Patent Publication Number: US-9433104-B2

Title: Method of manufacturing stretchable circuit assemblies

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to stretchable circuits. More specifically, the present invention relates to stretchable circuits that are polymer based and include conductive wires or flexible circuits embedded in the stretchable circuits. 
     2. Description of the Related Art 
     Known flexible circuits are implemented using flexible printed circuits. While flexible circuits can bend, they cannot stretch. To offer electrical connections that can elongate, flexible circuits are folded so that they can slide. Such known flexible circuits can slide back and forth with a specified bend gap and are intended to last for more than 200,000 sliding cycles. As the bend gap of the flexible circuit decreases, the number of cycles before failure occurs reduces exponentially. 
     U.S. Pat. No. 7,337,012 B2 teaches a stretchable circuit including a stretchable polymer body with micro-channels that are filled with conductive material. U.S. Pat. No. 7,337,012 B2 does not discuss the electrical terminals necessary to connect the stretchable circuit to other components. U.S. Pat. No. 7,337,012 B2 uses a conductor that is in liquid or paste form. The micro-channels are created in the substrate, and then the conductor is formed by forcing the liquid or paste into the micro-channels. Thus, the conductor takes the shape of the micro-channels. The liquids and pastes used in U.S. Pat. No. 7,337,012 B2 have a much higher bulk resistivity, in the range of three to ten times, than the bulk resistivity of copper wire. A higher resistance produces a lower performing circuit, which will not be suitable for many electronic applications. 
     U.S. Patent Application Publication No. 2009/0317639 A1 teaches conventional stretchable circuits using flexible circuits. The stretchable circuits are formed by laser cutting or die cutting the flexible circuits to form patterns in the flexible circuits. Portions of the flexible circuit are then removed to define stretchable conductive elements. This conventional stretchable circuit is then embedded in a polymer. However, in this conventional stretchable circuit, the flexible circuit and the conductive patterns are on the same plane, which causes the thickness of the polymer to be greater than optimal. Further, this stretchable circuit does not use conductive wires. 
     International Patent Application No. WO 2010/086034 A1 also teaches a conventional stretchable circuit. Portions of the stretchable circuit have different stiffnesses, which allows the stretchable circuit to stretch. To form the stretchable circuit, flexible circuits are laser cut, and the portions of the flexible circuits that are not needed are removed. The resulting stretchable circuit is then embedded in polymer. The conductive patterns are on the same plane as the body of the circuit, which causes the thickness of the polymer to be greater than optimal. 
     SUMMARY OF THE INVENTION 
     To overcome the problems described above, preferred embodiments of the present invention provide a stretchable circuit assembly including conductive wires or flexible circuits embedded within a stretchable interconnect. 
     In a first preferred embodiment of the present invention, a stretchable circuit assembly includes first and second printed circuit boards, discrete conductive wires including ends connected to the first and second printed circuit boards, and a stretchable interconnect in which the discrete conductive wires and a portion of the first and second printed circuit boards are embedded. 
     The stretchable circuit assembly preferably further includes a strain relief wire embedded in the stretchable interconnect and arranged to prevent the stretchable interconnect from being stretched such that the discrete conductive wires are damaged. The ends of the discrete conductive wires are preferably soldered to the first and second printed circuit boards. The discrete conductive wires preferably have an oscillating configuration. The discrete conductive wires preferably include semi-circular shaped portions connected by linear portions. 
     In a second preferred embodiment of the present invention, a stretchable circuit assembly includes first and second printed circuit boards, flexible circuits including ends connected to the first and second printed circuit boards, and a stretchable interconnect in which the flexible circuits and a portion of the first and second printed circuit boards are embedded. Main surfaces of the flexible circuits are perpendicular or substantially perpendicular to main surfaces of the stretchable interconnect. 
     The stretchable circuit assembly preferably further includes a strain relief circuit embedded in the stretchable interconnect and arranged to prevent the stretchable interconnect from being stretched such that the flexible circuits are damaged. Ends of the flexible circuits are preferably soldered to the first and second printed circuit boards. The flexible circuits preferably have an oscillating configuration. The flexible circuits preferably include semi-circular shaped portions connected by linear portions. 
