions of the circuit element substrate may be covered with one or more layers which remain from the forming of the circuitry and/or the contact pads.
The thin film electronic circuitry and conductive contact pads may be formed such that the contact pads face towards, and are in contact with, the circuit element substrate.
The circuit element substrate may be released from the auxiliary support substrate by directing ultraviolet laser light through the auxiliary support substrate at the circuit element substrate. The adhesive layer comprises a non-permanent adhesive, and the circuit elements and/or the portions of the circuit element substrate and the transparent cover layer (and any other layers formed therebetween) are released from the first support substrate by releasing the adhesive bond.
Further, the step of providing the first support substrate having the circuit elements may further comprise, before removing the portions of the circuit element substrate and the transparent cover layer: (a) applying a structural film over the circuit element substrate, the structural film comprising a structural layer and a releasable layer arranged over the structural layer; (b) forming holes in the structural film and the circuit element substrate to expose the contact pads and filling the holes with a curable conductive paste; and (c) removing the releasable layer from the structural layer of the structural film, and wherein portions of the structural layer overlying the circuit element substrate are removed at the same time as the portions of the circuit element substrate and the transparent cover layer are removed to leave the circuit elements having the contact pads.
The second substrate may be substantially elastic, in which case it may comprise a polyurethane film having a low hardness (for example, Shore A 85 or less). In a specific embodiment, the step of providing the second substrate having the conductive tracks comprises: (a) forming metallic tracks on a surface of a first film layer, ends of the conductive tracks having the conductive contact pads; (b) applying a second film layer over the metallic tracks, the contact pads and exposed areas of the surface of the first film layer; and (c) forming holes in at least one of the first and second film layers to expose the contact pads.
The metallic tracks may comprise copper. Each of the tracks may take a meandering path between the contact pads at its ends. For example, the tracks may define alternating horseshoe shapes, since tracks having this shape have been found to be highly extensible.
The invention also provides an electronic device obtained using the method described above.
According to another aspect of the invention, there is provided a sheet-like electronic device which is bendable and stretchable, the device comprising: (a) a bendable and stretchable substrate having arranged thereon a plurality of conductive tracks, ends of the conductive tracks having conductive contact pads; and (b) a plurality of spaced apart electronic circuit elements attached to the substrate, each circuit element having at least one conductive contact pad, (c) wherein the circuit elements are separated from the substrate by a structural layer, and wherein the contact pads of the conductive tracks are electrically connected to the contact pads of the circuit elements through holes formed in the structural layer.
It has been found that electrically connecting the contact pads of the conductive tracks to the contact pads of the circuit elements through holes formed in a structural layer is a particularly effective and reliable technique which may lead to manufacturing efficiencies.
The device may be one capable of supporting a radius of curvature of 500 mm or less, preferably 300 mm or less, and most preferably 100 mm or less. The device may be capable of 5 % stretch or more, preferably 10 % stretch or more, and most preferably 20 % stretch or more.
The electronic circuit elements may be spaced apart in two orthogonal directions. A ratio of the width of the spaces to the width of the circuit elements may have any positive value. In general, a higher ratio may provide a device having a greater degree of stretchability. The ratio may, for example, be at least 1.0 or at least 2.0.
Additional features and advantages of the invention will become apparent from the following detailed description of the invention. BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Figs. Ia and Ib are schematic drawings showing a known stretchable electronic device;
Fig. 2 shows the conductive tracks of the device shown in Figs. Ia and Ib;
Fig. 3 is a flow chart showing a first general method of manufacturing an electronic device according to the invention;
Fig. 4 shows a first substrate having electronic circuit elements for use in the first general method of manufacturing an electronic device according to the invention;
Figs. 5a to 5d show processing steps for the first substrate shown in Fig. 4;
Fig. 6 shows a second substrate having conductive tracks for use in the first general method according to the invention;
Fig. 7 shows the combining of the first and second substrates according to the first general method according to the invention;
Figs. 8a and 8b show the electronic device manufactured by the first general method according to the invention;
Figs. 9a to 9f show the processing steps of a first specific method according to the invention;
Figs. 10a to 10c illustrate a variation of the first specific method according to the invention; and
Figs. 1 Ia to 1 Ig show the processing steps of a second specific method according to the invention; and
Fig. 12 illustrates a variation of the second specific method according to the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The invention provides a method for manufacturing a sheet-like electronic device which is bendable and stretchable and can therefore be made to conform to surfaces having curvature in at least two different planes, such as a spherical surface. The method, in a general sense, is represented as a flow chart in Fig. 3.
