BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates generally to the field of printed circuit boards and is more particularly directed to more efficient methods of forming electrical conductors bonded to a plastic substrate than are presently known.

State of the Prior Art

Printed circuit boards are used extensively in the electronics industry. Printed circuits consist of a stable sheet of insulating material with thin conductor lines or traces bonded to the sheet to form the circuit paths.

Electronic components and devices are supported by the insulating substrate and are interconnected by the conductive traces. Typically, the board material is a 1/16 inch thick sheet of epoxy bonded fiberglass. The epoxy sheet is clad on one or both sides with a thin sheet of copper e.g. 0.0027 inch thick (this thickness is widely used and is called "2 ounce" thickness) . The first step in typical conventional processing of circuit boards is to drill the holes through which will be inserted the leads of the components to be mounted on the board. The mounting holes are usually "plated through" by a difficult multi-step copper plating process, creating continuous conducting paths from one side of the board to the other. If the board is clad with copper on both sides, then the plated through holes may be used to connect circuit traces on opposite sides of the board. Even if the holes are not used for this purpose, it is desirable to plate through the holes in order to achieve a superior solder joint between the component

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leads and the printed circuit traces on the board. A protective resist material is applied to the portions of the copper foil which are to be removed. The board is then immersed in a solder-plating bath, plating solder (a tin/lead alloy) everywhere the foil pattern is to remain, including the insides of the holes. Next the resist is removed chemically, exposing the copper foil that is to be removed, and the board is treated with a copper etching compound. That leaves the desired pattern of solder-plated copper complete with plated through holes. At this point, it is desirable to carry out a step known as "reflow soldering" which consists of heating the board to make the thin solder plating flow. This prevents the formation of tiny slivers of metal created by the undercutting action of the etching bath, that could otherwise cause conductive bridges between traces. The final step in board manufacture is to electroplate certain portions of the board with gold. Such portions normally include edge-connector fingers which make a mechanical connection with spring leaf members of a plug-in edge connector for interconnecting the circuit board to other circuits. The gold electroplating greatly improves the corrosion resistance of these critical portions of the printed circuit and also makes for smoother insertion of the board into the edge connector. After the circuit board is finished, it is "stuffed" with the various components such as integrated circuits, transistors, resistors and capacitors, etc... , which are to be interconnected by the printed circuit traces, and the component leads are soldered to the circuit braces, usually by floating the board on molten solder in a wave soldering machine.

Epoxy or phenolic sheet has been the traditional material chosen for the manufacture of circuit boards due to its chemical inertness, dimensional stability, flame resistance and relatively low absorption of moisture. All these are important considerations in the manufacture of circuit boards. The stability of electrical parameters in a

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circuit may be adversely affected by absorption of humidity in the board. Dimensional changes in the board due to expansion and contraction caused by temperature changes likewise can affect electrical circuit parameters. Even worse, dimensional instability of the insulating sheet may cause delamination of the foil patterns from the supporting insulating sheet and stress-cracking of the conductive traces, particularly in plated through holes.

In the past, thermoplastic materials were not considered for use as printed circuit board substrates because of deficiencies in their thermal, mechanical and/or chemical resistance properties. More recently, however, several polymers have been developed with properties which make injection molded circuit boards a feasible alternative to existing thermoset laminate technology.

Conventional thermoset materials, such as the epoxy/glass laminated boards, are resin systems which are cured by heating. Once cured, the epoxy material cannot be melted and reused. With thermoset technology, the circuit boards have to be cut from large sheets of stock material to the desired shape. Openings in the circuit board have to be punched out or cut out, and lettering and markings must be either printed on the board or formed during the etching process of the conductor foil. This results in considerable quantities of non-reusable scrap produced by the cutting and forming of the circuit boards.

Injection molding of printed circuit boards offers significant advantages over more conventional technology. The thermoplastic material can be processed on conventional injection molding and extrusion equipment. Injection molded circuit boards can be mass produced in finished shape and with all component mounting holes molded in. This results in significant cost reduction by eliminating drilling, routing, punching and deburring, and by reducing scrap

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losses. Three dimensional features such as as stand-offs, snap fits, structural ribs, and integral connectors can also be designed in, further enhancing the cost effectiveness of the board. Any scrap generated from thermoplastic substrates may be readily ground up and reused.

Further, certain thermoplastic resins exhibit properties superior to those of conventional epoxy/glass laminates. For example, Polyetherimide is an amorphous thermoplastic which exhibits an electrical resistivity (a measure of insulating capability) approximately 1,000 times higher than epoxy/glass laminates. It has been granted a continuous use temperature rating of 338° F (170° C) with and without impact by Underwriters Laboratories Inc. The unmodified plastic has excellent flame resistance, a low thermal expansion coefficient which significantly improves the integrity of plated-through holes, and exceptional mechanical properties. The Polyetherimide is also available with glass reinforcement. The reinforced resin has even better mechanical properties than the unmodified plastic, resulting in excellent warp resistance and dimensional stability.

Thermoplastic such as polysulfone has been used as a substrate for printed circuit boards either through conventional copper-clad laminate technology or by using plated injection molded boards. Copper-clad thermoplastic in sheet form offers advantages of high performance characteristics over epoxy/glass without departing from standard processing techniques. Injection molded boards, however, offer significant cost savings potential for high volume production of printed circuit boards.

