Abstract:
A method and materials for drilling through-holes in printed circuit boards with a drilling tool is disclosed. The method involves the use of a lubricating entry material placed on the top surface of a stack of printed circuit boards and a lubricating backup board placed beneath the bottom surface of the stack of printed circuit boards. The lubricating entry material has a core with skins attached on both sides by a lubricant/adhesive. Similarly, the backup board has a core with skins attached on both sides by a lubricant/adhesive. The skins are hard enough to support the top and bottom surfaces of the printed circuit boards and thereby reduce burring at the entry point and exit point of the through-hole. The lubricant/adhesive coats the drilling tool during the drilling operation to reduce friction and thereby reduce the temperature of the drilling tool.

Description:
FIELD OF THE INVENTION 
     This invention relates to drilling through-holes in printed circuit boards, and more particularly to the use of entry materials and backup boards in drilling through-holes in printed circuit boards. 
     BACKGROUND OF THE INVENTION 
     Multi-layer printed circuit boards typically include, in cross-section, a copper outer layer, followed by an epoxy-glass resin layer, a copper inner layer, another epoxy-glass resin layer, and an outer copper layer. The number of alternating copper inner layers and epoxy-glass resin layers can vary. In some printed circuit boards, the copper inner layer may not be used. In other printed circuit boards, the dielectric layer may be made from a material other than epoxy-glass resin. 
     Multi-layer printed circuit boards are made by a multi-stage process. Once the circuit pattern is etched onto each copper inner layer, the inner layer or layers are sandwiched between the epoxy layers and the copper outer layers, and heat and pressure are then applied to cure the epoxy and bond the layers together. A large number of small through-holes are then drilled through the cross-section of the multi-layer printed circuit board by a high speed drilling tool, in preparation for forming electrical contacts between the top and bottom outer copper layers and the inner copper layers of the printed circuit board. Usually, the contacts are made by first plating an electrically conductive metal, such as electroless copper, to the surface of the through-hole. The wall of the through-hole is then further plated with electrolytic copper to complete the contact between the copper layers of the printed circuit board. 
     It is not uncommon to drill thousands of through-holes in a single printed circuit board. As a result, abrasion to the drill bit is a problem because of the high drilling speed and the large amount of heat generated, especially because of drilling through the metal layers and the abrasive epoxy-glass layers. In conventional drilling operations, the drilling tool can easily reach temperatures in the neighborhood of 500.degrees. F. to 700.degrees. F. 
     Such drilling temperatures encountered during the drilling of through-holes in the epoxy-glass and copper layers often causes the epoxy to smear over the copper inner layers. The dielectric material also can smear onto the conductive layers when drilling panels having insulating layers made from other materials. If the through-holes are left untreated (i.e., if the smeared dielectric material is not completely removed) prior to copper plating the walls of the through-holes, a dielectric barrier can be left between the conductive path of the copper plated through-holes and the copper conductive layers. This interferes with the electrical connection and results in unreliable electrical contacts between the copper inner and outer layers (or between the conductive outer layers if an inner layer is not used). The high drilling temperatures also shortens the useful life of the drill bit. 
     Another problem often encountered during the high speed drilling of printed circuit boards is the creation of a burr on the periphery of the through-hole where the drill exits the copper-clad outer surface. Such a burr may form a dam-like condition during plating of the through-hole which can entrap contaminants or air. Such entrapped contaminants or air may later result in cracking of the through-hole plating and a resultant electrical failure. Such burrs thus must be removed, either by a separate sanding operation or prevented by the use of a suitable backup board. Suitable backup boards, for example are disclosed in Block U.S. Pat. Nos. 3,700,341 and 4,269,549. 
     Yet another problem is countered when the drilling tool enters the printed circuit board. The high speed drill bit upon encountering the surface of the printed circuit board tends to create burrs at the entry point of the through-hole. Conventionally, the problem of burrs at the entry point of the through-hole is reduced by the use of an appropriate entry material. Typically, the entry material used is a thin metal foil, such as aluminum. 
