METHOD OF PROCESSING THROUGH-HOLES

Abstract: A optimum hole wall surface is created by cuttings carried up by a drill bit (18) from a reformable back up plate (16). The nature of the cuttings cleans, abrades and coats the hole wall surface with a film (17) of back up material. Such material is adherent and can be used for mechanical and electrical interconnections or removed for conventional catalytic processing. Simplified mechanical and electrical interconnections are made possible by virtue of the surfaces coated with the back up material. The reformable nature of the back up material allows it to be continuously re-used.

Description: METHOD FOR PROCESSING OF PLATED-THROUGH-HOLES IN PRINTED CIRCUIT BOARDS FIELD OF INVENTION This invention relates to the processing of plated-through-holes in conductive and non-conductive materials and the (specific) structures capable of being manufactured with respect to this processing. BACKGROUND OF THE INVENTION In the prior art to produce a through-the hole conductive path in a printed circuit board having internal copper conductive layers as well as conductive layers of a copper on both sides, the board, after lamination is first passed through a drilling operation where all the holes to be utilized as conductive holes through the board are drilled. Thereafter a series of steps are taken in which debris, damage and resin smear created during the drilling operation are removed either mechanically, chemically or by gaseous plasma discharge in order to present a clean and acceptable surface for the subsequent deposition of a metallic coating. Subsequently, a series of steps are taken in which a heavy layer of copper is ultimately deposited on the walls of each of the holes. This is done by first subjecting it to a catalyst seeding operation wherein microdeposits of metal are placed on the walls of each of the holes. Thereafter, copper is plated onto the walls of the holes. After all of the holes have thus been plated, the printed circuit pattern is laid out on the two sides of the board and the copper is etched leaving the lead path intact. Finally the components are mounted on the circuit board and a step which fills the holes containing the component leads with solder completes the task of securing the component to the board. In U.S. Patent 84,183,137 the walls of the hole are metalized simultaneously with the drilling operation by forcefully and intimately applying the metal from the drill back up plate directly onto the hole walls as the rotating drill bit is withdrawn thus eliminating the need for catalysts and associated conditioners and plating solutions. The metalized hole can be used as is or overplated with a metal such as copper. SUMMARY OF THE INVENTION The present invention eliminates many of the foregoing steps. -The material may be drilled, plated and/or soldered and etched with these operations occuring in any order. The hole cleaning operations are eliminated by the wiping and smearing action of the cuttings from the back up drill plate that are propelled through the hole by the twisting action of the drill. This action results in smearing a film of this back up material onto the conductive and non-conductive layers that comprise the surface of the hole wall. More specifically this invention relates to a method of directly conditioning the surface of the hole wall in such a manner as to displace or remove any resinous material that is normally smeared onto the hole wall surface during the hole generating operation. In addition it relates to a method that creates an optimum hole wall surface by the coating action of the cuttings that increases drill life, reduces direct friction and the resultant drilling damage while simultaneously abrading the hole wall to clean and remove debris created during the passage of the drill. If the resulting film is non-conductive, it may be subsequently removed by solvents or chemicals thus presenting excellent resin free surfaces that may be further processed by the use of conventional plated-through-hole techniques. A further embodiment of this invention relates to the ability to utilize interconnecting conductive elements that are generally impractical or impossible to use in conjunction with the conventional platedthrough-hole technology. The use of a metal alloy back up plate results in the deposition of a metal film directly onto the surface of the hole wall including all conductive layers that engage the drill bit during its passage. Metals such as aluminum which are extremely difficult to solder to or electrolessly or electrolitically plate on due to near instantaneous oxide formation, are coated with a metal film simultaneously with their exposure by the cutting action of the drill. This newly created film is in direct metalurgical contact with the base metal conductor and thus prevents oxidation of the otherwise highly critical exposed surface. Future operations such as plating, soldering etc. can then be carried out without the specialized processing normally required to prepare these materials for plating, soldering