     In a third preferred embodiment of the present invention, a method of making a stretchable circuit assembly includes the steps of providing electrical interconnects, a first printed circuit board, and a second printed circuit board; shaping the electrical interconnects to have an oscillating configuration; and forming a stretchable interconnect such that the electrical interconnects, a portion of the first printed circuit board, and a portion of the second printed circuit board are embedded within the stretchable interconnect. 
     The electrical interconnects preferably include one of conductive wires and flexible circuits. The step of forming a stretchable interconnect is preferably performed by injection molding. The step of forming a stretchable interconnect preferably uses a polymer. The method of making a stretchable circuit assembly preferably further includes the step of attaching ends of the electrical interconnects to the first printed circuit board and the second printed circuit board. 
     The above and other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top plan view of a stretchable circuit assembly according to a first preferred embodiment of the present invention. 
         FIG. 1B  is a bottom plan view of a stretchable circuit assembly according to a first preferred embodiment of the present invention. 
         FIG. 1C  is a top plan view of a stretchable circuit assembly showing the conductive wires according to a first preferred embodiment of the present invention. 
         FIG. 2A  is a perspective view of a stretchable circuit assembly according to a first preferred embodiment of the present invention. 
         FIG. 2B  is a perspective view of a stretchable circuit assembly showing the conductive wires according to a first preferred embodiment of the present invention. 
         FIG. 3A  is a top plan view of a stretchable circuit assembly according to a second preferred embodiment of the present invention. 
         FIG. 3B  is a bottom plan view of a stretchable circuit assembly according to a second preferred embodiment of the present invention. 
         FIG. 3C  is a top plan view of a stretchable circuit assembly showing the flexible circuits according to a second preferred embodiment of the present invention. 
         FIG. 4A  is a perspective view of a stretchable circuit assembly according to a second preferred embodiment of the present invention. 
         FIG. 4B  is a perspective view of a stretchable circuit assembly showing the flexible circuits according to a second preferred embodiment of the present invention. 
         FIG. 4C  is a close-up view of a flexible circuit according to a second preferred embodiment of the present invention. 
         FIGS. 5A and 5B  show an injection mold used in a method of manufacturing a stretchable circuit assembly according to a third preferred embodiment of the present invention. 
         FIG. 6  shows a flexible printed circuit used in a method of manufacturing a stretchable circuit assembly according to a third preferred embodiment of the present invention. 
         FIGS. 7A-7C  show top plan views of a mold core used in a method of manufacturing a stretchable circuit assembly according to a third preferred embodiment of the present invention. 
         FIGS. 8A-8C  shows side views of a mold core used in a method of manufacturing a stretchable circuit assembly according to a third preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIGS. 1A-2B  show a stretchable circuit assembly  10  with conductive wires  13  according to a first preferred embodiment of the present invention.  FIGS. 3A-4B  show a stretchable circuit assembly  20  with flexible circuits  23  according to a second preferred embodiment of the present invention.  FIGS. 5A-8B  show a method of manufacturing a stretchable circuit assembly according to a third preferred embodiment of the present invention. 
     Stretchable Circuit Assembly with Conductive Wires 
       FIGS. 1A-2B  show a stretchable circuit assembly  10  according to a first preferred embodiment of the present invention. The stretchable circuit assembly  10  includes two printed circuit boards  11   a ,  11   b , a stretchable interconnect  12  connected between the two printed circuit boards  11   a ,  11   b , and conductive wires  13  connected to the two printed circuit boards  11   a ,  11   b  and embedded within the stretchable interconnect  12 . 
     The stretchable interconnect  12  is preferably formed such that the conductive wires  13  and the two printed circuit boards  11   a , 11   b  are embedded within the stretchable interconnect  12 . The stretchable interconnect  12  is made of a material that is stretchable so that the distance between the two printed circuit boards  11   a ,  11   b  can be increased by stretching the stretchable interconnect  12 . The stretchable interconnect  12  is preferably a polymer such as polydimethylsiloxane (PDMS); however, other suitable stretchable materials, such as urethane, polyurethane elastomers, hydrocarbon rubber/elastomers, and polyether block amides (PEBA), can also be used. 
