PROCESS FOR THE MANUFACTURE OF SUBSTRATES TO INTERCONNECT ELECTRONIC COMPONENTS AND ARTICLES MADE BY SAID PROCESS

Abstract: An improved process for making substrates for mounting and interconnecting high density components comprising a substrate core and at least two printed circuit con­ ductor networks and wire scribed conductor filaments wherein connection between selected conductors of the printed circuit networks are formed by providing cavi­ ties reaching from a conductor in one network to a con­ ductor in another network and depositing conductive ma­ terial covering the cavity wall and reaching from one to the other conductor; encapsulating the surface with a resin composition and providing its surface with wire scribed conductors; encapsulating the thus formed sur­ face and,at predetermined locations, providing cavities reaching from the encapsulant surface to the respective wire scribed conductor; and applying a conductive metal layer to said cavity and the core of said wire scribed conductor thus forming an electrical connection from said conductor core to said surface of said encapsulant layer.

Description: This invention relates to substrates comprising printed circuit conductor patterns and wire scribed conductors suitable for supporting and interconnecting components and a method for making such substrates. US Patents 3 674 602 and 3 674 914 and US Application Serial No. 9,082 disclose and describe wire scribed cir­ cuit boards and methods and apparatus for making such boards. In accordance with this prior art technology a wire is scribed onto and affixed to the surface of a panel in a predetermined pattern. In the past, leads of components, e.g., of packages, commonly called dual-in-line packages, have been mounted onto and connected to wire scribed or printed circuit boards by inserting the leads into holes with metal­ lized walls, commonly known as plated through holes. Extremely compact microelectronic packages have been de­ veloped to accommodate very complex integrated circuits. One such familiy of packages, commonly called chip carriers, may be surface mounted rather than being connected by leads passing through holes in the board. These packages are square and have terminals on all four edges rather than on two sides as in the dual-in-line packages. The terminals are spaced on centers of 50 mils, 40 mils or less, thus substantially reducing the area of the packages as compared to the dual-in-line package. This substantially increases the number of terminals per unit area. The reduction in package area made possible by the chip carrier substantially increases the number of micro­ electronic packages which may be mounted and electrically connected by an interconnection substrate of given area. However, such increase in number of packages or com­ ponent package density and closer terminal spacing in­ creases the terminal density requirements on the board or substrate and heat density of the system. This in­ creased terminal density generates and requires higher conductor density. Contact pads, rather than through holes, are employed on the surface of the substrate for connecting such components to respective conductors. Electrical contact between such surface pads and the substrate conductors can be made in dimensions which approach that of the conductor itself thus leaving space for higher conductor density. A process for scribing filaments onto a carrier in a predetermined pattern in a high density horizontal and vertical grid is described in US patent 4 500 389. The conductor filaments contain at least one conductive portion, e.g., an electrical conductor, an optical con­ ductor or the like. The conductive portion of the fila­ ments may or may not have a dielectric coating. The filaments are bonded to the carrier in a high density horizontal and vertical grid. A layer of coating material capable of being hardened at a temperature below 100°C is applied over the filaments forming a smooth, planar surface over the carrier and filaments. The coating material is then cured. The carrier is attached to a table movable in a controlled pattern along "X" and "Y" axes so as to locate preprogrammed points in the carrier under a high energy beam, such as a coherent laser (e.g. CO₂ or other suitable laser), controlled depth mechanical drills, a modulated stream of abrasive particles, a water jet or stream of chemicals or solvents. The beam is directed perpendicular to the table and to the carrier mounted thereon. Alternatively, the carrier may be fixed and the beam moved in "X" and "Y" axes. The location of the filaments in the hardened, planar ma­ terial are targeted optically so as to substantially eliminate location errors. The carrier is moved relative to the high energy beam so that the desired filament is in line with the beam at a predetermined point. The fila­ ment is then exposed by the beam at such point. If the filament