With reference to the Figure, in a first step 1 of the method, a first support substrate is provided. The first substrate has arranged thereon a plurality of spaced apart electronic circuit elements, each circuit element having at least one conductive contact pad facing away from the substrate. The circuit elements each constitute a part of a complete electronic circuit.
In a second step 3 of the method, a second substrate which is bendable and stretchable is provided. The second substrate has arranged thereon a plurality of conductive tracks for interconnecting the circuit elements. Ends of the conductive tracks are provided with conductive contact pads.
In a third step 5 of the method, the first and second substrates are brought together such that the contact pads of the circuit elements face the contact pads of the conductive tracks. The circuit elements are then attached to the second substrate such that the contact pads of the circuit elements are electrically connected to the contact pads of the conductive tracks.
In a fourth step 7 of the method, the circuit elements are released from the first support substrate, leaving them attached to the second substrate. Since the second substrate is bendable and stretchable, the resulting device can be made to conform to different curved surfaces.
The method according to the invention will now be described in greater detail with reference to Figs. 4 to 8b. Fig. 4 shows the first substrate 11 having the electronic circuit elements in a part-finished state. Figs. 5a to 5d illustrate subsequent processing steps for the first substrate 11. Fig. 6 shows the second substrate 51 having the conductive tracks 53. Fig. 7 shows the combining of the first and second substrates 11, 51. Figs. 8a and 8b show the finished electronic device 101 manufactured by the method.
The first substrate 11 having the electronic circuit elements is manufactured using a variation on the so-called EPLaR (electronics on plastic by laser release) process. The EPLaR process is described in greater detail in WO 2005/050754, but a brief description of the process will be provided herein for completeness.
The EPLaR process is a technique whereby a substrate arrangement is manufactured comprising a rigid support substrate and a flexible plastic substrate over the rigid support substrate. According to the normal EPLaR process, the flexible plastic substrate is released from the rigid support substrate after thin film circuitry has been formed over the plastic substrate. This enables substantially conventional substrate handling and processing to be employed, while at the same time providing a sheet-like electronic device which has some bendability (but very limited stretchability).
With reference to Fig. 4, according to the invention, a polymer layer in the form of polyimide is spin coated over the surface of a first support substrate 11 to define a substrate 13 for the circuit elements. The first support substrate 11 is formed of glass to be rigid. The polyimide is typically applied in a thickness of 3 to 30 μm and has a low coefficient of thermal expansion.
Thin film circuitry 15 is formed over the circuit element substrate 13 using conventional techniques. For example, the thin film circuitry 15 may comprise a TFT display array or other semiconductor device. Contact pads 17 formed of copper are also deposited over the circuit element substrate 13 to face away from the first support substrate 11. The contact pads 17 are patterned using known techniques and are electrically connected to the thin film circuitry 15.
The thin film circuitry 15 and contact pads 17 are formed over the substrate 13 in a plurality of spaced apart areas. The areas are typically 1 to 10 mm wide, although other sizes are possible. The spaced apart areas define a regular array of orthogonal rows and columns, although it should be noted that other arrangements are possible. The spacings between the areas may be such that a ratio between the width of the spaces and the width of the circuit elements may be at least 1.0 and preferably at least 2.0.
The substrate arrangement described above is then processed further, as illustrated in Figs. 5a to 5 d. In these Figures, the circuit element substrate 13, thin film circuitry 15 and contact pads 17 are represented as a single layer referred to hereinafter as the circuit element layer 19. The arrangement shown in Fig. 5a corresponds to that shown in Fig. 4 and represents the starting point for the further processes shown in Figs. 5a to 5d.