Injection molded substrates are presently processed by additive rather than subtractive methods, i.e., copper or equivalent electrically conductive material is chemically plated onto the substrate surface after the board

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has been molded. Prior to plating, preparation of the surface is necessary to promote adhesion of the copper to the thermoplastic. Preparation of the substrate surface is accomplished through "swell and etch" techniques. An aggressive organic solvent such as dimethylformamide is used to increase the surface area of the board, followed by treatment with chromic acid to improve the wetting characteristic of the thermoplastic. Standard electroless copper plating solutions may then be used to complete the processing cycle. The known processes for electroless plating of copper onto a thermoplastic surface typically involve many processing steps using various corrosive solutions which are difficult to dispose of due to pollution control requirements imposed on processing plants. Further, the various solutions must be carefully controlled in terms of chemical concentrations, pollutant content, and temperature.

Once the substrate surface has been prepared, a thin coating of metal is deposited onto the thermoplastic by electroless plating. This initial thin coating is then used for building up additional thickness of metal onto the substrate by faster and more economical electroplating methods. The thin initial metal coating is masked to leave exposed only the areas which are to define the conducting circuit paths and areas. Additional metal is then electroplated onto these exposed areas to increase the thickness of the conductors. This additional metal is bonded to the plastic by the thin initial metallic coating. The masking material is then removed and the now exposed thin metal coating on the previously masked area is removed by differential etching.

A typical electroless metal plating process requires 16 separate steps. Attempts have been made to develop more direct methods of forming electrical conductors bonded to thermoplastic substrates to eliminate the need for

such elaborate preparation and electroless plating of the plastic substrate, while retaining the advantages of injection molding the thermoplastic substrates of printed circuit boards. Such alternate processes include the use of conductive material such as copper foil which is precut to the desired circuit pattern and then is directly laminated to the substrate by means of suitable adhesives. In yet other known processes conducting wires are laid down on the substrate surface by numerically controlled machines in a predetermined pattern. The substrate surface is precoated with an adhesive, to prevent the wires from shifting position and the entire circuit pattern is then encapsulated in an epoxy. These attempts have not been fully successful in providing a truly low cost process for mass production of printed circuits on injection moldable thermoplastic substrates.

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SUMMARY OF THE INVENTION

The present invention is directed to a preferred and several alternate methods of forming an electrical conductor bonded to a plastic substrate. The substrate material may be any substance capable of being shaped or molded by heat or pressure or a combination of heat and pressure, and having suitable insulating properties.

According to the invention, electrically conductive lines or areas are formed and bonded to a moldable substrate by partially embedding granules of an electrically conductive material into the substrate along the desired lines or areas, leaving a portion of each embedded granule exposed above the substrate surface, and binding the exposed portions of the embedded granules in a matrix of additional .electrically conductive material. The binding matrix assures continuity of the conductive path thus formed, while the partially embedded granulated or powdered material provides a root system for the conductor and serves to bond the conductor to the substrate in a fundamental mechanical linkage. This bonding of the conductor to the substrate is achieved without preliminary chemical treatment of the substrate surface.

In a presently preferred form of practicing the invention, the substrate is injection molded thermoplastic with shallow female channels for the desired circuit paths or other conductive areas. The granulated conductive material to be embedded onto the substrate i.e., the root or bonding material has a relatively high melting temperature and is mixed with powdered matrix material which is selected to have a relatively low melting temperature. The powder mixture is deposited within the preformed channels molded in the substrate surface. Heat and pressure are then simultaneously applied to the powder mixture. When sufficient heat is applied and the powder mixture is

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simultaneously pressed into the substrate, the low melting point matrix material melts, leaving only the granules of the higher melting point material in the solid state. At the same time a layer of substrate material immediately underlying the heated powder mixture softens. Under the influence of pressure being exerted against the powder, the solid granules of the higher melting material are compacted against the substrate and some portion of the hot granules are partially embedded into the layer of soft substrate material leaving portions of the granules exposed above the substrate. Solid granules of the bonding material not embedded into the thermoplastic are nonetheless pressed ' together into a compacted layer of granulated conductive material. The molten matrix material wets the exposed portions of the higher melting temperature granules and fills the spaces between the solid granules compacted above the layer of soft thermoplastic. The compacted granules are bound to each other and to the exposed portions of the partly embedded granules together by the conductive matrix material after the article has been allowed to cool and both the thermoplastic and the matrix substance have solidified forming continuous electrical conductors bonded to the substrate within the channels.

Heat and pressure may be applied do the powder mix in the preformed channels in the substrate by means of a heated male tool with lands formed thereon which closely mate with the female channels in the substrate. When the heated tool is pressed against the substrate containing the powder mix in the female channels, the male land portions of the heated tool contact the powder mixture in the substrate channels. The powder mixture will normally have good thermal conductivity. Heat from the tool is therefore quickly carried through the powder mix to the bottom and side walls of the substrate channels, reflowing a shallow layer of substrate material within the channel. As pressure is exerted by the tool, at least some of the granules of the

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higher melting temperature material are pressed into the fluid thermoplastic layer. Some granules of the solid root material may be completely embedded in the thermoplastic, but an interface will be formed between the solid granulated material and the fluid thermoplastic at which granules of conductive root material will be only partially embedded into the substrate leaving portions of individual granules exposed above the substrate surface within the channels. Additional granules of root material may lie above the partially embedded granules and not be embedded in the thermoplastic at all. Simultaneously with the pressing of the granulated root material, heat is readily and evenly transferred from the tool to the matrix substance admixed with the granulated root material. The matrix substance melts and wets the exposed granules or surfaces of the bonding material, thereby to form a layer of fluid matrix material above the fluid thermoplastic layer. The heated tool is then separated from the substrate, withdrawing the male lands from the substrate channels. The contact between the tool and the substrate may be brief so that only a minimum amount of heat necessary to reflow the plastic in immediate contact with the powder mix and for reflowing the matrix substance is imparted. The remainder of the plastic substrate is not heated to any significant extent, although some reflowing of the thermoplastic may occur in a very shallow ^* layer on the face of the substrate facing the heated tool. Such reflowing is not detrimental to the process since this layer will readily solidify as soon as the tool is separated from the substrate. In general, heating of the substrate is kept to a minimum to prevent any significant permanent dimensional change or warping of the substrate.

