WO2018152672A1

WO2018152672A1 - Flexible multilayer system with stiffening features

Info

Publication number: WO2018152672A1
Application number: PCT/CN2017/074212
Authority: WO
Inventors: Bin Lan; Hengyuan ZHOU; Zheng Liu; Tongyong LI; Alejandro Aldrin Agcaoili Ii Narag
Original assignee: 3M Innovative Properties Company
Priority date: 2017-02-21
Filing date: 2017-02-21
Publication date: 2018-08-30

Abstract

A flexible multilayer system (100) for being cut into a plurality of flexible multilayer constructions (10) is described. Each multilayer construction (10) includes an electronic circuit (77). The flexible multilayer system (100) includes an electrically conductive first stiffening feature (20) integrally formed on the flexible multilayer system (100) and electrically isolated from the electronic circuit (77) of each flexible multilayer construction (10). The first stiffening feature (20) extends along a first direction across at least a majority of a length of the flexible multilayer system (100) between opposite edges of the flexible multilayer system (100). The first stiffening feature (20) increases a stiffness of the flexible multilayer system against a bend along the first direction.

Description

FLEXIBLE MULTILAYER SYSTEM WITH STIFFENING FEATURES

Background

Light emitting semi-conductor device constructions may include a polymeric dielectric layer having a conductive layer disposed on a surface of the dielectric layer. The conducive layer may include an electrical circuit configured to power one or more light emitting semiconductor devices.

Summary

In some aspects of the present description， a flexible multilayer system for being cut into a plurality of flexible multilayer constructions is provided. Each flexible multilayer construction includes an electronic circuit. The flexible multilayer system includes an electrically conductive first stiffening feature integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit of each flexible multilayer construction. The first stiffening feature extends along a first direction across at least a majority of a length of the flexible multilayer system between opposite edges of the flexible multilayer system. The first stiffening feature increases a stiffness of the flexible multilayer system against a bend along the first direction.

In some aspects of the present description， a flexible multilayer system is provided that includes a flexible substrate comprising opposing first and second major surfaces； a plurality of spaced apart electronic circuits arranged on and along a length of the flexible substrate； and parallel electrically conductive first and second stiffening ribs formed integrally on the respective first and second major surfaces of the substrate. Each electronic circuit includes first and second electrodes on the respective first and second major surfaces of the substrate； and an electrically conductive via extending between and connecting the first and second electrodes. The first and second ribs are electrically connected to each other and electrically isolated from each electronic circuit. At least one of the first and second stiffening ribs extends substantially along the entire length of the substrate and increases a stiffness of the flexible multilayer system against a bend along the length of the substrate.

In some aspects of the present description， a flexible multilayer system is provided that includes a flexible substrate comprising opposing top and bottom major surfaces； a plurality of spaced apart parallel electrically conductive first electrodes disposed on the top or bottom major surface of the substrate and extending along a first direction； and a plurality of spaced apart parallel electrically conductive second electrodes disposed on the top or bottom major surface of the substrate and extending along a different second direction， such that in a plan view of the flexible multilayer system， the first and second electrodes form a two-dimensional grid defining a plurality of grid cells. Each grid cell includes an electronic circuit electrically isolated from the first and second electrodes and including electrically conductive first and second pads for electrically connecting to respective electrically conductive first and second terminals of a light emitting semiconductor device (LESD) mounted on the flexible multilayer construction entirely within the grid cell.

Brief Description of the Drawings

FIG. 1 is a schematic plan view of a flexible multilayer system；

FIG. 2 is a schematic cross-sectional view of a multilayer construction including a light emitting semiconductor device (LESD) mounted on a circuit in the multilayer construction；

FIG. 3 is a schematic cross-sectional view of a flexible multilayer system；

FIG. 4 is a schematic plan view of a flexible multilayer system；

FIG. 5 is a schematic plan view of a comparative flexible multilayer system； and

FIG. 6 schematically illustrates the relative bending of a flexible multilayer system and a comparative flexible multilayer system.

Detailed Description

In the following description， reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description， therefore， is not to be taken in a limiting sense.

An array of circuits integrally formed in a flexible substrate is described， for example， in U.S. Pat. Appl. Pub. No. 2013/0320390 (Palaniswamy et al. ) and U.S. Pat. No. 9,179,543 (Palaniswamy et al. ) ， each of which is hereby incorporated herein by reference to the extent that it does not contradict the present description. In some cases， the flexible substrate may be sufficiently thin (e.g.， less than 0.1 mm thick) that the flexibility of the substrate creates processing challenges during mass production. For example， it is often desired to store the substrates in a metal cassette such that a push stick will push the substrate to the corresponding track one by one. If the substrate is not rigid enough， its front side can bend down and not move appropriately into the track. As another example， an operator inspecting the array of circuits may grasp the substrate at one side and the bending of the soft substrate may cause the bonding force of the attached circuit chip to drop allowing the circuit to be damaged. According to some embodiments of the present description， techniques for increasing the stiffness of the flexible substrate are provided which reduce or eliminate the processing problems associated with the mass production of the circuits without requiring additional processes and while maintaining a desired thinness.

FIG. 1 is a schematic top view of a flexible multilayer system 100 for being cut into a plurality of flexible multilayer constructions 10. Each flexible multilayer construction 10 includes an electronic circuit 77. In the illustrated embodiment， the electronic circuit 77 includes spaced apart electrically conductive first and

second pads

11 and 12 on a major surface of the substrate 40. The electronic circuit 77 may include other elements， such as those illustrated in FIG. 2. In some embodiments， the electronic circuit 77 is configured to support and electrically connect a light emitting semiconductor device (e.g.， a flip-chip die) . The flexible multilayer system 100 includes a first feature 20 integrally formed on the flexible multilayer system 100. In some embodiments， first feature 20 is an electrically conductive first stiffening feature and in some embodiments the electrically conductive first stiffening feature includes a plurality of laterally overlapping electrically conductive ribs as described further elsewhere herein. In some embodiments， first feature 20 is or includes an electrically conductive first electrode. In some embodiments， the first feature 20 is electrically isolated from the electronic circuit 77 of each flexible multilayer construction 10. The first feature 20 extends a length L2 along a first direction (x-direction) across at least a majority (i.e.， greater than 50 percent) of a length L1 of the flexible multilayer system 100 between

opposite edges

102 and 104 of the flexible multilayer system 100. In some embodiments， L2/L1 is greater than 0.6， or greater than 0.7， or greater than 0.8， or greater than 0.9. In some embodiments， the feature 20 is an electrically conductive first stiffening feature extending substantially across the entire flexible multilayer system 100 between

opposite edges

102 and 104 of the flexible multilayer system 100.