In Figs. 5b and 5c, which are sectional and plan views of the substrate arrangement, respectively, the spaced apart areas 21 in which the thin film circuitry 15 and contact pads 17 have been formed are cut out by dicing or a laser, for example a CO2 laser. The cuts 23 may follow the circumference of the spaced apart areas 21 only, or for practical reasons the cuts 23 may be arranged to run the entire length or width of the substrate arrangement, as shown in the Figures. In either case, the cuts 23 delimit the spaced apart areas 21 from portions of the circuit element layer 25 which are arranged between the spaced apart areas 21.
In Fig. 5d, the portions of the circuit element layer 25 which are arranged between the spaced apart areas 21 are removed from the first support substrate 11. In particular, the rear surface of the polyimide circuit element substrate 13 is exposed to a laser that can pass through the glass of the first support substrate 11, but which is strongly absorbed in polyimide. The laser therefore typically emits with a wavelength in the range 300 to 410nm. The laser is strongly absorbed within a very thin layer of the polyimide circuit element substrate 13, which is ablated. This leaves a very thin layer of polyimide on the first support substrate 11 and releases most of the circuit element layer 19. The spaced apart areas 21 of the circuit element layer 19 are not exposed to the laser, for example by being masked, and therefore remain attached to the first support substrate 11.
The first support substrate 11 shown in Fig. 5d is suitable for use in the first step 1 of the method of the invention shown in Fig. 3. The spaced apart areas 21 are the circuit elements.
Fig. 6 shows the second substrate 51 having the conductive tracks 53, for use in the second step 3 of the method shown in Fig. 3. The second substrate 51 is prepared separately from the first support substrate 11 described above.
The second substrate 51 is a flexible (bendable and substantially elastically stretchable) substrate formed of a polyurethane (PU) material a low hardness (Shore hardness of A85 or less). As shown in Fig. 6, a thin layer of copper is formed over a stretchable layer of the polyurethane material which defines the second substrate 51. The copper layer is patterned to form the conductive tracks 53, ends of which terminate in enlarged contact pads 55. The copper layer may be patterned using conventional techniques known to those skilled in the art. The second substrate typically has a thickness of lOOμm.
The conductive tracks 53 are arranged to take a meandering path between the contact pads 55 and define alternating horseshoe shapes, similar to those illustrated in Fig. 2. The conductive tracks 53 and contact pads 55 are also arranged such that, when first and second substrates 11, 51 are brought together and the facing contact pads 17, 55 are electrically connected, the conductive tracks 53 serve to interconnect the circuit elements 21 of the first substrate 11 to thereby make a spatially distributed electrical circuit.
An additional thin layer 57 of the stretchable polyurethane material is formed over the conductive tracks 53, the contact pads 55 and exposed areas of the second substrate 51 to provide vertical electrical isolation and to prevent corrosion. The additional polyurethane layer 57 is typically formed to a thickness of around 25 μm. Holes 59 are formed in the additional polyurethane layer 57 at the positions of the contact pads 55 to enable electrical connections to be made. The holes 59 may be formed using a CO2 laser, which is able to form the holes 59 without penetrating the metallic contact pads 55.
Once the first and second substrates 11, 51 have been manufactured, as described above, they are brought together in a facing relationship in the third step 5 of the method shown in Fig. 3. This step is illustrated in more detail in Fig. 7. As shown in Fig. 7, the substrates 11, 51 are brought together so that the circuit elements 21 attached to the first support substrate 11 face the exposed contact pads 55 of the second (flexible) substrate 51. The substrates 11, 51 are also spatially aligned such that the contact pads 17 of the circuit elements 21 are in registration with the contact pads 55 of the second substrate 51.
The spaced apart circuit elements 21 are then attached to the second substrate 51, for example by adhesion, and the facing contact pads 17, 55 are electrically connected such that the circuit elements 21 are interconnected by the conductive tracks 53 of the second substrate 51 to form a spatially distributed electrical circuit.
In a final step 7 of the method shown in Fig. 3, the spaced apart circuit elements 21 are released from the first support substrate 11 to leave a sheet-like electronic device which is bendable and stretchable. The circuit elements 21 are released by directing a laser at the polyimide of the circuit element substrate 13, using essentially the same technique which is described above for releasing the portions of the circuit element substrate 13 arranged between the circuit elements 21.