etc. Such surfaces are selective in their placement and thus open the way for variations in processing that are generally not possib,le by ordinary or conventional processing steps. A further embodiment of this invention provides a method for the direct attachment of pins, plugs and/or conductors to these holes to create a completed electrical circuit. This may be accomplished in several ways depending on the character and/or composition of the back up material. Very thick coatings of an alloy may be placed directly on the hole wall by selecting a rotational speed, feed, and withdrawal rate for the drill bit that creates a degree of melting in the alloy that permits a "fluidized" layer of this material to be coated on the walls within the hole. In the case of very small holes, this would essentially fill the hole with the alloy. Likewise this coating can be added to and/or remelted in order to completely fill the hole. In the case of a thin metallic film that covers the entire hole wall, it has been possible to directly fill the hole with solder thus creating a solid metal plug that makes a direct solder joint to the metal conductive layers that engage or surround the hole. while a eutectic 63/37 solder has been successfully used, there are cases where a positive expansion alloy might be desirable in order to maintain a compressive force on the metal conductive layers at all times. Such an alloy might contain Bismuth and have a composition such as 52.5% Bismuth, 32.0% lead and 15.5% tin. Similarly, it may be advantageous to utilize alloys with higher and lower melting points for specific applications; there are numerous alloys that can be used for this application. Subsequent heating may also be utilized to bring about alloying or diffusion of the interfaced conductive materials. Solid or tubular metal elements can likewise be directly soldered into these holes by several means. The simplest method involves the use of a preform of solder that encircles the metal element and is so placed that upon melting it collapses due to surface tension and comes into intimate contact with the metal element where capillary action pulls it into the hole and into contact with the previously metalized metal conductive laye#rs. The resultant structure creates a direct solder contact from the metal element by way of the solder to the metal conductive layers that is very strong and reliable. Likewise a variety of solders can be used to accomplish this task. Another method involves the use of pre-tinned solid or tubular metal elements that have edges that come into direct contact with the hole wall. The preferred geometry of these metal elements has two or more edges that are designed to incise directly into the conductive metal layers or the overlying metalized surface. Subsequent re-melting results in a solder fillet being created between the edges of the metal elements and the metal conductive layers. Additional solder may be added to further fill the hole. A further embodiment of this invention relates to the reformable and reuseable nature of this back up material and the apparatus required to reform it. Panels of masonite and other wood pulp materials are currently used to support the printed circuit board during drilling operation. These materials are rigid, not reformable and must be disposed of after use. Various low and intermediate melting alloys, waxes and plastics possess characteristics that make them ideally suited for use as a reformable back up plate for drilling and machining operations. Metallic alloys such as those discussed in U.S. Patent #4,183,137 are easily reformed by remelting and molding or extruding. Similarly waxes waxes such as parafin, polyethylene and other rigid / perform this same function and can be further enhanced by the addition of a filler material such as Silica flour. The ability to reform a back up plate is a distinct advantage in'itself since it requires only the addition of sufficient material to replace that lost as cuttings and the power.to reform it. Reformable material has been successfully produced by extrusion, casting and molding. BRIEF DESCRIPTION OF THE DRAWINGS The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as further objects and advantages thereof, will best be und & stood by reference to the following detailed description of an illustrative embodiment taken in conjunction with the accompanying drawings in which: FIG. 1 is a side view of a drill bit, panel of material and back up plate. FIG. 2 is a side view of a drill bit, a panel consisting of alternating layers of a conductive and non-conductive material and a back up plate. FIG. 3 is a side view of a metalized hole in a multilayered printed circuit board that has subsequently been dipped in molten metal. FIG. 4 is a side view of a multilayered printed circuit board showing the ref lowed connections between the metal conductive layers and an axial metal element. FIG. 5 is a top view showing an interference fit tubular metal element in place and soldered within a printed circuit board. FIG. 6 is a top and edge