The granulated root or bonding conductive material is selected to have a relatively high melting point such that its solid state is substantially unaffected by contact with the heated tool. On the other hand, the matrix material is chosen to have a relatively low melting

temperature, one which may be actually lower than the melting temperature of the material used for the substrate. It is preferable, but not essential that the melting temperature of the material be lower than the melting or softening point of the substrate so that the substrate is not heated excessively prior to melting of the matrix material. Desirably the balance between heat and pressure applied to the powder mix is such that the granulated root material may be mechanically compacted into the substrate during pressing of this powder mix, without excessive heat transfer to the thermoplastic so as not to reflow the substrate to a depth much greater than necessary for embedding the solid granular material into the substrate. Such compacting of the granulated root material against the substrate is desirable to improve the electrical conduc¬ tivity characteristics of the resulting bonded conductor trace. This is because the granulated root material may be powdered copper, gold, graphite or other substance having very good electrical conductivity. The matrix material however must have a relatively low melting point e.g. tin/lead eutectic mixtures with a somewhat lesser electrical conductivity. It is therefore desirable to maximize the contribution of the granulated root material to the overall conductivity of the bonded trace conductor. This may be obtained by mechanically compacting the powder to the point of obtaining physical contact between adjacent granules of the higher melting point root material, thereby to establish an electrically conductive path through the compacted granules, including even those granules which are fully embedded within the thermoplastic substrate. By pressing together the granulated high melting point conductive material to obtain maximum mechanical contact between the granules the overall conductivity of the trace may be maximized, with minimum reliance being placed on the binding matrix substance to serve as a conductor and primarily as a cementing matrix.

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From the foregoing it will be appreciated that the above method of forming finished electrical conductors bonded to a thermoplastic substrate material can be carried out essentially in a single manufacturing step once the substrate has been molded with the preformed female trace channels and the powder mixture has been deposited into the channels.

In a first alternate method of practicing the present invention, only the granulated high melting point conductive material is pressed into the thermoplastic substance by applying heat and pressure in a manner similar to that described in connection with the preferred embodiment above. The result of this first step is the creation of a bare root system consisting of a compacted granulated conductive material having a relatively high melting point. Part of this granulated material may be fully embedded in the thermoplastic while a portion of the granulated material will be only partially set into the thermoplastic leaving exposed portions of individual granules. Still further, layers of fully exposed granules may be compacted against underlying partially embedded granules. A cementing matrix may be added to bind the exposed granules or portions of embedded granules by one of several possible processes including electroplating, electroless plating, application of conductive inks' or epoxies, or any other method for depositing conductive temperature which can be tolerated without permanent deformation by the thermoplastic substrate. As it melts, the low melting point powder flows over the embedded granular root material wetting the exposed surfaces of the compacted granules, filling the spaces between the the granules and binding the compacted material in a continuous solid electrically conducting matrix after the article has been allowed to cool.

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In a third alternate method of practicing the present invention no female channels are formed in the substrate surface prior to pressing the granulated material into the thermoplastic. Instead, reliance is placed entirely on the land elements of a male tool to selectively press granular or powdered material into the substrate in a pattern corresponding to the desired conductor layout. The powdered electrically conductive material may be initially deposited in an even layer over the entire surface of the substrate facing the male tool. The male land elements of the tool are then brought to bear against the powder to press into . the thermoplastic only the granulated material directly underlying the lands. The granulated material not underlying the lands of the tool is not pressed and may be subsequently wiped off or otherwise removed from the substrate surface. The lands of the tool are not pressed into the substrate to their full depth so as to avoid pressing the recessed surfaces of the tool against the excess granular material on substrate areas which are to be devoid of conductive material, and also to minimize reflowing of the substrate surface outside the channels. Additional conductive material may then be added over the embedded granules by electroplating, electroless plating, application of a conductive ink or paste, or by melting of low melting temperature powdered conductive material, to form a binding matrix and to improve the electrical conductivity characteristics of the conductor traces. The traces may be built up if desired by any of the additive methods already discussed and others which will be apparent to those skilled in the art, including electroplating and electroless plating processes.

In the earlier described, presently preferred method of practicing this invention, conductor lined component mounting holes may be formed in the substrate simultaneously with the formation of the bonded circuit traces. This is accomplished by preforming blind hole

cavities at the desired mounting hole locations in the thermoplastic substrate as part of the network of female channels during injection molding of the substrate. The blind hole cavities, which may have different cross- sectional shapes adapted to receive particular component mounting pins, e.g., standard rectangular dual-in-line integrated circuit packages, cylindrical leads of various diameters, etc. have inwardly sloping walls, so that the hole cavity diameter diminishes towards the closed end of the hole. For example, for a mounting hole of circular cross section, the cavity may be of frus.to-conical shape and has its largest diameter at the substrate surface. Preformed channels which correspond to circuit braces that are to be connected to component leads inserted through the mounting holes terminate at corresponding hole cavities; that is, the trace channel opens into the sidewall of the tapered hole cavity. The hole cavities formed in the substrate are closed by a temporary transverse wall or dam which serves to keep the powder within the cavity preliminary to the pressing of the granular powdered material into the substrate. The powdered conductive material is deposited into the hole cavities at the same time that the powder is deposited into the channels in the substrate. The tool is provided with male pin elements disposed in register with the closed hole cavities. The pin elements are tapered to mate with the inclined walls of. the hole cavities when the heated tool is applied against the substrate. The male pin elements of the tool press the powder mixture placed in the hole cavities into the sloping walls of each hole cavity. The matrix material contained in the powder mixture melts on contact with the hot tool and flows around the male pin element, carrying the still solid higher melting point material. Some of the solid particles are embedded in a substantially even layer over the cavity walls while additional granules are compacted over the partially embedded granules. The result is that after the article has been allowed to cool, the cavity is lined with

