A Flexible Circuit and a Method of Manufacture

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under 35 U.S.C. § 1 19 of Provisional Application Serial No. 62/499,868, filed February 6, 2017, and Provisional Application Serial No. 62/602,364, filed April 20, 2017, each of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0002] This invention was made with government support under U.S. Army No. W81XWH- 16-C-0186. The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0003] The invention relates to a flexible circuit. More specifically, the invention relates to a flexible circuit that incorporates rigid components such as resistors, capacitors, wireless-enabled microprocessors, or other integrated circuit components. The invention further relates to a method of making the flexible circuit.

[0004] Flexible circuits have many uses, including as sensors adapted to be worn in contact with the skin of a user. For example, a wearable, flexible circuit can be used as a sensor in medical applications, providing physiological metrics in real time, such as heartrate, temperature, blood pressure, and blood oxygenation. In addition, being able to collect physiological data noninvasively and transmit that data wirelessly would enable big-data machine learning algorithms to autonomously identify sick and healthy patients. With rigid devices, the lack of flexibility can cause issues with data acquisition, since the interface between the non-flexible device and flexible skin is not ideal. For example, wrist-worn general wellness devices, such as fitness watches, can suffer from inaccurate heart rate measurements due to movement of the device relative to the skin.

[0005] Attempts have been made to manufacture flexible sensors and circuits to address the problems associated with non-flexible devices. In one example, a flexible circuit was created using a mold having recesses defining a conductive path, where the recesses are filled with a conductive elastomer then coated with a plain elastomer to support the conductive layer. In this example, rigid circuit components, if present, are placed directly onto the conductive elastomer material before the plain elastomer is added and the elastomers cured. However, the contact resistance between the integrated circuit components and conductive elastomer can be prohibitively high, particularly after stretching the device. Other attempts have used liquid conductors or meandering wires rather than a conductive elastomer, but these techniques can require expensive manufacturing processes, such as the sputter deposition of copper. While these examples demonstrate flexible circuits, the ability to incorporate non-flexible circuit components is marginal and the costs of these techniques can be prohibitive to commercial- scale manufacturing. Therefore, it would be advantageous to develop a flexible circuit that incorporates non- flexible integrated circuit components and a low-cost method of manufacturing the circuits.

BRIEF SUMMARY

[0006] According to embodiments of the present invention is a flexible circuit comprising a conductive polymer layered with a polymer base. In one embodiment, rigid circuit components are connected directly to the conductive polymer through elongated electrical contacts, which allow an electrical connection between the conductive polymer and the rigid component with negligible contact resistance. To manufacture the flexible circuit, the conductive polymer is placed in the recesses of a mold, which define the electrical pathway of the circuit. The rigid integrated circuit components, with the elongated electrical contacts, are then placed into contact with the conductive polymer. In some embodiments, a conductive epoxy is used in the junction between the electrical contact of the component and the conductive polymer. A flexible polymer base is then poured over the mold and cured. Once cured, the flexible circuit comprising the conductive polymer, integrated circuit components, and flexible base is removed from the mold. In alternative embodiments, the method of manufacture allows multiple layers to be combined in a single circuit, with the conductive elements of each layer electrically connected, such as through the use of a via, enabling the manufacture of complex circuits. The main advantage of this approach over other methods is that it's low-cost and produces a highly flexible and stretchable device which is ideal for comfort and usability when worn next to the skin.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0007] Figs. 1A- 1B show examples of the flexible circuit according to certain embodiments.

[0008] Figs. 2A-2D depicts a method of manufacturing the flexible circuit, according to one embodiment.

[0009] Fig. 3 shows a mold used to manufacture the flexible circuit, with recesses etched into the surface of the mold.

[0010] Figs. 4-6 show integrated circuit components fitted with elongated electrical contacts, or solder cones, according to some embodiments.

[001 1] Figs. 7A-7D depicts a method of manufacturing a multi-layer circuit.

[0012] Fig. 8 shows an example of a multiple-layer flexible circuit.

DETAILED DESCRIPTION

[0013] According to certain embodiments, the flexible circuit 100 comprises a conductive polymer 101, a flexible base 102, and at least one integrated circuit component 103. Fig. 1A shows an example of a flexible circuit 100 used as a heart monitor, where the conductive polymer 101 is visible on a surface of the flexible base 102. Fig. IB demonstrates the flexibility of another example circuit 100, with the flexible base 102 and conductive polymer 101 bending in a curve. During the manufacturing process, the conductive polymer 101 and flexible base 102 are bonded together, providing support for the relatively thin lines of conductive polymer 101. Further shown in Figs. 1A- 1B are several integrated circuit components 103, in electrical contact with the conductive polymer 101.

[0014] In the example embodiments shown in Figs. 1A- 1B, an electrically conductive silver/ polydimethylsiloxane material is used as the conductive polymer 101, where silver filler-particles are dispersed within the polydimethylsiloxane polymer. The filler-particles provide conductivity and the polymer (or elastomer) provides flexibility and the ability to stretch. Alternative filler-particles such as carbon black, carbon nanotubes, silver flakes, silver nanotubes, and eutectic gallium indium (eGaln) may be used, for example. Similarly, the filler-particles may be combined with alternative polymers such as polymethylmethacrylate or polytetrafluoroethylene, for example.