     The prior art methods of drilling printed circuit boards fail to adequately address the three identified problems in an integrated fashion. For example, Hatch et al. U.S. Pat. Nos. 4,781,495 and 4,929,370 disclose a drilling method in which a lubricating sheet is placed between each printed board in a stack of printed circuit boards prior to drilling through-holes in the printed circuit boars. Each sheet consists of a porous, pulp-based paper which is saturated with a water soluble dry lubricant. While the lubricating sheet provides lubrication for the drilling tool during the drilling process, the porous, pulp-based paper does not contribute to the elimination or reduction of burrs. Moreover, because the outer surface of the lubricating sheet is coated with a layer of the lubricant, the lubricant directly contacts the surface of the printed circuit board and consequently contaminates the printed circuit board so that a subsequent cleaning step is required. 
     Another prior art entry material consists of a thin aluminum foil with a lubricating film coated on one surface. The entry material is placed on the top surface of the stack of printed circuit boards to be drilled with the aluminum foil in contact with the top surface of the stack of printed circuit boards. While such an entry material provides lubrication to the drilling tools, the thin aluminum foil lacks dimensional stability in that it bows and twists. Such bowing and twisting results in lost of contact between the top surface of the printed circuit board and the aluminum foil. Without such contact, the entry material cannot perform the function of reducing burring. 
     SUMMARY OF THE INVENTION 
     The present invention includes methods and materials for use in drilling printed circuit boards. The method for drilling printed circuit boards of the present invention provides an integrated approach to solving the problems of excessive heat and burring described above. In one embodiment of the method of the present invention, a lubricating, dimensionally stable entry material is placed on top of a stack of printed circuit boards and a lubricating, dimensionally stable backup board is placed beneath the stack of printed circuit boards. During drilling, the lubricating entry material and the lubricating backup board serve to lubricate the drill bit and thereby reduce drilling temperatures and extend the life of the drill bit. The dimensionally stable lubricating entry material and dimensionally stable lubricating backup board also support the top and bottom surfaces of the printed circuit board and thereby serve to reduce burrs. 
     In another embodiment of the method of the present invention where top surface burring is not a serious concern, the lubricating backup board is used alone without the lubricating entry material. Because the drill bit penetrates the lubricating backup board at the end of the first pass through the stack of printed circuit boards, the drill bit retains sufficient lubricant for withdrawal and for the subsequent pass through stacks of printed circuit boards. 
     The materials of the present invention include entry materials and backup boards. In one embodiment of the present invention, the entry material comprises a cellulose core and an outer skin on both faces of the cellulose core. The outer skins are bonded to the cellulose core by layers of lubricant/adhesive. The cellulose core consists of consolidated cellulose fibers. The cellulose core has limited porosity to the lubricant/adhesive and is not impregnated or saturated by the lubricant/adhesive. In another embodiment of the entry material, the cellulose core may be eliminated, and the outer skins may be bonded directly to each other by means of a layer of the lubricant/adhesive. The laminated construction of the entry materials assures dimensional stability both with respect to bowing and twisting. 
     The layer of lubricant/adhesive consists of a water dispersion of polyethylene oxide resin plasticized with a high molecular weight polyethylene glycol. The lubricant/adhesive layer is formed by coating either the outer skin or directly coating the cellulose core and then drying the solution to form the lubricant/adhesive film for lamination. 
     The outer skin of the entry material may include metal foil, epoxy coated paper, uncoated paper, or plastic. The outer skin serves several functions. The outer skin of the entry material keeps the lubricant/adhesive from directly contacting the top surface of the circuit board which reduces surface contamination of the circuit board during drilling. The outer skin eliminates the need for slip sheets between successively stacked sheets of the entry material to isolate the lubricant/adhesive on adjacent sheets of entry material during the manufacturing, storing, and shipping of the entry material. The outer skin also serves the purpose of burr prevention/reduction by providing a hard, dimensionally stable surface to support the top surface of the circuit board during the entry of the drill bit. The outer skin, however, has sufficient resilience so that when initially “hit” by the drill bit, the outer skin dents to absorb the shock of the drill bit and to center the drill bit at the point of impact. 
     One embodiment of the backup board of the present invention comprises a pressed wood core and an outer skin on both faces of the wood core. The outer skins are bonded to the wood core by layers of lubricant/adhesive. The wood core consists of a pressed wood fiber board. The pressed wood fiber board is not porous, i.e. the lubricant/adhesive does not penetrate appreciably into and does not saturate the wood fiber board. The lubricant/adhesive is a same as the lubricant/adhesive used with the entry material. 
     The outer skin of the wood core backup board may include metal foil, epoxy coated paper, uncoated paper, or plastic. The outer skin of the wood core backup board serves several functions. The outer skin keeps the lubricant/adhesive from directly contacting the bottom surface of the circuit board which reduces surface contamination of the circuit board. The outer skin eliminates the need for slip sheets to isolate the lubricant/adhesive between successively stacked backup boards in the manufacturing, storing, and shipping of the wood core backup boards. The outer skin serves the purpose of burr prevention/reduction by providing a hard, dimensionally stable surface to support the bottom surface of the stack of circuit boards. 
     A second embodiment of the backup board of the present invention comprises a ventilated backup board. The ventilated backup board is used in the method of the present invention and comprises a sinuous core member having opposing antinode portions of metallic foil sandwiched between opposing metallic foil sheets. One of the antinode portions faces against one of such foil sheets and the other antinode portions faces against the other of the foil sheets. The antinode portions are preferably elongated running in substantially parallel lines between opposite edges of the foil sheets so as to form elongated channels therebetween and may be approximately in the form of a sine wave. The pair of opposing metallic foil sheets may be of copper or steel but are preferably of aluminum. The sinuous core may likewise be foil sheets of copper or steel but are preferably of aluminum. A layer of adhesive bonds each of the aluminum foil sheets to the top and bottom of the sinuous core member. An outer skin is bonded to each of the aluminum foil sheets by a layer of lubricant/adhesive. The outer skins of the ventilated backup board may include metal foil, epoxy coated paper, uncoated paper, or plastic. The outer skins of the ventilated backup board serve several functions. The outer skins keep the lubricant from directly contacting the bottom surface of the circuit board which reduces surface contamination. The outer skins eliminate the need for slip sheets in the manufacturing, storing, and shipping of the ventilated backup boards. The outer skins serve the purpose of burr prevention/reduction by providing a hard, dimensionally stable surface to support the bottom of the circuit board. The layer of lubricant/adhesive is a same as the layer of lubricant/adhesive used with the entry material and the wood core backup board. 
     In connection with the method of the present invention for drilling through-holes in printed circuit boards, the entry material is placed on the top surface of the circuit board or stack of circuit boards to be drilled, and the backup board is placed on a support surface beneath the bottom surface of the stack of circuit boards to be drilled. The top surface of the circuit board or stack of circuit boards abuts against the outer skin of the entry material, and the bottom surface of the circuit board or stack of circuit boards abuts against the outer skin of the backup board. The stack is then pressed down by a pressure foot prior to drilling to assure that the stack is flat and secure when the drill bit makes its initial contact with the entry material. During the drilling, the drill bit passes through the first outer skin of the entry material, through the first layer of lubricant/adhesive, through the cellulose core (if present), through the second layer of lubricant/adhesive, and through the second outer skin of the entry material. The drill bit then passes through the stack of printed circuit boards and into the backup board. In the case of the wood core backup board, the drill bit passes through the outer skin of the backup board, through the layer of lubricant/adhesive, and into the wood core of the backup board. In the case of the ventilated backup board, the drill bit passes through the outer skin, through the lubricant/adhesive, through the aluminum sheet, through the core lamination adhesive, and into the ventilated core of the backup board. After the drill bit enters the core space, the drill bit goes no further and momentarily dwells in the core of the backup board. For the ventilated backup board, air flow is provided in the core of the backup board to facilitate the removal of cuttings therefrom and to aid in the cooling of the drill bit. As the drill bit passes through the layer of lubricant/adhesive of the backup boards, the drill bit picks up lubricant from the layer of lubricant/adhesive. The lubricant remains on the drill bit and reduces the friction as the drill bit is withdrawn from the through-hole and reduces friction as the drill bit drills the next through-hole. 
     It is therefore the object of this invention to provide a method for drilling printed circuit boards which reduces the temperature of the drill bit and thereby extends the life of the drill bit and reduces the smearing of the dielectric material in the through-hole of the printed circuit board. 
     It is a further object of the present invention to provide a method for drilling printed circuit boards which reduces burring both at the entry point of the through-hole and at the exit point of the throughhole. 
     It is also an object of the present invention to provide an entry material for use in drilling printed boards, which entry material reduces the temperature of the drill bit and also reduces burring around the entry point of the through-hole. 
     It is likewise an object of the present invention to provide a backup board for use in drilling printed boards, which backup board reduces the temperature of the drill bit and also reduces burring around the exit point of the through-hole. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects of the invention will become apparent as the description proceeds in connection with the accompanying drawings of which: 
     FIG. 1 is a cross-sectional view of a lubricating entry material; 
     FIG. 2 is a cross-sectional view of one embodiment of a lubricating backup board; 
     FIG. 3 is a cross-sectional view of another embodiment of a lubricating backup board; 
     FIG. 4 is a cross-sectional view illustrating one method for drilling printed circuit boards using a lubricating entry material and a lubricating backup board; and 
     FIG. 5 is a cross-sectional view illustrating another method for drilling printed circuit boards using a conventional entry material and a lubricating backup board. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 4, there is shown an embodiment of a lubricating entry material  10  used in connection with drilling through-holes in a stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d . The lubricating entry material  10  comprises of first hard outer entry skin  12 , a second hard outer entry skin  14 , a first lubricant/adhesive layer  16 , a second lubricant/adhesive layer  18 , and a core  20 . The hard outer entry skins  12  and  14  are bonded to the core  20  by means of the lubricant/adhesive layers  16  and  18 . 
     The core  20  may consist of consolidated, randomly oriented cellulose fibers. Other suitable materials for the core  20  may include solid resin films including polyester films. The cellulose core  20  typically has a thickness of from about  3  mils to about  8  mils. The cellulose core  20  has limited porosity to the lubricant/adhesive of the lubricant/adhesive layers  16  and  18 . The cellulose core  20  is not impregnated or saturated by the lubricant/adhesive. Instead, the cellulose core  20  only absorbs enough lubricant/adhesive to insure a bond between the lubricant/adhesive layers  16  and  18  and the cellulose core  20 . The cellulose core  20  is resilient and thereby functions as a cushion against the impact of the drill bit  22  of a high speed drilling tool  24  (FIG. 4) when the entry material  10  is “hit” by the drill bit  22  during the drilling of through-holes in a printed circuit boards  30   a ,  30   b ,  30   c , and  30   d.    
     Alternatively, the core  20  may be eliminated from the laminated structure of the entry material  10  where cushioning requirements are reduced. In that alternative embodiment without the core  20 , the hard outer entry skins  12  and  14  are bonded directly to each other by a layer of lubricant/adhesive. 
     Each of the layers  16  and  18  of lubricant/adhesive consists of a water dispersion of polyethylene oxide resin plasticized with a high molecular weight polyethylene glycol. The lubricant/adhesive layers  16  and  18  are formed by coating either the outer skins  12  and  14  or directly coating the cellulose core  20  and then drying the solution to form the lubricant/adhesive film for lamination. The resulting lubricant/adhesive layers  16  and  18  are typically from about 3 mils to about 7 mils in thickness. The dried lubricant/adhesive layers  16  and  18  have a softening point of less than 250° F. so that during drilling of the through-holes in the printed circuit boards, the lubricant/adhesive layers  16  and  18  can transfer lubricant to the drill bit  22 . 