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an electrical conductor analogous to the conductors formed in the substrate channels and consisting of electrically conductive partially embedded and compacted granules cemented in an electrically conductive matrix. The temporary transverse wall closures of the hole cavities may be punched out simultaneously with the pressing of the powder mixture by appropriate punch elements provided at the ends of the pin elements of the male tool which mate with the preformed hole cavities. Alternatively, the holes may be opened subsequently to the pressing operation by a separate punching step.

This method of forming through-conductive mounting holes is also adaptable to the first and second alternate methods of practicing this invention. The remarks made above in connection with the conductivity of the bare embedded granular root material as a precondition of electroplating of additional material onto the embedded bare root material holds likewise for the construction of through plated component mounting holes by the first alternate method. If the additional matrix material is deposited by electroless plating instead of electroplating, or by melting a powdered low melting temperature conductive material as in the second alternate method, such conductivity of the bare root system is not essential.

In the case of double side circuit boards, i.e. circuit boards having conductors bonded to both sides of the substrate sheet, conductor lined holes are formed in two steps. A first heated male tool has lands corresponding to the preformed, trace channels on one side of the substrate while a second heated tool has lands corresponding to the preformed trace channels on the other side of the substrate. Each hole cavity consists of a pair of opposing cavities formed on opposite sides of the substrate and separated by a common transverse wall located between the two substrate surfaces. The powder mix is deposited first in the channels

and cavities on the one side of the substrate and the first heated male tool is applied to form bonded conductive traces and to line the closed hole cavities on the one substrate ^■ side with a conductive layer. The substrate is then turned over with the other side up and powder mix is then deposited in the channels and cavities of this side. The second male tool, unlike the first male tool, is provided with punch elements at the ends of the male pins which mate with the still closed hole cavities. When the second tool is applied the hot punch elements pass through the transverse wall to open the mounting hole and at the same time a conductive lining is formed on the cavities opposing the cavities already lined by the first tool, thereby completing an open, conductor lined mounting hole.

It will be further understood that conventional hole through-plating techniques may be employed with a molded substrate on which conductive traces have been formed by the method of this invention. It is known that small diameter component mounting holes can be formed during injection molding of thermoplastic circuit board substrates. Thus conventional cylindrical or otherwise shaped mounting holes may be formed in the substrate (open to both sides of the substrate) and the circuit traces formed on the substrate interconnecting the component mounting holes by any of the methods disclosed in the present specification. The through-plating of the component mounting holes may be omitted altogether or may be carried out by conventional techniques.

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These and other characteristics of the present invention are better understood by reviewing the following figures, which are submitted for purposes of illustration only and not limitation, wherein like elements are referenced by like numerals, in light of the detailed description of the. preferred and alternate embodiments.

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BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a fragmentary cross section showing a preformed trace channel defined in a substrate.

Fig. 2 shows in fragmentary cross section a male tool element aligned with a trace channel for pressing into the substrate powdered conductive material deposited in the preformed channel.

Fig. 3 shows ^t e powdered conductive granulated material being pressed into the substrate by the tool.

Fig. 4 illustrates the tool separated from the substrate and the resulting conductor structure bonded to the substrate within the preformed channel.

Fig. 5 is a magnified section of the bonded conductor structure of Figure 4 illustrating the granular root material set in the substrate and the matrix binding the exposed portions of the granular material.

Fig. 6a illustrates a preformed blind hole cavity defined ^' in the substrate with powdered conductive material deposited therein, and a male pin element positioned in alignment with the substrate for making a through-plated component mounting hole.

Fig. 6b is a plan view of a tapered blind hole cavity preformed in the substrate and a trace channel terminating at the hole cavity.

Fig. 7 is an elevational section of a finished through plated mounting hole and connecting trace taken along line 7-7 of Fig. 6b.

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Fig. 8 is an elevational section taken along line 8-8 in Fig. 6b showing the through conductive hole open at both ends and the uninterrupted conductive transition between the trace and the mounting hole and a connecting trace.

Fig. 9 is a fragmentary plan view of a typical tool used in the method of this invention.

Fig. 10 is an elevational section taken along line 10-10 in Fig. 9.

Fig. 10a is an elevational section taken along line lOa-lOa in Fig.- 9.

Fig. 11 illustrates preformed substrate channels having different depth but comparable width at the substrate surface such that variations in the conductor cross section may be obtained without significant change in the trace area.

Fig. 12 is an isometric view opened up to show a typical substrate sheet with preformed trace channels and mounting hole and the male elements of a heated tool mateable to the substrate.

Fig. 13 illustrates a preformed hole cavity structure in a substrate for a double sided circuit board and the pin elements of each of the two male tools corresponding to the two sides of the substrate.

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DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows in fragmentary cross section an insulating substrate 10 typically made in relatively thin sheet form. The insulating substrate has a component side 12 and a trace side 14. A trace channel 16 is preformed in the substrate 10 as by injection molding. The trace channel 16 is of substantially rectangular cross section and includes a channel bottom 18 and channel side walls 20. The thickness of the substrate measured between the component side 12 and the trace side 14 may be varied to suit the particular . application. The substrate may be relatively thick if rigidity is important, or it may be only slightly thicker than the depth of channel 16 if it is desired to make a flexible printed circuit board.