id metallic, a high energy beam, such as a CO₂ laser, is used. The beam is aligned with the de­ sired predetermined point on the filament, and then the beam is pulsed or modulated to direct energy at the filament so as to vaporize and remove the hardened ma­ terial and the filament insulation, leaving a precisely formed cavity with the filament substantially exposed. The impoved process of this invention provides even greater density and better electrical properties and enable the production of interconnection substrates which can accommodate high pin-count, high speed electronic packages. It further affords the electronics industry new ways in which to construct high-density interconnection circuits. According to the process of this invention, the material which forms the core of the substrate base used should have good heat dissipation properties; it should be dimensionally stable and preferably have coefficients of thermal expansion in the X-, Y-plane which match the coefficient of thermal expansion of the chip carrier or other circuit component materials to be mounted on the substrate. Another improvement over known processes is the se­ quential layering of circuit patterns which provide circuitry for two or more functions on one interconnect­ ion circuit board. Still another improvement is a pro­ cess providing means for wiring a substrate base on both sides and providing side-to-side communications. In accordance with the invention, there is provided a process for making substrates for mounting and inter­ connecting high density components comprising a sub­ strate core, a printed circuit conductor pattern and wire scribed conductors (hereinafter also referred to as 'interconnection circuit boards') comprising the steps of forming on said substrate core at least a first and a second printed circuit pattern electrically insulated from each other; forming at least one cavity in pre­ determined locations penetrating said second pattern and substantially exposing said first pattern; pro­ viding interfacing means between the said circuit patterns for establishing electrical conduct between respective conductors of the first and second pattern; applying and affixing filaments over said printed cir­ cuit patterns, said filaments comprising a conductive core, thus forming a wire scribed circuit pattern; applying a substantially smooth, flat, planar coating over the surface provided with said wire scribed circuit pattern thus maintaining the position of the filaments and providing a surface suitable for mounting surface-­ mounted components; forming, at predetermined locations on said planar surface, at least one cavity penetrating said coating and substantially exposing the conductive core of a respective conductor filament while maintain­ ing the integrity of said wire scribed pattern; and providing conductive interfacing means between said core of said conductor filament and the external sur­ face of said planar coating. Fig. 1 is a perspective view with partial cut-away of an interconnection substrate made according to the pro­ cess of this invention having a multiple-layer substrate core and plated through hole communication between both sides of the board. Fig. 2 is a cross-sectional view of an interconnection circuit board having multiple levels of power and ground planes. In the instant invention, a high density interconnection circuit may be formed and processed on a substrate core which will form an integral part of the article. There­ fore, said substrate core must be selected from a material which is suitable for incorporation into said high density interconnection circuit and dimensionally stable for the process to be employed in its fabri­ cation. Preferably, the substrate core may be made of an alloy of nickel and iron comprising, e.g., between 55-65% iron and 30-35% nickel, and more preferably 57-60% iron and 40-43% nickel. The alloy sheet is preferably clad with a thin layer of conductive metal such as copper to provide improved thermal and electrical conductivity. The core may also be a thin laminate composed of a thin fiberglass-reinforced epoxy resin sheet bonded on both sides with composite metal sheets. The composite metal sheets themselves should have thin metal alloy cores of about 60-65% iron and 35-40% nickel clad with very thin copper sheets. Also suitable for use as core materials are, e.g., copper/molybdenum alloys, polyimides and epoxy resins which may be reinforced with glass, quartz, Kevlar® or the like. Preferably, the coefficients of thermal ex­ pansion should be between 5 and 20 x 10⁻⁶ in/in/°C. Some examples of possible core materials and their coeffi­ cients of thermal expansion (TCE) in the X-,Y-plane are shown in Table I. The present invention may be used to accommodate compact microelectronic packages. The interconnection circuit boards made in accordance to the process of this in­ vention may take the form of certain preferred embodi­ ments. One preferred process of making the interconnection cir­ cuit boards according to the present invention is to apply thin copper foil to each side of a substrate core using a suitable dielectric bonding material. Circuit patterns should be formed on one or both of the copper foil surface(s) according to methods known to those skilled in the art. The substrate base should be treated so as to promote adhesion. The surface of the substrate base should then be encapsulated with a resin so as to obtain a flat, smooth, planar surface. The coating should be covered with an adhesive and the surface scribed with filaments, such as wire conductors. The formed wire scribed circuit pattern should then be encapsulated with resin to form a smooth, flat, planar surface and to pro­ vide a means for maintaining the positional relation­ ship of the filaments. It should then be further coated with adhesive. The thus prepared substrate should be mounted on an X-, Y-positioning table and cavities formed at predetermined locations. The cavities should be cleaned and treated to promote subsequent metal plating. The substrate should then be plated with con­ ductive material and circuit patterns should be formed in the conductive material according to methods known to those skilled in the art. In another preferred embodiment, a sequence of layers may be produced which provide circuitry for two or more desired functions on one board. In this embodiment, con­ ductive material should first be laminated to a sub­ strate core using a dielectric bonding material. A cir­ cuit pattern should be formed on the resulting substrate base. The surface should be treated to promote adhesion. Thin sheets of conductive material should be laminated to the base such that the conductive material is iso­ lated from the base and is on the external surface. Additional circuit patterns can then be formed on the conductive material. The substrate base should then be placed on an X-, Y-positioning table for cavity form­ ation between the two conductive layers in predetermined areas. The substrate base should then be plated to de­ posit conductive metal in the cavities. If desired, additional circuit layers may be formed over these layers in accordance with the process of this invention. The substrate base is then treated as described herein­ before including the steps of encapsulation, wire scribing, further encapsulation, cavities formation, metal plating and formation of circuit patterns accord­ ing to methods known to those skilled in the art. This embodiment provides for at least two circuit levels, which can be used for power and ground distribution, logical functions, or the like. Obviously, more than two or three circuit levels may be built on the substrate. Using the encapsulating method of this invention in order to maintain the position of the wires and thus en­ able the conductors to be scribed in high density, another embodiment may be produced. The substrate may be wired on both sides and provided with plated through holes in order to provide side-to-side communications between the sides of the circuit board. The conductors should be positionally stabilized with an encapsulation layer according to this invention to enable very close clearances. In this embodiment, one or more cavities may be made in addition to the through holes. In the preferred embodiments of this invention, the di­ electric substrate base is mounted on and affixed to a rigid stable base while the wire is scribed, the liquid coating is applied, levelled, flattened and cured in accordance with the teachings of US patent 3 607 380. The cavities in the circuit which are used to expose the wires or conductors are formed by high energy beams and the holes are electrolessly plated or electroplated. In the preferred embodiments of this invention, the con­ ductor or wire extends through the high energy formed cavity at the time of plating. This provides better electrical and mechanical strength between the conductor and cavity plating metal than could be achieved when attempting to drill through a 2 to 4 mil diameter wire with a 6 to 12 mil diameter drill. Fig. 1 depicts the substrate core 10, which is also described in Example 4 hereinafter. The substrate core 10 comprises an epoxy resin sheet 15 bonded on each side to composite metal sheets 30 consisting of very thin metal alloy cores 25 and cladding 20. The metal alloy is comprised of 64% iron and 36% nickel and the cladding material is copper. A layer of flat planar coating 40 is placed over clad­ ding 20. Over this coating layer 40 is laminated a copper sheet 45 to both sides of the structure. This copper sheet 45 is etched to form a desired pattern and wire filaments 50 are scribed in a predetermined pattern onto both sides of the base structure. A smooth, flat, planar coating 60 is applied over the entire surface comprising said filaments 50. Cavities 70 are made by exposing pre­ determined areas along the base structure to a CO₂ laser and oblating the smooth, flat, planar coating at those points. This procedure leaves the conductive core 55 of said filaments intact. At other predetermined points, holes 80 are drilled to afford communication between both sides of the inter­ connection circuit board. A copper layer 90 may be attached and surface patterns provided. Fig. 2 is a cross-sectional view of an improved interconnection circuit board made in accordance with this invention. The substrate core comprises a support sheet 110 provided on both sides with copper foil 115 and an epoxy resin sheet 120. A printed circuit conductor pattern 125 is formed on the surface of epoxy resin sheet 120 by known processes. The entire surface is provided with a smooth, flat, planar coating 130 thus encapsulating said conductor pattern 125. Coating 130 is cured and a circuit pattern 138 is formed on its surface, e.g., by first providing it with a copper cladding and subsequently removing it from areas 140 not re­ presenting circuit conductors. The formed laminate is exposed to a CO₂ laser in predetermined locations 135 thus forming a cavity at each such location exposing the respective areas of conductor pattern 125. Subsequently, metal is deposited in cavities 135 thus connecting re­ spective conductors of patterns 125 and 136 or 138. Next, the entire surface is encapsulated with a layer of resin 150 and wire filaments 160 are scribed onto the resulting resin surface in a predetermined pattern. Subsequently, filaments 160 of the scribed circuit pattern are encapsulated thus forming a smooth, flat, planar surface 170. Cavities 180 are formed in prede­ termined locations using a CO₂ laser and metallized thus providing connections between conductor level 125 and the external surface of the board by means of cavity 135 or cavities 135 and 185. Metallization of cavity 188 contacts conductor pad 136 and metal core of conductor wire 189 with plated overcoat 190. In the practice of this invention cavity forming with a high energy beam , such as a laser, has been found to be particularly useful. However, other means such as a drill bit, abrasive, chemical or high speed jet might also be employed. Other high energy beams absorbable by the material being drilled, but not the conductor or wire, and capable of volatilizing the material and in­ sulation on the conductor or wire, may also be employed. The following examples serve to illustrate certain pre­ ferred embodiments of this invention. EXAMPLE 1 A 0.45 mm thick metal alloy sheet core of 64% iron and 36% nickel was clad on both of its planar surfaces with copper metal sheets having a thickness of 0.15 mm. The copper surfaces were treated to form a black oxide layer suitable for promoting adhesion. The substrate base was laminated on each side with copper foil 0.035 mm thick and a semi-cured fiberglass-reinforced epoxy resin sheet. The lamination took place at a pressure of 375 psi and 175°C for a period of 25 minutes. The substrate base was then baked at a temperature of 175°C for 1 hour in order to effect full curing of the epoxy resin. After brushing the surface, a circuit pattern was formed on one side of the substrate by providing it with a dry film photoresist, exposing the resist layer to an UV light source through suitable artwork and developing thus forming an etching mask. The other side of the sub­ strate base was encapsulated by photoresist material. The substrate base was then exposed to a ferric chloride-­ water solution to remove exposed copper metal from its surface. The remaining photoresist masking layer was re­ moved from the surfaces and the substrate base treated to form a black oxide layer for adhesion promotion. The patterned surface was then coated with an encapsulat­ ing resin according to the following procedure: A pair of rigid rollers, mounted parallel to each other, were adjusted to provide a controlled encapsulation thickness. A filled, thermosetting epoxy resin which cures at room temperature was thoroughly mixed with an amount of curing agent sufficient to ensure complete poly­ merization. This mixture was placed into a bell jar and the bell jar evacuated with a vacuum pump. The mixture was uniformly applied to the patterned surface of the substrate base. The substrate base was then placed in a vacuum chamber to remove any air in the mixture. After removal from the vacuum chamber, one edge of the substrate base was placed into the preset gap between the rigid rollers. One end of a 0.1 mm thick release-treated polyester carrier sheet on which was coated a 0.07 mm thick sheet of wiring adhesive was placed in contact with and parallel to the gap egde of the substrate base. The substrate base, carrier sheet and adhesive were then drawn through the gap in order to bond the adhesive to the resin surface such that the surface of the adhesive was substantially flat. The encapsulating resin was allowed to cure at room tempe­ rature for a period of 8 hours and subsequently baked for a period of 30 minutes at a temperature of 65°C. After baking, the substrate base was placed onto a computer-­ controlled X-, Y-positioning table with the adhesive-­ coated surface exposed and a wire scribed conductor pattern applied to it employing insulated copper wire 0.064 mm in diameter. The thus formed article was then baked for a period of 90 minutes at a temperature of 95°C and the wire scribed circuit pattern coated with an encapsulating resin coating and adhesive sheet to stabilize the position of the scribed wire and to form a flat, planar surface in accordance with the procedure described above. The adhesive sheet in this procedure was specifically adapted for processing using semi-­ additive plating techniques. It was 0.03 mm thick. After being drawn through the rollers, the substrate base was allowed to cure at room temperature for 8 hours and then baked at 65°C for 90 minutes. The substrate base was then placed onto an X-, Y-position­ ing table with the adhesive surface exposed and the beam of a CO₂ laser was directed to the adhesive surface at predetermined locations. The laser beam vaporized and removed the materials surrounding the copper wire and formed cavities which were between 0.25 and 0.30 mm in diameter. After baking in an oven for 30 minutes at a temperature of 120°C and for 90 minutes at 175°C, the surface was cleaned by an ultrasonically-agitated cleaning solution and rinsed in order to remove any dirt or debris from the laser-generated cavities. It was further cleaned by immersion for 40 minutes in a solution of potassium per­ manganate in water which was maintained at a pH of about 13.5, followed by immersion for 3 minutes in a stannous chloride and a dilute hydrochloric acid solution at room temperature. The substrate base was rinsed in running water for 6 minutes. A palladium/tin catalyst was applied to the substrate surface to render it active for metal deposition and subsequently a very thin layer of copper metal was de­ posited by electroless deposition. After rinsing in running water for 60 seconds, the thin copper layer was enhanced by electroplating. The surface was then brushed and coated with a dry film photoresist which was exposed and developed as described above to form a circuit pattern on the substrate base. It was then again electroplated in order to increase the thickness of the copper deposit in the areas not masked by the photoresist, rinsed in running water and immersed in a commercially available tin/lead electro­ plating solution in order to deposit solder on all ex­ posed copper surfaces. The photoresist was removed and the surfaces exposed to a copper etching solution which did not dissolve the tin/­ lead plating thus preserving the desired circuit pattern while all copper metal in areas not protected by the tin/lead overplate was dissolved and removed. The thus formed article was rinsed in running water, baked for 30 minutes at 65°C and and additional 120 min. at 175°C. It was then subjected to conventional fabri­ cation operations such as cutting to size and drilling mounting holes thus producing the finished wire-scribed circuit board suitable as the substrate for mounting and connecting high density components. EXAMPLE 2 The procedure of Example 1 was repeated up to and inclu­ ding the step of removing the photoresist masking layer from the surfaces and treating the substrate base to form a black oxide layer for adhesion promotion as described hereinbefore. Subsequently, thin, single-sided copper-clad laminated sheets were applied to each side of the substrate base by a lamination step employing semi-cured fiberglass rein­ forced epoxy sheets as the bonding medium. Circuit patterns were formed on the substrate base as described above in Example 1, except that they consisted of circular pad areas of between 0.2 and 0.25 mm in dia­ meter. In predetermined locations, the cured epoxy resin layer was evaporated and thus removed between the two copper layers using a CO₂ laser as in Example 1. After cleaning and rinsing, the surface was rendered receptive to metal deposition using a palladium/tin solution, immersed in an acidic copper sulfate plating solution and electroplated to deposit copper metal in the said cavities. Subsequently, the substrate was re­ moved from the solution, rinsed in running water for 2 minutes, dried and the surface provided with an en­ capsulated, wire scribed conductor pattern and pro­ cessed in accordance with Example 1. EXAMPLE 3 The procedure of Example 1 were followed except that the metal alloy sheet used as the core was composed of 58% iron and 42% nickel, and it was not clad with copper metal sheets. Mechanical roughening techniques were em­ ployed in order to promote