In some embodiments， the first feature 20 is a first stiffening feature that increases a stiffness of the flexible multilayer system 100 against a bend along the first direction. To illustrate a bend along a first direction， if edge 102 were held fixed while opposite edge 104 were moved out of the x-y plane， the flexible multilayer system 100 would bend along the x-direction.

In some embodiments， the electronic circuits 77 of the flexible multilayer constructions 10 and the electrically conductive first stiffening feature are integrally formed on the flexible substrate 40. In some embodiments， the first feature 20 is disposed on a top or on a bottom surface of a flexible substrate 40. In some embodiments， the first feature 20 includes portions on both the top the bottom surfaces of a flexible substrate as described further elsewhere herein.

In some embodiments， the flexible multilayer system 100 includes one or more first features extending in a first direction and one or more second features extending in a different second direction. In some embodiments， the flexible multilayer system 100 includes a plurality of

first features

20， 24 and 25， which may be a plurality of electrically conductive first stiffening features. Alternatively， the flexible multilayer system 100 may be described as including an electrically conductive first stiffening feature which is or includes the

first features

20， 24 and 25. Each first stiffening feature may be integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit 77 of each flexible multilayer construction 10. In some embodiments， each first stiffening feature extends along the first direction substantially across the entire flexible multilayer system 100 between the

opposite edges

102 and 104 of the flexible multilayer system 100. In some embodiments， the

first features

20， 24 and 25 are first stiffening features that increase the stiffness of the flexible multilayer system 100 against a bend along the first direction. In some embodiments， the first feature 20 is or includes an electrically conductive rib 21 extending continuously along the first direction substantially across the entire flexible multilayer system 100 between

opposite edges

102 and 104 of the flexible multilayer system 100. Similarly， in some embodiments， each of the

first features

24 and 25 is or includes an electrically

conductive rib

22 and 23， respectively， extending continuously along the first direction substantially across the entire flexible multilayer system 100 between

opposite edges

102 and 104 of the flexible multilayer system 100.

An element (e.g.， a stiffening feature， a rib or an electrode) is described herein as extending across substantially an entire length or width of an article (e.g.， a flexible multilayer system or a substrate) if the element extends across at least 80 percent of the length or width， respectively. In some embodiments， an element described as extending across substantially an entire length or width of an article， extends across at least 85 percent， or at least 90 percent， or at least 95 percent of the length or width， respectively.

In some embodiments， the flexible multilayer system 100 further includes second feature 50， which may be an electrically conductive second stiffening feature and/or an electrode， integrally formed on the flexible multilayer system 100 and electrically isolated from the electronic circuit of each flexible multilayer construction 10. In some embodiments， the flexible multilayer system 100 includes a plurality of

second features

50， 52， 54 and 56. Alternatively， the flexible multilayer system 100 may be described as including a second electrically conductive second stiffening feature which is or includes second features 50， 52， 54 and 56. In some embodiments， the second feature 50 extends along a second direction (y-direction in the illustrated embodiment) ， different than the first direction， across at least a majority of a width W1 of the entire flexible multilayer system between

opposite edges

105 and 106 of the flexible multilayer system 100. In some embodiments， the second feature 50 is a second stiffening feature that increases a stiffness of the flexible multilayer system against a bend along the second direction. In some embodiments， the flexible multilayer system 100 has a maximum width W1 along the second direction and the second feature 50 has a length L5 along the second direction， where L5/W1 is greater than 0.6， or greater than 0.7， or greater than 0.8， or greater than 0.9. In some embodiments， the second feature 50 extends substantially across the entire flexible multilayer system 100 between

opposite edges

105 and 106 of the flexible multilayer system 100. In some embodiments， the first and

second features

20 and 50 physically intersect one another.

In some embodiments， the flexible multilayer system 100 further includes at least one electrically conductive feature 34 disposed between a longitudinal end 20a of the first feature 20 and the edge 104 of the multilayer system corresponding to the longitudinal end 20a. In some embodiments， the flexible multilayer system 100 further includes at least one electrically conductive feature 35 disposed between the first feature 20 and an edge 105 of the multilayer system 100 closest to the first feature 20 and substantially parallel to the first feature 20. In some embodiments， the flexible multilayer system 100 further includes at least one electrically conductive feature 35 disposed between a longitudinal end 50a of the second feature 50 and the edge 105 of the multilayer system 100 corresponding to the longitudinal end 50a. In other words， in some embodiments， the at least one electrically conductive feature 35 is closer to the edge 105 which is closest to the longitudinal end 50a than the longitudinal end 50a is to the edge 105. The flexible multilayer system 100 can cut into individual flexible multilayer constructions 10 or strips or rows of flexible multilayer constructions 10 by slitting or stamping the substrate 40， for example. In some embodiments， the at least one electrically conductive features 34 and/or 35 are alignment features that facilitates alignment of a die cutter or the like with the flexible multilayer constructions 10.

FIG. 2 which is a schematic cross-sectional view of an exemplary multilayer construction 110 which may correspond to multilayer construction 10. A light emitting semiconductor device (LESD) 30 mounted on the flexible multilayer construction 110 is illustrated in FIG. 2. In some embodiments， the electronic circuit 150 of the flexible multilayer construction 110 includes spaced apart electrically conductive first and

second pads

111 and 112， which may correspond to first and

second pads

11 and 12， electrically isolated from each other for electrically connecting to respective electrically conductive first and second 31 and 32 terminals of the light emitting semiconductor device (LESD) 30 mounted on the flexible multilayer construction 110. In the embodiment illustrated in FIG. 2， the first and

second pads

111 and 112 are disposed on a first major surface 141 of the flexible substrate 140， which may correspond to flexible substrate 40， and the electronic circuit 150 of the flexible multilayer construction 110 further includes： spaced apart electrically conductive third and

fourth pads

113 and 114 electrically isolated from each other and disposed on an opposite second major surface 142 of the flexible substrate 140； an electrically conductive first via 115 extending between and connecting the first and

third pads

111 and 113； and an electrically conductive second via 116 extending between and connecting the second and

fourth pads

112 and 114. Solder bumps 47， for example， may be used to attach first and

second pads

111 and 112 to respective first and second 31 and 32 terminals. Other known bonding methods (e.g.， eutectic bonding) may alternatively be used.

