WO2017134331A1

WO2017134331A1 - Method for the manufacture of an extremely thin flexible multilayer printed circuit board and a flexible multilayer printed circuit board

Info

Publication number: WO2017134331A1
Application number: PCT/FI2016/050062
Authority: WO
Inventors: Timo Tarvainen
Original assignee: Elcoflex Oy
Priority date: 2016-02-01
Filing date: 2016-02-01
Publication date: 2017-08-10
Also published as: EP3412121A1; EP3412121A4

Abstract

In the manufacturing method of a multilayer printed circuit board and in a multilayer printed circuit board of the invention, at least part from insulating, conductive or semi-conductive layers remaining inside the multilayer printed circuit board there are manufactured a printed circuit board by using a roll-to-roll manufacturing method for two or several semi-finished printed circuit boards. Between the conductive printed layers of a semi-finished printed circuit board it is possible to manufacture electric vias connecting these by printing. A flexible multilayer printed circuit board is manufactured by connecting at least two semi-finished printed circuit boards face to face so that the pure surfaces of the metal foils of the semi-finished printed circuit boards remain outermost. After this holes can be manufactured through the flexible multilayer printed circuit board, which are metal-plated to produce an electric via.

Description

Method for the manufacture of an extremely thin flexible multilayer printed circuit board and a flexible multilayer printed circuit board

The invention relates to a method for the manufacture of an extremely thin flexible multilayer printed circuit board. The invention also relates to a flexible multilayer printed circuit board manufactured by the method.

Level of technology

Two- or multilayer flexible printed circuit boards are manufactured using roughly the same methods as conventional inflexible printed circuit boards. Conventional printed circuit boards have a base, a so-called substrate functioning both as mechanical support material and insulating material. In flexible printed circuit board solutions, generally either a polyimide-based film (PI) or polyester-based film (PET) or polyethersulphone film (PES) is used as the flexible substrate. A polyi- mide film withstands temperatures of over 300°C. Some polyimide brands are Kapton®, Apical® and Upilex®. Polyester-based films withstand temperatures of approximately 100°C. Examples of polyester-based brands are Mylar® and Melinex®.

Either a roll-to-roll manufacturing method or a panelization process can be used in the manufacture of flexible printed circuit boards. In the roll-to-roll manufacturing method, a film-type printed circuit board material is processed as long strips rolled onto reels. The different manufacturing steps are performed in the manufacturing apparatus on the straight section arranged between the starting and receiving roll. There can be numerous successive manufacturing steps. The roll-to-roll technology is well applicable when the manufacturing batches are big. The panelization process is generally used for manufacturing inflexible printed circuit boards.

In the publications US2006/0240364 and US2003/0059578 there are presented manufacturing methods, in which a pure metal film is used as an initial material in the roll-to-roll manufacturing method. The film is coated with an insulant, which is patterned. In both specifications, the metal film functions as a mechanical support element in the final product.

The desired conductive pattern can be fabricated onto the surface of the printed circuit board substrate, for example, by growing or etching. In the growing method, the desired conductive pattern is formed onto the thin metal layer on the substrate electrolytically. After the growing step, the original thin metal layer remaining around the conductors is etched off.

A more conventional way to fabricate a conductive layer onto the printed circuit board substrate is to use an etching method, in which the extra metal on the sur- face of the printed circuit board substrate is removed by a chemical etching process. In this case, a metal foil covering the entire surface has been laminated onto at least one surface of the printed circuit board substrate. The metal foil can be, for example, a copper laminate. If this concerns a two-sided printed circuit board, an electric connection can be made between the copper laminates on different sides of the printed circuit board substrate by means of through holes.

The electric connection between conductive metal films on different sides of the printed circuit board substrate can be achieved, for example, by first drilling through holes to desired points. In the second manufacturing step, copper is grown to the drilled through holes in a chemical process. When copper is grown to the through holes made to the printed circuit board substrate for making vias between different layers, also the layer thickness of the copper conductors manufactured of copper laminate grows simultaneously. Thus, the total thickness of the copper conductors and surfaces in different layers of the multilayer printed circuit board increases due to the number of copper-containing lay- ers of the printed circuit board. The total thickness of a multilayer printed circuit board with several layers can grow to be so big that its utilization in the thought object of use becomes difficult.

Laser is used in a second method for the manufacture of vias. In the first manufacturing step, a wiring is fabricated on both sides of the printed circuit board sub- strate. After this, a pit is made through the conductor on the one side of the substrate by using laser, the pit extending also through the printed circuit board substrate; however, so that it does not penetrate the conductive pattern on the second side. In the next manufacturing step, the pit is filled, for example, by pressing conductive silver paste into it. This manufacturing method is more unreliable, because a hole made by laser can be vague and, in this case, its sufficiently good filling is unsure.

It is also possible to make use of so-called printable electronics in the manufacture of electric circuit entities. In this manufacturing method, the printing plate or the ink material in the printing plate touches and adheres to the material functioning as the printing base. An electrically insulating material is used as the printing base, onto which the desired circuit entities are fabricated by printing. Electrically functional, liquid or powdery materials are available both for the manufacture of electrically conductive, semi-conductive or optical circuit elements. If a two-sided printed circuit board is manufactured using this technique, the necessary vias must be fabricated by the techniques described above.

In different industrially manufactured devices, the thickness of either the inflexible or flexible multilayer printed circuit board used has an impact on the mechanical structure of the device. Excessive thickness of the multilayer printed circuit board can become a problem, which limits the size or shape desired for the device under fabrication.

Objects of the invention

It is an object of the invention to present a method for the manufacture of a flexible multilayer printed circuit board, with which it is possible to cost-effectively manu- facture a printed circuit board containing flexible electric circuit elements, advantageously using a roll-to-roll manufacturing apparatus. Part of the electric vias between two or several conductive layers of a flexible multilayer printed circuit board of the invention can be fabricated in connection with the patterning of insulation, semi-conductive or conductive materials. The multilayer printed circuit board of the invention can advantageously also comprise such electric vias, which have been fabricated by a chemical copper coating made through the walls of the through holes fabricated to the multilayer printed circuit board.

The objects of the invention are achieved with a manufacturing method, in which at least two advantageously flexible semi-finished printed circuit boards are placed against each other advantageously by gluing or laminating so that both outer surfaces of the finished multilayer printed circuit board are mainly conductive metal film of semi-finished printed circuit boards, which enclose the wirings and components in the multilayer printed circuit board.

Onto the first surface of the pure conductive metal foil of at least two semi-finished printed circuit boards contained in the multilayer printed circuit board of the invention there is added a first insulation layer advantageously by printing, laminating or coating. A pure metal film without a support film can be, for example, a pure copper foil. Apertures are fabricated into the insulation layer advantageously, for ex- ample, by photolithography, mechanically, laser, plasma etching or chemical processing.

When needed, metal can be added to the apertures fabricated into the insulation layer for advantageously strengthening the connection points made onto the one surface of the copper foil in the later manufacturing steps or ensuring the electrical connection of the conductors of the conductive layer with the copper foil.

In the next steps, one or several conductive, semi-conductive or insulating material layers can be printed onto the patterned insulation layer in order to achieve the manufacture of the desired circuit entity. The vias of the multilayer printed circuit board of the invention are advantageously carried out either by pressing conductive material into the apertures patterned to the first insulation layer of the semi-finished printed circuit board so that they form an electric contact through the insulation layer to the copper foil or by chemically adding copper to the walls of the holes fabricated into the multilayer printed circuit board.

When all advantageously printing-technically manufactured circuit entities have been fabricated, two or several advantageously flexible semi-finished printed circuit boards of the multilayer printed circuit board of the invention are attached against each other so that the mainly unbroken metal foils belonging to two semi- finished printed circuit boards form the planar outer surfaces of the multilayer printed circuit board of the invention.

It is an advantage of the method of the invention that the vias of an advantageously flexible multilayer printed circuit board with three or several layers can be manufactured so that only the thickness of two outermost metal layers must be grown in connection with the coating process of through holes according to the state of the art. Using the manufacturing method of the invention, the thickness of the multilayer printed circuit board can be reduced by tens of percentages compared to respective multilayer printed circuit boards manufactured using a state-of-the-art method. In addition, it is an advantage of the invention that the process steps are roll-to-roll processes so that the turnaround time required by the manufacture of a multilayer printed circuit board is short with big volumes, thus achieving savings in costs. It is further an advantage of the invention that by printing the circuit elements directly onto the surface of the metal foil of the semi-finished printed circuit board so that the plastic film supporting the conventional copper laminate can be excluded, it is possible to gain more savings in costs. It is still an advantage of the invention that, compared to other printing-technical manufacturing methods, in the manufacturing method of the invention the metal foil of the semi-finished printed circuit board stays uniform during the printing processes so that the metal foil also functions as a support element during printing. This way shrinkages can be avoided, which impede the reaching of the desired manufacturing accuracy.

It is characteristic of the manufacturing method of the flexible multilayer printed circuit board, in which at least part of the electric circuit elements of the multilayer printed circuit board are printed onto the pure metal foil of the semi-finished printed circuit board included in the multilayer printed circuit board using a roll-to-roll man- ufacturing apparatus, that further in the manufacturing method:

- an insulating glue layer is printed or laminated onto the surface of the semifinished printed layer, onto which at least one conductive layer has been manufactured;

- at least two semi-finished printed circuit boards are joined against each other, of which at least one is provided with an insulating layer and at least one conductive layer so that the pure metal layers of the semi-finished printed circuit boards remain as outer surfaces, thus forming a laminate structure, of the metal foils of which part of the conductors of the flexible multilayer printed circuit board are processed; and that

- at least part of the electric vias between the conductive layers of the multilayer printed circuit board are fabricated by coating the through holes made to the multilayer printed circuit board chemically with metal.

It is characteristic of the flexible multilayer printed circuit board of the invention, which comprises at least two semi-finished printed circuit boards manufactured with a roll-to-roll manufacturing apparatus, that the multilayer printed circuit board also comprises:

- at least two semi-finished printed circuit boards joined together, of which at least one is provided with an insulating layer and at least one conductive layer so that the pure metal surfaces of the semi-finished printed circuit boards are outermost, forming a laminate structure, part of the metal foils of which forming the outer sur- faces are configured to be processed of the conductors of the flexible multilayer printed circuit board, and that

- at least part of the electric vias between the conductive layers of the multilayer printed circuit board are fabricated by coating the through holes made into the mul- tilayer printed circuit board chemically with metal.

Some advantageous embodiments of the invention are presented in the dependent patent claims.

The basic idea of the invention is the following: In the manufacturing method of the invention, advantageously when connecting components of at least two conductive layers of a multilayer printed circuit board with three or several layers with each other at desired points, using electrically conductive vias through an insulation layer, is advantageously carried out in a printing-technical manner. These vias are manufactured in connection with the printing of insulating and conductive layers of two or several semi-finished printed circuit boards included in the multilayer printed circuit board of the invention.

In the manufacturing method of the invention, the first patterned insulation layer is advantageously fabricated from an insulation layer processed onto the first surface of a pure metal foil, which advantageously is a copper foil. Apertures are made into the insulation layer at least by the vias to be manufactured between different conductive layers. In the next processing steps, one or several conductive, semi- conductive or insulating material layers can be printed onto the patterned insulation layer so that the desired electric circuit entity can be manufactured for the semi-finished printed circuit board.

The vias of at least two semi-finished printed circuit boards included in the multi- layer printed circuit board of the invention are advantageously carried out by pressing conductive material also into the apertures patterned into the first insulating layer in connection with the printing of the conductor patterns. The conductive material pressed into the apertures then achieves an electrical contact from the printed conductive layer to the metal foil and/or the second conductive layer. The multilayer printed circuit board of the invention can also comprise electric vias, which are carried out by chemically coating the walls of the through holes made through the multilayer printed circuit board using, for example, copper.

The requirement for continuous current in the intermediate layers of the multilayer printed circuit board is often very small. In this case, it is possible to use conduc- tive printable polymers as electric conductors in the intermediate layers conducting electricity. Separate copper foils can also be laminated into the intermediate layers; the copper foils being connected to desired conductors/lines in other conductive layers in connection with the through-coppering process. In this way, it is pos- sible to form conductor rails or ground planes for protection purposes inside the multilayer printed circuit board. These copper foils can advantageously be perforated before they are connected to the multilayer printed circuit board so that unnecessary electric contacts to other electric circuits of the multilayer printed circuit board are not generated. The invention is next explained in detail. In the explanation, reference is made to the attached drawings, in which

Figure 1 a illustrates as an exemplary flow chart the main steps of the manufacturing method of a semi-finished printed circuit board of a multilayer printed circuit board of the invention; Figure 1 b illustrates as an exemplary flow chart the final manufacturing steps of the multilayer printed circuit board of the invention;

Figures 2a-2i are exemplary cross-sections of the multilayer printed circuit board of the invention in the different manufacturing steps of Figures 1 a and 1 b;

Figure 3 illustrates as an exemplary cross-section a second multilayer printed cir- cuit board of the invention; and

Figure 4 illustrates as an exemplary cross-section a third multilayer printed circuit board of the invention.

The embodiments in the following description are only exemplary, and one skilled in the art can carry out the basic idea of the invention also in some other way than the one described in the description. Even though reference may be made to an embodiment or embodiments in several places of the description, this does not mean that the reference would only be targeted at one described embodiment, or that the described feature would only be feasible in one described embodiment. Individual features of two or several embodiments can be combined and thus achieve new embodiments of the invention.

Figure 1a illustrates as an exemplary flow chart the main steps of the method for manufacturing an advantageously flexible multilayer printed circuit board 2 of the invention. It is obvious for one skilled in the art that some steps presented in the flow chart in Figure 1 a can be implemented also in another order than what is shown in the exemplary flow chart in Figure 1 a. In connection with the description of the flow chart in Figure 1 a the reference numbers presented in Figures 2a-2d are used as clarifying reference numbers.

Actual manufacturing steps of the flexible multilayer printed circuit board 2 is preceded by the step 100. In the step 100 there is ready a pure metal foil 21 , which is advantageously a copper foil, aluminium foil or nickel silver, to which no support film according to the state of the art has been attached. Later in the description, copper foil is used as the exemplary metal foil.

The thickness of the copper foil 21 in Figure 1 a is advantageously about 12-25 pm. The copper foil 21 is packed onto a reel, which can be used in a roll-to-roll printing machine. The pure copper foil 21 has two surfaces, which are later referred to as the first surface and the second surface. In the exemplary structure in Figures 2a-2d, the first surface can also be called as the lower surface of the copper foil 21 and the second surface as its upper surface.

In the step 101 , the first insulation layer 22 is laminated, pressed, printed or grown onto the first surface (lower surface in Figure 2b) of the copper foil 21 . The thickness of the insulation layer 22 can advantageously be approximately 25 pm. As the pure copper foil 21 functions as a substrate in the manufacturing method of the invention, high processing temperatures can be used in the manufacture of a flexible multilayer printed circuit board so that the selection of processable materials to be added onto the surfaces of the copper foil increases. In one embodiment of the invention, this makes possible the coating of the copper foil 21 on one or both surfaces in the step 100, for example, by using the ALD process (Atomic Layer Deposition) with aluminium oxide (AI2O3) (not shown in Figures 2a and 2b). The coating can be made either onto the entire surface of the copper foil 21 or only to desired points. The coating process can advantageously be used for facilitating the making of the vias to be manufactured and/or for ensuring the functionality of connection points of separate components to be installed to the multilayer printed circuit board in further processing.

The aluminium oxide layer can advantageously be used also as moisture insulation for the multilayer printed circuit board or its components. By using the pure copper foil 21 , a processing temperature of, for example, 300°C is possible in the manufacturing method of the invention. In this case, for example, the manufacture of organic and inorganic semi-conductors is possible by printing technology onto the multilayer printed circuit board of the invention. In the step 102, apertures 221 are fabricated to the insulation layer 22, through which the electric vias of the multilayer printed circuit board 2 between conductive layers can be at least partly achieved. The apertures 221 can be fabricated to the insulation layer 22, for example, by photolithography, mechanically by die cutting, laser, plasma etching or chemically by drilling. The number, locations, shapes and magnitude of the apertures 221 in the insulation layer 22 vary advantageously in accordance with the requirements set for an electric circuit entity to be manufactured onto the multilayer printed circuit board 2.

In an advantageous embodiment of the invention, when needed, a nickel-plated layer 231 and 232 with a thickness of approximately 4-10 pm can be printed into the apertures 221 and 222 of the insulation layer 22 in the steps 103 and 104 onto the pure lower surface of the copper foil 21 . The nickel-plated layers 231 and 232 are used either for strengthening the connection areas to be formed onto the second surface (upper surface) of the pure copper foil 21 or for improving the connection of the printable conductors fabricated on the side of the first surface of the copper foil 21 with the copper foil 21 .

In the step 105, the first patterned conductor layer 24, which can be, for example, silver paste or ink is printed onto the insulation layer 22 and advantageously into at least part of the apertures 221 in the insulation layer 22. The thickness of the patterned conductor layer 24 is advantageously approximately 10 pm. At least part of the wirings between electric components to be integrated or installed into the first side of the semi-finished printed circuit board 20a can be advantageously manufactured to the conductor layer 24. It is also simultaneously possible to fabricate the electric vias going through the insulation layer 22 from the conductor layer 24 onto the first surface of the copper foil using the material of the conductor layer 24. After the printing of the first conductor layer 24, in the inspection step 106 it is decided, for example, if a new insulation layer is to be printed onto the fabricated conductor layer 24.

If the decision in the inspection step 106 is "No", then the step 1 1 1 is adopted, in which a glue layer 25 is printed or laminated onto the insulation layer 22 and con- ductor layer 24 so that the glue layer entirely covers these layers, the thickness of the glue layer being advantageously 35 m. The layer 25 can also be fabricated by laminating a so-called "Bond-Ply" film onto the outer material surfaces 22 and 24 of the semi-finished printed circuit board 20a. If the decision in the inspection step 106 is "Yes", the next advantageously patterned insulation layer is printed in the step 107 onto the insulation layer and conductor layer already fabricated.

After the manufacture of the printed insulation layer in the step 107, in the inspection step 108 it is decided whether to print a new conductor layer or a material lay- er of different electric circuit components, passive or active, onto the fabricated insulation layer.

If the decision in the inspection step 108 is "No", then in the step 1 1 1 a glue layer or a glue film with a thickness of 25 Mm covering entirely the last printed insulation layer and conductor layer is printed or laminated onto these layers. If the decision in the inspection step 108 is "Yes", advantageously the next patterned conductor layer or a material layer of an electric circuit component is then printed in the step 109.

The manufacturing method of a flexible printed circuit board of the invention can also be used to fabricate circuit components based on semi-conductors manufac- tured by printing technology on the flexible multilayer printed circuit board 2. In this case in the step 108, it is advantageously checked, if also semi-conductor based components or passive circuit elements, such as resistors, capacitors or coils are to be printed to the multilayer printed circuit board 2 by pressing or printing.

If the result of the inspection step 108 is the decision "Yes", then one material lay- er required by the component to be manufactured is manufactured in the step 109 by pressing or printing the component material 21 onto the insulation and conductor layers manufactured onto the first surface of the copper foil 21 . The manufacture of passive components or semi-conductor components can comprise several separate successive pressing or printing times with different material mixtures. In addition to passive components and semi-conductor components, also different kinds of sensors can be manufactured to the multilayer printed circuit board 2 by printing. After the manufacture of a printed conductor layer or a material layer of an electric, passive or active circuit component in the step 109, it is decided in the inspection step 1 10, if yet another conductor layer or a next material layer for an electric circuit component is to be printed. If the decision in the inspection step 1 10 is "No", then the step 1 1 1 is adopted, in which a glue layer or a glue film with a thickness of advantageously 25 pm is printed or laminated onto the insulation or conductor layers or material layers of electric circuit components printed last, covering them entirely.

If the decision in the inspection step 1 10 is "Yes", then the manufacturing process of the multilayer printed circuit board returns to the step 107, in which the next advantageously patterned insulation layer is printed.