Various presently known thermoplastic materials are suitable for use as substrate material in the making of printed circuit boards by the method of this invention. Exemplary of such known products are polyetherimide sold under the tradename ϋlte , polyphenylene sulfide sold as Ryton, polyethersulfone sold under the tradename Victrex, polysulfone sold as Udel, and thermoplastic polyester sold as Valox. These thermoplastics have varying mechanical, chemical and thermal properties and a choice may be made according to the requirement of the particular application contemplated. Additionally, the thermoplastics may be reinforced with glass or other materials in a manner known in the plastics industry to modify or improve the character¬ istics of the thermoplastic. These materials are given only by way of example and still other insulating materials, not necessarily thermoplastics, may be suitable for use as substrate material for the methods of this invention. For example, Teflon, a fluoropolymer which softens at 621°F. and has superior insulating, chemical and flame resistance properties, may be used to make printed circuit substrates by cold forming the substrate rather than injection or

extrusion molding. Yet other materials and substrate forming processes will be apparent to persons skilled in the art.

Turning now to Figure 2, a granulated or powdered conductive material 22 is deposited within the preformed channel 16 of the insulating substrate 10, uniformly along the length of the channel. The quantity of powdered material 22 placed in the channel may be varied according to the desired finished conductor cross section. The powder 22 includes a first granulated high melting temperature electrically conductive material such as copper, aluminum, gold, graphite or other material having low electrical resistivity, copper being presently preferred. The high melting point granulated material, which may be referred to as the root or substrate bonding material is admixed with a second powdered electrically conductive substance, which may be referred to as the matrix or binding material. The second powdered material is selected to have a relatively low melting point compared to the first granulated material, which low melting point may be somewhat higher or lower than that of the insulating material selected for the substrate. The second powdered material may consist of a eutectic mixture of tin and lead such as is commonly used in the electronics industry as a solder. It is presently preferred that the eutectic tin/lea material be a tin/lead alloy, rather than a mixture of the two etalic powders. It has been found that the alloyed solder has superior wetting characteristics and yields a conductor trace having negligible electrical resistivity. For purposes of the following description and only by way of example, a thermoplastic is chosen as the substrate material, copper is selected as the high melting point root material, while tin/lead alloy is used as the low melting point matrix material.

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While only a fragmentary section of a substrate is illustrated in Figs. 1 through 4, it will be understood that the trace channel 16 may be part of a trace network of any desired shape and complexity extending over the surface of the insulating substrate 10. An example of a complete substrate 10 is shown in Fig. 12. The trace channels may be of varying widths and also of varying depth. By varying the cross sectional area of the channels, by increasing either its width depth, or both, it is possible to obtain greater or lesser conductor cross sections. Similarly, for a given channel cross section, the cross section of the resulting conductor may be adjusted by depositing greater or lesser quantities of the powder mix into the channel. Conductive areas can be formed on an insulating substrate not only for conducting electricity, but also for use as heat sinks, to dissipate heat produced by circuit components.

A male tool 24 includes a lower tool face 26 from which extend male trace elements 28 configured to mate closely with the preformed trace channel 16 in the substrate 10. The tool 24 can be made of machine steel by known milling techniques presently used for the making of tools and mold cavities in the injection molding of thermo¬ plastics. Fig. 12 shows an example of a tool 24 attached to a shank 25 for mounting in a suitable press (not shown in the drawings) such as a drill press so that the tool may be applied against the trace side 14 of the substrate 10. For small quantity production of printed circuit boards according to the present invention the tool 24 may be mounted in a drill press while the substrate 10 is supported on a table 60 in register with the male elements of the tool 24, as indicated by the dotted lines in Fig. 12. For larger quantity production of printed circuit boards, the tool 24 may be mounted in an automated stamping press. The tool 24 may be heated by an electric resistance heater 62 inserted into a bore 64 drilled in the tool, and regulated by means of a rheostat 66. The tool 24 is heated to a temperature at

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least equal to the melting temperature of the low melting point component of the powder mixture 22 or the melting temperature of the thermoplastic substrate, whichever is higher.

The thermoplastic substrate 10 may have a melting temperature higher than that of certain available conductive materials such as eutectic mixtures of tin/lead. A mixture of 63% tin and 37% lead has a melting temperature of 374° F which is lower than the softening temperature of unrein- forced Victrex, ϋltem or Ryton, for example and is, of course, lower than the melting point of copper powder. While the actual melting temperature of these thermoplastics varies among the various compounds and also varies with the glass content of the plastic, generally the softening or melting temperatures of the plastics fall in the range of 350° F to 500° F. The temperature of the tool 24 is adjusted to exceed the melting or softening point of the particular substrate material being used and is typically in the range of 490° F to 575° F for Victrex, Ultem or Ryton. This temperature suffices to quickly melt the tin/lead powder in the powder mix 22. The tool is next brought into contact with the substrate 10 such that the male lands 28 enter into the preformed channels 16 and the heated tool 28 presses the powder mix 22 against the substrate while simultaneously heating the powder in the channel. The copper powder is a much better heat conductor than the thermoplastic substrate so that heat flows rapidly through the powder mix 22 to the bottom 18 and side walls 20 of the channel, reflowing the thermoplastic immediately underlying the powder mix 22. At the same time, the tin/lead component of the powder mix 22 readily melts such that the tool now exerts pressure only against the solid copper granules. The copper granules are compacted against the substrate while the tin/lead material is in a fluid state and some of the copper granules are forced into the soft thermoplastic at the bottom and side walls of the channel 16. The pressure

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exerted by the tool 24 is carefully controlled so as not to fully embed all of the solid granules into the thermo¬ plastic. Instead, it is desirable that only a minimal number of the copper granules be fully embedded in the thermoplastic since the thermoplastic acts as an insulator and therefore does not promote electrical conductivity between adjacent copper granules. Optimally, an even layer of partially embedded copper granules is formed lining at least the bottom 18 of the preformed channel, and possibly extending partially along the side walls 20 of the channels as the powdered material is squeezed around the male element 28 of the tool. The molten tin/lead component of the powder mix wets the exposed surfaces of partially embedded copper granules as well as those copper granules which are not embedded in the thermoplastic at all but are compacted against the partially embedded granules by pressure of the tool. The molten electrically conductive tin/lead material fills the spaces and crevices between the solid copper granules forming a continuous electrically conductive trace within the substrate channel. The tool 26 is then separated from the substrate. The circuit board quickly cools and both the substrate and the tin/lead matrix solidify, leaving an electrical conductor bonded to the substrate material within the channel.

It is desirable to minimize the duration of contact between the tool 24 and the substrate 10 in order to minimize the depth of the reflowed layer of thermoplastic, consistent with adequate compaction of the copper granules. With the heated tool mounted in a drill press, good results have been obtained with several short, quick taps of the heated tool against the plastic substrate. The powdered copper in the powder mix 22 rapidly conducts heat from the tool to the bottom and side walls of the channel 16 and the thermoplastic reflows almost immediately upon contact of the tool 24 with the powder mix 22. In fact, heat is conducted by the powder even as the tool draws near the powder and

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before physical contact occurs. The channel structure in which the powder mix 22 is deposited also aids in containing the flow of heat to the thermoplastic surfaces which are in contact with the powder and helps minimize flow of heat to other portions of the substrate. While some reflowing of the substrate surface 14 may occur due to the proximity of the tool face 26 during the pressing operation, the depth of thermoplastic which is reflowed is kept to a minimum by extending the height of the male trace elements 28 so as to allow for a spacing "d" illustrated in Figs. 3 and 6a between the tool face 26. and the substrate surface 14, and also by minimizing the contact time of the tool with the substrate. This surface layer of thermoplastic quickly returns to the solid state when the tool is separated from the substrate without any significant permanent effect on the substrate.

The resulting electrically conductive trace 30 bonded to the substrate 10 is illustrated in Figure 4. The conductive trace 30 extends in width across the bottom 18 of the channel 16 and typically also extends upwardly along part of the side walls 20, defining a somewhat U-shaped structure. The trace 30 is firmly anchored to the substrate along its full width including the bottom and side walls of the channel 16. The detailed structure of the conductive trace 30 is better appreciated by reference to the magnified cross section of Figure 5. The bonded trace formed by the aforedescribed process is seen to include compacted granules 34 and 36 of the relatively high melting point granulated copper. The depth of the compacted layer will normally comprise at least several granule diameters. The lowermost copper granules 34 will be either fully or partially embedded in the substrate 10. Additional copper granules 36 are compacted onto these lowermost granules 34. The random physical contact between adjacent copper granules obtained by applying pressure to the powder mixture while the tin/lead material is fluid can establish by itself a

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continuous electrically conductive path along the channel 16. The tin/lead component of the powder mix 22 melts almost immediately upon contact with the hot tool 24 and forms a slurry with the solid copper particles. This slurry facilitates compaction of the solid copper granules and also readily and evenly conducts heat from the hot tool to the thermoplastic material at the bottom and side walls of the preformed channels. The tin/lead material fills the spaces between the irregularly shaped granules 34 and 36 to form a binding matrix 38 which serves the dual purpose of mechanically binding those particles 36 which are not embedded in the substrate material to the exposed portions of partially embedded copper particles, and also to ensure reliable electrical connections between adjacent copper granules. As may be appreciated in Figure 5 the volume of the bonded conductive trace 30 is largely composed of discrete copper granules 34 and 36. The tin/lead matrix 38 acts as a filler and accounts for a lesser proportion of the conductor cross section. This will normally be a desirable result since as has been explained there is a choice of materials for the higher melting point conductive material having low electrical resistivity such as copper, gold, silver or various alloys of these and other metals having a relatively high melting point, that is a melting point sufficiently higher than tha of the thermoplastic so that the granules of this material remain in the solid state while the substrate is being reflowed. However, the choice of the- matrix material is presently limited to tin/lead combinations which have a higher electrical resistivity than copper and it is therefore presently desirable to minimize the conductor cross section comprised of the low melting point substance.

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Example 1

A conductive circuit was made on a substrate of polyethersulfone, glass filled Grade 520P (20% glass fiber content) by preforming channels in a sheet of substrate material, the channels being .030 inches wide and .015 inches deep. The substrate thickness was 3/32 of an inch. The channels were filled with a powder mixture consisting of granulated copper of -200 mesh mixed with a powdered eutectic alloy of 63% tin and 37% lead. The eutectic alloy powder was -325 mesh. The composition by weight of the powder mixture was 50% granulated copper and 50% powdered eutectic tin/lead alloy. A machine steel tool was mounted in the chuck of a conventional drill press and heated to approximately 600° F. The substrate was placed on the table of the drill press and prealigned with the tool to ensure mating registry of the male elements of the tool with the preformed channels in the substrate. The substrate channels were filled with the aforesaid powder mix and any excess of powder on the substrate surface outside the channels was wiped off. The hot tool was then brought down into contact with the substrate in several short, quick taps. After the substrate was allowed to cool it was found that continuous electrical conductors had been formed in the channels, the conductors being firmly bonded to the substrate along their entire length. The conductor thickness obtained was approximately .006 inches.

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Example 2

The procedure of Example 1 was repeated sub¬ stantially as described but using Polyetherimide (unfilled) Ultem 1000 as the substrate material. Results similar to those of Example 1 were obtained.

Example 3

The procedure _, of Example 1 was repeated but using Ryton-R4 (polyphenylene sulfide) 40% glass fiber filled, as the thermoplastic substrate material. Results similar to those of Example 1 were obtained.

The composition of the powder mixture 22 may vary both in terms of the proportion of high melting point to low melting point material and also as to the absolute and relative mesh size of the powders. Mixtures of copper and tin/lead powder have been used ranging in composition from 15% to 50% tin/lead alloy by weight. It has been found that a proportion of tin/lead alloy in the mixture closer to the 50% figure yields better wetting of the copper powder and therefore produces conductors having lower electrical resistance. The copper powder presently preferred is -200 mesh, although the mesh size may vary. In general, larger copper granules pack down more into the substrate channels and cavities than finer mesh copper powders when the powder mix is impacted with the tool. Smaller mesh copper is easier to mix with the tin/lead powder in a homogeneous mixture but the powder also becomes more difficult to handle and to wipe off from the substrate surface. For given channel dimensions the thickness of the conductor structures obtained by the present method can be controlled by changing the mesh size of the copper granules and also by adjusting the quantity of powder deposited in the channels.

Figures 6a-8 illustrate a method for making conductor lined component mounting holes in the thermo¬ plastic substrate simultaneously with the making of conductive circuit traces. A frustro-conical cavity 40 is preformed in the thermoplastic substrate 10, as part of the network of trace channels 16, at the location of each desired component mounting hole, as shown in Fig. 12. For cylindrical component leads the hole cavity 40 will normally be of cylindrical cross section and have an inwardly sloping or conical wall 42, closed at the bottom by a transverse dam wall 44 as seen in Fig. 6b. The tool 24 is provided with a male conical pin element 46 mateable with each hole cavity 40. The pins 46 extend downwardly from the tool face 26 and will normally be connected to a male trace element 28 as illustrated in Figs. 9, 10 and 10a. Likewise, each hole cavity 40 in the substrate 10 will normally, but not necessarily, be connected to a channel 16 in the substrate, as illustrated in Fig. 6b and Fig. 12. A typical fragment of a tool 24' illustrating both pins 46 and trace lands 28 is shown in Figures 9, 10 and 10a.

The hole cavities 40 are filled with the same powder mix 22 deposited in the trace channels 16 described in connection with Figures 1-4. The powder mix becomes a relatively fluid slurry almost immediately upon contact with the hot pin 46 when the heated tool 24 is pressed against the substrate 10. The slurry readily flows around the pin 46 and offers minimal resistance to penetration of the pin into the cavity 40. When sufficient pin penetration has occurred, the solid copper particles in the cavity 40 are pressed and compacted by the pin 46 against the sloping side walls 42 of the cavity. The thermoplastic defining the sidewalls 42 is reflowed by the heat of the pin 46, and the copper powder is pressed into the soft plastic, thus lining the side walls 42 with a coating of embedded and compacted granules of conductive material. The molten tin/lead component of the powder mix 22 wets the copper particles

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lining the side walls 42 of the cavity and fills the spaces between granules to ensure a continuous electrically conductive lining extending over the hole wall. As the tool 24 is separated from the substrate, the pin 46 is withdrawn from the cavity 40 and both the thermoplastic and the tin/lead solder solidify, an electrically conductive lining remains bonded to the sidewall 42 within the hole. The lining, in magnified cross section, is similar to the bonded conductor structure illustrated in Figure 5. The walls of the hole cavities 40 may slope at an angle of 30 to 60 degrees relative to a line perpendicular to the substrate. The pins 46 have a conical surface 49 which tapers at an angle similar to that of the cavity walls and seats agains the conical sidewall 42 of the hole cavity. The conical pins may be extended to form punching tips 47 which pass through the transverse bottom wall 44 of the closed hole cavity when the pin is fully seated in the cavity, thereby to open the mounting holes by creating an opening 54 on the component side 12 of the substrate to allow insertion of component leads through the substrate. The conical portion 49 of the pin 46 which seats into the hole cavity is spaced from the tool surface 26 by a base portion 45 which provides for a clearance "d" of at least 0.030 to 0.040 inches between the tool face 26 and the surface 14 of the substrate as shown in Fig. 6a. The male trace elements 28 are similarly extended from the tool face as illustrated in Fig. 3 to avoid contact of the tool face 26 with the substrate, in order to limit the melt area of thermoplastic to those areas directly underlying the cppper powder. The pins 46 and other portions of the tool 24 may be Teflon coated to prevent adhesion of metal or plastic to the tool.

During the powder pressing operation the substrate

10 may rest on a resilient, heat resistant surface, e.g. , a sheet of silicone rubber, which yields to the punching tips

47 of the pins, but provides adequate support against the

-pressure of the tool. Alternately, the substrate 10 may

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rest on a die having cavities disposed for receiving the tips of the pins of the tool 24. If desired, the hole cavities 40 may be left closed during the powder pressing operation and can be opened by a subsequent separate punching step. This could be accomplished by means of a second tool having punching pins disposed in a pattern which is a mirror image of the pins 46 on tool 24. The mirror image tool is pressed against the component side 12 of the substrate to punch out the transverse walls 44 of each closed hole cavity 40 in a single step.

Figures 7 and 8 illustrate a typical conductor lined component mounting hole obtained by the just described process. The conductive lining 50 circumferentially covers the wall 42 of the now open hole 40 and is physically and electrically connected to the conductive trace 30 in a channel 16 terminating at the mounting hole 40.