     Although only one stretchable interconnect  12  is shown in  FIGS. 1A-2B  connecting the two printed circuit boards  11   a ,  11   b , it is possible to have more than one stretchable interconnect connecting two printed circuit boards. Although only two printed circuit boards  11   a ,  11   b  are shown in  FIGS. 1A-2B , it is also possible to connect more than two printed circuit boards. For example, two stretchable interconnects could connect three printed circuit boards together in a chain, with two sides of a middle printed circuit board connected to one of the stretchable interconnects that is connected to a printed circuit board at the end of the chain. Another example is for two stretchable interconnects to be connected at one end to the same side of the same printed circuit board and at the other end to separate printed circuit boards. It is also possible to use an adhesive to more securely attach the stretchable interconnect  12  to the two printed circuit boards  11   a ,  11   b . Any suitable adhesive can be used. 
     The conductive wires  13  are preferably attached to the printed circuit boards  11   a ,  11   b  by using solder  14 . However, the conductive wires  13  could be attached to the printed circuit boards  11   a ,  11   b  using any suitable method. For example, the conductive wires  13  could be attached to the printed circuit boards  11   a ,  11   b  by bonding with electrically conductive epoxy adhesive, clamping, pressure fittings, and crimping. The conductive wires  13  can be made of any suitable conductive material. The conductive wires  13  are preferably made of a conductive metal such as copper, silver, gold, or aluminum. The conductive wires  13  can be coated or uncoated. If the conductive wires  13  are not coated, then the stretchable interconnect  12  acts as a dielectric between the individual conductive wires  13  to prevent shorting between adjacent conductive wires  13 . The conductive wires  13  are preferably discrete wires that are fabricated in bulk for commercial sale. That is, the conductive wires  13  are shaped and formed before being embedded in the stretchable interconnect  12  as compared to the conductors formed in U.S. Pat. No. 7,337,012 B2 by forcing a liquid or paste into micro-channels in a substrate. 
     Although not shown in the figures, it is possible to have more than one conductive line  13  that have the same shape when viewed in plan view but that are vertically separated from each other when viewed in a cross-sectional view. For example, two conductive lines  13  could be used, with one of the conductive lines  13  soldered to the top of the printed circuit boards  11   a ,  11   b  and with the other of the conductive lines  13  soldered to the bottom of the printed circuit boards  11   a ,  11   b . Of course, having more than one conductive line  13  spaced apart from each other when viewed in cross-section requires that the stretchable interconnect  12  be thicker than when only one conductive line  13  is used. 
     In  FIGS. 1C and 2B , the stretchable interconnect  12  is shown as see-through so that the shape of the conductive wires  13  can be seen. The conductive wires  13  preferably have an oscillating or meandering shape as shown in  FIGS. 1C and 2B  so that shape of the conductive wires  13  changes as the stretchable circuit assembly  10  is stretched in the direction between the printed circuit boards  11   a ,  11   b . For example, the conductive wires  13  can have semi-circular portions connected with linear portions as shown in  FIGS. 1C and 2B . Instead of semi-circular portions, the conductive wires  13  could have triangular portions or other similar shaped portions. It is also possible to not include linear portions in the conductive wires  13  and to have the semi-circular portions, or other suitable shape connected to each other. Other configurations are also possible as long as the conductive wires  13  are not damaged and the signal integrity of the signals transmitted through the conductive wires  13  is maintained when the stretchable interconnect  12  is stretched. The stretch range of the stretchable circuit assembly  10  is typically between 0% and about 125% of the length of the stretchable interconnect  12 . 
