Abstract:
A novel process for fabricating metal clad laminates used in the manufacture of printed wiring boards. The process which eliminates multiple use separator plates within laminate books during buildup and pressing, utilizes a sacrificial separator sheet between copper sheets and wherein the separator sheet remains intact throughout the pressing and cutting steps and is finally removed after the laminates have been sized to the desired dimensions. This process provides time and cost savings, as well as a significant increase in quality and quantity of the laminates produced.

Description:
FIELD OF THE INVENTION 
     The invention is in the field of metal clad laminates used to manufacture printed wiring boards. More particularly, the invention relates to a process for fabricating laminates to desired dimensions from a block of laminates. 
     BACKGROUND OF THE INVENTION 
     In its simplest form, a printed wiring board laminate includes, as a component, a dielectric layer of either a reinforced or non reinforced resin that is known in the art as a prepreg. On the opposite sides of the prepreg are bonded conductive metal foil sheets. Subsequently, after curing, the metal on the laminate, through a number of photolithographic processes, is etched to produce conductive paths, or circuits, on the surface of the cured dielectric layer. This layer may be used individually as a two sided printed wiring board, or assembled with other etched layers to produce a multilayer printed wiring board. 
     These laminates are typically assembled by alternately stacking conductive metal foil sheets, prepreg sheets and separator sheets into a press “book”. The book is then inserted into a lamination press where it is cured under heat and pressure into a printed wiring board laminate. After pressing, the cured, metal-clad laminates are typically detached from the separator sheets and are subjected to further processing. 
     Of extreme importance in the manufacture of these laminates is the requirement for preventing and eliminating, to the greatest possible extent, contamination of the metallic foil sheets and maintaining cleanliness during the manufacturing process. It is known that the main cause of contamination is the presence of resin dust, fiberglass various other types of foreign material resulting from earlier manufacture and cutting of the laminates. During the manufacturing process, great care is taken to prevent resin dust and other contaminants from settling on the metallic surfaces. 
     It is of additional importance in the manufacture of laminates for printed wiring boards to increase the quantity of laminates produced while reducing the costs associated with materials and labor. 
     Laminates can be produced with metallic or composite separator sheets which inhibit laminates from adhering to each other and imparting their surface finish to the laminate. Generally, books of laminate containing separator sheets are separated or broken down prior to being cut into panels of desired sizes and dimensions. The laminates are generally pressed between thick metallic plates to maintain flatness and to assure the application of even pressure. The metallic plates represent a significant capital investment. Typically, 8-30 laminates are pressed in a book inserted into each opening within a multi opening press. Laminates may also be formed using a thin, sacrificial separator sheet. After pressing, the laminates are sent to a breakdown system, which may be manual or automatic, during which the metallic plates and/or separator sheets are removed. Following breakdown, the laminates either proceed directly to or are stacked prior to an automated or manual fabrication system to be cut one at a time. 
     The process of breaking down the books prior to cutting makes the laminates vulnerable to handling damage, contamination and staining as discussed hereinabove. Handling laminates in full sheet size also magnifies the chances of damage from bending or edge impacts during cutting. Additionally, handling the laminates in full sheet size requires additional process steps which decreases quantity output and overall productivity. 
     It is obvious from the foregoing that the elimination of handling damage and contamination incurred during the breakdown of the books, as well as increasing the quantity and quality output of laminates, in addition to reducing labor and costs of production by simplifying the process, is extremely important with regard to the manufacture of laminates used in the manufacture of printed wiring boards. 
     SUMMARY OF THE INVENTION 
     The present invention offers productivity and quality advantages associated with the production of laminates used in the manufacture of printed wiring boards. More particularly, the present invention is directed to a process for producing laminates in good quantity and with a significant decrease in imperfections. As used herein, the term “laminate” refers to a single metal-dielectric-metal layered sheet that has been bonded together in a press during which the cure reaction of the resin is completed. The process according to the present invention is generally directed to cutting stacks comprising a plurality of laminates separated from one another by a separator component, generally in the form of separator sheets, without removing the separator sheets upon the transfer of the laminate stack from the press to the cutting apparatus. In this manner, the laminates are pressed with the separator sheets intact and the separator sheets will be sacrificial. By cutting an entire book comprised of numerous stacks with the separator sheets intact, the chances for damaging the surface of the laminate or the chances of introducing handling damage are significantly reduced. Cutting the book of laminates with the separator sheets retained therein significantly reduces the time and labor required to break down the separator sheet from the laminates as was required in prior art laminate cutting processes. Furthermore, cutting the books, or partial, or multiple books, with the separator sheet intact stabilizes the laminate during cutting. The stabilization of the laminate results in very high edge quality on the panels, which allows for the introduction into a clean room environment without secondary cleaning. 