[0015] The flexible base 102 in the example embodiments is polydimethylsiloxane. As will be described in further detail, the flexible base 102 is comprised of a curable polymer which is initially applied in the manufacturing process as a liquid. As such, while polydimethylsiloxane is used as the flexible base 102 in the examples depicted in Figs. 1A- 1B, any of several flexible polymers that can be cured or thermoset may be used. In addition, while polydimethylsiloxane is used for the flexible base 102 and in the conductive polymer 101 in the example embodiments, the flexible base 102 may or may not be the same material as the polymer used in the conductive polymer 101.

[0016] To create the flexible circuit 100, a molding process is used. Figs. 2A-2D show the various steps in this process. At step 201, as shown in Fig. 2A, the mold 210 is etched, cut, or engraved with recesses 21 1 forming a pattern that replicates the desired conductive pathway in the flexible circuit 100. The conductive pathway is similar to the conductive traces and pads in a printed circuit board. The recesses 21 1 can vary in depth, width, and shape depending on the intended application of the flexible circuit 100 or patterning process used to create the mold 210. For example, in one embodiment, a laser is used to cut the pattern in an acrylic mold material. The laser provides a fast cutting process, but produces a lower quality mold 210. In an alternative embodiment, a CNC milling machine cuts the recesses 21 1 in a block of acrylic, metal, wax, or other similar material. A CNC mill can produce a higher quality mold 210, but requires more cutting time than a laser cutter. In addition, a CNC mill may require additional programming to convert the circuit design into data readable by the mill. Fig. 3 depicts a finished mold 210 with recesses 21 1 etched onto the surface of the mold 210.

[0017] At step 202, as shown in Fig. 2B, the conductive polymer 101 is deposited into the recesses 21 1. In one embodiment, the conductive polymer 101 is spread across the surface of the mold 210, forcing the conductive polymer 101 into the recesses 21 1 and on the surface of the mold 210. Excess conductive polymer 101 is then removed from the surface, resulting in the conductive polymer 101 remaining only in the recesses 21 1.

[0018] In step 203, as shown in Fig. 2C, integrated circuit components 103 are placed in contact with the conductive polymer 101. In one embodiment, the components 103 with elongated electrical contacts 1 10 are simply placed on the surface of the mold 210 so that the contacts 1 10 of the component 103 are engaged with the conductive polymer 101. In an alternative embodiment, the integrated circuit component 103 is placed in contact with the conductive polymer 101, the conductive polymer 101 is cured, and then the integrated circuit component 103 is removed, leaving an indentation 104 formed by the elongated contacts 1 10 extending from the component 103. The indentation 104 is then filled with a conductive epoxy 105, such as a silver epoxy, and the component 103 re-positioned on the conductive polymer 101 and cured. In this embodiment, the epoxy 105 provides a more robust connection between the integrated circuit component 103 and the conductive polymer 101. [0019] In step 204, the polymer base 102 is poured over the mold, at least partially encapsulating the integrated circuit components 103. When cured, the polymer base 102 bonds to the conductive polymer 101, allowing the circuit 100 to be removed from the mold 210 as a unitary circuit.

[0020] As discussed in step 203, in certain embodiments the integrated circuit component 103 is pressed into the conductive polymer 101 to create an indentation in the conductive polymer 101. Figs. 4-6 depict various integrated circuit components 103 that have been fitted with elongated contacts 1 10. Often, integrated circuit components 103 have a flat profile and would therefore sit on the surface of the layer of conductive polymer 101, preventing a robust mechanical or electrical connection. The elongated contacts 1 10 extend from a surface of the component 103, allowing the contact 1 10 to penetrate into the conductive polymer 101 during the manufacturing process. The elongated contacts 1 10 can be added to standard integrated circuit components 103, such as by adding a cone-shaped bead of solder to the component 103. Alternatively, the elongated contact 1 10 can be integrated into the component 103 when it is manufactured. The elongated contacts 1 10 enable high quality electrical and mechanical interfacing between the component 103 and the conductive polymer 101, particularly when an epoxy 105 is used at the interface 106 between the two.

[0021] In an alternative manufacturing method, a multilayer approach is used. As shown in Figs. 7A-7D, rather than using a single mold 210, two molds 210 are patterned with recesses 211, filled with conductive polymer 101, fitted with integrated circuit components 103, and positioned opposite each other with a polymer base 102 disposed between the two molds 210. By using two molds 210, each side of the base 102 in the completed flexible circuit 100 has electrical pathways.

[0022] As further detailed in Fig. 7A, at step 301, the two molds 210 are aligned. In one embodiment, a pin that is adapted to fit in a hole on each mold 210 is used to ensure proper alignment. At step 302, shown in Fig. 7B, the molds 210 are removed from the flexible circuit 100 after curing. At this stage, the flexible circuit 100 has multiple layers of conductive polymer 101, but each layer is not electrically connected. As such, at step 303, as shown in Fig. 7C, each layer of conductive polymer 101 and the flexible polymer base 102 is pierced with tool 310, creating a void traversing the depth of the circuit 100. In step 304, shown in Fig. 7D, conductive polymer 101 or epoxy 105 is used to fill the void, creating a via 31 1. With the via 31 1, the electrical pathways on each side of the polymer base 102 are electrically connected. Fig. 8 shows an example of a multiple-layer circuit 100, with the top layer electrically connected to the bottom layer through the use of vias 31 1.

[0023] In yet another alternative manufacturing method, the mold 210 has recesses 21 1 passing through the depth of the mold 210. In a process similar to screen printing, the conductive polymer 101 can be deposited onto the surface of the flexible polymer base 102. The integrated circuit components 103 can then be added in a manner similar to the processes described in step 203, with another layer of polymer base 102 used to cover the conductive polymer 101.

[0024] While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.