     One or more of the conductive wires  13  can be replaced by strain relief wires  15 . The strain relief wires  15  prevent the stretchable interconnect  12  from being over stretched, which protects the conductive wires  13  from being damaged. Preferably, as shown in  FIGS. 1A-2B , the top and bottom conductive wires  13  are replaced by strain relief wires  15 . However, no strain relief wires  15  can be used; one strain relief wire  15 , e.g., in the middle of the stretchable interconnect  12 , can be used; and more than two relief wires  15 , e.g., alternating with the conductive wires  13 , can be used. 
     The strain relief wires  15  can be made of any suitable material, including, for example, a metal or carbon fiber. The strain relief wires  15  preferably have the same shape as the conductive wires  13  as shown in  FIGS. 1A-2B ; however, it also possible for the strain relief wires  15  to have a shape different from the conductive wires  13 . 
     Any suitable printed circuit board can be used for the printed circuit boards  11   a ,  11   b . Although not shown in  FIGS. 1A-2B , the printed circuit boards  11   a ,  11   b  can include active or passive components for processing and/or modifying the signals transmitted through the stretchable circuit assembly  10 . Although not shown in  FIGS. 1A-2B , the printed circuit boards  11   a ,  11   b  preferably include an electrical connector for connecting the stretchable circuit assembly  10  to electrical devices with a corresponding electrical connector. The printed circuit boards  11   a ,  11   b  can be attached to an electronic device via surface mounted connectors such as board-to-board, clamping, pressure fittings, or spring pins, or can be embedded within a secondary printed circuit board, drilled, and copper plated to create a via interconnect. 
     Stretchable Circuit Assembly with Flexible Circuits 
       FIGS. 3A-4B  show a stretchable circuit assembly  20  according to a second preferred embodiment of the present invention. The stretchable circuit assembly  20  includes two printed circuit boards  21   a ,  21   b , a stretchable interconnect  22  connected between the two printed circuit boards  21   a ,  21   b , and flexible circuits  23  connected to the two printed circuit boards  21   a ,  21   b  and embedded within the stretchable interconnect  22 . 
     The stretchable interconnect  22  is preferably formed such that the flexible circuits  23  and the two printed circuit boards  21   a ,  21   b  are embedded within the stretchable interconnect  22 . The flexible circuits  23  are preferably embedded within the stretchable interconnect  22  such that the main surfaces of the flexible circuits  23  are perpendicular or substantially perpendicular to the main surfaces of the stretchable interconnect  22 . By arranging the main surface of the flexible circuits  23  perpendicular or substantially perpendicular to the main surface of the stretchable interconnect  22 , it is possible to increase the length of the flexible circuits  23  without increasing the thickness of the stretchable interconnect  22 .  FIG. 4B  shows axes for the thickness T 1 , width W 1 , and length L 1  of the stretchable interconnect  22  and for the thickness T 2 , width W 2 , and length L 2  of the flexible circuits  23 . The thicknesses T 1 , T 2  are in the smallest dimensions of the stretchable interconnect  22  and flexible circuits  23 , respectively. Because the thicknesses T 1 , T 2  are perpendicular or substantially perpendicular (i.e., the thickness T 1  of the stretchable interconnect  22  and the width W 2  of the flexible circuits  23  are parallel or substantially parallel), the width W 2  of the flexible circuit  23  and the thickness T 1  of the stretchable interconnect  22  can be substantially the same. Because of the oscillating shape of the flexible circuits  23 , when the length of the flexible circuits  23  is increased, it might be necessary to increase the width of the stretchable interconnect  22 . By increasing the length of the flexible circuits  23 , the stretch range of the stretchable circuit assembly  20  is increased. 
     The stretchable interconnect  22  is made of a material that is stretchable so that the distance between the two printed circuit boards  21   a ,  21   b  can be increased by stretching the stretchable interconnect  22 . The stretchable interconnect  22  is preferably a polymer such as polydimethylsiloxane (PDMS); however, other suitable stretchable materials, such as urethane, polyurethane elastomers, hydrocarbon rubber/elastomers, and polyether block amides (PEBA), can also be used. 