This process loop 107-1 10 is repeated so long that the inspection step 1 10 produces the decision "No", in which case the manufacturing process proceeds to the manufacturing step 1 1 1 of the glue layer 25. With the described procedure, a semi-finished printed circuit board 20a and 20b can be produced with the manufacturing method of the invention by repeating the process loop 106-1 10 several times.

Finally, the manufacturing process of an individual semi-finished printed circuit board 20a or 20b included in the multilayer printed circuit board 2 of the invention terminates in the step 1 12.

Figure 1 b illustrates as an exemplary flow chart how the flexible multilayer printed circuit board 2 according to the invention is manufactured by two semi-finished printed circuit boards 20a and 20b, the thickness of the multilayer printed circuit board being clearly smaller than that of a multilayer printed circuit board of the state of the art, which contains as many conductive layers and which is manufactured with the same manufacturing method.

In the step 120, at least two semi-finished printed circuit boards 20a and 20b to be joined with each other are ready. At least one of the semi-finished printed circuit boards 20a or 20b has a glue layer or glue film 25 covering it entirely printed on the processed side of the semi-finished printed circuit board.

In the step 121 , the semi-finished printed circuit boards 20a and 20b are attached to each other by the glue layer 25. In this case, the metal foils 21 and 21 a of the semi-finished printed circuit boards 20a and 20b enclose the insulation and con- ductor layers fabricated onto the semi-finished printed circuit boards 20a and 20b. The reference 200 in Figure 2e illustrates the semi-finished printed circuit board in this manufacturing step.

In the step 122, through holes 26 are made through the semi-finished printed cir- cuit board 200. The through holes can advantageously be fabricated by drilling, laser, die cutting or etching. The fabricated through holes 26 are coated advantageously chemically with copper 26a so that they form electric vias between the perforated copper foils 21 1 and 21 1 a. The reference 200a in Figure 2f illustrates the semi-finished printed circuit board in this manufacturing step. In the step 123, exposure masks 27 and 27a covering entirely the copper foils 21 and 21 a are laminated onto the second pure surfaces of both copper foils 21 1 and 21 1 a of the semi-finished printed circuit board 200a.

In the step 124, apertures 27b1 and 27b2 are made to the exposure masks to the points in which copper of the copper foils 21 1 or 21 1 a is intended to be etched off. It is obvious for one skilled in the art that the shape of the apertures 27b1 and 27b2 is arbitrary, because the apertures are advantageously used to form conductors from the copper foils 21 1 and 21 1 a. The reference 200b in Figure 2g illustrates the semi-finished printed circuit board in this manufacturing step.

In the step 125, copper is etched off from the place of the apertures 27b1 and 27b2 so that the apertures 28a and 28b in Figure 2h generated into the copper foils 21 1 and 21 1 a in Figure 2g define the copper conductors manufactured from the copper foils 212 and 212a and other surfaces remaining metallic in the multilayer printed circuit board 2. The reference 200c in Figure 2h illustrates the semifinished printed circuit board in this manufacturing step. In the step 126, when needed, a solder mask is advantageously fabricated onto at least one outer surface of the semi-finished printed circuit board 200c. With the patterning of the solder mask 29 and 29a it is possible to coat at least part of the outer surfaces (references 29b1 and 29b2) of the multilayer printed circuit board 2 with a metal layer or organic coating preventing oxidization. For example, a nickel- gold mixture can be used as the coating.

In the step 127, when needed, a coating with a thickness of 5 nm - 20 pm ensuring the joining of separate components is processed onto at least one outer surface of the semi-finished printed circuit board 200c, making use of the solder mask. After the coating process, the advantageously flexible multilayer printed circuit board 2 of the invention is finished in the step 128. Figure 2i illustrates the finished advantageously flexible multilayer printed circuit board of the invention.

Figures 2a-2i illustrate an exemplary flexible multilayer printed circuit board 2 of the invention containing four flexible layers (Figure 2i). In Figures 2a-2i, an exemplary copper foil is used as the metal foil 21 of the printed circuit board 2 of the invention. However, the invention is not limited to the four-layer printed circuit board of the example, but the multilayer printed circuit board of the invention can contain several layers. In this example, the exemplary finished multilayer printed circuit board 2 does not include semi-conductor components made by pressing or printing.

In Figure 2a there is illustrated a pure copper foil 21 , to which no separate plastic film has been laminated as a support element. In the manufacturing process of the invention, the copper foil 21 functions in the initial steps of the process also as the support element required by the printing processes so that there is no need for a separate plastic support element. The thickness of the copper foil 21 is advantageously in the range of 12-35 pm. The use of the pure copper foil 21 as a substrate in the processing makes possible a high processing temperature, for example of +300°C so that the selection of materials to be processed/added to the sur- faces of the copper foil 21 increases.

In an advantageous embodiment of the present invention, at least the one surface of the copper foil 21 has been coated with aluminium oxide (AI2O3) using the ALD process (Atomic Layer Deposition) (not shown in Figure 2a). The aluminium oxide coating can be made either onto the entire surface of the copper foil 21 or to de- sired points only. The coating process can advantageously improve the moisture protection properties of the printed circuit board.