To avoid any risk of damage to the second substrate 51 during the laser release process, the polyurethane material of the second substrate 51 may be selected to be transparent to the wavelengths output by the laser.
The resulting device 101 is illustrated in Figs. 8a and 8b, which are sectional and plan views, respectively.
The circuit elements 21 of the device 101 are bendable to some degree because of use of the modified EPLaR process for their manufacture. However, the circuit elements 21 themselves are not capable of being stretched to any useful degree. Instead, stretchability is provided by the second substrate 51 to which the circuit elements 21 are bonded. The substantially elastic nature of the second substrate 51 enables the flat, sheet-like device 101 to confirm to surfaces which have curvature in at least two different planes, such as spherical surfaces. Unlike the circuit elements 21, the meandering conductive tracks 55 formed on the second substrate 51 are capable of being stretched (and contracted) with the second substrate 51 without suffering significant degradation.
It is important in the step of attaching the circuit elements 21 to the second substrate 51 that a large proportion of the available surface of each circuit element 21 is positively attached, to thereby limit the stress on the thin film circuitry 15 and the electrical connections between the contact pads 17, 55. Attaching substantially the whole of the available surface of the circuit elements 21 to the second substrate 51 may also allow for convenient thicknesses to be used for the circuit element substrate 13 of the circuit elements 21 and for the second substrate 51, as will now be explained. When the sheet- like electronic device 101 is stretched, the strain in the circuit elements 21 must remain below a failure limit for the thin film circuitry, which is typically approximately 1 %. If we assume that the force across a circuit element 21 must be equal to the force across an adjacent area of second substrate 51, the following equation must be satisfied:
KP PI = KuϋPU (!)
where hpi and σpi are the thickness of the circuit element substrate 13 (formed of polyimide) and the stress across the circuit element substrate 13, respectively, and hpu and σpu are the total thickness of the second substrate 51 (formed of polyurethane) and the stress across the second substrate 51, respectively. The mechanical effects on the circuit element 21 of the thin film circuitry 15 and the attached portion of the second substrate 51 are relatively small and have therefore been neglected in equation (1).
Hooke's law applies to the material of the circuit element substrate 13, which provides the following relationship:
Cn = ZnEn (2)
where εpi is the strain across the circuit element substrate 13 (formed of polyimide) and Epj is the Young's modulus of the material of the circuit elements substrate 13.
Equations (1) and (2) can be combined into the following equation:
Figure imgf000014_0001
The polyurethane material of the second substrate has a Shore hardness of A85 or less. Provided this material is stretched by no more than 50 %, it can be demonstrated that stress in the material remains below 10 MPa. The Young's modulus for this material is 8.5 GPa. Substituting these values, together with the above-mentioned failure limit of 1 %, into equation (3) gives the following requirement:
hn > 0A2hpu (4) Thus, for a second substrate 51 having a typical thickness of lOOμm, the circuit element substrate 13 of each circuit element 21 must have a thickness of at least 12μm, which is technically feasible.
This above-described requirement assumes that the second substrate 51 is cut away in the areas which do not have circuit elements 21 or conductive tracks 55 (see Fig. 8b), since only then is the stress across each circuit element 21 transmitted to a portion of the second substrate 51 having the same width. In practice, the circuit elements 21 are spaced apart, and the stress across a circuit element 21 is therefore transmitted to a wider portion of the second substrate 51. For a spacing between the circuit elements 21 that is equal to the width of the circuit elements the thickness requirement for the circuit element substrate would be no more than double that which has been calculated above, which thickness remains technically feasible.
Specific methods according to the invention will now be described in greater detail. These methods include steps for providing a reliable attachment of the circuit elements 21 to the second substrate 51 and for making reliable electrical connections between the contact pads 17, 55.
Figs. 9a to 9f show the processing steps of a first specific method according to the invention. The first specific method is similar to the method described above, except that the first support substrate 11 is processed further before it is combined with the second (flexible) substrate 51. Only the additional processing steps will be described in detail herein. Like reference numerals are used to refer to like elements in the drawings.
