GB2050702A - Printed circuits - Google Patents

Abstract

Printed circuits produced by an additive technique. A resin 26, preferably a metal-loaded resinous ink is printed on a board 10 to map out those areas which are to be conductive. A metal powder 28 is added with pressure to the resin, and the resin is then cured. A layer of solder 40 is then alloyed to the powder. After this, a solder resist 42 may be applied selectively over the board. More than one layer of circuits can be built up in this way. The ink and the solder may be applied to the beard by silk-screening. <IMAGE>

Description

SPECIFICATION Printed circuits This invention relates to printed circuits, and to a method of making, changing and repairing printed circuits.

Conventional printed circuits are made by a subtractive procedure, in which circuit boards laminated with one or more conductive strata are etched to remove some portions of a conductive layer, while other portions remain, according to predetermined circuit design. The technique involves relatively complex photographic procedures requiring a large capital investment for etching and plating equipment and related apparatus, such as cameras. In general, the time required to design and produce a printed circuit with conventional techniques is quite long and requires a rather large number of skilled technicians. As a result, relatively few companies are capable of producing printed circuits using conventional techniques.Also, because of the time and expense involved in preparing a printed circuit, standard procedures are not appropriate for short production runs, even though the printed circuit itself may be more desirable.

We have now discovered a method of producing improved printed circuits, which can be cheap for both short and long production runs.

Novel materials and associated apparatus can be used to produce such printed circuits by means of additive procedures.

Thus the present invention consists in a method of making a circuit board, which comprises (a) applying a thermosetting resin, preferably a resinous ink, more preferably a metal-loaded resinous ink, to that part or those parts of a surface which are to be conductive; (b) applying electrically conductive metal particles to the part; -(c) curing the resin; (d) applying and alloying an electrically conductive solder to the metal particles; and, optionally, applying a dielectric stratum selectively over the solder.

This method can be used to produce a circuit board having more than one layer of electrically conductive circuits. In this case the optional layer of insulating solder resist is applied and the method of the invention is repeated.

The surface also consists in an electrically conductive ink suitable for use in printing an electrically conductive circuit on a dielectric surface which comprises: (a) a thermosetting resin preferably an epoxy resin, and preferably a resin loaded with conductive metal powder, such as copper powder; (b) conductive metal particles dispersed in the resin; and (c) a catalyst, the resin, particles and catalyst being present in the ink by weight percent respectively of approximately 23%, 71% and 6%.

The solder paste preferably used with this ink is preferably a lead-tin alloy plus powdered antimony suspended in a binder together with a strong flux.

The invention further consists in an apparatus suitable for use in making a printed circuit board, which comprises: (a) a frame (b) a plurality of pairs of rollers mounted to the frame; and (c) means for driving at least one roller; the rollers being mounted to define a linear path between each pair for a board to be fed therethrough; the rollers being of progressively increasing hardness along the path.

The invention is further illustrated by the accompanying drawings, in which: Figure 1 is a flow diagram setting forth the steps involved in a method of producing printed circuits; Figure 2 is a view in perspective of a board that may be used for making a printed circuit; Figure 3 is a view in perspective showing a screen suitable for use in printing a circuit onto a board; Figure 4 is a sectional view in side elevation illustrating a preferred technique for applying a conductive ink to a board; Figure 5 is a view in perspective showing a board printed with a conductive ink; Figure 6 is a view in perspective showing a board being dusted with a conductive powder; Figure 7 is a sectional side elevation showing a pressing apparatus; Figure 8 is a view in perspective showing a screen for use in printing a solder resist onto a board;; Figure 9 is a detail cross-sectional view on an enlarged scale showing a portion of a printed circuit; and Figure 10 is a view similar to Figure 9, but showing a modification.

An electrically conductive circuit is printed on a supporting surface, such as a board 10, by adding circuit materials to the board. The board 10 may be made from any suitable material. Boards of this type must be electrically insulating and commonly are relatively thin and stiff, although for some applications flexible supporting surfaces may be used. For this purpose boards of, for example, phenolic resins, fibreglass laminates or impregnated paperboard may be used. The board should preferably be capable of withstanding heat generated during soldering and it should be resistant to physical shock and vibration. In any event, the board 10 is first formed into the desired shape; in the illustrations the shape of the board is reectangular and it is formed with a pair of integral tongues 12 and 14 at one edge, by means of which the board may be plugged into a chassis in any usual manner.This shape is only by way of example, and many different shapes are used, with or without tongues. If a large number of boards are to be produced, a number of circuits may be printed on a large sheet and this may be cut subsequently to give individual boards. The board is generally formed with a number of small holes 16 through which the leads of circuit components are passed after the circuit has been printed. The components are then soldered to these leads. The plain board 10 is preferably cleaned by a suitable solvent, such as isoproponol, or methyl ethyl ketone, before the method of the invention is carried out.

In one method a silk screen 18 is superimposed over the board 10 after cleaning preparation as illustrated in Figures 3 and 4. The silk screen 18 bears the desired circuit pattern and, in practice, a silk screen fabricated from 1 60-1 80 mesh stainless steel has been found to give particularly satisfactory results. The silk screen 18 is open at 20 where the circuit is to be printed on the board and is solid in those areas where no printing is to take place. With the silk screan 18 in position over the board 10, and ink 22 preferably a conductive ink, is applied through the screen 1 8 onto the surface of the board 10. The ink passes through the open mesh areas 20 of the screen so that the circuit pattern is transferred onto the board as illustrated in Figure 5.

The ink 22 is preferably a metal-loaded resin system, and it preferably includes a phenolic based resin loaded with a conductive metal powder, preferably a 325 mesh copper powder.

We prefer that the ink also includes an acid which catalyses curing and serves to remove any oxides that may be present with the copper.

The ink may alternatively be applied to the board by hand, although for production purposes silk screening has been found to give the more satisfactory results since it produces a clean, sharp image and is cheaper. In the Figure 4 an applicator 24 is shown spreading the ink 22 over the screen to ensure that the image passes through the screen onto the board. As shown in Figure 5, the printed board 10 displays a circuit image of circuit segments 26 corresponding to the circuit design on the silk screen 18, each circuit segment being printed by the ink 22.

While the ink in the circuit segments 26 printed on the board is still wet the circuit segments are covered with a conductive metal powder 28. This can be done by dusting, for example, and for this purpose 325 mesh copper powder has been found to give satisfactory results. The powder 28 should be dusted over the circuit pattern within a few minutes, preferably within five minutes, after the printing operation. What appears to be an excess of powder should preferably be added to ensure that the ink is fully covered by the powder, even though excess powder may be present on the board surface. Once the board has been fully dusted with the copper powder it is passed through a roller press 30 shown in Figure 7, which presses the copper powder 28 intimately into the ink of the circuit segments 26.

The press 30 includes a plurality of rolls 32 arranged in pairs, the pairs increasing in durometer hardness from one end of the press to the other. Thus, for example, the first pair of rollers may have a durometer hardness of, perhaps, 45 while the next pair may have a hardness of 50, followed by a pair at 55, and a final pair at 60. The rollers are mounted to a rigid frame 34, with the lowermost rolls being driven by a common driving system to advance the board from left to right, as viewed in Figure 7. The upper rolls are inwardly adjustable by means of screws 36 loaded by springs 38 applying pressure to the upper set of rolls. Thus, as the board i O is fed into the bite of the first pair of rolls, the copper powder will be pressed down into the ink and the pressure gradually increases as the board is fed through the press.The pressing operation produces an intimate contact between the powder and the ink and, once the board has passed once through the press, excess copper dust may be removed and the board may be passed through again. By providing excess copper powder on the initial dusting, all of the printed circuit is covered by the powder and this ensures that no ink is picked up and transferred to the rolls of the press. The result is that the pressed circuit image remains crisp and clean. Further, there is a relation between the surface tension of the resin and the pressure applied by the rollers. A uniform surface can be produced without distortion of the image by the application of progressive pressure.

When the pressing operation is complete the ink is cured. In practice, the printed circuit may be preliminarily cured at 700C to 90do. for a period of 10 to 15 minutes and a finally cured at 1 250C. for one to two hours. The curing may be carried out by various means such as use of a circulating air oven or of infra-red lamps. When the ink has been cured fully, the circuit segments 26 bond tightly to the board 10 and the copper powder 28 becomes an integral part of the circuit segments 26. Any loose or excess copper powder 28 may be removed by a vacuum or other suitable means.

The next step in the manufacture of the printed circuit is to apply a solder paste over those circuit segments 26 which were dusted with copper powder and cured. A silk screen may be used for this purpose, similar to the screen 1 8 used to apply the printed ink initially onto the board, except a wider mesh screen is preferred such as a 90-100 SS mesh in a slightly larger image to ensure full coverage of the circuit. The solder paste is preferably a lead-tin alloy with antimony in powder form, in a flux and a binder such that the metal powders remain suspended in the paste.

Two solder pastes that have been found satisfatory are a 60/40 solder with one percent ethylene glycol sold by Bow Solder Products Co., Inc, and a 60/40 paste produced by Electronic Fusion Devices, Inc.

and identified as No. 2037. The paste has a creamy consistency and may be printed onto the circuit in the fashion illustrated in Figures 3 and 4 using the silk screen and applicator. The flux in the solder preferably should be very active, and a strong inorganic acid can be used. A preferred solder paste has the following composition in weight%: zinc-chloride 3-6, pre-alloyed lead-tin antimony 80, and a binder, the balance. Once the solder paste has been applied it forms a solder stratum 40 on top of the circuit segments 26.

Once the solder printing is complete, the solder is alloyed with the underlying ink and copper powder by heating the circuit to the melting point of the solder, typically in the range of 3250C. to 5500C., depending upon the particular solder composition.

The heating may be carried out by various methods, such as by infra-red lamps in association with a moving belt carrying the board at a rate of, for example, 10 feet per minute. In any event, the solder is heated to its melting point causing it to alloy with the ink and copper powder to form a conductive printed circuit on the board. The application of the solder onto the underlying ink and copper powder greatly increases the conductivity of the circuit and serves to mate the solder tightly to the board.

Once the alloying operation is complete a pattern of solder resist 42 may be applied selectively over the printed circuit and the board.

The solder resist is a dielectric material which serves to insulate those portions of the board which are not to be exposed to whatever contacts, such as leads, that are subsequently to be applied to the board. The solder resist may be an epoxy material and it prevents solder from, for example, a wave solder machine adhering to other parts of the circuit when items are being soldered to the board. The solder resist may be applied by silk screening, as already described. Normally, the solder resist pattern will differ from that of the circuit board since certain portions of the circuit are to be covered while others are not.

The solder resist stratum may also serve as an insulator between successive layers of circuits which may be built up on one face of the board as suggested in Figure 10. Different circuits may be applied over a solder resist substrate by repeating the technique used to apply the first circuit directly to the board. In practice, where a multiple circuit pattern is to be built up on one side of a board in successive layers, the solder paste layer 40 on the circuit nearest the board should have a higher melting point than the solder paste layers on the outer circuits. This ensures that the lowermost solder strata will not reflow or melt when the outermost circuits are being alloyed.

In Figure 10 a board is shown with two layers of circuits built up one upon the other. Figure 9, on the other hand, shows a board with a single layer of circuit. The first circuit includes a circuit segment 26 which has been dusted with the copper powder, cured and alloyed with a stratum of solder paste 40 with the circuit then covered with a solder resist stratum 24. Printed on top of the solder resist stratum 42 is another circuit segment 26' of conductive ink to which has been applied a dusting of copper powder and on top of which is a stratum 40' of solder paste alloyed with the ink and powder. The various superimposed printed circuits may cross over one another, or be built up in whatever number of layers may be required for the particular circuit.Using similar techniques the process may be employed to repair conventional printed circuit boards or to carry out engineering change orders on such boards.

The conductive ink employed in the process is a thermosetting resin, and this is loaded with electrically conductive metal powders. A preferred composition is a two-part system comprising a phenolic based resin (such as one based on resorcinol) loaded withc copper powder to which is added a catalyst, such as anhydrous isoproponol and phosphoric acid. The anhydrous isoproponol is preferably pure, for example at least 99% pure.

Similarly the phosphoric acid is preferably of reagent grade. The anhydrous isoproponol not only acts as a catalyst but also serves as a partial thinner so as to provide viscosity control over the ink. Although, phosphoric acid is preferred, other inorganic acids may be used. The acid not only serves as a catalyst but also removes any oxides that may be present in the copper powder. Since the oxides inhibit conductivity, it is desirable to eliminate any that may be present.

A conductive ink of a preferred embodiment may be prepared by first heating a phenolic resin (resorcinol) at 1 300C. until the degree of polymerization is such that the resin has a toffylike consistency. The viscosity of the resin may be adjusted using anhydrous isoproponol. Next, the copper powder is added and preferably this should be a 325 mesh, untreated, 200 RL grade copper powder. Sufficient copper powder is added to achieve 7(#75% loading by weight. For example, for 100 grams total weight of ink, 25 grams would be resin, and 75 grams copper powder. If the material is not to be used immediately, it should be stored in a refrigerator at about 400F. to inhibit further polymerization. When the material is to be used for printing, it is brought to room temperature and then the catalyst is added.The catalyst is preferably 6% by weight of the material. A preferred catalyst mixture contains 3% phosphoric acid and 3% isoproponol.

Other materials used in the process may include anhydrous isoproponol as a thinner, as previously indicated, and also as a cleaner for the printing screens, as required. A cleaner that has been found satisfactory is a mix of isoproponol and methyl ethyl ketone in equal proportions.

The solder paste may comprise a 60/40 composition of lead and tin with a 1% ethylene glycol additive. The solder may be made in a number of different compounds depending upon the desired melting point. The solder paste should be of such a consistency that it can be screened through a stainless mesh of the sort described.

The temperatures, times and other parameters above have been as examples, and variations may be made with satisfactory results. For example, the flexibility of the printing ink may be controlled by adjusting the ratio between the epoxy and the phenols. Also, in lieu of the copper powder, other conductive metals, such as silver or aluminium may be used. Further, instead of applying the copper powder by manual dusting it could be applied by vibratory feeders over a moving belt, electrostatically, or by other suitable means.

Likewise, the solder stratum may be applied by, for example, wave solder techniques instead of by printing with a silk screen, in which case the solder would alloy immediately to the copper powder stratum without a separate heating step. A further modification may be made by using a plain resinuous ink without a metal filler, in which case all of the conductivity would be provided by the powder and solder strata.

Claims (25)

1. A method of making a circuit board, which comprises: (a) applying a thermosetting resin to that part of a surface which is to be conductive; (b) applying electrically conductive metal particles to the part; (c) curing the resin; and (d) applying and alloying an electrically conductive solder to the metal particles.

2. A method according to Claim 1, in which the resin is an ink.

3. A method according to Claim 1 or Claim 2, in which the resin, the metal particles and the solder are applied as strata.

4. A method according to any one of Claims 1, 2 and 3, in which the surface is a dielectric surface.

5. A method according to any one of the preceding Claims, in which the metal particles are pressed into the resin prior to curing.

6. A method according to Claim 5, in which the particles are pressed into the resin by progressively increasing pressure.

7. A method according to any one of the preceding Claims, in which the resin and solder are applied by printing.

8. A method according to any one of the preceding Claims, in which the resin is electrically conductive.

9. A method according to any one of the preceding Claims, in which the solder is applied by wave soldering.

10. A method according to any one of the preceding Claims, which additionally comprises: (e) applying a dielectric stratum selectively over the solder,

11. A method of making a circuit board having more than one layers of circuits, which comprises applying a method according to any one of the preceding Claims to a board produced by a method according to Claim 10.

12. A method according to Claim 11, in which the melting point of the solder of any layer is higher than that of the preceding layer.

13. An electrically conductive ink suitable for use in printing an electrically conductive circuit on a dielectric surface which comprises: (a) a thermosetting resin; (b) conductive metal particles dispersed in the resin; and (c) a catalyst; the resin, particles and catalyst being present in the ink by weight percent respectively of approximately 23%, 71% and 6%.

14. An electrically conductive ink according to Claim 13, in which the resin is a phenolic resin.

15. An electrically conductive ink according to Claim 14, in which said resin is resorcinol.

16. An electrically conductive ink according to any one of Claims 13, 14 and 15 in which the particles are copper having an average particle size of approximately 325 mesh.

17. An electrically conductive ink according to any one of Claims 13-1 6, in which the catalyst comprises substantially equal parts by weight of anhydrous isopronol and an organic acid.

18. An electrically conductive ink according to Claim 17, in which the acid is phosphoric acid.

19. Apparatus suitable for use in making a printed circuit board, which comprises: (a) a frame; (b) a plurality of pairs of rollers mounted to the frame: and (c) means for driving at least one roller.

20. Ä printed circuit, coriiprising (a) a support providing a dielectric surface; and (b) at least one circuit segment bonded to the surface; said segment including a stratum of resinous ink, a stratum of electrically conductive metal powder bonded to the surface of the ink, and a stratum of electrically conductive solder alloyed to the stratum of metal powder.

21. A circuit according to Claim 20, in which the ink is a metal-loaded ink.

22. A printed circuit, according to Claim 20 or Claim 21, which additionally comprises a stratum of electrically insulating material bonded over at least a portion of said surface and said segment.

23. A method of making a circuit board, substantially as herein described with reference to the accompanying drawings.

24. A circuit board when produced by a method according to any one of Claims 1-12 and 23.

25. A method of making an electrically conductive circuit on a dielectric surface, comprising the steps of (a) applying a stratum of thermosetting resinous ink on said surface in a predetermined circuit pattern, (b) applying a stratum of electrically conductive metal particles over said ink stratum, (c) curing said ink statum, and (d) applying and alloying a stratum of electrically conductive solder over said metal particle stratum