Referring again to FIG. 1， in some embodiments， a flexible multilayer system 100 includes a flexible substrate 40 having opposing top and bottom major surfaces； a plurality of spaced apart parallel electrically conductive first electrodes (corresponding to

first features

20， 24 and 25) disposed on the top or bottom major surface of the substrate 40 and extending along a first direction (x-direction in the illustrated embodiments) ； and a plurality of spaced apart parallel electrically conductive second electrodes (corresponding to

second features

50， 52， 54， and 56) disposed on the top or bottom major surface of the substrate and extending along a different second direction (y-direction in the illustrated embodiment) ， such that in a plan view of the flexible multilayer system 100， the first and second electrodes form a two-dimensional grid defining a plurality of grid cells (e.g.， grid cell 64) ， each grid cell comprising an electronic circuit 77 electrically isolated from the first and second electrodes and including electrically conductive first and

second pads

11 and 12 for electrically connecting to respective electrically conductive first and

second terminals

31 and 32 of a light emitting semiconductor device (LESD) 30 mounted on the flexible multilayer construction entirely within the grid cell. In some embodiments， each grid cell includes a plurality of spaced apart electronic circuits 77. In some embodiments， the plurality of spaced apart electronic circuits 77 forms a regular array of spaced apart electronic circuits.

In some embodiments， the pluralities of the spaced apart electrically conductive first and second electrodes are integrally formed on the top major surface of the substrate (e.g.， corresponding to ribs 220 described elsewhere herein) ， and the first and second pads of each electronic circuit are integrally formed on the top major surface of the substrate (e.g.， corresponding to

electrodes

211 and 212 described elsewhere herein) . In some embodiments， the pluralities of the spaced apart electrically conductive first and second electrodes are integrally formed on the bottom major surface of the substrate (e.g.， corresponding to ribs 226 described elsewhere herein) ， and the first and second pads of each electronic circuit are integrally formed on the top major surface of the substrate (e.g.， corresponding to

electrodes

211 and 212 described elsewhere herein) .

A stiffening feature may be disposed on one or both sides of a flexible substrate. In some embodiments， stiffening features are formed on a same side of a flexible substrate as an electronic circuit and in some embodiments， stiffening features are formed on an opposite side of a flexible substrate as an electronic circuit. Is some embodiments， the stiffening feature and the electronic circuit each includes elements on both opposing sides of a flexible substrate. In some embodiments， stiffening features are disposed on opposing sides of a substrate and the opposing stiffening features are connected to each other by an electronically conducive via. In some embodiments， a first stiffening rib (e.g.， rib 21) is formed on a first major surface of a flexible substrate and a second stiffening rib is formed on an opposing second major surface of the substrate.

FIG. 3 is a schematic cross-sections view of a flexible multilayer system 300 including a flexible substrate 240 having opposing first and second

major surfaces

241 and 242 and a plurality of spaced apart electronic circuits 277 arranged on and along a length (e.g.， L1 depicted in FIG. 1) of the flexible substrate 240. Each electronic circuit 277 includes first and

second electrodes

211 and 213 on the respective first and second

major surfaces

241 and 242 of the substrate 240 and includes an electrically conductive via 215 extending between and connecting the first and

second electrodes

211 and 213. In the illustrated embodiment， each electronic circuit 277 further includes third and

fourth electrodes

212 and 214 on the respective first and second

major surfaces

241 and 242 of the substrate 240 and includes an electrically conductive via 216 extending between and connecting the third and

fourth electrodes

212 and 214. Multilayer system 300 may correspond to multilayer system 100 and first， second， third and

fourth electrodes

211， 213， 212 and 214 may correspond to first， third， second and

fourth pads

111， 113， 112 and 114， respectively. The flexible multilayer system 300 further includes parallel electrically conductive first and

second stiffening ribs

220 and 226 formed integrally on the respective first and second

major surfaces

241 and 242 of the substrate 240. The first and

second ribs

220 and 226 are electrically connected to each other and electrically isolated from each electronic circuit. In some embodiments， at least one of the first and

second stiffening ribs

220 and 226 extends substantially along the entire length of the substrate 240 (e.g.， the length L1 of FIG. 1) and increases a stiffness of the flexible multilayer system 300 against a bend along the length of the substrate 240. In some embodiments， each of the first and

second stiffening ribs

220 and 226 extends substantially along the entire length of the substrate 240 and increases a stiffness of the flexible multilayer system 300 against a bend along the length of the substrate 240. In some embodiments， more than one pair of first and

second stiffening ribs

220 and 226 are included. For example， three pairs of stiffening ribs are depicted in the embodiment of FIG. 3. In some embodiments， the first and

second stiffening ribs

220 and 226 are electrically connected to each other by an electronically conductive via 227 extending between and connecting the first and

second stiffening ribs

220 and 226. In other embodiments， the via 227 may be omitted.

In some embodiments， the flexible multilayer system 300 further includes at least one electrically conductive feature 135 which may correspond to the at least one electrically

conductive features

34 or 35， for example.

In some embodiments， a flexible multilayer system (e.g.，

flexible multilayer system

100， 300 or 400) includes a plurality of spaced apart electronic circuits that extend substantially across the entire length and width of the flexible multilayer system. In some embodiments， in a plan view， the plurality of the spaced apart electronic circuits occupies at least 50％or at least 70％of a maximum area of the flexible multilayer system. For example， referring to FIG. 1， the maximum area of the flexible multilayer system 100 is L1 times W1 and， in some embodiments， the spaced apart electronic circuits 77 included in the flexible multilayer constructions 10 occupy an area of at least 50％or at least 70％of L1 times W2. In some embodiments， the plurality of spaced apart electronic circuits form a regular array of spaced apart electronic circuits. In some embodiments， this regular array is a two-dimensional array.

In some embodiments， the electronic circuit 277 of the flexible multilayer system 300 has a maximum height H1 and the first rib 220 has a maximum height of H2.