In Figure 2b there is illustrated advantageously a uniform first insulation layer 22 laminated, pressed or printed onto the first surface (lower surface) of the copper foil 21 . The thickness of the insulation layer 22 is advantageously approximately 25 m. Aperture 221 have been fabricated into the insulation layer 22, through which the vias leading from one conductive layer to a second conductive layer can be realized in the finished multilayer printed circuit board 2 of the invention. Support material is processed into one exemplary aperture 222 of the insulation layer 22 in later process steps to support the connection to be carried out to the second surface (upper surface) of the copper foil 21 .

The apertures 221 and 222 in the insulation layer can advantageously be made to the insulation layer 22 photolithographically, mechanically by die cutting, laser, plasma etching, chemically by drilling or by printing a patterned insulation layer. The number, locations, sizes and shapes of the apertures 221 and 222 in the insulation layer can vary.

In Figure 2c there are illustrated the nickel coatings 231 and 232 fabricated to the apertures 221 and 222 of the insulation layer 22, the thickness of the coatings being in the range of 2-5 pm.

By the coating treatment of apertures 221 on the lower surface of the copper foil used in the realization of electric vias, the reliability of fabricated vias, especially vias with a small diameter, and the installation of couplings or separate compo- nents made to the second surface (upper surface) of the copper foil in the final configuration of the printed circuit board is improved.

The nickel coating 232 processed into the aperture 222 is used to support the coupling to be realized to the second surface (upper surface) of the copper foil 21 .

In Figure 2d there is illustrated a first patterned conductor layer 24, which can, for example, be silver paste or ink and pressed or printed onto the insulation layer 22 and into the apertures 221 intended for the making of vias. The thickness of the conductor layer is advantageously in the range of 10-15 pm. In the conductor layer 24 fabricated, there is located advantageously at least part of the wiring of the electric components to be integrated or installed into the multilayer printed circuit board 2 of the invention.

The requirement for current carrying capacity requirement of the intermediate layers of the multilayer printed circuit board 2 is often small, and in this case, printable conductive polymers can advantageously be used as conductors of the multilayer printed circuit board to be manufactured. Also, vias 231 leaving from the conductor layer 24 through the insulation layer 22 and extending to the first surface of the copper foil 21 are included in the insulation layer 24.

It is obvious for one skilled in the art that a new non-patterned insulation layer or a patterned insulation layer can be processed onto the conductor layer 24 and fur- ther a conductive material layer can be processed onto this layer to manufacture a printed circuit board with several layers.

In an advantageous embodiment of the invention, by using a low-loss printable dielectric material and a well conducting conductor material, it is possible to manu- facture a low-loss RF transfer line inside the multilayer printed circuit board.

When all insulating, conductive or semi-conductive material layers have been printed, a glue layer is advantageously printed or a glue film is laminated (reference 25 in Figure 2e) onto the last processed material layer.

In Figure 2e there is illustrated a semi-finished printed circuit board 200 of the in- vention, in which the glue film 25 is advantageously laminated onto the insulation layer 22 and conductive circuit elements 24 manufactured onto the first surface of the copper foil 21. The semi-finished printed circuit board 200 comprises two preferably different printed circuit board preforms 20a and 20b. The semi-finished printed circuit board 20a is connected to the second semi-finished printed circuit board 20b with the glue film 25, the copper foil of which has the reference number 21 a and the insulation the reference number 22a. The semi-finished printed circuit boards 20a and 20b to be connected can advantageously have different electric circuit elements 24.

In Figure 2f there is illustrated the semi-finished printed circuit board 200a after the through holes 26 have been fabricated to it through the copper foils 21 1 and 21 1 a, for example, by drilling. The through holes 26 have advantageously been chemically coated with copper 26a. Along with the chemical coating 26a of the holes, also the thickness of the copper foils 21 1 and 21 1 a increases.

In Figure 2g there is illustrated the semi-finished printed circuit board 200b after the etching masks 27 and 27a have been fabricated onto its both outer surfaces. The etching masks 27 and 27a have apertures 27b1 and 27b2, at the place of which copper is etched off from the copper foils 21 1 and 21 1 a.

In Figure 2h there are illustrated the parts of the copper foils 212 and 212a remaining on the semi-finished printed circuit board 200c after etching. In the exam- pie in Figure 2h, copper has been removed by etching from the copper foil 212 at the points 28a and from the copper foil 212a at the points 28b. The parts of the copper foil remaining after the etching can be, for example, wirings between the components to be integrated or installed into the multilayer printed circuit board 2. After the etching, part of the remaining copper 212 or 212a can also be a ground plane needed by a circuit entity.

In Figure 2i there is illustrated the finished flexible multilayer printed circuit board 2 of the invention after its both outer surfaces have been coated with the solder mask 29 and 29a. The solder mask 29 and 29a advantageously has apertures 29b1 and 29b2, which are advantageously coated with a metal layer or organic coating preventing oxidization. For example, nickel-gold can be used as the coating.