As shown in Fig. 9a, a structural film 27 is applied to the first support substrate 11 described above with reference to Fig. 4, prior to carrying out the subsequent processing steps illustrated in 5 a to 5 d. The structural film 27 comprises a structural layer 29 of semi-cured epoxy resin in contact with the circuit elements 21 and exposed areas of the circuit element substrate 13 and a releasable layer 31 of silicone coated PET film arranged in contact with the structural layer 29. The semi-cured resin of the structural layer 29 is sufficiently cured to retain its own shape while at the same time being mouldable under pressure and sticky enough to ensure reliable adhesion to adjacent layers (other than the releasable layer 31).
In a subsequent step, as shown in Fig. 9b, holes 33 are formed through the structural film 27 to expose the contact pads 17 of the circuit elements 21. The holes 33 may be formed using a CO2 laser, which is able to penetrate the structural film 27 but does not penetrate the metallic contact pads 17. The holes 33 are then filled with a curable conductive paste 35 of the type conventionally used for via filling (or similar). The holes 33 are filled with the paste 35 using a squeegee 37, as shown in Fig. 9c.
The first support substrate 11 having the filled holes 33 is then processed to cut out and remove portions of the circuit element substrate 13 arranged between the circuit elements 21, as described above with reference to Figs. 5a to 5d. The laser release process not only removes portions of the circuit element substrate 13 but also removes overlying portions of the structural film 27. Before or after the cut-out and/or laser release processes are carried out, the releasable layer 31 is removed from the structural layer 29 overlying the circuit elements 21, as shown in Fig. 9d. The portions of conductive paste 35 stand proud in the resulting arrangement.
Fig. 9d also shows the step of bringing the first support substrate 11 and the second substrate 51 together in preparation for attaching the circuit elements 21 to the second substrate 51 and electrically connecting the contact pads 17, 55 of the substrates 11, 51. As shown in the Figure, the substrates 11, 51 are aligned such that the portions of conductive paste 35 are in registration with the holes 59 in the second substrate 51 through which the contact pads 55 are exposed.
The circuit elements 21 are attached to the second substrate 51 by pressing the first and second substrates 11, 51 together and heating. As shown in Fig. 9e, the conductive paste 35 is compressed between the substrates 11, 51 and displaces the moldable structural layer 29 formed of part-cured resin in a lateral direction. The structural layer 29 cures to thereby bond the circuit elements 21 to the second substrate 51. The conductive paste 35 cures to provide a reliable electrical connection between the contact pads 17, 55 of the substrates 11, 51. The curing temperature is below the melting temperature of the polyurethane material of the second substrate 51.
In a final step of the method, as shown in Fig. 9f, the circuit elements 21 are laser released from the first support substrate 11 to leave the finished sheet-like electronic device 101, which is bendable and stretchable.
When the substrates 11, 51 are pressed together, the lateral displacement of the structural layer 29 causes some of the structural layer 29 to spread beyond the edge of the circuit element 21 and contact the first support substrate 11. The cured structural layer 29 may then become adhered to the first support substrate 11, making release of the circuit elements 21 from the first support substrate 11 difficult. This problem may be avoided by selecting a resin material for the structural layer 29 which can be laser released at the same time as the circuit element substrate 13 formed of polyimide. Alternatively, the method may be modified as illustrated in Figs. 10a to 10c to prevent the structural layer 29 contacting the first support substrate 11.
As shown primarily in Fig. 10a, the method is modified so that the structural layer 29 is removed from around the circumference 39 of the circuit elements 21. In this way, when the structural layer 29 is laterally displaced by the conductive paste 35, it does not spread beyond the edges of the circuit elements 21. Fig. 10b shows the position of the structural layer 29 after it has cured.
The structural layer 29 may be removed from around the circumference 39 of the circuit elements 21 using a CO2 laser, for example at the same time that portions of the circuit element substrate 13 are cut out from between the circuit elements 21. A metallic band 41 may be formed around the circumference of each circuit element 21, for example at the same time as the contact pads 17 are formed, to prevent the CO2 laser penetrating the circuit element substrate 13 of the circuit elements 21 when the structural layer 29 is removed. A suitable metallic band 41 is shown in Fig. 10c, which is a plan view of the first support substrate 11 having the circuit elements 21.
