[1]0 324732993 United States Patent Office Patented Oct. 21, 1969 2 It is another object of this invention to provide a flexible printed circuit composition having a single sheet of fibrous base impregnate d and covered with afully cured synthetic resin insulating material. It is another object of this invention to provide a flexible printed surface composite which does not deform when subjected to heat, which has satisfactory cold-flow characteristics, and which has excellent dimensional stability. It is another object of this invention to provide Ek 10 metho d for makin a flexibl e printe d circuit compo sites includi ng the compl ete penetr ation of a sheetlike fibrou s flexibl e web with a synthe tic fire retard ant and therea fter fully curin.- the resin to provid e a therm ostabil e coatin g. Other object s and advant ages will appea r herein after. 15 Brie fly, the present invention consists essentially of a base of a flexible fibrous material having at least one smooth surfaced coating of insulatin@ thermoset synthetic resin permeatin@ the interstices of and impregnatin g the fibrous material, the synthetic resinous coatings being 20 fully cured, the resin containi ng fireretardan t additive s, a resinous adhesiv e on the sur@fac e of the fully cured resin coated base and at least one layer of a metallic foil bonded to the base of the adhesiv e. The inventi on also includ es a metho d for produ cing 25 a flexible metal foil clad member includin g.the steps of applyin.- a plurality of coatings of a fiteretardan t containi ng syntheti c resin to a single sheet of flexible fibrous material, applying a resinous adhesiv e coating to at least one surface of the resin, heat treatinthe adhesiv e coat30 ing to a tackfree state short of complet e curing, applying to a metal foil a coating of a resinous adhesiv e solution and driving off the solvent therefro m by heatin@ -, and hot pressing the fibrous sheet mater-ial and the foil togethet and heating the composit e of the member and the foil 35 to complet ely cure the Tesin and adhesiv e and to bond the foil to the flexible sheet. For a better under standi n.- of the nature and object s of this inventi on refere nce is made to the drawin .-s in which: 40 FIGUR E 1 is a diagram matic view of a process; FIG. 2 is an enlar.-ed sectional view showing the manner in which successi ve, resinous coatings adhe-re to glass fiber strands; FIG. 3 is an enlarg ed sectio nal view showi ng adja45 cent layers of metal foil, resino us adhesi ve, and a fibrou s insulati ng base materi al; FIG. 4 is an enla rged pers pecti ve@ view of a frao ment ary porti on sho wing a porti on of a print ed circu it mem ber aftet the surr oun din.- meta l foil has bee n etch ed awa y, and 50 showi ng an additio nal bondi ng.resi n layer on the under side of the fibrou s base materi al; and FIG. 5 is a frag ment ary verti cal secti onal view thro ugh a plur ality of laye rs of print ed circu its after they have bee n pres sed toget her into one com pact unit. 55 As shown in FIG. 1 a strip -14) of a fibroug carrier web or substr ate of backin g materi al is discha rged from a coil 12 and is subjec ted to a numbe r of coatin gs of insulati ng materi al before it is united with a strip 14 of metalh c foil. The strip 10 is a flexibl e sheetlike memb er which is com60 posed of a fibrou s materi al such as woven glass fibdr, dacron mats, non-woven glass mats, asbestos cloth, cotton cloth, and paper. The strip 10 is preferably provided with at least one coating of resinous material such as shown in containers or tanks 16, 18, and 20 which are disposed at 65 the lower side of a &ying tower 22 at the upper end of which are the disposed spaced rollers 24. Similar rollers 26 are provided in each tank 16, 18, and 20 for guiding the strip into the tank. An additio nal tank 28 is provid ed for applyi ng an ad70 hesive coatin g 34 to the strip 10 after the resino us coatin gs 30 and 31 have been applie d. Wher e the strip 10 is compo sed of glass fiber cloth, it 3,473,993 MANUFACITURE OF FLEXIBLE FLANM RETARDANT FOIL CLAD LAMINATES Charles Kepple, Pepn Hills Township, Verona, Norman E. Martel!o, Turtle Creek, 2nd Clarence L. Zeise, Jr., Penn Township, Irwin, Pa., assi-,nors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 9, 1966, Ser. No. 533.026 Int. Ci. C09j 5104; B32b 7/00 7 Claims U.S. Cl. 156-314 ABSTRACT OF THE DISCLOSURE A substrate of ilexible fibrous material havin@ at least one smooth coatin.