the present invention relates , generally , to the fabrication of precision dies and , more especially , to such dies which are destined for the manufacture of circuit boards . further along these lines , the present invention resides in part in the identification of heretofore unappreciated and undetected sources of problems in die fabricating techniques and resides in equal part in a solution therefor . accordingly , the invention will now be described with reference to certain preferred embodiments within that context ; although those skilled in the art will appreciate that such a description is meant to be exemplary only and should not be deemed limitative . a pervasive problem in the processing of precision dies by chemical milling is the lack of control of the etching process which can occur when an etchant - resistant fill disposed within cavity to protect the same is not bonded properly to the face of that cavity . this can allow etchant to migrate within a void between the metal die block and the fill , whereby the die element being formed by the etching procedure loses its dimensional tolerance . it can be appreciated that this problem becomes especially critical in fabricating circuit stamping dies , where it is attempted to form the edge portions of the individual die elements within quite close tolerances ( e . g ., on the order of one thousandth of an inch or less ) so that these can properly perform their function of separating the quite small circuit elements from the foil , which circuit elements may be as small as 0 . 010 &# 34 ; wide . not only is this precision of importance in the initial formation of the die , but also with regard to future maintenance of the die . as the edge portions of the individual die elements become worn , resharpening becomes necessary and with the very small dimensions involved this resharpening can best be accomplished by means of etching . such resharpening can only be accomplished properly if the initial formation of the die elements is within proper tolerances . in one aspect of the present invention , the uncontrollable etching due to void formation or imperfect adherence between an epoxy fill and a cavity in a metallic die block is minimized by first applying to selected areas of the die block a film of an etchant - resistant material as an underlayment for the epoxy fill . thus , even if etchant is able to penetrate the void , it will encounter a passive surface notwithstanding the presence of that void . turning to the figures of drawing , in all of which like parts are identified with like reference numerals , fig1 - 3 illustrate cross - sections of a die block at various stages in a chemical milling procedure to produce raised die elements which may be destined for stamping circuit boards . for the sake of brevity , reference is made to the present inventor &# 39 ; s u . s . pats . no . 3 , 758 , 350 , no . 3 , 911 , 716 , no . 4 , 053 , 348 , and no . 4 , 102 , 735 , for a complete discussion of the manner in which the chemical milling process is utilized for the formation of the die elements shown in these figures . briefly stated , a die block designated generally as 10 is formed with a plurality of raised die elements designated generally as 12 , a separating cavity 14 disposed intermediate the die elements and background cavities 16 outwardly bounding the region of die elements . each die element includes a die element recess 18 separating opposing edges or shoulders 20 . this profile of die elements and cavities is preferably formed in a step - back sequence of etching -- first , where the overall region of die elements is masked and the background cavities formed ; second , where the individual die element regions are masked and the separating cavities formed ; and , third , where the background and separating cavities are masked and the die element cavities formed . this step - back procedure is disclosed in the aforementioned patents , forms no part of the present invention , and thus further background information can be obtained by reviewing those references . fig1 illustrates a die 10 nearing the last stages of formation of die elements 12 . the background and separating recesses within metal die block 10 are loaded with a filler designated generally as 22 , such as an epoxy filler . the epoxy fill , which is an etchant - resistant material , serves to protect the cavities 14 and 16 during etching , while the upper surface of the fill serves to support a photo resist layer 24 used to mask selected areas of the die block during the chemical milling procedure . the die block 10 is exposed to an etchant , such as ferric chloride , which , due to the resist layer 24 and epoxy fill 22 , is capable of removal of material only from the die element cavities 18 since the remaining areas are protected by the film 24 . as etching proceeds , the die element edges 20 become progressively thinner as the cavity 18 is etched toward the separating and background cavities 14 and 16 , respectively . when this step has proceeded to an appropriate extent , the die is washed and the resist layer 24 is removed to yield the intermediate configuration of fig2 . as can be seen in fig2 the die elements 12 still retain a thickness at the upper working surface of the die where the edges 20 are being formed . when it is desired to form a knife edge element , such as the one shown in detail in fig4 the die block is subjected to a further etching treatment with , e . g ., ferric chloride . again , due to the epoxy fill 22 , etching proceeds within the die element cavities 18 and material is removed from the walls thereof to reduce further the transverse dimension of the die element edges 20 . this reduction occurs both laterally and transversely and , upon completion , yields the die block illustrated in fig3 . at this stage , the etching procedure is terminated , the die is washed and cleaned thoroughly , and the epoxy fill 22 is stripped therefrom to yield a precision die having raised die elements 12 shown in enlarged view in fig4 . the foregoing general description of the etching procedure is now somewhat conventional and is well disclosed in the