Similarly， in some embodiments， the electronic circuit 77 of the flexible multilayer system 100 has a maximum height H1 and the first feature 20 has a maximum height of H2. In some embodiments， H1 is substantially equal to H2. Two heights are described herein as substantially equal if the heights differ by less than 10 percent (i.e.， (larger height-smaller height) divided by the larger height times 100 percent is less than 10 percent) . In some embodiments， H2/H1 is greater than 1. In some embodiments， H2/H1 is at least 1.2， or at least 1.3， or at least 1.4.

In some embodiments， the feature 20 is a first stiffening feature that includes a plurality of laterally overlapping electrically conductive ribs.

FIG. 4 is a schematic top view of a flexible multilayer system 400 for being cut into a plurality of flexible multilayer constructions 310. Each flexible multilayer construction 310 includes an electronic circuit 377 such as any of those described elsewhere herein. The flexible multilayer system 400 includes an electrically conducive first stiffening feature 420 integrally formed on the flexible multilayer system 400. The first stiffening feature 420 has a length L2 and extends along a first direction (x-direction in the illustrated embodiment) across at least a majority of a length L1 of the flexible multilayer system 400 between

opposite edges

302 and 304 of the flexible multilayer system 400. The first stiffening feature 420 includes laterally overlapping electrically

conductive ribs

121， 122 and 123. In some embodiments， each rib is electrically isolated from the electronic circuit of each flexible multilayer construction 310. Each rib extends continuously along the first direction across a different portion of the flexible multilayer system 400 such that each rib partially laterally overlaps at least one other rib along a second direction (y-direction in the illustrated embodiment) perpendicular to the first direction. For example， rib 123 has a length L3 along the x-direction. Ribs 122 and 123 overlap each other along the y-direction and have a lateral overlap length of L4. In some embodiments， a maximum length of the individual ribs is L3 and a minimum lateral overlap length between the ribs is L4 where L4/L3 is greater than 0.1， or greater than 0.2， or greater than 0.3， or greater than 0.4， or greater than 0.5. It has been found that utilizing even a relatively small overlap ratio (L4/L3) provides a substantial stiffening effect while allowing more space for additional circuits， for example. In some embodiments， L4/L3 is in a range of 0.1 to 0.9， or in a range of 0.2 to 0.8， or in a range of 0.2 to 0.6.

The first stiffening feature 420 may be disposed on a major surface of flexible substrate 440 or may include portions disposed on opposite major surfaces of the flexible substrate 440 as illustrated in FIG. 3. The flexible multilayer system 400 may further include an electrically conductive second stiffening feature 450 integrally formed on the flexible multilayer system 400 and electrically isolated from the electronic circuit of each flexible multilayer construction 310. In some embodiments， the second stiffening feature 450 extends along a second direction (y-direction in the illustrated embodiment) ， different than the first direction， across at least a majority of a width of the entire flexible multilayer system between

opposite edges

305 and 306 of the flexible multilayer system 400. In some embodiments， the second stiffening feature 450 comprises a plurality of spaced apart stiffening ribs. The second stiffening feature may be provided to increase a stiffness of the flexible multilayer system 400 against a bend along the second direction.

The flexible multilayer system 400 may further include at least one electrically conductive feature 434 disposed between a longitudinal end 23a of a rib 123 closest to an edge 304 of the multilayer system 400. The flexible multilayer system 400 may further include at least one electrically conductive feature 435 disposed between an edge 305 of the multilayer system 400 and the rib 121 closest to the edge 305. The at least one electrically conductive feature 434 and/or 435 may be alignment features used to aid in singulating the flexible multilayer system 400 into individual multilayer construction 310.

The flexible multilayer systems of the present description can be made by applying a conductive layer on one or both major surfaces of a flexible dielectric substrate and etching a desired pattern for the circuits and stiffening features into the conductive layer or layers. In embodiments where a via connects an element on a top surface of the substrate to an element on a bottom surface of the substrate， the via may be etched into the substrate and then plated with a conductive material prior to applying the conductive layers onto the major surfaces of the substrate. In some embodiments， the circuits included in the flexible multilayer systems include at least one cavity containing a conductive material including electrically separated first and second portions configured to support and electrically connect a light emitting semiconductor device to the conductive layer on a major surface of the substrate as described in U.S. Pat. No. 9,179,543 (Palaniswamy et al. ) .

Suitable dielectric substrates for the present invention include polyesters， polycarbonates， liquid crystal polymers， and polyimides. Polyimides are preferred. Suitable polyimides include those available under the trade names KAPTON， available from DuPont； APICAL， available from Kaneka Texas corporation； SKC Kolon PI， available from SKC Kolon PI Inc， and UPILEX and UPISEL， available from Ube Industries. Most preferred are polyimides available under the trade designations UPILEX S， UPILEX SN， and UPISEL VT， all available from Ube Industries， Japan. These polyimides are made from monomers such as biphenyl tetracarboxylic dianhydride (BPDA) and phenyl diamine (PDA) .

Cavities or vias may be formed in the dielectric substrates using any suitable method such as chemical etching， plasma etching， focused ion-beam etching， laser ablation， embossing， microreplication， injection molding， and punching. Chemical etching may be preferred in some embodiments. Any suitable etchant may be used and may vary depending on the dielectric substrate material. Suitable etchants may include alkali metal salts， e.g. potassium hydroxide； alkali metal salts with one or both of solubilizers， e.g.， amines， and alcohols， such as ethylene glycol. Suitable chemical etchants for some embodiments of the present invention include KOH/ethanol amine/ethylene glycol etchants such as those described in more detail in U.S. Pat. Pub. No. 2007/0120089 (Mao et al. ) . Other suitable chemical etchants for some embodiments of the present invention include a KOH/glycine etchants such as those described in more detail in U.S. Pat. Pub. No. 2013/0207031 (Palaniswamy) . Subsequent to etching， the dielectric substrates may be treated with an alkaline KOH/potassium permanganate (PPM) solution， e.g.， a solution of about 0.7 to about 1.0 wt％KOH and about 3 wt％KMnO4.

The side wall angle resulting from chemical etching varies， and is most dependent on etch rate， with slower etching rates resulting in shallower side wall angles. Typical side wall angles resulting from chemical etching are about 5° to 60°， and in at least one embodiment， about 25° to about 28°. As previously mentioned as an alternative to chemical etching， cavities in the dielectric substrate may be formed by punching， plasma etching， focused ion-beam etching， and laser ablation. With these methods of forming a cavity， the side walls typically have a steeper angle， e.g.， up to 90°. A sloped side wall means a side wall that is not perpendicular to the horizontal plane of the dielectric layer. Cavities or vias with sloped sidewalls could also be made using methods such as punching， embossing， microreplication， and injection molding. If a via is initially formed， but a cavity is desired， a dielectric coating， such as a polyimide coating， may be added to electrically insulate the cavity from a conductive layer on the bottom side of the dielectric substrate. The dielectric coating is preferably thermally conducting to facilitate transfer of heat away from the LESD. The dielectric coating may be any suitable material that is electrically insulating and， preferably， thermally conducting. One such suitable coating is a polyimide resin formed by first applying a thin layer of polyamic acid resin in the cavity. The polyamic acid is preferably precision-coated such that the dielectric coating formed at the bottom of the cavity provides the desired thickness for the cavity floor. Subsequently， an imidization process is carried out to form a uniform polyimide coating in the cavity. The polyimide/polyamic acid resin can be applied using precision coating， knife coating， or other methods known in the art. The thickness of the cavity floor may be no more than about 5％to about 25％of the thickness of the dielectric substrate layer.

A conductive layer may be applied to the bottom side of the dielectric substrate before the cavity or via is formed if the cavity-or via-forming method would not destroy the conductive layer， e.g.， because the etching depth can be controlled and/or because the cavity-forming method will not etch or degrade the conductive layer， such as with plasma etching， or it may be added after the cavity is formed if the cavity-forming method would destroy the conductive layer， such as with punching.

The dielectric substrates may be clad on one or both sides with a conductive layer which may be patterned to form circuits and stiffening features. A multilayer flexible substrate (having multiple layers of dielectric and conductive material) may also be used as a substrate. The conductive layers may be any suitable material， such as elemental metals or metal alloys. In some embodiments， the conductive layer (s) are copper layer (s) .

In some embodiments， when the flexible multilayer system (e.g.， 100， 300 or 400) has a length L1 of at least 80 centimeters (or at least 90 centimeters， or at least 100 centimeters， or at least 110 centimeters， or at least 120 centimeters) between the opposite edges (e.g.， 102 and 104， or 302 and 304) and the flexible multilayer system is held along one， but not the opposite， edge， the flexible multilayer system bends at the held edge at least 5 degrees (or at least 10 degrees) less than a comparative flexible multilayer system having the same construction except that it does not have the first stiffening feature. For example， in the embodiment illustrated in FIG. 1 includes a first stiffening feature (corresponding to

first features

20， 24 and 25) . The comparative flexible multilayer system 100C is illustrated in FIG. 5 which does not include the first stiffening feature but is otherwise equivalent to flexible multilayer system 100. The relative bending of a flexible multilayer system 600 and a comparative flexible multilayer system 600C is schematically illustrated in FIG. 6. The flexible multilayer system 600， and similarly for the comparative flexible multilayer system 600C， is held fixed at a held edge having the smallest x-coordinate in the figure. The held edge is held horizontally at a fixed height and the opposite edge (largest x-coordinate) is not held but is allowed to deflect under the influence of gravity. The flexible multilayer system 600 bends at an angle of θ at the held edge， while the comparative multilayer system 600 bends at and an angle of θC at the held edge. In some embodiments， θ is less than θC by at least 5 degrees or by at least 10 degrees.

The following is a list of exemplary embodiments of the present description.

Embodiment 1 is a flexible multilayer system for being cut into a plurality of flexible multilayer constructions， each flexible multilayer construction comprising an electronic circuit， the flexible multilayer system comprising an electrically conductive first stiffening feature integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit of each flexible multilayer construction， the first stiffening feature extending along a first direction across at least a majority of a length of the flexible multilayer system between opposite edges of the flexible multilayer system， the first stiffening feature increasing a stiffness of the flexible multilayer system against a bend along the first direction.

Embodiment 2 is the flexible multilayer system of Embodiment 1， wherein the electronic circuit of each flexible multilayer construction comprises spaced apart electrically conductive first and second pads electrically isolated from each other for electrically connecting to respective electrically conductive first and second terminals of a light emitting semiconductor device (LESD) mounted on the flexible multilayer construction.

Embodiment 3 is the flexible multilayer system of Embodiment 1 comprising a flexible substrate， wherein the electronic circuits of the flexible multilayer constructions and the electrically conductive first stiffening feature are integrally formed on the flexible substrate.

Embodiment 4 is the flexible multilayer system of Embodiment 1， wherein the electrically conductive first stiffening feature comprises an electrically conductive rib extending continuously along the first direction substantially across the entire flexible multilayer system between opposite edges of the flexible multilayer system.

Embodiment 5 is the flexible multilayer system of Embodiment 1 having a maximum length L1 along the first direction and the electrically conductive first stiffening feature having a length L2 along the first direction， L2/L1 being greater than 0.6.

Embodiment 6 is the flexible multilayer system of Embodiment 5， wherein L2/L1 is greater than 0.7.

Embodiment 7 is the flexible multilayer system of Embodiment 5， wherein L2/L1 is greater than 0.8.

Embodiment 8 is the flexible multilayer system of Embodiment 5， wherein L2/L1 is greater than 0.9.

Embodiment 9 is the flexible multilayer system of Embodiment 1， wherein the electrically conductive first stiffening feature comprises a plurality of laterally overlapping electrically conductive ribs， each rib extending continuously along the first direction across a different portion of the flexible multilayer system， each rib partially laterally overlapping at least one other rib along a second direction perpendicular to the first direction， each rib being electrically isolated from the electronic circuit of each flexible multilayer construction.

Embodiment 10 is the flexible multilayer system of Embodiment 9， wherein a maximum length of the individual ribs is L3 and a minimum lateral overlap length between the ribs is L4， L4/L3 being greater than 0.1.

Embodiment 11 is the flexible multilayer system of Embodiment 10， wherein L4/L3 is greater than 0.2.

Embodiment 12 is the flexible multilayer system of Embodiment 10， wherein L4/L3 is greater than 0.3.

Embodiment 13 is the flexible multilayer system of Embodiment 10， wherein L4/L3 is greater than 0.4.

Embodiment 14 is the flexible multilayer system of Embodiment 10， wherein L4/L3 is greater than 0.5.

Embodiment 15 is the flexible multilayer system of Embodiment 1 comprising a plurality of electrically conductive first stiffening features， each first stiffening feature integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit of each flexible multilayer construction， each first stiffening feature extending along the first direction substantially across the entire flexible multilayer system between the opposite edges of the flexible multilayer system， the first stiffening features increasing the stiffness of the flexible multilayer system against a bend along the first direction.

Embodiment 16 is the flexible multilayer system of Embodiment 1 further comprising an electrically conductive second stiffening feature integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit of each flexible multilayer construction， the second stiffening feature extending along a second direction， different than the first direction， across at least a majority of a width of the entire flexible multilayer system between opposite edges of the flexible multilayer system， the second stiffening feature increasing a stiffness of the flexible multilayer system against a bend along the second direction.

Embodiment 17 is the flexible multilayer system of Embodiment 16 having a maximum width W1 along the second direction and the electrically conductive second stiffening feature having a length L5 along the second direction， L5/W1 being greater than 0.6.

Embodiment 18 is the flexible multilayer system of Embodiment 17， wherein L5/W1 is greater than 0.7.

Embodiment 19 is the flexible multilayer system of Embodiment 17， wherein L5/W1 is greater than 0.8.

Embodiment 20 is the flexible multilayer system of Embodiment 17， wherein L5/W1 is greater than 0.9.

Embodiment 21 is the flexible multilayer system of Embodiment 16， wherein the first and second stiffening features physically intersect one another.

Embodiment 22 is the flexible multilayer system of Embodiment 16 further comprising at least one electrically conductive feature disposed between a longitudinal end of the second stiffening feature and the edge of the multilayer system corresponding to the longitudinal end.

Embodiment 23 is the flexible multilayer system of Embodiment 1 comprising a flexible substrate， wherein the electronic circuit of at least one flexible multilayer construction comprises：

spaced apart electrically conductive first and second pads electrically isolated from each other and disposed on a first major surface of the flexible substrate for electrically connecting to respective electrically conductive first and second terminals of a light emitting semiconductor device (LESD) mounted on the flexible substrate； and

spaced apart electrically conductive third and fourth pads electrically isolated from each other and disposed on an opposite second major surface of the flexible substrate， an electrically conductive first via extending between and connecting the first and third pads， an electrically conductive second via extending between and connecting the second and fourth pads.

Embodiment 24 is the flexible multilayer system of Embodiment 1， wherein a maximum height of the electronic circuit is H1 and a maximum height of the first stiffening feature is H2， H1 being substantially equal to H2.

Embodiment 25 is the flexible multilayer system of Embodiment 1， wherein a maximum height of the electronic circuit is H1 and a maximum height of the first stiffening feature is H2， H2/H1 being at least 1.3.

Embodiment 26 is the flexible multilayer system of Embodiment 1 further comprising at least one electrically conductive feature disposed between a longitudinal end of the first stiffening feature and the edge of the multilayer system corresponding to the longitudinal end.

Embodiment 27 is the flexible multilayer system of Embodiment 1， wherein the flexible multilayer system comprises at least one electrically conductive feature disposed between the first stiffening feature and an edge of the multilayer system closest to the first stiffening feature and substantially parallel to the first stiffening feature.

Embodiment 28 is the flexible multilayer system of Embodiment 1， wherein when the flexible multilayer system has a length of at least 80 centimeters between the opposite edges and the flexible multilayer system is held along one， but not the opposite， edge， the flexible multilayer system bends at the held edge at least 5 degrees less than a comparative flexible multilayer system having the same construction except that it does not have the first stiffening feature.

Embodiment 29 is the flexible multilayer system of Embodiment 28， wherein when the flexible multilayer system has a length of at least 90 centimeters between the opposite edges.

Embodiment 30 is the flexible multilayer system of Embodiment 28， wherein when the flexible multilayer system has a length of at least 100 centimeters between the opposite edges.

Embodiment 31 is the flexible multilayer system of Embodiment 28， wherein when the flexible multilayer system has a length of at least 110 centimeters between the opposite edges.

Embodiment 32 is the flexible multilayer system of Embodiment 28， wherein when the flexible multilayer system has a length of at least 120 centimeters between the opposite edges.

Embodiment 33 is the flexible multilayer system of Embodiment 28 bending at least 10 degrees less than the comparative flexible multilayer system.

Embodiment 34 is a flexible multilayer system comprising：

a flexible substrate comprising opposing first and second major surfaces；

a plurality of spaced apart electronic circuits arranged on and along a length of the flexible substrate， each electronic circuit comprising：

first and second electrodes on the respective first and second major surfaces of the substrate； and

an electrically conductive via extending between and connecting the first and second electrodes； and

parallel electrically conductive first and second stiffening ribs formed integrally on the respective first and second major surfaces of the substrate， the first and second ribs electrically connected to each other and electrically isolated from each electronic circuit， at least one of the first and second stiffening ribs extending substantially along the entire length of the substrate and increasing a stiffness of the flexible multilayer system against a bend along the length of the substrate.

Embodiment 35 is the flexible multilayer system of Embodiment 34， wherein the first and second stiffening ribs are electrically connected to each other by an electronically conductive via extending between and connecting the first and second stiffening fibs.

Embodiment 36 is the flexible multilayer system of Embodiment 34， wherein each of the first and second stiffening ribs extends substantially along the entire length of the substrate and increases a stiffness of the flexible multilayer system against a bend along the length of the substrate.

Embodiment 37 is the flexible multilayer system of Embodiment 34， wherein the plurality of the spaced apart electronic circuits extend substantially across the entire length and width of the flexible multilayer system.

Embodiment 38 is the flexible multilayer system of Embodiment 34， wherein in a plan view， the plurality of the spaced apart electronic circuits occupies at least 50％of a maximum area of the flexible multilayer system.

Embodiment 39 is the flexible multilayer system of Embodiment 34， wherein in a plan view， the plurality of the spaced apart electronic circuits occupies at least 70％of a maximum area of the flexible multilayer system.

Embodiment 40 is the flexible multilayer system of Embodiment 34， wherein the plurality of spaced apart electronic circuits form a regular array of spaced apart electronic circuits.

Embodiment 41 is the flexible multilayer system of Embodiment 40， wherein the regular array is a two-dimensional array.

Embodiment 42 is a flexible multilayer system comprising：

a flexible substrate comprising opposing top and bottom major surfaces；

a plurality of spaced apart parallel electrically conductive first electrodes disposed on the top or bottom major surface of the substrate and extending along a first direction； and

a plurality of spaced apart parallel electrically conductive second electrodes disposed on the top or bottom major surface of the substrate and extending along a different second direction， such that in a plan view of the flexible multilayer system， the first and second electrodes form a two-dimensional grid defining a plurality of grid cells， each grid cell comprising an electronic circuit electrically isolated from the first and second electrodes and comprising electrically conductive first and second pads for electrically connecting to respective electrically conductive first and second terminals of a light emitting semiconductor device (LESD) mounted on the flexible multilayer construction entirely within the grid cell.

Embodiment 43 is the flexible multilayer system of Embodiment 42， wherein the pluralities of the spaced apart electrically conductive first and second electrodes and the first and second pads of each electronic circuit are integrally formed on the top major surface of the substrate.

Embodiment 44 is the flexible multilayer system of Embodiment 42， wherein the pluralities of the spaced apart electrically conductive first and second electrodes are integrally formed on the bottom major surface of the substrate， and the first and second pads of each electronic circuit are integrally formed on the top major surface of the substrate.

Embodiment 45 is the flexible multilayer system of Embodiment 42， wherein each grid cell comprises a plurality of spaced apart electronic circuits.

Embodiment 46 is the flexible multilayer system of Embodiment 45， wherein the plurality of spaced apart electronic circuits forms a regular array of spaced apart electronic circuits.

Examples

Example 1

Finite Element Analysis (FEA) was used to determine the angles θ and θC (see FIG. 6) for a flexible multilayer system similar to flexible multilayer system 100 depicted in FIG. 1. The flexible multilayer system had a length L1 of 104 mm and a width W1 of 52 mm.The flexible substrate 40 was modeled as polyimide having a thickness of 0.05 mm. Three equally spaced ribs extending in the x-direction (length direction) across the length of the substrate were included and six equally spaced ribs extending in the y-direction (width direction) across the width of the substrate were included. The ribs included portions on the upper and lower surfaces and internal portion connecting the upper and lower portions (as in FIG. 3) . The upper and lower portions had a thickness of 0.035 mm and a width of 2.2 mm， and the internal portion had a width of 1.1 mm. One end of the flexible multilayer system was held fixed with clamped boundary conditions and the other end was allowed to deflect due to gravity. The angle θ was determined to be 17.34 degrees when held along an end in the length direction (e.g.， edge 102) and was determined to be 2.83 degrees when held along an end in the width direction (e.g.， edge 105) . A comparative system equivalent to the flexible multilayer system but without any ribs was also modeled. The angle θC was determined to be 31.39 degrees when held along an end in the length direction and was determined to be 6.06 degrees when held along an end in the width direction. The difference between θC and θ in the length direction was due primarily to the ribs extending in the length direction， and the difference between θC and θin the length direction was due primarily to the ribs extending in the width direction.

Example 2

Finite Element Analysis (FEA) was used to determine the angles θ and θC (see FIG. 6) for a flexible multilayer system similar to flexible multilayer system 400 depicted in FIG. 4. The flexible multilayer system was modeled as described in Example 1 except that the three ribs extending in the x-direction (length direction) extended only over portions of the flexible substrate. Various degrees of overlap (specified by L4/L3) were considered. The angle θ was determined to be 23.04 degrees for L4/L3＝0.4 and 23.53 degrees for L4/L3＝0.2 when held along an end in the length direction (e.g.， edge 302) and was determined to be 3.80 degrees for L4/L ＝ 0.4 or 0.3 when held along an end in the width direction (e.g.， edge 305) . The comparative system equivalent to the flexible multilayer system but without any ribs is as described for Example 1.

Example 3

A flexible multilayer system was prepared by laminating a copper foil having a thickness of 0.035mm to a chemically etched polyimide film having a thickness of 0.05 mm. Subtractive etching of the copper foil was used to form the desired pattern and the etched vias were directly filled as thermal conduits. The flexible multilayer system had a length L1 of 104 mm and a width W1 of 52 mm and included a two-dimensional array of 10 by 23 circuits disposed within an approximately rectangular feature having a uniform width of 2.4 mm disposed near the edges of the flexible multilayer system. The rectangular feature was a first stiffening feature including two ribs extending along the length direction near the long edges of the sample and a second stiffening feature including two ribs extending along the width direction near the short edges of the sample.

A comparative multilayer system was prepared similarly but with the rectangular feature broken into an array of squares of width 1.23 mm with a gap of 0.07 mm between adjacent squares so that that the array of squares did not substantially stiffen the comparative multilayer system.

The flexible multilayer system and the comparative system were taped at one end to a flat surface and the opposite end in the length direction was allowed to deflect due to gravity. The angle θ was measured and determined to be 22 degrees and the angle θC was measured and determined to be 37 degrees.

Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures， unless indicated otherwise. Although specific embodiments have been illustrated and described herein， it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore， it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

A flexible multilayer system for being cut into a plurality of flexible multilayer constructions， each flexible multilayer construction comprising an electronic circuit， the flexible multilayer system comprising an electrically conductive first stiffening feature integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit of each flexible multilayer construction， the first stiffening feature extending along a first direction across at least a majority of a length of the flexible multilayer system between opposite edges of the flexible multilayer system， the first stiffening feature increasing a stiffness of the flexible multilayer system against a bend along the first direction.
The flexible multilayer system of claim 1， wherein the electronic circuit of each flexible multilayer construction comprises spaced apart electrically conductive first and second pads electrically isolated from each other for electrically connecting to respective electrically conductive first and second terminals of a light emitting semiconductor device (LESD) mounted on the flexible multilayer construction.
The flexible multilayer system of claim 1 comprising a flexible substrate， wherein the electronic circuits of the flexible multilayer constructions and the electrically conductive first stiffening feature are integrally formed on the flexible substrate.
The flexible multilayer system of claim 1， wherein the electrically conductive first stiffening feature comprises an electrically conductive rib extending continuously along the first direction substantially across the entire flexible multilayer system between opposite edges of the flexible multilayer system.
The flexible multilayer system of claim 1 having a maximum length L1 along the first direction and the electrically conductive first stiffening feature having a length L2 along the first direction， L2/L1 being greater than 0.6.
The flexible multilayer system of claim 1， wherein the electrically conductive first stiffening feature comprises a plurality of laterally overlapping electrically conductive ribs， each rib extending continuously along the first direction across a different portion of the flexible multilayer system， each rib partially laterally overlapping at least one other rib along a second direction perpendicular to the first direction， each rib being electrically isolated from the electronic circuit of each flexible multilayer construction.
The flexible multilayer system of claim 6， wherein a maximum length of the individual ribs is L3 and a minimum lateral overlap length between the ribs is L4， L4/L3 being greater than 0.1.
The flexible multilayer system of claim 1 comprising a plurality of electrically conductive first stiffening features， each first stiffening feature integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit of each flexible multilayer construction， each first stiffening feature extending along the first direction substantially across the entire flexible multilayer system between the opposite edges of the flexible multilayer system， the first stiffening features increasing the stiffness of the flexible multilayer system against a bend along the first direction.
The flexible multilayer system of claim 1 further comprising an electrically conductive second stiffening feature integrally formed on the flexible multilayer system and electrically isolated from the electronic circuit of each flexible multilayer construction， the second stiffening feature extending along a second direction， different than the first direction， across at least a majority of a width of the entire flexible multilayer system between opposite edges of the flexible multilayer system， the second stiffening feature increasing a stiffness of the flexible multilayer system against a bend along the second direction.
The flexible multilayer system of claim 9 having a maximum width W1 along the second direction and the electrically conductive second stiffening feature having a length L5 along the second direction， L5/W1 being greater than 0.6.
The flexible multilayer system of claim 9， wherein the first and second stiffening features physically intersect one another.
The flexible multilayer system of claim 9 further comprising at least one electrically conductive feature disposed between a longitudinal end of the second stiffening feature and the edge of the multilayer system corresponding to the longitudinal end.
The flexible multilayer system of claim 1 comprising a flexible substrate， wherein the electronic circuit of at least one flexible multilayer construction comprises：

spaced apart electrically conductive first and second pads electrically isolated from each other and disposed on a first major surface of the flexible substrate for electrically connecting to respective electrically conductive first and second terminals of a light emitting semiconductor device (LESD) mounted on the flexible substrate； and

spaced apart electrically conductive third and fourth pads electrically isolated from each other and disposed on an opposite second major surface of the flexible substrate， an electrically conductive first via extending between and connecting the first and third pads， an electrically conductive second via extending between and connecting the second and fourth pads.
The flexible multilayer system of claim 1， wherein a maximum height of the electronic circuit is H1 and a maximum height of the first stiffening feature is H2， H1 being substantially equal to H2.
The flexible multilayer system of claim 1， wherein a maximum height of the electronic circuit is H1 and a maximum height of the first stiffening feature is H2， H2/H1 being at least 1.3.
The flexible multilayer system of claim 1 further comprising at least one electrically conductive feature disposed between a longitudinal end of the first stiffening feature and the edge of the multilayer system corresponding to the longitudinal end.
The flexible multilayer system of claim 1， wherein the flexible multilayer system comprises at least one electrically conductive feature disposed between the first stiffening feature and an edge of the multilayer system closest to the first stiffening feature and substantially parallel to the first stiffening feature.
The flexible multilayer system of claim 1， wherein when the flexible multilayer system has a length of at least 80 centimeters between the opposite edges and the flexible multilayer system is held along one， but not the opposite， edge， the flexible multilayer system bends at the held edge at least 5 degrees less than a comparative flexible multilayer system having the same construction except that it does not have the first stiffening feature.
A flexible multilayer system comprising：

a flexible substrate comprising opposing first and second major surfaces；

a plurality of spaced apart electronic circuits arranged on and along a length of the flexible substrate， each electronic circuit comprising：

first and second electrodes on the respective first and second major surfaces of the substrate； and

an electrically conductive via extending between and connecting the first and second electrodes； and

parallel electrically conductive first and second stiffening ribs formed integrally on the respective first and second major surfaces of the substrate， the first and second ribs electrically connected to each other and electrically isolated from each electronic circuit， at least one of the first and second stiffening ribs extending substantially along the entire length of the substrate and increasing a stiffness of the flexible multilayer system against a bend along the length of the substrate.
The flexible multilayer system of claim 19， wherein the first and second stiffening ribs are electrically connected to each other by an electronically conductive via extending between and connecting the first and second stiffening ribs.
The flexible multilayer system of claim 19， wherein each of the first and second stiffening ribs extends substantially along the entire length of the substrate and increases a stiffness of the flexible multilayer system against a bend along the length of the substrate.
The flexible multilayer system of claim 19， wherein the plurality of the spaced apart electronic circuits extend substantially across the entire length and width of the flexible multilayer system.
A flexible multilayer system comprising：

a flexible substrate comprising opposing top and bottom major surfaces；

a plurality of spaced apart parallel electrically conductive first electrodes disposed on the top or bottom major surface of the substrate and extending along a first direction； and

a plurality of spaced apart parallel electrically conductive second electrodes disposed on the top or bottom major surface of the substrate and extending along a different second direction， such that in a plan view of the flexible multilayer system， the first and second electrodes form a two-dimensional grid defining a plurality of grid cells， each grid cell comprising an electronic circuit electrically isolated from the first and second electrodes and comprising electrically conductive first and second pads for electrically connecting to respective electrically conductive first and second terminals of a light emitting semiconductor device (LESD) mounted on the flexible multilayer construction entirely within the grid cell.
The flexible multilayer system of claim 23， wherein the pluralities of the spaced apart electrically conductive first and second electrodes and the first and second pads of each electronic circuit are integrally formed on the top major surface of the substrate.
The flexible multilayer system of claim 23， wherein the pluralities of the spaced apart electrically conductive first and second electrodes are integrally formed on the bottom major surface of the substrate， and the first and second pads of each electronic circuit are integrally formed on the top major surface of the substrate.