SE440844B - SET AND DEVICE FOR MANUFACTURING PRINTED CIRCUITS - Google Patents

Description

a) printing a layer of electrically conductive ink, containing a curable phenolic resin, electrically conductive metal powder and an oxide reducing acid catalyst on the surface in question in a predetermined circuit pattern with relatively low electrical conductivity, b) applying and embedding a layer of electrically conductive - * the metal particles over and in the ink layer before it cures, c) cure the paint layer and d) press and alloy a layer of electrically conductive solder over the metal particle layer and apply sufficient heat thereto to alloy the solder to the metal particle layer to form a solderable circuit pattern with relatively high electrical conductivity.

In the accompanying drawings, Fig. 1 shows a flow chart illustrating the steps for producing printed circuits according to the invention, Fig. 2 is a perspective view of a representative card used to manufacture a printed circuit according to the invention, Fig. 3 is a perspective view showing a wire cloth to be used in printing a circuit on the card, Fig. 4 is a sectional side sectional view showing the preferred technique for applying the conductive color to the card; Fig. 5 is a perspective view illustrating a card printed with the conductive color. Fig. 6 is a perspective view showing the card being powdered with conductive powder; Fig. 7 is a side sectional view showing a press device according to the invention; Fig. 8 is a perspective view showing a wire cloth to be used for printing a loasky ash layer. Fig. 9 is a cross-sectional view in detail on an enlarged scale showing a part of a pressure circuit made according to the invention, and Fig. 10 finally shows a view similar to Fig. 9 but a modification hence. According to the present invention, an electrically conductive circuit is printed on a support surface, such as a card 10, by supplying the circuit with circuit materials. The board 10 can be made of any of different materials suitable for printed circuit boards.

Cards of this type must be electrically insulating and are generally relatively thin and rigid, although for some applications flexible support surfaces can be used. For this purpose, you can use cards made of phenolic resins, fiberglass laminate, impregnated paper cards or the like. Ideally, the card should be able to withstand heat from soldering operations, exhibit electrical insulation properties as well as resistance to physical shocks and vibrations. In any case, the card 10 is first formed into the desired shape and in the illustrated embodiment the shape of the card is generally rectangular and formed with a pair of tongues 12 and 14 in one piece at an edge by means of which the card can be plugged into a chassis in ordinary way. This form is only an example because many different forms were used with and without tongues. If a large number of cards are to be produced, a number of circuits can be printed on a large disc or sheet and then cut into individual cards.

The board is formed with a number of small holes 16 through which the wires for the circuit components are passed after the circuit has been pressed and to which the components will be soldered. The smooth card 10 is initially cleaned by means of a suitable solvent such as isopropanol or methyl ethyl ketone.

With cleaned card 10, a silk screen cloth 18 is placed over the card 10 as indicated in Figs. 5 and 4. The wire cloth 18 carries the desired circuit pattern and in practice the wire cloth is made of stainless steel with a mesh size of 160 - 180 mesh (..... am fl The pattern on the wire cloth 18 is open at 20, where the circuit is to be printed on the card and is solid in the areas where no printing is to take place.With the wire cloth 18 in operative position over the card 10, a conductive color is applied 22 through the cloth 18 on the surface of the card 10, the paint passing through the open cloth areas 20 on the cloth, so that the circuit pattern is transmitted on the card as indicated in Fig. 5.

The paint 22 is in accordance with the present invention a metal-laden resin system and the paint comprises a phenol-based resin mixed with conductive metal powder, preferably. afniezsßfs-s 4 copper powder with a grain size of 525 mesh (0.044 mm). The paint also includes an acid, which acts as a catalyst in curing the paint, as well as serving to remove any oxides that may be present with the copper. The color can be applied to the card by hand, although for production purposes application.with wire cloth has been found to give the most satisfactory results because you thereby obtain a clean, sharp image and which is relatively inexpensive. In Fig. 4, an applicator 24 can be used to spread the paint 22 across the screen so that the image passes through the screen to the card. As shown in Fig. 5, the printed circuit board 10 shows a circuit image composed of circuit segments 26 corresponding to the circuit structure of the wire cloth 18, where each circuit segment is printed with the color 22.

While the color of the circuit segments 26 printed on the card is still wet, the circuit segments are covered with a conductive metal powder 28 such as e.g. by powdering and for this purpose it has been found that copper powder with a grain size of 525 mesh (0.044 mm) has been found to give satisfactory results. The powder 28 should be powdered over the circuit pattern within a few minutes after the printing operation and should preferably be performed within 5 minutes. An excess amount of powder should be added to ensure that the paint is completely covered by the powder, even though excess powder may be present on the surface of the card. When the card is fully powdered with copper powder, it is allowed to pass a roller press 50 shown in Fig. 7, which presses the copper powder 28 intimately into the color of the circuit segments 26.