     Each of the outer skins  12  and  14  of the entry material  10  may consist of metal foil, epoxy coated paper, uncoated paper, or plastic. Each of the hard outer skins has a hardness, the value of which depends on the material used and which combined with the lubricant/adhesive is sufficient to suppress entry burring on the top printed circuit board  30   a . For example, the aluminum used for the outer skins  12  and  14  may be type 3003-H19 aluminum foil having a thickness from about 0.001 inch to about 0.002 inch. 
     The outer skins  12  and  14  serve several functions. The outer skin  12 , which is in contact with the top surface  32  of the stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  during drilling (FIG.  4 ), keeps the lubricant/adhesive layer  16  from directly contacting the top surface  32  of the stack  28  and thereby eliminates surface contamination of the circuit boards during drilling. The outer skin  14 , which is in contact with the pressure foot  26  during drilling (FIG.  4 ), keeps the pressure foot  26  of the drilling machine  24  from contacting the lubricant/adhesive layer  18  and thereby eliminates build up of lubricant on the pressure foot  26  of the drilling machine  24 . The outer skins  12  and  14  also eliminate the need for slip sheets between successively stacked sheets of the entry material  10  by isolating the lubricant/adhesive layers  16  and  18  on adjacent sheets of entry material  10  during the manufacturing, storing, and shipping of the entry material  10 . The outer skin  12 , which is in contact with the top surface  32  of a stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  (FIG.  4 ), also serves the purpose of burr prevention/reduction by providing a hard, dimensionally stable outer surface to support the top surface  32  of the circuit board  30   a  during the entry of the drill bit  22 . The outer skin  14 , however, has sufficient resilience so that when the outer skin  14  is initially “hit” by the drill bit  22 , the outer skin  14  in cooperation with the core  20  dents to absorb the shock of the drill bit  22  and to center the drill bit at the point of impact. 
     With or without the core  20 , the laminated construction of the lubricating entry material  10  gives the lubricating entry material  10  dimensional stability both with respect to bowing and twisting. Typically the dimensional stability exhibited by the entry material  10  of the present invention is less that 2% bow and twist for the entry material  10  with a cellulose core  20  and is less that 2% bow and twist for the entry material  10  without the cellulose core  20 . 
     Referring to FIGS. 2 and 4, there is shown an embodiment of a lubricating backup board  34  used in connection with drilling through-holes in a stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d . The backup board  34  comprises a first hard outer backup skin  36 , a second hard outer backup skin  38 , a first lubricant/adhesive layer  40 , a second lubricant/adhesive layer  42 , and a solid core  44 . The hard outer backup skins  36  and  38  are bonded to the solid core  44  by means of the lubricant/adhesive layers  40  and  42 . 
     The core  44  may consist of pressed wood fiber board. Other suitable materials for the core  44  may include solid resin and paper-resin blends. The wood core  44  typically has a thickness from about 0.050 inch to about 0.125 inch. The wood core  44  has limited porosity to the lubricant/adhesive and is not impregnated or saturated by the lubricant/adhesive. Instead, the wood core  44  only absorbs enough lubricant/adhesive at the surface to ensure a bond between the lubricant/adhesive layers  40  and  42  and the wood core  44 . 
     Each of the layers  40  and  43  of lubricant/adhesive consists of the same water dispersion of polyethylene oxide resin plasticized with a high molecular weight polyethylene glycol as previously described in connection with lubricant/adhesive layers  16  and  18  of the entry material  10 . Likewise, the lubricant/adhesive layers  40  and  42  are formed by coating either the outer skins  36  and  38  or directly coating the wood core  44  and then drying the solution to form the lubricant/adhesive for lamination. The resulting lubricant/adhesive layers  40  and  42  are typically from about 3 mils to about 7 mils in thickness. Generally the dried lubricant/adhesive layers  40  and  42  have a softening point of less than 250° F. so that during drilling of the through-holes in the printed circuit boards, the lubricant/adhesive layers  40  and  42  can transfer lubricant to the drill bit  22 . 
     Each of the outer skins  36  and  38  of the entry material  10  may consist of metal foil, epoxy coated paper, uncoated paper, or plastic. Each of the hard outer skins has a hardness, the value of which depends on the material used and which combined with the lubricant/adhesive is sufficient to suppress entry burring on the top printed circuit board  30   a . For example, the aluminum used for the outer skins  12  and  14  may be type 3003-H19 aluminum foil having a thickness of about 0.005. 
     The outer skins  36  and  38  of the wood core backup board  34  serves several functions. The outer skin  36 , which is in contact with the bottom surface  46  of the stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  during drilling (FIG.  4 ), keeps the lubricant/adhesive layer  40  from directly contacting the bottom surface  46  and thereby eliminates surface contamination of the circuit board  30   d  during drilling. The outer skin  38 , which is in contact with a support surface  48  during drilling (FIG.  4 ), keeps lubricant/adhesive from building up on the support surface  48 . The outer skins  36  and  38  of the wood core backup board  34  also eliminate the need for slip sheets between successively stacked backup boards  34  for isolating the lubricant/adhesive layers  40  and  42  on adjacent backup boards  34  during the manufacturing, storing, and shipping of the backup boards  34 . The outer skin  36 , which is in contact with the bottom surface  46  of the stacked  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  during drilling (FIG.  4 ), also serves the purpose of burr prevention/reduction by providing a hard, dimensionally stable surface to support the bottom surface  46  of circuit board  30   d  (FIG. 4) during the drilling process. 
     The laminated construction of the wood core backup board  34  gives the backup board  34  dimensional stability both respect to bowing and twisting. Typically the dimensional stability exhibited by the backup board  34  of the present invention is less that 2% bow and twist. 
     Referring now to FIGS. 3 and 5, there is shown another embodiment of a lubricating backup board  50  used in connection with drilling through-holes in a stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d . The backup board  50  comprises a first hard outer backup skin  60 , a second hard outer backup skin  62 , a first lubricant/adhesive layer  64 , a second lubricant/adhesive layer  66 , and a ventilated core  51  comprising an upper metallic foil sheet  52 , a lower metallic foil sheet  54 , and a sinuous core member  56 . The hard outer backup skins  60  and  62  are bonded to the metallic foil sheets  52  and  54  of the ventilated core  51  by means of the lubricant/adhesive layers  64  and  66 . 
     The sinusoidally undulating core member  56  is sandwiched between the upper metallic foil sheet  52  and the lower metallic foil sheet  54  to form the ventilated core  51  for the backup board  50 . The core member  56  has opposing antinode portions  56   a  and  56   b  which face against the inside surfaces of sheets  52  and  54  respectively. Antinodal portions  56   a  and  56   b  are bonded to sheets  52  and  54  by means of a suitable bonding material  58  and run between opposite edges of the sheets, the apices of these nodal portions abutting against the sheets and running in substantially parallel lines. The pair of opposing metallic foil sheets  52  and  54  may be of copper or steel but are preferably of aluminum. The sinuous core member  56  may likewise be foil sheets of copper or steel but are preferably of aluminum. It has been found that aluminum foil is particularly suitable for sheets  52  and  54  and core member  56 . 
     In one embodiment of the core  51  of the ventilated backup board  50  is constructed of type 3003-H19 aluminum foil having a thickness of about 0.005 inch for the upper and lower foil sheets  52  and  54  and a thickness of about 0.002 inch for the sinuous core member  56 . When using aluminum of this type with 0.005 inch thickness, the spacing between the inner walls of the two foil sheets  52  and  54  should typically be approximately 0.053 inch. In another embodiment of the core  51  of the backup board  50 , the upper and lower foil sheets  52  and  54  have a thickness of about 0.007 inch, the core member  56  has a thickness of about 0.002 inch, and the spacing between the inner walls of the two foil sheets should be approximately 0.079 inch. In yet another embodiment of the core  51  of the backup board  50 , the upper and lower foil sheets  52  and  54  have a thickness of about 0.009 inch, the core member  56  has a thickness of about 0.002 inch, and the spacing between the inner walls of the two foil sheets  52  and  54  should be approximately 0.106 inch. 
     Each of the layers  64  and  66  of lubricant/adhesive consists of the same water dispersion of polyethylene oxide resin plasticized with a high molecular weight polyethylene glycol as previously described in connection with lubricant/adhesive layers  16  and  18  of the entry material  10  and lubricant/adhesive layers  40  and  42  of the wood core backup board  34 . Likewise, the lubricant/adhesive layers  64  and  66  are formed by coating either the outer skins  60  and  62  or directly coating the foil sheets  52  and  54  and then drying the solution to form the lubricant/adhesive for lamination. The resulting lubricant/adhesive layers  64  and  66  are typically from about 3 mils to about 7 mils in thickness. Generally the dried lubricant/adhesive layers  64  and  66  have a softening point of less than 250° F. so that during drilling of the through-holes in the printed circuit boards, the lubricant/adhesive layers  64  and  66  can transfer lubricant to the drill bit  22  (FIG.  5 ). 
     Each of the outer skins  60  and  62  of the entry material  10  may consist of metal foil, epoxy coated paper, uncoated paper, or plastic. Each of the hard outer skins has a hardness, the value of which depends on the material used and which combined with the lubricant/adhesive is sufficient to suppress entry burring on the top printed circuit board  30   a . The aluminum used for the outer skins  60  and  62  may be type 3003-H19 aluminum foil having a thickness of about 0.005 inch. This type of aluminum has the following constituents added to pure aluminum: silicon—6%; iron—7%; copper—0.05-0.20%; manganese—1.0-1.5%; and zinc—0.10%. 
     The outer skins  60  and  62  of the ventilated core backup board  50  serves several functions. The outer skin  60 , which is in contact with the bottom surface  46  of the stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  during drilling (FIG.  5 ), keeps the lubricant/adhesive layer  64  from directly contacting the bottom surface  46  and thereby eliminates surface contamination of the circuit board  30   d  during drilling. The outer skin  62 , which is in contact with a support surface  48  during drilling (FIG.  5 ), keeps lubricant/adhesive from building up on the support surface  48 . The outer skins  60  and  62  of the ventilated core backup board  50  also eliminate the need for slip sheets between successively stacked backup boards  50  to isolate the lubricant/adhesive layers  64  and  66  on adjacent backup boards  50  during the manufacturing, storing, and shipping of the backup boards  50 . The outer skin  60 , which is in contact with the bottom surface  46  of the stack  28  of printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  during drilling (FIG. 5) also serves the purpose of burr prevention/reduction by providing a hard, dimensionally stable surface to support the bottom surface  46  of circuit board  30   d  (FIG. 5) during the drilling process. 
     The laminated construction of the ventilated core backup board  50  gives the backup board  50  dimensional stability both respect to bowing and twisting. Typically the dimensional stability exhibited by the backup board  50  of the present invention is less than 1% bow and twist. 
     Referring now to FIG. 4, the operation of a method in accordance with the present invention for drilling through-holes in printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  is schematically illustrated. A lubricating backup board such as the backup board  34  is placed on support surface  48 . A stack  28  of circuit boards  30   a ,  30   b ,  30   c , and  30   d  is placed on the lubricating backup board  34  so that the bottom surface  46  of the stack  28  abuts the hard outer backup skin  36  of the backup board  34 . It should be understood that the lubricating backup board  50  could be used instead of the lubricating backup board  34  as shown in FIG.  4 . 
     The lubricating entry material  10  is placed on the top surface  32  of the stack  28  of circuit boards  30   a ,  30   b ,  30   c , and  30   d  to be drilled. The top surface  32  of the stack  28  of circuit boards  30   a ,  30   b ,  30   c , and  30   d  abuts against the hard outer skin  12  of the entry material  10 . The stack  28 , of course, may include more or less printed circuit boards than the four illustrated in FIG.  4 . The exaggerated spacing between the backup board  34 , the circuit boards  30   a ,  30   b ,  30   c , and  30   d , and the entry material  10  is shown in FIG. 4 for purpose of clarity whereas in reality all of the components would be in tight, intimate contact with each other. 
     Once the backup board  34 , the stack  28 , and the entry material  10  have been positioned as shown in FIG. 4, the stack is then pressed down by a pressure foot  26  prior to drilling to assure that the stack is flat and secure when the drill bit  22  makes its initial contact with the entry material  10 . During the drilling, the drill bit  22  passes through the first outer skin  14  of the entry material  10 , through the first layer of lubricant/adhesive  18 , through the cellulose core  20  (if present), through the second layer of lubricant/adhesive  16 , and through the second outer skin  12  of the entry material  10 . The drill bit  22  then passes through each of the circuit boards  30   a ,  30   b ,  30   c , and  30   d  of the stack  28 . The drill bit  22  then continues through hard outer skin  36  of the backup board  34 , through the lubricant/adhesive layer  40 , and into the wood core  44 . After the drill bit  22  enters the wood core  44 , the drill bit goes no further and momentarily dwells in the core  44  of the backup board  34 . As the drill bit  22  passes through the lubricant/adhesive layers  16  and  18  of the entry material  10  and lubricant/adhesive layer  40  of the backup board  34 , the drill bit picks up lubricant from the layer of lubricant/adhesive. The lubricant remains on the drill bit  22  and reduces the friction as the drill bit  22  drills the through-holes and as the drill bit is withdrawn from the through-holes. 
     Referring now to FIG. 5, another method in accordance with the present invention for drilling through-holes in printed circuit boards  30   a ,  30   b ,  30   c , and  30   d  is schematically illustrated. A lubricating backup board such as the backup board  50  is placed on support surface  48 . A stack  28  of circuit boards  30   a ,  30   b ,  30   c , and  30   d  is placed on the lubricating, ventilated backup board  50  so that the bottom surface  46  of the stack  28  abuts the hard outer backup skin  60  of the backup board  50 . It should be understood that the lubricating backup board  34  could be used instead of the lubricating backup board  50  as shown in FIG.  5 . 
     A conventional entry material  68  may be placed on the top surface  32  of the stack  28  of circuit boards  30   a ,  30   b ,  30   c , and  30   d  to be drilled. The conventional entry material may consist of a single ply of aluminum skin, or the conventional entry material  68  may consist of outer skins  70   a  and  70   b  bonded to a cellulose core  72  with laminating adhesive  70   c  and  70   d . In any case, the conventional entry material  68  does not have lubricating properties. The top surface  32  of the stack  28  of circuit boards  30   a ,  30   b ,  30   c , and  30   d  abuts against the hard outer skin  70   b  of the conventional entry material  68 . The stack  28 , of course, may include more or less printed circuit boards than the four illustrated in FIG.  5 . The exaggerated spacing between the backup board  50 , the circuit boards  30   a ,  30   b ,  30   c , and  30   d , and the entry material  68  is shown in FIG. 5 for purpose of clarity whereas in reality all of the components would be in tight, intimate contact with each other. 
     Once the backup board  50 , the stack  28 , and the entry material  68  have been positioned as shown in FIG. 5, the stack is then pressed down by a pressure foot  26  prior to drilling to assure that the stack is flat and secure when the drill bit  22  makes its initial contact with the entry material  68 . During the drilling, the drill bit  22  passes through the entry material  68  and through each of the circuit boards  30   a ,  30   b ,  30   c , and  30   d  of the stack  28 . The drill bit  22  then continues through outer skin  60  of the backup board  50 , through the lubricant/adhesive layer  64 , through the metallic foil sheet  52 , through the core laminating adhesive  58 , and into the ventilated core member  56 . After the drill bit  22  enters the ventilated core member  56 , the drill bit goes no further and momentarily dwells in the core member  56  of the backup board  56 . A supply of air flows through the core member  56  of the backup board  50 . The supply of air removes cuttings and cools the drill bit  22 . 
     As the drill bit  22  passes through the lubricant/adhesive layer  64  of the backup board  50 , the drill bit picks up lubricant from the lubricant/adhesive layer. The lubricant remains on the drill bit  22  and reduces the friction as the drill bit  22  is withdrawn from the through-hole and remains on the drill bit  22  during the subsequent drilling of the next set of through-holes. 
     While the invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the following claims.