     The process of the present invention provides for the buildup of a laminate which may comprise a plurality of copper-clad laminates separated by sheets of aluminum, steel, composite or other materials, which may be attached to the separator sheet using for example, CAC™ (copper/aluminum/copper, which is described in U.S. Pat. No. 5,153,050), or other joining methods or positioned freely between the laminates. CAC™ is commercially available from Gould Electronics Inc., Foil Divisision, Eastlake, Ohio The book of laminates is then placed in a curing press and pressed into a cured block. Without breaking down the cured block of laminates and removing the separator components, the book is transferred to a cutting apparatus, such as a paneling saw or router, which is capable of fabricating multiple laminates simultaneously. One or more books, or partial books, can be fabricated at the same time. Finally, the laminates are sized to predetermined consumer-selected dimensions. 
    
    
     The above description sets forth rather broadly the more important features of the present invention in order that the detailed description thereof that follows may be understood, and in order that the present contributions to the art may be better appreciated. Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In the drawings in which like reference characters denote similar elements throughout-the several views: 
     FIG. 1 is a schematic illustration of a prior art process for pressing, stacking and cutting copper clad laminates; 
     FIG. 2 is a schematic illustration of the novel process according to the present invention; 
     FIG. 3 is a close-up side view of a laminate indicated as  10  in FIGS. 1 and 2; 
     FIG. 4 is a side view of the layers of a prior art laminate book prior to removal from a curing press; 
     FIG. 5 is a side view of the layers of a prior art laminate sheet prior to cutting; 
     FIG. 6 is a side view of the layers of a laminate book in accordance with the present invention prior to removal from a curing press; and 
     FIG. 7 is a side view of the layers of a laminate book in accordance with the present invention prior to cutting the book; 
     In the figures of the present application, two-digit numerals refer to parts involved in the process of the invention, and three-digit numerals refer to steps used to describe the process of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Presently, the process of preparing laminates for use in the manufacture of printed wiring boards proceeds as illustrated in FIG. 1, wherein a plurality of laminates  10  are stacked and positioned between press platens (not shown) of a curing press. Laminates  10  are generally comprised of two copper foil sheets  14  having one or more epoxy coated fiberglass layers or prepregs,  16 , sealed between the copper sheets  14  (see FIG.  3 ). 
     Laminates  10  have metallic separating plates  12  positioned between each individual laminate  10  (indicated at  110 ) and on the top and bottom of the laminates adjacent each press platen as in FIG.  1 . The plurality of laminates  10  that are positioned in an opening of the curing press is referred to as a “book”  18 . Book  18  is typically comprised of 8-30 laminates. The number of laminates and the height of book  18  are more precisely determined by the size of the press opening, and is indicated by arrow “A”. A more detailed view of book  18  prior to removal from the opening in the curing press is illustrated in FIG.  4 . The number of laminates  10  pressed in an opening is limited by the presence of metallic separating plates  12 , which occupy a significant amount of space in book  18 . After pressing, book  18  is removed from the opening in the curing press, the metallic separating plates  12  are removed and the laminates  10  are stacked (indicated at  112 ) and then transferred to a cutting apparatus to be sized. A more detailed view of the stack of laminates after removal of the metallic plates and prior to cutting is illustrated in FIG.  5 . The laminates  10  are then conveyed to the cutting apparatus one at a time and are cut to final, predetermined panel dimensions (indicated at  114 ). Alternatively, the laminates may be individually fed directly to the cutting apparatus after breakdown. The step of removing the metallic plates requires costly automatic breakdown system, or manual labor resulting from the handling of each sheet of laminate individually. 