Figs. 1 Ia to 1 Ig show the processing steps of a second specific method according to the invention. The second specific method is similar to the first specific method described above, except that the first support substrate 11 is processed differently so that the circuit element substrate 13 does not face outwardly in the finished sheet-like electronic device 101. This may be advantageous for devices which emit or sense light, such as display devices, since the polyimide material of the circuit element substrate 13 is typically yellow in color and not fully transparent. Only the modified processing steps will be described in detail herein. Like reference numerals are used to refer to like elements in the drawings.
Fig. 11a shows a support substrate 43 which is substantially the same as the first support substrate 11 described above with reference to Fig. 4, prior to carrying out the subsequent processing steps illustrated in 5a to 5d. The support substrate 43 shown in Fig. 11a differs from the substrate 11 shown in Fig. 4 only in that the contact pads 17 are formed underneath the thin film circuitry 15 so that they face towards the substrate 43 and are adjacent to the circuit element substrate 13. As shown in Fig. 1 Ia, a transparent cover layer 45 is also formed over the thin film circuitry 15 and the exposed portions of the circuit element substrate 13.
The support substrate 43 shown in Fig. 1 Ia is an auxiliary (temporary) support substrate which, like the first support substrate 11 described above, is rigid and formed of glass. Substantially conventional substrate handling and processing techniques may therefore be employed.
Once the circuit element substrate 13, the contact pads 17, the thin film circuitry 15, and the transparent cover layer 45 have been formed, the first support substrate 11 is attached to the transparent cover layer 45 with a (non-permanent) adhesive film 47, as shown in Fig. 1 Ib. The whole of the substrate arrangement is then released from the auxiliary support substrate 43 using the EPLaR laser release process described above and the substrate arrangement is inverted, as shown in Fig. l ie.
Fig. l ie also shows the application of a structural film 27 to the circuit element substrate 13. The structural film 27 is the same as that described above with reference to Fig. 9a and comprises a structural layer 29 of semi-cured epoxy resin in contact with the circuit element substrate 13 now facing outwards and a releasable layer 31 of silicone coated PET film arranged in contact with the structural layer 29. The semi-cured resin of the structural layer 29 is sufficiently cured to retain its own shape while at the same time being moldable under pressure and sticky enough to ensure reliable adhesion to adjacent layers (other than the releasable layer 31).
In a subsequent step, as shown in Fig. 1 Id, holes 33 are formed through the structural film 27 and the circuit element substrate 13 to expose the contact pads 17 of the circuit elements 21. The holes 33 may be formed using a CO2 laser, which is able to penetrate the structural film 27 and the circuit element substrate 13 but does not penetrate the metallic contact pads 17. The holes 33 are then filled with a curable conductive paste 35 of the type conventionally used for via filling (or similar). The holes 33 are filled with the paste 35 using a squeegee 37, as shown in Fig. l ie.
The first support substrate 11 having the filled holes 33 is then processed to cut out and remove portions of the circuit element substrate 13 arranged between the circuit elements 21, as described above with reference to Figs. 5a to 5d. The laser release process not only removes portions of the circuit element substrate 13 but also removes the other layers overlying the first support substrate 11. Before or after the cut-out and/or laser release processes are carried out, the releasable layer 31 is removed from the structural layer 29 overlying the circuit elements 21, as shown in Fig. 1 If. The portions of conductive paste 35 stand proud in the resulting arrangement.
Fig. 1 If also shows the step of bringing the first support substrate 11 and the second substrate 51 together in preparation for attaching the circuit elements 21 to the second substrate 51 and electrically connecting the contact pads 17, 55 of the substrates 11, 51. As shown in the Figure, the substrates 11, 51 are aligned such that the portions of conductive paste 35 are in registration with the holes 59 in the second substrate 51 through which the contact pads 55 are exposed.
The circuit elements 21 are attached to the second substrate 51 by pressing the first and second substrates 11, 51 together and heating. The conductive paste 35 is compressed between the substrates 11, 51 and displaces the mouldable structural layer 29 formed of part-cured resin in a lateral direction. The structural layer 29 cures to thereby bond the circuit elements 21 to the second substrate 51. The conductive paste 35 cures to provide a reliable electrical connection between the contact pads 17, 55 of the substrates 11, 51. The curing temperature is below the melting temperature of the polyurethane material of the second substrate 51.
In a final step of the method, as shown in Fig. 1 Ig, the circuit elements 21 are released from the first support substrate 11 by releasing the adhesive layer 47 to leave the finished sheet-like electronic device 101, which is bendable and stretchable.
As with the first specific method described above with reference to Figs. 9a to 9f, when the substrates 11, 51 are pressed together, the lateral displacement of the structural layer 29 causes some of the structural layer 29 to spread, which can make release of the circuit elements 21 from the first support substrate 11 difficult. Again, this problem may be avoided by selecting a resin material for the structural layer 29 which can be laser released at the same time as the circuit element substrate 13 formed of polyimide. Alternatively, the method may be modified as illustrated in Fig. 12 to prevent the structural layer 29 contacting the first support substrate 11.
As shown in Fig. 12, the method is modified so that the structural layer 29 and the circuit element substrate 13 are removed from around the circumference 39 of the circuit elements 21. In this way, when the structural layer 29 is laterally displaced by the conductive paste 35, it does not spread beyond the edges of the circuit elements 21.
The structural layer 29 may be removed from around the circumference 39 of the circuit elements 21 using a CO2 laser, for example at the same time that portions of the circuit element substrate 13 are cut out from between the circuit elements 21. A metallic band 41 may be formed around the circumference of each circuit element 21, for example at the same time as the contact pads 17 are formed, to prevent the CO2 laser penetrating beyond the circuit element substrate 13 of the circuit elements 21 when the structural layer 29 is removed. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims.
For example, the circuit elements described above are prepared using the EPLaR (electronics on plastic by laser release) process, but circuit elements prepared by other processes are suitable. Wholly rigid circuit elements may be used. The second substrate may be formed of materials other than polyurethane. For example, synthetic rubber materials may be used.
In the claims, the word "comprising" does not exclude other elements or steps, and the definite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

CLAIMS:
1 A method for manufacturing a sheet-like electronic device which is bendable and stretchable, the method comprising: providing a first support substrate (11) having arranged thereon a plurality of spaced apart electronic circuit elements (21), each electronic circuit element (21) having at least one conductive contact pad (17) facing away from the first support substrate (11); providing a second substrate (51) having arranged thereon a plurality of conductive tracks (53), ends of the conductive tracks having conductive contact pads (55); bringing the first (11) and second (51) substrates together such that the contact pads (17) of the circuit elements face the contact pads (55) of the conductive tracks (53) and attaching the circuit elements (21) to the second substrate (51) such that the contact pads (17) of the circuit elements are electrically connected to the contact pads (55) of the conductive tracks; and releasing the circuit elements (21) from the first support substrate (11), wherein the second substrate (51) of the device, to which the circuit elements(21) are attached, is bendable and stretchable.
2. A method as claimed in claim 1, wherein: the first substrate (11) is a rigid substrate; and/or the second substrate (51) is attached to a further rigid support substrate, the second substrate being released from the further rigid support substrate after the circuit elements have been attached to the second substrate.
3. A method as claimed in claim 1, wherein providing the first support substrate (11) having the circuit elements (21) comprises: applying a polymer coating over the first support substrate to define a circuit element substrate (13); forming thin film electronic circuitry (15) and conductive contact pads (17) over the circuit element substrate, the thin film circuitry and the contact pads defining a plurality of spaced apart areas; and removing portions of the circuit element substrate (13) from between the spaced apart areas to thereby leave the circuit elements having the contact pads.
4. A method as claimed in claim 3, wherein the portions of the circuit element substrate (13) between the spaced apart areas are cut out by dicing or laser cutting.
5. A method as claimed in claim 3, wherein the polymer of the circuit element substrate (13) is polyimide, and wherein the circuit elements (21) and/or the portions of the circuit element substrate (13) are released from the first support substrate (11) by directing laser energy through the first substrate at the circuit element substrate.
6. A method as claimed in claim 3, wherein providing the first support substrate (11) having the circuit elements (21) further comprises, before removing the portions of the circuit element substrate: applying a structural film (27) over the thin film circuitry, contact pads and the circuit element substrate, the structural film comprising a structural layer (29) and a re leasable layer (31) arranged over the structural layer; forming holes (33) in the structural film to expose the contact pads and filling the holes with a curable conductive paste (35); and removing the releasable layer (31) from the structural layer (29) of the structural film, and wherein portions of the structural film (27) overlying the circuit element substrate (13) are removed at the same time as the portions of the circuit element substrate are removed to leave the circuit elements having the contact pads.
7. A method as claimed in claim 6, wherein the structural layer (29) of the structural film (27) comprises a part-cured thermosetting resin and/or the releasable layer (31) of the structural film (27) comprises a silicone-coated polymer.
8. A method as claimed in claim 7, wherein attaching the circuit elements (21) to the second substrate (51) comprises applying heat and/or pressure to the curable conductive paste (35) and the structural layer (29) to thereby cure the paste and the structural layer, wherein the cured conductive paste (35) electrically connects the contact pads of the circuit elements to the contact pads of the conductive tracks.
9. A method as claimed in claim 1, wherein providing the first support substrate having the circuit elements comprises: applying a polymer coating over an auxiliary support substrate (43) to define a circuit element substrate (13); forming thin film electronic circuitry (15) and conductive contact pads (17) over the circuit element substrate, the thin film circuitry and the contact pads defining a plurality of spaced apart areas; forming a transparent cover layer (45) over the thin film circuitry, contact pads and exposed areas of the circuit element substrate; attaching the first support substrate (11) to the transparent cover layer with an adhesive layer (47); releasing the circuit element substrate (13) from the auxiliary support substrate (43); and removing portions of the circuit element substrate (13) and the transparent cover layer (45) from between the spaced apart areas to thereby leave the circuit elements having the contact pads.
10. A method as claimed in claim 9, wherein: the polymer of the circuit element substrate (13) is polyimide and the circuit element substrate is released from the auxiliary support substrate (43) by directing laser light through the auxiliary support substrate at the circuit element substrate; and/or the adhesive layer (47) comprises a non-permanent adhesive, and the circuit elements and/or the portions of the circuit element substrate and the transparent cover layer are released from the first support substrate (11) by releasing the adhesive bond of the adhesive layer.
11. A method as claimed in claim 9, wherein providing the first support substrate (11) having the circuit elements (21) further comprises, before removing the portions of the circuit element substrate and the transparent cover layer: applying a structural film (27) over the circuit element substrate (13), the structural film comprising a structural layer (29) and a releasable layer (31) arranged over the structural layer; forming holes (33) in the structural film and the circuit element substrate to expose the contact pads and filling the holes with a curable conductive paste (35); and removing the releasable layer (31) from the structural layer (29) of the structural film, and wherein portions of the structural layer (31) overlying the circuit element substrate (13) are removed at the same time as the portions of the circuit element substrate and the transparent cover layer are removed to leave the circuit elements having the contact pads.
12. A method as claimed in claim 1, wherein the second substrate (51) comprises a polyurethane film.
13. A method as claimed in claim 1, wherein providing the second substrate (51) having the conductive tracks comprises: forming metallic tracks (53) on a surface of a first film layer, ends of the conductive tracks having the conductive contact pads (55); applying a second film layer (57) over the metallic tracks, the contact pads and exposed areas of the surface of the first film layer; and forming holes (59) in at least one of the first and second film layers to expose the contact pads.
14. A method as claimed in claim 1, wherein each of the conductive tracks (53) takes a meandering path between the contact pads at its ends.
15. A sheet-like electronic device which is bendable and stretchable, the device comprising: a bendable and stretchable substrate (51) having arranged thereon a plurality of conductive tracks (53), ends of the conductive tracks having conductive contact pads (55); and a plurality of spaced apart electronic circuit elements (21) attached to the substrate, each circuit element having at least one conductive contact pad (17), wherein the circuit elements are separated from the substrate by a structural layer (27), and wherein the contact pads of the conductive tracks are electrically connected to the contact pads of the circuit elements through holes (33) formed in the structural layer.
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