@ of a flame retatdant synthetic resin permeatin- the material and, on at least one side thereof, a resinous adhesive on the surface of the resin coated substrate, and a metal foil to form a circuit attached to the adhesive and embedded therein. This invention relates to flexible foil clad laminates suitable for use in printed electrical circuits or components, such as disclosed in application Ser. No. 533,002, filed Mir. 9, 1966. More particularly it pertains to the manufacture of extremely thin flexible fire retardant foil clad laminates. Just a few years a,ao the electronics industry launched an intensive effort to reduce electronic circuits to smaller dimensions. In most respects the results have been spectacular. However, the trend towatd micro-electronics has created a need for an improvement in reliability, performance, and cost. Indeed, the development of microelectronic, technolo.@y has been so remarkable that at the present time miniaturization is usually of secondary importance. One contribution to the micro-electronic industry has been the socalled "printed" circuit in which some or all of the components of an eleetrical circuit are mounted on an insulated base by adaption of conventional printin-, methods. Some advanta-es of that type of circuit are compactness, li.-htweight, duplication, andeconomy. Most insulating bases or carrier-webs are composed of a plurality of superimposed layers of a sheet-like material such asfiber,-lass or woven cotton cloth that is impre-nated and coated with an electrically insulatin.th,rmosettin- resin. An electrically conductin.- patterned foil of metal is bonded to the base by an adhesive. Most of such composites, however, have disadvantages including poor dimensional stability durin.- processin.-I poor cold-ffow characteristics, and separation of the base and metal pattern. Moreover, a base composed of resin impte.-nated fiberglass loses its ffexibility with a.-e. That is particularly true where the base is flexed repeatedly. Associated Nvith the foregoin- is the problem of providin@ a flexible metal clad laminate that is flame retardant. It has been found that the f(>re.-Oin.- problems may be overcome by providing a laminated composite of one or more layers of circuits composed of a base of a singie flexible sheet-like fibTOUS material havin.- at least one coatin.- of a flame retardant synthetic resin permeatin.the material and, on at least one side thereof, a resinous adhesive on the surface of the resin coated base, and a metal foil to form a circuit attached to the adhesive and embedded therein. Accordin.-ly, it is an object of the present invention to provide a flexible flame retardant printed citcuit composition havin.- one or more printed circuits adhesively bonded to a single sheet of a flexible fibrous base material.
[2]3 is preferably provided with a thin sizing coating 29 of synthetic material such as a polyvinyl alcohol or a polyvinyl butyral to improve flexiblity of the fibers. Though the sizing 29 and coatings 30 and 31 of resin are applied to both sides of the surfaces of the strip 10 as shown in FIG. 2, the coatings of sizing and resin impregnate the fabric and cover the fibers 32. The resins are preferably synthetic and suitable examples are alkyd, phenolic and epoxy resin varnishes with added flame retardants. After each coating is applied, the strip passes through the drying tower 22 and over the rolls 24 at a speed of from 2 to 9 feet per minute. The drying tower 22 is a housing in which means for heating are provided to dry the solvent out of the resinous coatings. More than one coating of the resin is preferably applied to avoid the existence of pin holes in the outer surface of the strip on the initial coating of resin and to produce a smooth surface. A temperature range of 135' to 165' C. and preferably about 150' C. is maintained within the tower is substantially fully cure the applied resinous coatings on the strip 10. The temperature within the drying tower 22 is closely controlled so that the successive coatings of resin, which completeiy saturate and impregnate the fibrous carrier web or backing, are fully cured before the coated strip 10 leaves the tower. The carrier web or strip 10 has an original thickness of from 1 to 10 mils, depending upon the type of material used. After the resinous coatings including, for example, a thin polyvinyl alcohol precoating, have been applied and cured the thickness may vary from 31/2 to 20 mils. After the final coating of resin is applied, a coating 34 of adhesive is applied to one side of the strip 10 by passing over one roller 25 in the tank 28, whereby the strip is immersed on one side into a bath of liquid adhesive 34. To control the thickness (about 1/2 mil) of the coating of adhesive the strip 10 passes over a wiping bar 36 above the adhesive bath 34 comprising a synthetic adhesive resin such as epoxy, polyester, or phenolic nitrile rubber composition. The strip coated with the adhesive is partly cured by passing it through the drying tower 22 and from where it passes out of the tower over guide rollers 38 and 40. At least one and preferably al of the coatings of sizing 29 and of resin 30 and 31 comprise a composition containing a fire retardant or flame repeflent material. Such material,may be composed of a major portion of a retardant @uch as a chlorendic alkyd vamish having the fr,,Ilowing composition in which parts are by weight: Parts Chlorendic alkyd ----------------------------- 272 Chlorinated wax (Halowax 0077) -------------- 20 Antimony oxide ------------------------------ 10 Whiting-CaCO3 ------------------------------ 20 Iron oxide ---------------------------------- 3 The chlorendic alkyd is prepared by reacting the following: Grams Chlorendic anhy&ide ------------------------- 282 Soya fatty acid ------------------------------- 270 Glycerol ------------------------------------ 80 The soya fatty acid and glycerol are heated to about 80-901 C. The chlorendic anhydride is then added while sparging. No condenser is used. The mixture is then heated for eight hours at 175'-185' C. until an acid value of 3 is obtained. Reacting to a viscosity value of "S" on the Gardner-Holdt scale is preferred. Other alkyds may be prepared by reacting other fatty acids and one or more polyhydric alcohols with chlorendic anhydride in a similar manner. The foregoing composition dissolved in a solvent is used to impregnate and permeate an electric grade glass fiber fabric having a thickness of about 2 mils. By using conventional dip coating techniques a flexible product having a thickness of about 4,5 mils was obtained. The glass fiber 3)473,993 4 coated with fire retardant material is cured by heating in the tower at a tempearture ranging from 50-180' C. ana preferably 175' C. for 25-45 minutes. The material may be folded at least 12 times (180' crease) without breaking or cracking and stih retains good dielectric strength. On testing for fire retardancy, the insulation material easily passed standard tests for self-extinguishment. The dielectric strength is about 1800 volts per mil. The coated glass fiber is then provided with an adhesive 10 coating for subsequent bonding of metal foil thereto. The metallic foil 14, such as copper foil, is dischar.-ed from a coil 42 and an adhesive coating 43 of about 0.-; mil thickness (FIGS. 3-5) is applied to one side of the foil by an adhesive applicator 44 which is disposed be15 tween guide rollers 46 and 48. The adhesive coating 43 preferably has a composition similar to the adhesive 34 on the strip 10. Both adhesives 34 and 43 are composed of a synthetic resin such, for example, as a phenolic-nitrile rubber copolymer. 20 The phenolic-nitrile rubber adhesi-ve comprises a 20% solution methyl ethyl ketane of a mixture of equal parts by weight of acr ylonitrile-rubber and B stage phenofic resin. The proportions of the nitrile rubber can be varied from 25% to 75% and the phenolic resin constitutes the 25 balance. The phenolic resin comprises the reaction product of a phenol such as cresol and formaldehyde in substantially equimolecular proportions. As the strip of foil 14 continues to move it passes over heating means such as infrared lamps 50 whereby the 30 adhesive coating 43 is dried and partially cured. For that purpose an exhaust hood 52 is provided over the area of the infrared lamps 50. The strip 14 of metal foil then passes over a heated metal roll 54 which is heated to atemperature of approximately 200' C. for the purpose of 35 further drying the adhesive 43 on the foil by evaporating more solvent. The metal roll 54 operates in conjunction with an elastomer roll 56 for pressing the adhesive coated metal strip 14 to the adhesive treated strip 10@ under pressure of up to 300 p.s.i. 40 An excellent adhesive bond occurs when the strips 10 and 14 move between the rolls 54 and 56 with the metal foil strip 14 uppermost and the base strip 10 lowermost and adjacent to the lower roll 56. A pressure, preferably within the range of from 15 to 250 pounds per square 45 inch, is apphed to the strips 10 and 14 for joining their adhesive coatin-S 34 and 43 together into a single laminate 58 which is further heated by moving it in contact with a heating shoe 60 at a temperature of approximately 200' C. for finahy setting the adhesive layers. Thereafter 