previously mentioned united states patents . fig5 and 6 show how the presence of an imperfect bond between the epoxy fill 22 within the cavities separating a given die element can contribute to a loss of tolerance of the knife edges 20 . fig5 is an enlarged view of a portion of the die shown in fig2 . as can be seen , the knife edges 20 have a top profile which is to be removed in a sharpening operation by etching with ferric chloride . in the die of fig5 the epoxy fill 22 in the background recess 16 is shown to be separated from the side face of the cavity proximate the die element edge 20 , to yield a void or space 26 ( the dimension of which is somewhat exaggerated for the sake of clarity ) leading from the top surface of the die 10 . thus , when the die is exposed to etchant , that etchant may seep within the void 26 and remove the portion of die element edge 20 on the side adjacent the void 26 , that edge being identified as 20 &# 39 ;. this will yield a low , rounded , dull edge 20 &# 34 ;, as shown in fig6 . in turn , the die resulting from this operation will not exhibit the high precision needed for stamping , e . g ., circuit boards . fig7 and 8 parallel the die represented in fig5 and 6 , respectively . however , the cavities 14 and 16 are first imparted with an underlayment of a film of an etchant - resistant composition 28 . accordingly , where an inadvertent void 26 results from an imperfect bonding of the epoxy fill or from subsequent shrinkage thereof during a cure or for any other reason , any etchant which may migrate within the void will find passive surface which may not be etched . consequently , sharp , well defined uniform edges are imparted to the die elements 12 as etching and removal of material from the die can proceed only from within the die element cavity 18 . the preferred material for film 28 is gold since it is highly resistant to attack by ferric chloride , the most preferred etchant for dies of the variety with which the present invention is involved . after the background and separating cavities are formed but prior to filling these cavities with epoxy , the die is first thoroughly scrubbed with an abrasive such as aluminum oxide ( e . g . 240 grit ) to remove carbon deposits or other contaminants which might be present at the surfaces of the die . all of the surfaces of the die , save the cutting or working face , are masked and gold is then plated in accordance with conventional procedures to yield a film preferably from about 0 . 00005 to about 0 . 00015 &# 34 ;, and most preferably about 0 . 0001 &# 34 ;. the passive film is then removed from those areas where etching is to occur and the die is processed in accordance with the foregoing procedures for chemical milling to yield the sharp cutting edges 20 . a particularly preferred method for removing the passive film from the working areas ( e . g . those where die elements 12 are to be formed ) is by surface grinding the top face of the die , although any other desirable method might be used to achieve this end . while gold is the most preferred composition for the film underlayment , any material which is passive to the etchant employed might equally well be used to good advantage . for example , platinum might be utilized in lieu of gold . and , even though these are fairly expensive materials , the thin layer employed minimizes the overall cost of using such precious metals . for example , at current prices , a six inch by nine inch die can be electroplated with gold having a value of little more than about $ 5 . 00 . the metallic films are also preferred for ease of deposition , since the same may be electroplated directly onto the steel substrate . although plating gold onto steel normally requires an underlying flash of other metal ( such as nickel ), since the purpose of the plating here is to provide an etchant passive surface and not necessarily an integral permanent bond between the the gold film and the steel substrate , the electroplating procedure need not be as complicated as to require intermediate layers of compatible constituents . while electroplating is the preferred technique for depositing the metallic film , other techniques might be used . for example , a standard electroless plating process where a solution or paste is applied and then reduced to metallic form might be employed . likewise , the film might be sputtered onto the die face if that be desirable . other techniques will occur to those skilled in the art , as well as other suitable compositions for the underlayment . regardless of the deposition technique , it is somewhat surprising that such thin films of passive elements such as gold or platinum have the demonstrable , beneficial effects realized by practicing the present invention . for example , were a film of gold to be plated to a thickness of about 0 . 0001 - 0 . 00015 &# 34 ; on an open area of a die , that film would not be expected to function in an acceptable manner as a resist film . to the contrary , it would be expected to break down at least locally and fail to provide an efficient , etchant - resistant mask . nonetheless , as an underlayment film , these materials are found exceedingly beneficial in obtaining high quality precision dies through chemical milling techniques . when the preferred underlayment film of gold is employed , it not only serves to guard against unwanted etching as aforesaid , the retained gold film is believed to impart better operational characteristics to the die ultimately formed . for example , it is believed that the retained gold film will provide a measure of improved lubricity to the die when used in a subsequent stamping operation . this is an important advantage in providing dies with better stamping characteristics and better longevity . while the invention has now been described with reference to certain preferred embodiments , those skilled in the art will appreciate the various substitutions , modifications , changes and omissions that may be made without departing from the spirit thereof . accordingly , it is intended that the scope of the present invention be limited solely by that of the claims granted herein .