view of a portable frame for receiving the reformable material. DETAILED DESCRIPTION Referring now to Fig. 1, drill bit 18 passes through panel of material 10 and into back up plate 16. Film 17 is generated from plate 16 by the turning action of drill bit 18 and is smeared onto the wall surface of the hole. Referring now to Fig.2,drill bit 28 passes through a multilayer printed circuit board panel 20 consisting of conductive layers 21, 22 and 23 and insulating layers 24 and 25 and into back up plate 26. Film 27 is generated from plate 26 by the turning action of drill bit 28 and is smeared onto the wall surface of the hole. Referring now to Fig. 3, the metalized multilayer panel 30 has been dipped in molten solder to further build up the solder 36 on conductive layers 31, 32, and 33. The metalized film 37 that has been'previously smeared across layers 31, 32, 33, 34, and 35 may underlie the layer of solder. Referring now to Fig. 4, the metal element 48 is situated within the metalized multilayer panel 40 and subjected to a temperature above the melting point of the filler material 46 thus bringing about a reflow and subsequent joining of metal element 48 to layers 44, 45 and especially 41, 42, and 43 by the alloying of the filler material with the previously applied film 47. Referring now to Fig. 5, 56 is a multi-edged tubular metal element precoated with-filler material 55 that has been inserted into previously metal ized hole 51 in printed circuit board 50 so that upon heating the filler. material 55 forms a reflowed connection 54 between the tubular metal element 56 and the metalized film 53 coating metal conductive layer 52 within hole 51. Referring now to Fig. 6, metal frame 60 is of sufficient size and thickness to retain and accomodate reformable material 61 that is poured or inject ed into frame. Bottom plate 62 and top plate 63 closely mate with frame 60 at border surface 64 to prevent excess remeltable material 61 from being extruded between plates 62, 63 and frame 60. Injection port 65 is located within frame 60 for the introduction of reformable material 61. Ejection port 66 is located within frame 60 for the elimination of excess reformable material 61. Top plate 63 is removable in order to expose surface of reformable material 61 for use as a back up plate for tooling operations such as drilling. Likewise, bottom plate 62 can also be removed to expose bottom surface of reformable material 61 in order to utilize both surfaces for back up purposes prior to reforming for subsequent use. Tooling holes 67 can be permanently located in frame 60 for precise mounting to tooling surfaces of equipment such as drill. Cut out area 68 is designed to accomodate secondary tooling holes 69 which are subsequently created in reformable material 61 as required to present a zero reference location for panels to be drilled. In operation, referring to Fig. 1, the drill bit 18 is forced through the panel of material 10 and into back up plate 16. Upon contacting the plate of back up material 16, the twisting, cutting action of the drill produces cuttings that are carried upwards by. the turning drill bit 18. Film 17 is created as these cuttings are abraded against the surface of the hole wall. The shape and nature of these cuttings causes them to pack and come into intimate contact with the hole wall; rotational force causes these cuttings to abrade the surface of the hole wall thus removing loose drill damaged material and results in frictional melting of the cuttings at the interface. This molten material creates a film on the drill bit that reduces friction, prevents direct interfacing of the drill bit with the material to be drilled and cools the drill bit by the cooling action of the back up plate and cuttings as they change from a solid to a liquid. Additionally, it is known that a large degree of heating of the drill bit occurs due to. friction while it is within the back up material. Heating of the drill bit above the melting point of the back up material 16 cannot occur due to loss of friction as the back up material 16 in contact with the drill bit 18 changes from a solid to a liquid state at the interface. In the case where the panel of material 10 is an insulator, the use of a soft metal back up plate 16 results in the smearing of a metallic film 17 onto the surface of the walls. This film is adherent and can be used as is, overplated with a metal or solder or removed to present a smooth clean hole wall. Similarly, the use of a non-conductive back up plate material results in the cleaning and deposition of a non-conductive film that can be removed to present a smooth clean hole wall. -In the case where the panel of material 10 is metallic, the use of a soft metal back up plate 16 results in the smearing of a metallic film 17 onto the surface of the walls. Again, this film is adherent and can be used as is, overplated with metal or solder or removed to present a smooth, clean hole wall. This in effect can create a solderable or plateable surface on an otherwise difficult to plate material such as