50 the composite laminate 58 is accumulated on a coil 62. The composite foil clad laminate 58 is subsequently converted into a printed circuit member by removing or etching away by known techniques unnecessary portions of the metal foil leaving circuit portions 66 of the fofl as 55 shown in FIG. 4. A plurality of composite foil clad laminates 58 in printed circuit form may be bonded together to provide multilayer circuit member as shown in FIG. 5. For that purpose a plurality of laminates 58 are stacked and ro pr?ssed together after the foil has been converted into a printed circuit on each. The adhesive layer 43 at the bottom of the base strip 14 is applied in extra thickness to enable the circuit portions to be embedded when pressed together. For that purpose an adhesive layer 67 65 may be applied to the side of each laminate 58 opposite to that where ' the adhesive 43 has been applied. The nietallic foil 14 may be composed of any metal having suitable properties of electrical conductivity. Metals having good electricalconductivity include copper, 70 silver, aluminum, and base alloys thereof. Metals havin.higher electrical resistance include stainless steel and Kovar or other metals or alloys. These foils are from about 0.0005 to 0.003 mils in thickness. The adhesive nitrile rubber base resin is sold under the 75 trademark A-821-B.
[3]32473,993 5 The metals having hi.-h coefficients of resistance are used for the resistance portions of a circuit. The following examples are illustxative of the present invention. EXAMPLEI A 108 class cloth of 2 mils thickness is provided with a 1/2 mil coating of polyvinyl alcohol. One coatin.- of a phenolic alkyd vamish is applied to provide a relatively smooth pore-free coated surface to which a coating of a fire retardant vamish containing chlorendic alkyd resi@n of the composition previously set forth is applied. Then a coating of phenolic nitrile rubber adhesive is applied to the coated substrate as well as to a foil copper of 1.4 mils thickness. The adhesive is partly cured and the coated substrate and foil are bonded to.-ether by hot rolling att about 150 p.s.i. and the laminate is then substantially fully cured at about 200' C. The total thickness of the foil clad laminate is about 4.5 mils and it shows excellent flexibility properties, good thermal endurance up to 100' C., and exhibits -,Ood metal adherence. The copper foil has an avera.-e peel stren.- th of over 8 pounds per inch of width. EXAMPLE II In the fore- in- example a foil of 2 mils thick co per .0 @ p was substituted to provide a foil clad laminate havin.- a total thickness of about 5.0 mils. The properties of flexibility and foil adherence were similarly very good. The adherence or bond test involves puuing or peeling copper strips from the coated base by applying a load to a one inch width of foil at a 180' angle to the substrate. The load is increased until the foil be.-ins to peel away. This load is the peel strength. In all our laminates a peel strcn,-th averaging 8 pounds or higher is obtained. Each of the foil clad laminates of Examples I and 11 were converted into printed circuit members by applying a resist pattern to the foil surface, etchin.@ away the copper exposed through the pattem, and then removin.a the resist, thereby exposing the copper printed circuit pattern. The resultin.- printed circuit members can be employed in electrical apparatus, or they may be superimposed with a resinous adhesive between successive layers and the assembly consohdated under heat and pressure into a unitary inember providin.- a multi-layer printed circuit. Laminates havin.- metal foil on both sides of the substrate may be produced. After the desired circuits are obtained by a conventional method such as etchin- away the metal foil, a composite laminated circuit boar@d with ten superimposed circuit sheets having a total thickness Of 1/16 inch may be provided by the application of a pressure from 15 to 250 p.s.i. at a moldin.- temperature varying from 150 to 175' C. for a time from 5 minutes to 1 hour depending upon the de,-ree of curin.- of the resins. Each sheet had a layer of adhesive resin applied to its surface opposite the foil rlad surface to enable the sheets to bond to each other. Accoydin.-ly, the method of the present invention pro6 vides a multi-layered printed circuit which overcomes the disadvanta-es of prior printed circuits such as dimensional instability and poor solderability. Moreover, the base or fiberglass may be permeated with a fire retardant enamel coating to deter and minimize occurrences of fire. What is