In Figure 3 there is illustrated a flexible multilayer printed circuit board 3 of the invention, in which two semi-finished printed circuit boards have been joined to form the multilayer printed circuit board 3 with a glue layer or glue film 35. Between the two copper foils 31 and 31 a constituting the outer surfaces of the multilayer printed circuit board 3 there are four exemplary conductive layers 34a and 34b in the insulation layers 32 and 32a, which advantageously comprise several superimposed insulation layers (not shown in Figure 3). Both outer surfaces of the multilayer printed circuit board 3 are in this example coated with solder masks 39 and 39a. The solder masks 39 and 39a contain exemplary apertures 39b1 and 39b2, which are advantageously coated with a metal layer or organic coating preventing oxidization. For example, nickel-gold can be used as the coating. In Figure 4 there is illustrated a flexible multilayer printed circuit board 4 of the invention, which has seven conductive layers. Two of the conductive layers 44a and 44b are advantageously manufactured using the printing technology.

Five of the layers constitute of copper foils 41 , 41 a, 41 b, 41 c and 41 d. Inside the multilayer printed circuit board 4 of the invention the copper foil 41 c can function, for example, as the ground plane for the electric circuit elements to be installed into the multilayer printed circuit board 4.

The copper foils 41 and 41 a with tin-plated outer surfaces constitute the outermost surfaces of the multilayer printed circuit board 3, which in the example in Figure 3 are coated with solder masks 49 and 49a. The solder masks 49 and 49a contain exemplary apertures 49b1 and 49b2, which can advantageously be coated with a metal layer or organic coating preventing oxidization. For example, nickel-gold can be used as the coating.

In the example in Figure 4, these copper foils 41 and 41 a are connected by two electric vias 46, which have been manufactured by chemically coating a hole fabri- cated into the multilayer printed circuit board. In the example in Figure 4, the via on the left-hand side connects only the outermost copper foils 41 and 41 a. The via on the right-hand side in Figure 4 connects electrically all copper foils 41 , 41 a, 41 b, 41 c and 41 d with each other. A hole 40 has also been fabricated into the multilayer printed circuit board 4, through which separate wires can be led through the multilayer printed circuit board 4 of the invention.

Inside the multilayer printed circuit board 4, the copper foils 41 and 41 b are connected by conductive vias 46a. Two other copper foils 41 a and 41 d are also con- nected by conductive vias 46b.

The manufacturing method of the invention has the following technical advantages.

With the manufacturing method, it is possible to considerably reduce the thickness of the multilayer printed circuit board, because part of the vias of different conduc- tive layers of a conventional multilayer printed circuit board can be realized without chemical coppering of the through holes. In this case, chemical coppering can be avoided for vias between layers realized with a conductive polymer of the multilayer printed circuit board, which always increases also the thickness of the copper foils, which are electrically connected by vias. The more of the conductive layers are realized with conductive polymers instead of copper foils, the thinner the multilayer printed circuit board can be manufactured.

In addition, the thickness of the conductive layers of the multilayer printed circuit board realized using conductive polymers is approximately 10 pm, while the thickness of a conductive layer realized using copper foil is in the range of 12-35 pm. For example, the thickness of a multilayer printed circuit board of the invention, which has two copper foils and two conductive layers realized using conductive polymers, is approximately 195 pm. The thickness of a respective multilayer printed circuit board made in the traditional way is approximately 250 pm.

When manufactured with the method of the invention, the thickness of a multilayer printed circuit board with six conductive layers, i.e. two copper foils and four conductive layers made by printing, is approximately 300 pm. The thickness of a multilayer printed circuit board with seven conductive layers, of which five are copper foils, is approximately 400 pm, manufactured with the method of the invention. The thickness of a respective multilayer printed circuit board manufactured in the traditional way is approximately 560 pm. The conductive layers of the multilayer printed circuit board made by printing technology have clearly lower manufacturing costs than the conductive layers manufactured to the multilayer printed circuit board by using copper foils.

Some advantageous embodiments of the manufacturing method of the invention and of the flexible multilayer printed circuit board achieved by the manufacturing method have been described above. The invention is not limited to the solutions described above, but the inventional idea can be applied in numerous ways within the limits set by the patent claims.

Claims

Patent claims

1 . A manufacturing method of a flexible multilayer printed circuit board (2, 3, 4), in which at least part of electric circuit elements of the multilayer printed circuit board is printed to a pure metal foil (21 ) included in a semi-finished printed circuit board (20a, 20b) with a roll-to-roll apparatus, in which method:

- a first insulation layer (22) is added to a first pure surface of the pure metal foil (21 ) of the semi-finished printed circuit board (20a, 20b) by laminating or coating (101 );

- apertures (221 , 222) are manufactured (102) to the first insulation layer (22) pho- tolithographically, mechanically, by laser, plasma etching or chemically by drilling;

- a first patterned layer of conductive material is added (105) onto the insulation layer (22) and into the apertures (221 , 222) in it for fabricating wirings and electric vias of a first conductive layer (24) of the semi-finished printed circuit board (20a, 20b);

characterized in that further in the manufacturing method:

- an insulating glue layer (25) is printed or laminated (1 1 1 ) onto the surface of the semi-finished printed circuit board (20a, 20b), to which at least one conductive layer (24) has been manufactured;

- at least two semi-finished printed circuit boards (20a, 20b) are joined (121 ) op- posite each other, at least one of the printed circuit boards containing an insulation layer (22, 22a) and at least one conductive layer (24) so that the pure metal foils (21 , 21 a) of the semi-finished printed circuit boards (20a, 20b) remain as outer surfaces, forming a laminate structure, of the metal foils (21 , 21 a) of which part of conductors of the flexible multilayer printed circuit board (2, 3, 4) are processed, and that

- at least part of the electric vias between the conductive layers (21 1 , 21 1 a) of the multilayer printed circuit board (2, 3, 4) are manufactured by coating through holes (26) made to the multilayer printed circuit board (2, 3, 4) chemically with metal (26a).