Double sided circuit boards can be made by preforming trace channels on both sides 12 and 14 of the substrate and performing the steps of the present invention first on one side, then on the other, using different tools constructed to match the conductor patterns on each side of the substrate.

The punching tip 47a of tool 24b is preferably extended as a narrow cylindrical or tapering needle. The hot tip 47a melts through the wall 64 as the pin 46b seats into the cavity 60, but the. opening created is snug around the tip 47a and does not allow the powder to fall through the bottom of the cavity. When the pin 46b seats fully into the cavity, the flaring end of the pin fully opens the wall 64, but at this point in the process the powder mix has already been formed into the desired conductive lining and the wall 64 is no longer needed. The method of making conductor lined component mounting holes illustrated in Figs. 6a through 8 is suitable for the manufacture of single

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sided boards. Fig. 13 illustrates a method for making conductor lined holes adapted to double sided boards. Opposing hole cavities 60 are molded in the substrate on each surface 12 and 14 of the substrate 10. Each hole cavity has sloping walls 62 facing the corresponding side of the substrate 10. A common transverse wall or powder dam 64 located intermediate the substrate surfaces 12 and 14 separates the opposing cavities 60 and serves to contain the powder mix to be deposited in the cavities. The conductor lined hole is formed in two steps. Powder mix is first placed in the channels and hole cavities on one side of the substrate, e.g. , side 12 and a hot tool 24a is pressed against the upper side 12 of the substrate to form the conductive traces and cavity linings on that side. The male pins of the first tool 24a are not extended at the tips and therefore do not pierce the power dam 64. The substrate 10 is then reversed, powder mix is placed in the opposing cavity 60 on side 14 of the substrate, and a second hot tool 24b is pressed against this side of the board. The pins 46b of the second tool 24b have extended punching tips 47a which pierce the powder dam 64, thereby opening the mounting hole. The conductive lining of the resulting hole is formed in two steps, first on the wall 62 of cavity 60 on one side of the substrate and then on the wall 62 of the opposing cavity 60. The punching tip 47a and the flaring transition 49 open the transverse wall and help connect the conductive linings of the opposing cavities.

Different conductor cross sections may be obtained by varying the depth of the channels preformed in the substrate material, as illustrated in Figure 12. By making a channel deeper (with suitable adjustment of the tool) it is possible to deposit herein greater quantities of the powdered conductive materials, resulting in varying trace cross sections. It follows that the substrate surface area occupied by the trace is not necessarily related to the conductor cross sectional area. As a result, it is possible

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to pack more densely the conductive traces even for relatively high current traces which require larger conductor cross sections. In conventional circuit boards, both of the type having laminated copper foil and those made by plating copper on a plastic or other substrate this is not possible because the conductive foil is of uniform thickness over the entire substrate surface. Thus, if it is desired to create a larger conductor cross section, this necessarily requires that a greater area of copper foil be incorporated into the conductor. In addition, the copper foil in conventional laminated circuit boards is relatively thin, 1.5 mills in thickness typically. Such thin copper 'foil is desirable because the etching processes used to form the conductive pattern on such boards would be unduly prolonged if thicker foils are used. In the present method, relatively thick conductors, e.g. .050 inch thick, can be deposited and bonded to the substrate without difficulty. Further, the present method allows the full thickness of conductive material to be bonded to the thermoplastic substrate in one pressing step, and the time required by this step does not depend significantly on the thickness of the conductor being formed.

In printed circuits made by conventional methods the conductors lie on the surface of the substrate. A further advantage of printed circuit conductors formed by the method of this invention is that the conductors lie within channels in the substrate and are recessed from the substrate surface. The circuit boards can therefore be mounted directly against a conductive surface such as a metallic panel without short circuiting the recessed conductors. Conventional printed circuit boards are mounted by means of spacer elements ^* which support the circuit board away from conductive surfaces.

Variations on the above described process are possible whereby granulated copper is first embedded into

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the thermoplastic to form a bare root system and the binding matrix material is added onto the exposed portions of partly embedded copper particles in subsequent steps which may incorporate a variety of additive processes. For example, additional copper may be electroplated or electroless plated onto the exposed copper particles bonded to the substrate. Alternatively, a low melting temperature powdered conductive material may be deposited over the exposed copper particles. The entire board may be heated as in a hydrogen or inert gas to prevent oxidation furnace to melt the powdered conductive material over the copper particles thereby forming a binding conductive matrix over the root system of copper particles. The low melting temperature material may be commonly, available eutectic tin/lead alloy or powdered solder.- These materials have a melting temperature in the vicinity of approximately 374° F. which is lower than the melting temperature of certain ones of the plastic materials available for use as the substrate. The substrate may therefore be placed in a furnace controlled to a temperature lower than the melting temperature of the plastic but sufficient to melt the powdered solder material to create the cementing conductive matrix.

The matrix material may, however, be heated to a higher temperature than the melting point of the substrate material, .so long as no force is applied to the substrate. Thus powdered silver may be used as the matrix material in this alternate method. The powdered silver is deposited over the bare root system and heated by passing the substrate through an inert gas furnace to prevent oxidation of the metals. Even though the temperature required to melt the silver exceeds the softening point of thermoplastic substrates, the substrate will not deform significantly if no force is applied to the substrate.

It must be understood that many alterations and modifications may be made by those having ordinary skill in

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the art to the methods of fabrication of printed circuit boards disclosed herein without departing from the spirit and scope of the invention. Therefore, the presently illustrated embodiments have been shown only by way of example and for purposes of clarity and should not be taken to limit the scope of the following claims.

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