     Although only one stretchable interconnect  22  is shown in  FIGS. 3A-4B  connecting the two printed circuit boards  21   a ,  21   b , it is possible to have more than one stretchable interconnect connecting two printed circuit boards. Although only two printed circuit boards  21   a ,  21   b  are shown in  FIGS. 3A-4B , it is also possible to connect more than two printed circuit boards. For example, two stretchable interconnects could connect three printed circuit boards together in a chain, with two sides of a middle printed circuit board connected to one of the stretchable interconnects that is connected to a printed circuit board at the end of the chain. Another example is for two stretchable interconnects to be connected at one end to the same side of the same printed circuit board and at the other end to separate printed circuit boards. It is also possible to use an adhesive to more securely attach the stretchable interconnect  22  to the two printed circuit boards  21   a ,  21   b . Any suitable adhesive can be used. 
     The flexible circuits  23  are preferably attached to the printed circuit boards  21   a ,  21   b  by using solder. However, the flexible circuits  23  could be attached to the printed circuit boards  21   a ,  21   b  using any suitable method. For example, the flexible circuits  23  could be attached to the printed circuit boards  21   a ,  21   b  by bonding with electrically conductive epoxy adhesive, clamping, pressure fittings, and crimping. The ends of the flexible circuits  23  preferably have an L- or a reverse L-shape. The ends of the flexible circuits  23  are preferably inserted through holes  26  in the printed circuit boards  21   a ,  21   b . After the ends of the flexible circuits  23  are inserted through holes  26  in the printed circuit boards  21   a ,  21   b , the ends of the flexible circuits  23  are soldered to the printed circuit boards  21   a ,  21   b.    
     Typically, the flexible circuits  23  include a flexible plastic substrate with one or more conductive lines  27  for transmitting electronic signals.  FIG. 4C  is close-up view of one of the flexible circuits  23  with conductive lines  27 . The flexible plastic substrate can be a polyimide, a polyether ether ketone (PEEK), a transparent conductive polyester, or any other suitable flexible material. The conductive lines  27  can be made of any suitable electrically conducting material. The flexible circuits  23  can include passive and/or active components that process and/or modify the signals transmitted through the stretchable circuit assembly  10 . Although not shown in the figures, it is possible to have two or more flexible circuits  23  that have the same shape when viewed in plan view but that are vertically separated from each other when viewed in a cross-sectional view. For example, two flexible circuits  23  could be used, with one of the flexible circuits  23  soldered to the top of the printed circuit boards  21   a ,  21   b  and with the other of the flexible circuits  23  soldered to the bottom of the printed circuit boards  21   a ,  21   b . Of course, having more than one flexible circuit  23  spaced apart from each other when viewed in cross-section requires that the stretchable interconnect  22  be thicker than when only one flexible circuit  23  is used. 
     In  FIGS. 3C and 4B , the stretchable interconnect  22  and the printed circuit boards  21   a ,  21   b  are shown as see-through so that the shape of the flexible circuits  23  can be seen. The flexible circuits  23  preferably have an oscillating shape as shown in  FIGS. 3C and 4B  so that shape of the flexible circuits  23  changes as the stretchable circuit assembly  20  is stretched in the direction between the printed circuit boards  21   a ,  21   b . For example, the flexible circuits  23  can have semi-circular portions connected with linear portions as shown in  FIGS. 3C and 4B . Instead of semi-circular portions, the flexible circuits  23  could have triangular portions or other similar shaped portions. It is also possible to not include linear portions in the flexible circuit  23  and to have the semi-circular portions, or other suitable shape, connected to each other. Other configurations are also possible as long as the flexible circuits  23  are not damaged and the signal integrity of the signals transmitted through the flexible circuits  23  is maintained when the stretchable interconnect  22  is stretched. The stretch range of the stretchable circuit assembly  20  is typically between 0% and about 125% of the length of the stretchable interconnect  22 . 
