High capacitance sheet adhesives

#The present invention relates to thin adhesive composite films formed from filled or unfilled polymeric substrates that are imbibed with an adhesive or filler-adhesive mixture such that the film has high dielectric properties. The filler imparts high dielectric constants to the adhesive without loss of physical properties. Specifically, the present invention provides thin fluoropolymer adhesive composite films having high capacitance. The imbibed film composites of the present invention are suitable for use in the fabrication of electronic devices.

#The electronics industry is constantly challenged with the need to reduce the size and weight of electronic devices while increasing their performance. One method of providing the necessary increase in density is to combine functions of the different components. Printed Wiring Boards (PWB) are currently used to connect different devices in order to perform an electronic function. It would be desirable if the PWB had some of the functionality of some of those devices, eliminating the need to mount them to the surface. Resistors and capacitors are very simple devices that are required in almost every circuit. Because of the simplicity of their functions, they are prime candidates for incorporation into a PWB structure.

#The desire to incorporate a capacitor into a PWB is not a new concept. Limitations in materials, however, have prevented this from becoming common. There are presently several high dielectric materials which can be used as an internal capacitor, but they all have limitations in the amount of capacitance that they can supply. The capacitance of the layer is a function of the thickness and dielectric constant. An increase in dielectric constant and/or a reduction in thickness will increase the capacitance of the layer.

#FIG. 1 illustrates an expanded or stretched polytetrafluoroethylene film (A) containing nodes (B) and interconnected with fibrils (C) with a particulate dielectric filler (D) located within the node-and-fibril structure and adhesive (E) in the void volume of the film.

#FIG. 2 illustrates an expanded or stretched polytetrafluoroethylene film where the particulate dielectric filler and adhesive are located within the void volume of the film.

#FIG. 3 illustrates an expanded or stretched polytetrafluoroethylene film where a dielectric filler is located within the node-and-fibril structure and an adhesive with a filler are located in the void volume of the film.

#It has now been discovered that high capacitive, very thin, well-controlled thickness adhesives can be obtained by using filled expanded porous PTFE without producing team or pinholes in the thin films. Unusually low loadings of thermosetting or thermoplastic adhesive can be achieved by the invention described herein, while still providing exceptional adhesion. An added and unexpected result to the invention is that even small amounts of resin impart elevated voltage breakdown strength to the composite.

#The high dielectric adhesive films of the present invention are preferably formed from porous, expanded PTFE that contains a filler. The filler is present in the range of 5-80% of the final composition, preferably 7 to 63 volume percent, most preferably 15-55% in the form of particulate or fibers; 4-85 volume percent, and preferably 10 to 40 volume percent PTFE. The adhesive, high dielectric films of the present invention have a thickness of 0.2 mils or greater.

#An important aspect of the process invention lies in the use of the unusual feature of polytetrafluoroethylene (PTFE) to expand on stretching to form a porous material of interconnecting channels formed by nodes and fibrils. The stretching of polytetrafluoroethylene (PTFE) to form porous material is well known, and is taught in U.S. Pat. No. 3,953,566 to Gore and U.S. Pat. No. 4,482,516 to Bowman, et al., each of which is incorporated herein by reference. The void space in expanded PTFE comprises more than 30%, preferably at least 50% of the volume, and frequently more than 70% of the volume. Because of the expansion, the particulate filler particles are drawn apart from one another as the PTFE is expanded. This reduces the opportunity for tears or pinholes to form as the PTFE is compressed and results in a thin, highly filled film.