aluminum. Similarly, in Fig. 2, the drill bit forces its way through alternating layers of conductive material 21, 22, 23 and non-conductive material 24, 25, in panel 20 and upon contacting the back up plate 26 produces cuttings that are carried up the hole as previously described with similar results. A particular problem encountered in the drilling of multilayer printed circuit boards is the occurence of a film of resin that is smeared over the metal conductive layers by the heat and friction of the turning drill bit. This film is insulating and very tenaceous but must be removed in order that electrical contact can be made between these conductive layers and the subsequent plating that is to be deposited within the hole. The use of a metal back up plate 26 effectively prevents and removes this resin smear and further coats the conductive layers with a layer of this metal film 27 -that can be plated to, soldered to or removed for further processing. Similarly, the use of a non-conductive back up plate material 26, also prevents and removes resin smear and can also be subsequently removed for further processing. In operation, referring to Fig. 3, the metalized printed circuit board 30 consisting of conductive layers 31, 32, 33 and non-conductive layers 34 and 35 is dipped into molten solder to create a heavier, more solderable inter face 36 for subsequent assembly. Depending upon the nature and melting point of the solder used. The deposited metal film 37 may be added to 36 or completely removed except where it overlies and is alloyed to the metal conductive layers 31, 32 and 33. Likewise, in the case where a non-conductive back up plate material is used to prevent resin smear on the internal conductive surfaces, the board can be cleaned to remove deposited film 37 and solder dipped to add a layer of solder directly on the exposed metal conductive layers 31, 32 and 33 for subsequent processing. In operation, referring to Fig. 4, a metal element 48 is positioned within a hole in multilayered board 40. In the preferred embodiment, the metal element 47 is solder coated prior to insertion in the hole and additional solder, if required, is added by the use of solder preforms, solder paste or by bringing the structure into contact with a molten solder bath. Heat alone can be used to reflow the preforms, the solder paste or the solder on a close fitting previously solder coated metal element to create a mechanical and electrical junction to a conductive metal film 47 or solder coating overlying the metal conductive layers 41, 42 and 43. The metal film 47 promotes filling of the annular void between the metal element 48 and the hole wall by capillary action since it provides a wettable surface for the molten solder to move over. Direct attachment of a metal element to a multilayer structure similar to that shown in 40 but without metalized film 47 is inconsistent and very diff icult to achieve since the insulating portions of the hole wall are nor wetted by the molten solder and- thus retard capillary action. The bare metal conductive layers are similarly less wettable than ones having a metalized film or solder'coating on them and are thus more difficult to solder to. Again in operation referring to Fig. 4, the metal element 48 can take the form of a solder plug or a positive expansion alloy plug that has been formed in place. This approach is most attractive when used in high density printed circuit boards that utilize holes only for the electrical connection of the various metal conductive layers 41, 42, 43 and are generally of insufficient diameter to accept component leads. This small diameter creates a difficult problem in metal plating the holes. In practice, the multilayer structure 40 consisting of conductive layers 41, 42, 43 and non-conductive layers 44, 45 and metal film 47 has the hole filled with solder paste, solid pieces of solder or a positive expansion alloy. The printed circuit board containing the alloy within the holes is placed on a non-wettable plate and exposed to a sufficiently high temperature to cause the alloy to melt and subsequently upon cooling to form a mechanical and electrical connection between the adjoining metal layers within the hole. Similarly, a printed circuit board with metalized holes may be dip or wave soldered in order to fill the holes with solder, however in such a case it is advantageous to coat the deposited metal film on the surface of the holes with a thin layer of plating such as copper or tin to enhance the flow of the solder and,retar'd the solvent action of the molten solder thus preventing premature dissolution of the metalized film. A tin lead solder will normally contract on cooling and have a tendency to pull away from the metal conductive layers. A positive expansion alloy expands upon cooling and would continue to exert a compressive force against the internal edges of the conductive layers thus ensuring a good electrical connection. In operation, now referring to Fig. 5, a tubular metal element 56 that has been pre-coated with solder 55 is inserted in hole 51 of printed