     One or more of the flexible circuits  23  can be replaced by strain relief circuits  25 . The strain relief wires  25  prevent the stretchable interconnect  22  from being over stretched, which protects the flexible circuits  23  from being damaged. Preferably, as shown in  FIGS. 3A-4B , the top and bottom flexible circuits  23  are replaced by strain relief circuits  25 . However, no strain relief circuits  25  can be used; one strain relief circuit  25 , e.g., in the middle of the stretchable interconnect  22 , can be used; and more than two relief circuits  25 , e.g., alternating with the flexible circuits  23 , can be used. 
     The strain relief circuits  25  are typically made of flexible circuits just as the flexible circuits  23  but without any conductive lines. However, the strain relief circuits  25  can be made of any suitable material, including, for example, a metal or carbon fiber. The strain relief circuits  25  preferably have the same shape as the flexible circuits  23  as shown in  FIGS. 1A-2B ; however, it also possible for the strain relief circuits  25  to have a shape different from the flexible circuits  23 . 
     Any suitable printed circuit board can be used for the printed circuit boards  21   a ,  21   b . Although not shown in  FIGS. 2A-4B , the printed circuit boards  21   a ,  21   b  can include active and/or passive components for processing and/or modifying the signals transmitted through the stretchable circuit assembly  20 . Although not shown in  FIGS. 3A-4B , the printed circuit boards  21   a ,  21   b  preferably include an electrical connector for connecting the stretchable circuit assembly  20  to electrical devices with a corresponding electrical connector. The printed circuit boards  21   a ,  21   b  can be attached to an electronic device via surface mounted connectors such as board-to-board, clamping, pressure fittings, or spring pins, or can be embedded within a secondary printed circuit board, drilled, and copper plated to create a via interconnect. 
     Method of Making Stretchable Circuit Assembly 
       FIGS. 5A-8B  show a method of manufacturing a stretchable circuit assembly according to a third preferred embodiment of the present invention. Although the following discussion of the method of manufacturing a stretchable circuit assembly involves the use of flexible circuits, the discussion is equally applicable to manufacturing a stretchable circuit assembly using conductive wires instead of flexible circuits, except that conductive wires are used instead of flexible circuits. 
       FIGS. 5A and 5B  show an injection mold  30  according to a third preferred embodiment of the present invention.  FIG. 5A  shows an empty injection mold  30 , and  FIG. 5B  shows an injection mold  30  with the printed circuit boards  31   a ,  31   b  and the flexible circuits  32  loaded within the injection mold  30  but before any polymer is injected into the injection mold  30 . The injection mold  30  includes mold top  30   a , mold core  30   b , and alignment pins  30   c  for aligning the mold top  30   a  with the mold core  30   b . The mold top  30   a  and the mold core  30   b  define an injection area  30   d  into which polymer is injected. The injection mold  30  includes a seal  30   e  for sealing the injection area  30   d  when polymer is injected into the injection mold  30 . The injection mold  30  includes a hole  30   f  through which polymer is injected. Although only one hole  30   f  is shown in  FIGS. 5A and 5B , the injection mold  30  can have more than one hole. For example, the injection mold  30  could have a hole on each side of the injection mold  30 . 
     The steps of using the injection mold  30  to manufacture a stretchable circuit assembly will now be discussed. A flexible circuit assembly  32 ′ is manufactured as shown in FIG.  6 . Any suitable method can be used to manufacture the flexible circuit assembly  32 ′. As explained above with respect to individual flexible circuits  23 , the flexible circuit assembly  32 ′ includes a flexible plastic substrate with one or more conductive lines for transmitting electronic signals. The flexible circuit assembly  32 ′ can also include passive and/or active components. The flexible circuit assembly  32 ′ is divided into individual flexible circuits  32  and formed such that the ends of the flexible circuits  32  preferably have an L- or reverse L-shape. The ends of the flexible circuits  32  are preferably arranged such that the ends are perpendicular or substantially perpendicular to the portion of the flexible circuits  32  between the ends. 
       FIG. 7A  shows an empty mold core  30   b . The mold core  30   b  includes a first row of pins  30   b   1  and a second row of pins  30   b   2 . The first  30   b   1  and second  30   b   2  rows of pins are moveable with respect to each other along directions A, B that are anti-parallel to each other. The pins of the first  30   b   1  and second  30   b   2  rows of pins shown in  FIGS. 7A-7C  are shaped to form semi-circular portions in the flexible circuits  32 . However, the pins could have different shapes to form different shapes in the flexible circuits  32 , as explained above. 