adhesion. EXAMPLE 4 The procedures of Example 2 were followed except that the metal alloy core was replaced by a thin laminate con­ sisting of a 0.127 mm thick fiberglass reinforced epoxy resin sheet bonded on each side with composite metal sheets consisting of 0.12 mm thick metal alloy cores (64% iron and 36% nickel) both sides of which were clad with 0.02 mm thick copper metal. EXAMPLE 5 A metal alloy core comprising 64% iron and 36% nickel and being 0.15 mm thick was clad on both surfaces with copper metal sheets having a thickness of 0.05 mm each. The clad metal alloy core was placed onto an X-, Y-posi­ tioning table, the table moved to perdetermined locations, and 1.07 mm diameter holes were mechanically drilled through the core. The copper surfaces of the drilled metal alloy sheet were treated to promote adhesion by exposure to a commercially available treatment solution for pro­ ducing a black oxide layer. Several sheets of semi-cured fiberglass reinforced epoxy resin were laminated to the thus prepared surfaces thus filling the drilled holes, and a sheet of copper foil 0.035 mm thick was laminated to wach side as des­ scribed in Example 1.The resulting laminate was baked for 60 minutes at a temperature of 175°C to fully cure the epoxy resin. After cleaning, a dry film photoresist was applied to each side, exposed through a master and developed in order to form circuit pattern masks. The substrate was then exposed to ferric chloride so­ lution as described in Example 1 to dissolve copper metal in areas not protected by said mask. Subsequently, the remaining photoresist material was removed and the copper surface provided with a black oxide layer. The substrate was then provided with resin and adhesive layers and scribed with wire, one side at a time, accord­ ing to the procedures of Example 1, then provided with holes in the same locations in which holes were drilled earlier, said holes having a diameter of 0.5 mm and being produced by mechanical drilling through the layers on the substrate base. The substrate was then subjected to the cleaning/adhesion promotion treatment and plated in accordance with the process described in Example 1.

Claims: 1. A process for making substrates for mounting and interconnecting high density components comprising a substrate core,a printed circuit conductor pattern and wire scribed conductors, characterized in that it comprises the steps of - forming on said substrate core at least a first and a second printed circuit pattern electrically insulated from each other; - forming at least one cavity in predetermined locations penetrating said second pattern and substantially ex­ posing said first pattern; - providing interfacing means between the said circuit patterns for establishing electrical conduct between respective conductors of the first and second pattern; - applying and affixing filaments over said printed circuit patterns, said filaments comprising a conductive core, thus forming a wire scribed circuit pattern; - applying a substantially smooth, flat, planar coating over the surface provided with said wire scribed circuit pattern thus maintaining the position of the filaments and providing a surface suitable for mounting surface-­ mounted components; - forming, at predetermined locations on said planar surface, at least one cavity penetrating said coating and substantially exposing the conductive core of a re­ spective conductor filament while maintaining the inte­ grity of said wire scribed pattern; and - providing conductive interfacing means between said core of said conductor filament and the external surface of said planar coating. 2. The process of claim 1 wherein said inter­ facing means is a metal deposit. 3. The process of claim 2 wherein said metal deposit is formed by a metal plating operation. 4. The process of claim 1 wherein said substrate core is, or is provided with, an additional core layer which is dimensionally stable. 5. The process of claim 1 wherein the coefficient of thermal expansion of said substrate core substantially matches the coefficient of thermal expansion of the com­ ponents which are to be mounted on its surface. 6. The process of claim 1 wherein the coefficient of thermal expansion of said substrate core is between 5 and 20 x 10⁻⁶ in/in/°C. 7. The process of claims 1 to 6 further com­ prising the step of providing,at a predetermined lo­ cation,at least one through hole intersecting at least one of the conductors and having metallized walls. 8. The process of claim 1 wherein said substrate core comprises an epoxy resin impregnated core sheet which is provided on both sides with a metal alloy sheet comprising between 60 and 65% iron and between 35 and 40% nickel, the surface of said alloy sheets being clad with copper.

Inventors: LASSEN, CHARLES L.
Applicants: KOLLMORGEN CORPORATION
Application year: 1986
Main classification: H05K7
IPC codes: H05K3, H05K7, H05K1