circuit board 50. The dimension of the hole 51 and the tubular metal element 56 are such that an interference fit is generated at the edges of the tubular metal element 56 and the metalized film 53. Subsequent heating above the melting point of the interfacing alloys causes them to reflow and form a fillet 54 thus creating an electrical and mechanical junction at these points of intersection. A major advantage of this technique lies in the fact that no additional solder needs to be introduced thus preventing solder blocking of the tube. This feature is of prime importance if the tubular metal element 56 is to be used as a socket. The form of the tube may vary with the intended use as long as at least two edges create an interference fit with the hole. In operation, referring to Fig. 6, frame 60 is placed between bottom plate 62 and top plate 63 and clamped so that a seal occurs between them at the border seal area 64. Reformable material 61 is poured, injected or otherwise introduced by the way of injection port 65 in such'manner as to completely fill the cavity in frame 60 between plates 62 and 63. Entrapped air and excess reformable material 61 are forced out ejection port 66 by pressure of the incoming reformable material 61, by vacuum or a combination of both. In practice, upon cooling and solidification of the reformable material 61, top plate 63 is removed and frame 60 and bottom plate 62 are located on the drill table of the machine by means of tooling holes 67. Secondary tolling holes 69 are subsequently drilled in reformable material 61 in cut out areas 68 within frame for use as a zero reference for hole locations in the printed circuit boards that are to be drilled. Upon completion of the drilling operation, frame 60 and bottom plate 62 are removed. Frame 60 containing reformable material 6i may be reinstalled on the drill table as previously described in order to utilize the opposite face as a back up plate. In some cases it may be advantageous to completely remove the plate of reformable material 61 from frame 60. The surfaces of bottom plate 62 and top plate 63 facing frame 60 may be lined with foil to simplify removal of the frame from the plates. Further, either or both plates may be equipped for force cooling to prevent recrystallization or other undesired effects from occuring. Although the reformable material can be extruded in sheets, it is believed that the use of a frame as described above more suitably maintains the flatness and surface qualities desired in a back up material. Additionally, the frame, as described above, may be permanently attached to the drill and reformable material directly introduced by connecting it to a heated reservoir.

Claims: CLAIMS 1. A method for establishing a direct metallurgical interface on the surface of a passage within a body of material, characterised in that a mechanically spreadable, electrically conductive material is forced as a solid in sub-divided form into frictional contact with the walls of said passage to deposit a continuous layer of said material on said walls. 2. The method of claim 1, further characterised by positioning a solid back-up plate on the body and operating a cutting tool through said passage into the back-up plate to forcibly move mechanically spreadable cuttings in sub-divided from into intimate frictional contact with the surface of the passage. 3. A method-simultaneously conditions and cleans the surface of a passage within a body of material of generated debris and further displaces the deposition of a layer of unwanted material by means of a film that is created as the cutting tool forcibly moves mechanically spreadable cuttings in a sub-divided form upwards from a solid back-up plate into intimate frictional contact with the surface of the passage. 4. The method of any preceding claim, in which the back-up plate is of a re-formable and re-usable material, preferably a re-meltable material. 5. The method of claim 1 or claim 2, further including the steps of: A. introducing into said passage a conductive metal element; and B. establishing a soldered connection between said metal element and the conductive material layer. 6. The method of claim 5, in which the conductive metal element is dimensioned such that at least two edges thereof form an interference fit with the walls of the passage. 7. The method of claim 5 or claim 6, in which the conductive metal element is precoated with solder and, after insertion, the solder is subjected to a sufficient temperature to cause it to melt. 8. The method of claim 1 or claim 2, further including the steps of introducing metallic solder into said passage, #elting the solder to alloy with said conductive layer, and cooling to form a solid plug. 9. The method of claim 8, in which said solder is a positive expansion alloy. 10. The method of claim 3, including the further step of removing said created film by chemical or solvent action.

Inventors: LOMERSON, ROBERT BOGARDUS
Applicants: LOMERSON, ROBERT BOGARDUS
Application year: 1982
Main classification: H05K3
IPC codes: H05K3, B23B35