     After the flexible circuits  32  are manufactured, the flexible circuits  32  are loaded into the mold core  30   b  such that the flexible circuits  32  are arranged between the pins of the first  30   b   1  and second  30   b   2  rows of pins as shown in  FIG. 7B . It is possible to replace one or more of the flexible circuits with strain relief circuits. After the flexible circuits  32  are loaded in the mold core  30   b , the mold core  30   b  is closed by moving the first rows of pins  30   b   1  in direction B and by moving the second rows of pins  30   b   2  in direction A as shown in  FIG. 7C . By closing the mold core  30   b , the flexible circuits  32  are shaped to have an oscillating shape with semi-circular portions connected by linear portions, as discussed above. Preferably, the flexible circuits  32  are inserted into holes in the printed circuit boards  31   a ,  31   b  and are attached to the printed circuit boards  31   a ,  31   b  before any polymer is injected into the mold core  30   b . As discussed above, the flexible circuits  32  are preferably soldered to the printed circuit boards  31   a ,  31   b ; however, it is possible to use other suitable methods to attach the flexible circuits  32  to the printed circuit boards  31   a ,  31   b.    
       FIGS. 8A-8C  are side views of the mold core  30   b  according to a third preferred embodiment of the present invention. The mold core  30   b  shown in  FIGS. 8A and 8B  is simplified compared to the mold core  30   b  shown in  FIGS. 7A-7C  in that the mold core  30   b  shown in  FIGS. 8A and 8B  only has four rows of pins: two first rows of pins  30   b   1  and two second rows of pins  30   b   2 . The mold core  30   b  can have any number of first rows of pins  30   b   1  and of second rows of pins  30   b   2 . In addition to the first  30   b   1  and second  30   b   2  rows of pins, the mold core  30   b  also includes a base  30   b   3  and retraction plate  30   b   4 . 
     After the mold core  30   b  is closed, the mold core  30   b  is mated with the mold top  30   a  using alignment pins  30   c . For the sake of simplicity,  FIGS. 8A-8C  only show the mold core  30   b  and the injection area  30   d . After the mold core  30   b  and the mold top  30   a  are mated, a first shot of polymer is injected into injection area  30   d  of the injection mold  30  as shown in  FIG. 8A . The first shot of polymer is allowed to set. After the first shot of polymer sets, as shown in  FIG. 8B , the retraction plate  30   b   4  is moved in direction C so that the top of the pins of the first  30   b   1  and second  30   b   2  rows of pins are aligned with the top surface of the base  30   b   3 . 
     As shown in  FIG. 8C , after the top of the pins of the first  30   b   1  and second  30   b   2  rows of pins are aligned with the top surface of the base  30   b   3 , the mold core  30 , including the first  30   b   1  and second  30   b   2  rows of pins, the base  30   b   3 , and the retraction plate  30   b   4 , is moved in direction D so that a gap is formed between the top surface of the base  30   b   3  and the bottom surface of the set polymer from the first shot of polymer in the injection area  30   d.    
     After the gap is formed between the top surface of the base  30   b   3  and the bottom surface of the set polymer from the first shot of polymer, a second shot of polymer is injected into the injection mold  30 . The second shot of polymer is allowed to set. After the second shot of polymer is set, the injection mold  30  is opened and the stretchable circuit assembly is removed from the injection mold  30 . 
     After the stretchable circuit assembly is removed from the injection mold  30 , the stretchable circuit assembly can be cut into discrete circuits, can have secondary components assembled or connected to it, can be tested, and can have bonding operations performed on it. The bonding operations include, for example, bonding the printed circuit boards  31   a ,  31   b  of the stretchable circuit assembly to metal stiffeners, chassis, housings, or other flexible printed circuit/printed circuit board assemblies. 
     It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variances that fall within the scope of the appended claims.