The press 10 comprises a plurality of rollers 52 arranged in pairs, each pair having increasing durometer hardness from one end to the other. The first pair of rollers can e.g. have a durometer hardness of perhaps 45, while the next pair may have a hardness of 50, followed by a pair of hardness 55 and an end pair with a hardness of 60. The rollers are mounted in a rigid frame 54, with the lower rollers driven by a common drive system for advancing the card from left to right as seen in Fig. 7. The upper rollers are individually adjustable by means of screws 56 loaded by springs 58, which apply pressure to the upper set of rollers. Thus, when the card 10 is fed into the opening between the first pair of rollers, the copper powder will be pressed into the paint and the pressure will gradually increase as the card is fed through the press. The pressing operation produces an intimate contact between the powder and the paint and when the NMS 43-5 5 card has passed once through the press, excess copper powder is removed and the card can pass the press again. By applying excess copper powder to the first powder, the entire printed circuit is covered by the powder and this ensures that some of the fennel is not taken and transferred to the rollers of the press. The end result is that the pressed circuit image remains clear and clean. Furthermore, there is a relationship between the surface tension of the resin and the pressure applied by the rollers. By applying progressive pressure to the card, a uniform surface is produced without distorting the image.

When the pressing operation is completed, the paint hardens.

In practice, the printed circuit can be given a preliminary cure at? 0 ° C to 90 ° C for a period of 10 to 15 minutes and a cure at 125% for 1 or 2 hours. The hair can be performed by various means, such as a furnace with forced circulation of air or the use of infrared lamps e.g. When the paint has completely cured, the circuit segments 26 bind tightly to the board 10 and the copper powder 28 has become an integral part of the circuit segments 26. Any loose or excess copper powder 28 is removed by vacuum or some other suitable means.

The next step in the manufacture of the printed circuit is to apply a solder paste over the same circuit segment 26, which has been powdered with copper powder and which has now been cured. A wire cloth can be used for this purpose similar to the wire cloth 18 used to apply the ink to the card in the first step, except that a coarser wire cloth is used such as having a mesh size of 90-100 mesh (...... - 0.149 mm) with a slightly larger image to ensure full coverage of the circuit. The solder paste used here is preferably a composition of a lead-tin alloy with antimony in powder form in a flux and a binder, the metal powders remaining suspended in the paste. Two solder pastes that have been found to give satisfactory results include a 60/40 solder with 1% ethylene glycol, which is sold by Bow Solder Products Co., Inc. The other solder paste is a 60/40 paste made by Electronic Fusion Devices, Inc. and which has designated 2057. The paste has a creamy consistency and is printed on the circuit in the same manner as indicated in Figures 5 and 4 using wire cloth and applicator as shown.

The flux in the solder should preferably be very active and 6 should consist of a strong inorganic acid. A preferred solder paste composition comprises in weight percent zinc chloride 3 - 6%, pre-alloyed lead-tin antimony 80% and a binder constituting the remainder. Once the plague is applied, a layer 40 is applied to the top of the circuit segments 26. Once the solder is applied, the solder is alloyed with the underlying paint and copper powder by heating the circuit to the melting point of the solder, typically in the temperature range 525 ° C. to 550 ° C, depending on the particular solder composition. The heating can be carried out by various means, such as infrared lamps in conjunction with a moving band carrying the card at a speed of about 3 m / min.

In any case, the solder is heated to its melting point which causes it to alloy with the paint and copper powder and form a conductive printed circuit on the board. The application of the solder to the underlying paint and copper powder significantly increases the conductivity of the circuit and serves to join the solder tightly to the card.

Once the alloying operation is completed, a pattern of solder layer 42 can be selectively applied over the printed circuit and the board. The solder protection layer consists of a dielectric material, which serves to insulate the parts of the card which are not to be exposed to any contacts, wires, etc. which will subsequently be applied to the card.

The solder protection layer can representatively consist of an epoxy material and serves to prevent solder from a wave soldering machine from adhering to other parts of the circuit, when soldering parts to the card. The solder layer can be applied by means of silk screen techniques, which have already been described using a wire cloth 44 with a pattern which it is desired to appear on the circuit board. Normally, the plumbing protection layer differs from that in the circuit board in that certain parts of the circuit are to be covered while others are not to be covered.

The solder layer can also serve as an insulator between successive layers of circuits, which can be built up on a surface of the card as indicated in Fig. 10. Different circuits can be applied on top of a solder layer substrate using the same techniques as when applying of the first circuit directly on art. In practice, when a multiple circuit pattern is to be built up on one side of the card in successive layers, the solder paste layer 40 on the circuit closest to the card should have a higher melting point than the following solder paste layers on the outer circuits, so that the lowest solder layer does not will again become liquid or melt when the outermost circuits are alloyed.

Fig. 10 shows a card, with two circuit layers built on top of each other. The first circuit comprises a circuit segment 26, which is powdered with copper powder, hardened and alloyed with a layer of solder paste 40 and where the circuit is then covered with a solder layer 42. Printed on top of the solder layer 42 there is another circuit segment 26 'with le color on which a powdering of copper powder has taken place and on which a layer 40 'of solder paste has been alloyed with the paint and the powder. The different superimposed printed circuits can cross each other, or be built up in whatever number of layers may be required for the particular circuit. Using similar techniques, the method can be used to repair ordinary printed circuit boards or to execute change orders on such boards.

The conductive paint used in the process consists of a curable resin which is loaded with electrically conductive metal powder. A preferred composition is a bicomponent system composed of a phenol-based (resorcinol) resin amount of copper powder to which a catalyst such as anhydrous isopropanol or phosphoric acid is added. The anhydrous isopropanol should be pure and preferably at least 99% pure. Similarly, the phosphoric acid should be of reagent grade. The anhydrous isopropanol not only acts as a catalyst but also serves as a partial thinner, to provide viscosity control in the paint. While phosphoric acid is preferred, other inorganic acids may be used to advantage. The acid not only serves as a catalyst but also removes any oxides that may be present in the copper powder. Since the oxides prevent conductivity, it is desirable to eliminate any oxide that may be present.

The conductive paint according to the preferred embodiment is prepared by first heating the phenolic resin (resorcinol) to 15,000 until the degree of polymerization has progressed to a stage where the resin has a crack-like consistency. The viscosity of the resin can be adjusted by using anhydrous isopropanol using anhydrous. Then the copper powder is added and this should preferably be of a grain size of 525 mesh (0.044 mm) untreated copper powder of quality 200 RL. Sufficient copper powder is added to achieve 70 - 75% admixture by weight. For example, for 100 g of total weight of paint, 25 g would be resin while 75 g represents the copper powder. If the material is not to be used immediately, it should be stored in a refrigerator at about 4 ° C to prevent further polymerization. When the material is to be used for printing, it is first brought up to room temperature and 1 of the catalyst is added, and preferably the catalyst constitutes 6% by weight of the material. The preferred catalyst mixture contains 5% phosphoric acid and 5% isopropanol. Other materials used in the process may include anhydrous isopropanol to be used as a diluent, as previously indicated and also as a cleaning agent for the pressure wire cloths as needed. For this purpose, a cleaning agent containing a mixture of isopropanol and methyl ethyl ketone in equal proportions has been found to be satisfactory.

The solder paste may comprise a composition 60/40 of lead and fi e fl with the addition of 1% ethylene glycol. The solder can be manufactured in a number of different compounds depending on the desired melting point. The solder paste should have a consistency so that it can pass through a stainless steel cloth of the specified mesh size.

While the invention has been described with particular reference to the embodiments shown, it is obvious that several different modifications will be apparent to those skilled in the art.

Different areas, temperatures, times and other parameters such as! examples have been described in connection with the preferred embodiment, except that variations can be made with satisfactory results. The flexibility of the ink color can e.g. regulated by adjusting the ratio of the epoxy moiety to the phenols. Instead of copper powder, other conductive metals, such as silver, aluminum, etc. can also be used. Furthermore, instead of applying the copper powder L by manual powdering, it can be applied by means of a vibrator feeder over a moving conveyor belt, electrostatically or by means of other suitable devices. Likewise, the solder layer can be applied by wave soldering techniques or the like instead of printing with a wire cloth, in which case the solder would immediately alloy with the copper powder layer without any special heating step. A further modification can be envisaged by using a pure resin paint without metal filling in which case all the conductivity would be derived from the powder and solder layers.

Claims (7)

10 15 20 25k 50 35 '-H0 Ü i U i 10 Eatentkrav l 1. a method of manufacturing an electrically conductive circuit on a dielectric surface, characterized in that it comprises the steps of (a) printing a layer of electrically conductive ink containing a curable phenolic resin, electrically conductive metal powder and a non-reducing acid catalyst on the surface in question in a predetermined circuit pattern with relatively low electrical conductivity, I g (b) applying and embedding a layer of electrically conductive metal particles over and in the ink layer before curing, (c) curing the paint layer and S (d) pressing and alloying a layer of electrically conductive solder over the metal particle layer and applying sufficient heat thereto to alloy the solder to the metal particle layer to form a solderable circuit pattern with relatively high electrical conductivity. 2. A method according to claim 1, characterized in that it comprises applying a dielectric layer_selectively -over the solder layer. 3. A method according to claim 1, characterized in that the metal particles are pressed into the paint layer before it hardens. 4. H. A method according to claim 3, characterized in that the particles are pressed into the paint layer by progressively increasing the pressure. 5. A method according to claim 2, characterized in that it comprises the steps of forming a second electrically conductive circuit over the dielectric layer by applying a second electrically conductive ink to the dielectric layer in a predetermined pattern, applying a second layer of electrically conductive metal particles on the second paint layer, curing this second paint layer, applying a second layer of electrically conductive solder over the second metal particle layer and heating the second circuit sufficiently to melt the second solder layer alone . 6. A method according to claim 1, characterized in that the solder layer is applied by wave soldering. 7. A method according to claim 5, characterized in that the melting point of the first solder layer is higher than the melting point of the second solder layer.