2. The manufacturing method of claim 1 , characterized in that one or several insulating (32, 32a), conductive (34a, 34b) or semi-conductive layers are printed onto the first conductive layer (24) before a lamination of a glue layer (25).

3. The manufacturing method of claim 1 , characterized in that through holes (26) are manufactured by drilling, laser or etching.

4. The manufacturing method of claim 3, characterized in that the through holes (26) of the electric vias are manufactured between at least two metal foils (21 1 , 21 1 a, 31 , 31 a, 41 , 41 a, 41 b, 41 c, 41 d) included in the multilayer printed circuit board (2, 3, 4).

5. The manufacturing method of claim 1 , characterized in that nickel or nickel- gold coatings (231 and 232) are added into the apertures (221 , 222) of the insulation layer (22), the thickness of the coatings being 2-5 pm before the printing of the conductive layer (24).

6. The manufacturing method of claim 1 , characterized in that also at least one RF transfer line of low-loss dielectric substance and conductive polymer is manufactured to the multilayer printed circuit board (2, 3, 4) by printing.

7. The manufacturing method of claim 1 , characterized in that also at least one semi-conductor, sensor element, resistor, capacitor or coil is manufactured to the multilayer printed circuit board (2, 3, 4) by printing.

8. The manufacturing method of claim 1 , characterized in that at least one perforated metal foil (41 b, 41 c, 41 d) is laminated between the outermost metal foils (41 , 41 a) of the multilayer printed circuit board (4) as a grounding element or power conductor.

9. The manufacturing method according to any of the claims 1-8, character- ized in that a foil containing copper, aluminium or nickel silver is used as the metal foil (21 ).

10. A flexible multilayer printed circuit board (2, 3, 4), which is manufactured with a roll-to-roll apparatus from at least two semi-finished printed circuit boards (20a, 20b), the semi-finished printed circuit board (20a, 20b) comprising:

- a first patterned insulation layer (22) comprising apertures (221 , 222) on a first surface of a metal foil (21 );

- a patterned conductive layer (24) of conductive material added onto the first patterned insulation layer (22, 22a), the conductive layer comprising materials of electric conductors and electric circuit components;

- vias formed by conductive material from the conductors of the first patterned conductive layer (24) through the apertures (221 ) of the insulation layer (22) either to the copper foil (21 ) or to the conductors of a second patterned conductive layer; characterized in that the flexible multilayer printed circuit board (2, 3, 4) comprises: - at least two semi-finished printed circuit boards (20a, 20b) joined together, of which at least one has an insulation layer (22) and at least one conductive layer (24), which are connected face to face so that pure metal foils (21 , 21 a) of the semi-finished printed circuit boards (20a, 20b) are outermost, forming a laminate structure, part of conductors of the flexible multilayer printed circuit board (2, 3, 4) being configured to be processed from the metal foils (21 , 21 a) forming the outer surfaces; and that

1 1 . The multilayer printed circuit board according to claim 10, characterized in that one or several insulating (32, 32a), conductive (34a, 34b) or semi-conductive layers have been printed onto the first conductive layer (24) before lamination of a glue layer (25).

12. The multilayer printed circuit board according to claim 10, characterized in that the through holes (26) are manufactured by drilling, laser or etching.

13. The multilayer printed circuit board according to claim 12, characterized in that the through holes (26) of the electric vias are manufactured between at least two metal foils (21 1 , 21 1 a, 31 , 31 a, 41 , 41 a, 41 b, 41 c, 41 d) included in the multilayer printed circuit board (2, 3, 4).

14. The multilayer printed circuit board according to claim 10, characterized in that nickel or nickel-gold coatings (231 and 232) with a thickness of 2-5 pm are added to the apertures (221 , 222) of the insulation layer (22) before the printing of the conductive layer (24).

15. The multilayer printed circuit board according to claim 10, characterized in that the multilayer printed circuit board (2, 3, 4) also comprises at least one RF transfer line manufactured by printing from low-loss dielectric substance and conductive material.

16. The multilayer printed circuit board according to claim 10, characterized in that the multilayer printed circuit board (2, 3, 4) also comprises at least one semiconductor, sensor element, resistor, capacitor or coil manufactured by printing.

17. The multilayer printed circuit board according to claim 10, characterized in that at least one perforated metal foil (41 b, 41 c, 41 d) is laminated between the outermost metal foils (41 , 41a) of the multilayer printed circuit board (4) as a grounding element or power conductor.

18. The manufacturing method according to any of the claims 10-17, characterized in that a foil containing copper, aluminium or nickel silver is used as the metal foil (21 ).