the present invention relates to a prepreg substrate that is used for printed circuit boards and microwave substrates for use in the electronics industry . the prepreg comprises at least one layer of an expanded fluoropolymer , such as , porous expanded polytetrafluoroethylene . the expanded polytetrafluoroethylene is prepared in accordance with the teachings of u . s . pat . no . 3 , 953 , 566 incorporated herein by reference , and may contain up to 10 % by weight of a copolymer of perfluoroalkyl ether and tetrafluoroethylene ( pfa ), e . g ., perfluoropropylvinyl ether - tetrafluoroethylene or a copolymer of tetrafluoroethylene and hexafluoropropylene ( fep ). the polytetrafluoroethylene used in the present invention and containing the adhesive is considered &# 34 ; unfilled &# 34 ; because it does not contain fillers such as ceramics particles or fibers , metallic particles or fibers , etc . the polytetrafluoroethylene is expanded in accordance with the techniques of u . s . pat . no . 3 , 953 , 566 to have a void volume of at least 50 %, preferably 70 - 95 %, and most preferably 75 - 85 %. as seen in fig1 the uncompressed prepreg 10 is formed from expanded polytetrafluoroethylene having a matrix with fibrils 1 that interconnect nodes 2 and define therebetween a high void volume 8 . the high void volume 8 , fibrils 1 and interconnected nodes 2 form the scaffolding for the compliant prepreg layer 10 . the quantity of resin adhesive 3 impregnated into the void volume 8 of the polytetrafluoroethylene fibril node matrix is between 15 and at most 40 % by weight of the total matrix weight . it has unexpectedly been found that by reducing the adhesive to 15 to at most 40 % by weight , better adhesion by the resin itself and prevention of fracture and cracking can be obtained . this is because a change in the failure mechanism of laminated sheets has occurred . in the conventional design , a polytetrafluoroethylene sheet would be impregnated with a high to moderate quantity of resin and result in a layer of polytetrafluoroethylene that has impregnated adhesive as well as surface located adhesive . as a result , when multiple sheets of this prior art prepreg are laminated to one another or to another surface and failure occurs , a crack is propagated along the entire bond line between the contacting surface portions of the juxtaposed layers . on the other hand , when using the low resin content polytetrafluoroethylene matrix composite of the present invention , see fig2 composite 20 contains two layers 4 and 6 , each containing fibrils 1 interconnecting nodes 2 and containing resin 3 , that are laminated to layer 5 . according to the present invention , adhesive 3 is essentially only located within the void volume 8 of the polymer composite . as a result , nodes and fibrils on the outer most surface effectively interrupt the adhesive and provide what would be viewed as a discontinuous layer of adhesive . thus , when a crack occurs , it propagates until it reaches a fibril or node portion at which point a crack will be discontinued . the absence of the adhesive on the surface of the fibril or node terminates the propagation of the crack and thus provides for a more integral laminate . in fig3 composite 30 contains two layers 4 and 6 , each containing fibrils 1 , nodes 2 , resin 3 and void volume 8 and layers 4 of a polymeric film and layers 7 of a metal foil , such as copper . the composite 30 will be compressed to force the air from void volume 8 while at the same time allowing the uncured resin 3 to occupy a greater percentage of the remaining void volume . as a result of the compression , uncured resin contacts either the void volume in an adjacent layer or an inner surface of layers 7 . due to the low resin content of the prepreg , the exterior surfaces of the polytetrafluoroethylene will directly contact an inner surface of layer 7 without uncured resin being located therebetween . although the present invention prefers to use epoxy and cyanate ester resins , resins such as polyimide resins , polyester resins , acrylic resins , triazine resins , bismaleimide / triazine resins , polyolefin resins , a polysulfone resin , a polyether - sulfone resin , and other resins may be used . the invention will now be described with reference to the following non - limiting examples and comparisons . a first prepreg material for a printed circuit was made by solvent impregnating an expanded polytetrafluoroethylene membrane with a nelco n - 4205 - 2 epoxy resin . solvent impregnation was accomplished by submerging the ptfe membrane in a solution of 20 % solids and 80 % mek solvent followed by a dip in a solution of 54 % solids and 46 % mek solvent . the deposited resin made up 68 % of the composite weight of the prepreg . this material was tested and had the following properties : an expanded polytetrafluoroethylene membrane having a void volume of 80 % was impregnated with the same nelco n - 4205 - 2 epoxy resin of comparison 1 . the resin quantity amounted to 37 % by weight of the total prepreg composite and was prepared in the same manner as that of comparison example 1 . example 2 was tested and had the following properties : the above two examples demonstrate that a decrease in approximately 45 % resin content resulted in a 300 % increase in flexibility , commensurate with a nominal drop in dk and peel strength , respectively 17 % and 18 %. thus , it was unexpected that it would be possible to lower the percentage of resin and provide a more compliant prepreg of polytetrafluoroethylene for electrical and mechanical uses without significantly lowering the adhesion capability of the prepreg . a low resin prepreg was made by applying to the surface of an expanded polytetrafluoroethylene membrane hot melt cyanate ester resin . the deposited resin made up 37 % of the composite by weight . comparison - example 3 was tested and had a peel strength of 1 . 7 pli after a lamination of 450 psi at 220 ° c . for 90 minutes . in contrast to comparison - example 3 , expanded polytetrafluoroethylene was impregnated by a solvent impregnating process to incorporate cyanate ester resin , 37 % by weight , into the porous ptfe . as a result , the resin was evenly distributed throughout the membrane instead of being located solely on the surfaces . the material had a peel strength of 2 . 8 pli after a lamination of 450 psi at 220 ° c . for 90 minutes . as a result of the solvent impregnating process , example 4 had a peel strength of 2 . 8 pli after a lamination of 450 psi at 220 ° c . for 90 minutes in contrast to the 1 . 7 pli of comparison - example 3 . this gave over a 60 % increase in adhesion where the only difference was the position of the adhesive with respect to the polytetrafluoroethylene substrate . by distributing the adhesive resin within the polytetrafluoroethylene the failure mode of the adhesive changes and gives the composite better adhesion . while the invention has been disclosed herein in connection with certain embodiments and detailed descriptions , it will be clear to one skilled in the art that modifications or variations of such details can be made without deviating from the gist of this invention , and such modifications or variations are considered to be within the scope of the claims herein below .