Embedded electrical traces

#The present invention relates to fine, electrically conductive lines, grids or circuits embedded in a substrate and a method for making such devices. In one embodiment the conductive grid is sufficiently fine to be invisible to the unaided eye and the substrate is a transparent thermoplastic sheet. Transparent conductive sheets have a variety of uses including resistively heated windows, electromagnetic interference (EMI) shielding, static shielding, antennas, touch screens for computer displays, and surface electrodes for electrochromic windows and liquid crystal displays.

#The use of essentially transparent electrically conductive grids for such applications as EMI shielding is well known. The grid can be formed from a network or screen of metal wires that are sandwiched or laminated between transparent sheets or embedded in substrates (U.S. Pat. Nos. 3,952,152; 4,179,797; 4,321,296; 4,381,421; 4,412,255). The disadvantage of using wire screens is the difficulty in handling very fine wires or in making and handling very fine wire screens. For example, a 20 micron diameter copper wire has a tensile strength of only 1 oz (28 grams force) and is therefore easily damaged. Wire screens fabricated with wires of 20 micron diameter are available but are very expensive due to the difficulty in handling very fine wire.

#Rather than embed a preexisting wire screen into a substrate, a conductive pattern can be fabricated in situ by first forming a pattern of grooves or channels in a substrate and then filling the channels with a conductive material. This method has been used for making conductive circuit lines and patterns by a variety of means, although usually for lines and patterns on a relatively coarse scale. The grooves can be formed in the substrate by molding, embossing, or by lithographic techniques and then filling the grooves with conductive inks or epoxies (U.S. Pat. No. 5,462,624), with evaporated, sputtered, or plated metal (U.S. Pat. Nos. 3,891,514; 4,510,347; 5,595,943), with molten metal (U.S. Pat. No. 4,748,130), or with metal powder (U.S. Pat. Nos. 2,963,748; 3,075,280; 3,800,020; 4,614,837; 5,061,438; 5,094,811).

#FIG. 1 is a scanning electron micrograph showing a polycarbonate sheet having an embossed pattern of grooves.

#FIG. 2 is a scanning electron micrograph showing a polycarbonate sheet having an embossed pattern of grooves after the application of heat and pressure by a flat metal surface against the pattern of grooves.

#FIGS. 3, 4, and 5 are cross sectional drawings of grooves showing how they collapse under compression of the substrate by a flat plate.

#An embossed substrate comprising a polycarbonate sheet is shown in FIG. 1. The embossed grooves can be formed, for example, by hot embossing a flat substrate against a patterned die, by casting hot polymer resin against a patterned die, or by cutting into the surface with a sharp tool. The substrate can be selected from any family of materials that flow under the application of pressure, the application of heat, or the combined application of heat and pressure. Preferred substrates include any thermoformable or thermoplastic polymer, and may include elastomeric materials. The substrate may be a fluoropolymer, including perfluorinated and partially fluorinated polymers. For selected applications, it may be preferred that the substrate is transparent.

#As shown in FIG. 2, when the polycarbonate substrate shown in FIG. 1 is subjected to heat and pressure the embossed grooves partially collapse. When pressure is applied in a direction normal to the flat surface of the polymer substrate, the walls of the grooves collapse inward and the bottoms of the grooves collapse upward. The collapsing of the grooves is illustrated schematically in FIG. 3 and explained in greater detail below.

#As seen in FIG. 3, a polymer substrate 30 has a groove 31 with a generally rectangular cross-section. Groove 31 is defined by side walls 32 and bottom surface 33. As pressure is applied to flat surface 34 of the substrate 30, the original generally rectangular cross-section of groove 31 begins to deform, with side walls 32 and bottom surface 33 collapsing inward. After a moderate amount of collapse of the side walls 32 and bottom surface 33, groove 31 has a cross-sectional profile as illustrated by dashed line 35. Increased time, heat, and/or pressure results in groove 31 having a cross-sectional profile as illustrated by dashed line 36. Ultimately, in the absence of any foreign material in the groove 31, groove 31 will collapse shut completely. If groove 31 is sufficiently deep relative to its width, the side walls 32 of groove 31 will ultimately contact each other during the collapse.