     The process of the present invention as illustrated in FIG. 2, has reduced the time necessary to complete the process and results in an increased quantity and quality of laminate production. In FIG. 2, a book  18  comprised of a plurality of laminates  10  are stacked between upper metallic plate  20  and lower metallic plate  22  of a curing press (not shown). Upper and lower metallic plates  20 ,  22  maintain the flatness of laminates  10  and assure an even application of pressure from the press upon laminates. 
     Book  18  generally comprises 20-140 laminates between upper and lower metallic plates  20 ,  22  depending upon the size of the opening in the curing press, as indicated by arrow “B”. A more detailed view of book  18  prior to removal from an opening in the curing press is illustrated in FIG.  6 . Unlike the prior art, the process of the present invention comprises a book  18  comprised of a plurality of laminates  10  which are interleaved with thin sacrificial separator components or separator sheets  24 . Aluminum separator sheets are advantageous, however, it is essential to note that the invention is not limited to aluminum separator sheets, and separator sheets may also be fabricated from steel, composite, polymer film, copper and other sheet materials that are capable of withstanding the cure temperature of the laminate. Additionally, depending upon the type of laminate sheets utilized, the separator sheet may be freely positioned between the laminate sheets without the use of an adhesive. The separator sheet  24  may also be welded between laminate sheets. 
     A sheet of prepreg  16  is positioned between adjacent layers of copper when a plurality of laminate packages comprised of a single piece of aluminum separator sheet positioned between two copper sheets are stacked upon one another. These laminate packages are known in the industry as CAC™ and are commercially available from Gould Electronics Inc., Foil Division, Eastlake, Ohio. A prepreg  16  is a dielectric layer of an epoxy resin-impregnated woven glass fiber. After pressing between upper and lower metallic plates  22 ,  24 , the plurality of laminates  10  (comprising CAC™  20  and prepreg  16 ) are removed from the press and separated from upper and lower metallic plates  20 ,  22 , as indicated at  118 . The entire book, or a partial book or multiple books  18 , with the separator sheets  24  still intact, is then transferred as a mass to a cutting apparatus (not shown). A more detailed view of the book  18  of laminates  10  with separator sheets  24  intact is illustrated in FIG.  7 . Prior to cutting, an upper and lower rigid backing material or board such as a phenolic or non-phenolic backer (not shown) is advantageously positioned on the top and bottom of the stack. These backers improve the quality of the cutting. The cutting apparatus is advantageously a paneling saw, although the invention is not limited in this respect and a router or other cutting apparatus may be used. 
     With the separator sheets  24  remaining in place between copper sheets  14 , book  18  is cut on the cutting apparatus to final customer dimensions, indicated at  122 . Following cutting and sizing, the stacks of cut panels with the aluminum separator sheets  24  still intact, are inspected and the separator sheets  24  are then separated from the copper clad laminate, as indicated at  124 . 
     The inventive process of the present invention affords large gains in productivity, quality and reduction in labor and costs. The productivity advantages include: increased lamination capacity since time is not lost/wasted breaking down the books to remove the separator sheets and/or metallic plates prior to sizing/cutting of panels; increased fabrication capacity because individual laminates are not handled but rather, entire books are transferred from the press to the cutting and staging individual laminates for the cutting apparatus. Cutting with separator sheets intact and without the use for metallic plates increases the quality of the laminates and reduces the occurrence of imperfections in the laminates. Furthermore, cutting with the separator sheets intact stabilizes the laminate during the cutting step resulting in very high edge quality on the panels. This permits the introduction of the laminates into a clean room environment without the need for secondary cleaning. 
     It is to be understood that although the above inventive process is discussed with regard to CAC, the invention is not limited in this respect and other separators may be used with the same advantages accorded by the inventive process. 
     From the foregoing detailed description of the specific embodiments of the invention, it should be apparent that a unique process of preparing laminates for use in the manufacture of printed wiring boards has been described. Although particular embodiments have been disclosed herein in detail, this has been done by way of example for purposes of illustration only, and is not intended to be limiting with respect to the scope of the appended claims that follow. In particular, it is contemplated by the inventor that various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims.