GB2186433A - Producing multilayer circuits on a base board - Google Patents

Abstract

A method for producing multilayer circuits on one side of a copper laminated base board (3). The multilayer circuits which include a resistor circuit (13), are formed by; etching the copper lamination (8) to form a circuit pattern (C1); coating the said one side of the base board with a plating resistant resist (6) except the portions to be electrically connected to other circuits to be formed on the first circuits; coating an electrically conductive copper paste (9) on the said one side of the base board such that the first circuits may be divided into at least two portions to be electrically isolated from each other; heating the base board to harden the so processed base board; immersing the base board in a metal plating solution to provide a metal plating layer (10) on the face of the copper paste (9) so as to form a second circuit pattern (C2) of at least two electrically isolated portions of a second lamination; coating an electrically conductive paste (19) on a part of each electrically isolated portion of the second circuits; heating the base board to harden the paste (19) to form a pair of electric terminals (20) coating an electrically resistant paste (14) of a predetermined resistance value on a part extending between the two electric terminals; and heating the base board to harden the electrically conductive paste to form the resistor circuit (13). <IMAGE>

Description

SPECIFICATION A method for producing electric circuits on a base board The invention relates to a methodforforming elec tricallyconductive circuits, more particularly resistor circuits, on a base board.

It has been general practice to form a resistor circuit on a copper laminated print base board. In this case, a resistor with a lead ora chip-shaped resistor is soldered to a copper lamination circuit. The finished product is therefore bulky in addition to requiring so many processing steps and the resultant high cost including the cost ofthe resistor. Further according to such a conventional method, the loading density of the print base board is lower, and the reduction of praductweight and of production process is difficult. Moreover since the soldering operation is required,there have often been a misarrangement of leads and a misinsertion of resistors.

Further in the case of forming considerably complex circuits on the copper laminated print base board, it becomes necessary to electrically connect the circuits to each other. However, according to the prior art, since it was impossible to form the circuits of more than two layers on one side of the print base board, such circuits were divided and etched on both sides of the base board and electrically connected to each othervia holes through the base board.

Such prior art requiresto attach the copper lami nations on both sides of the base board, to etchthe copper laminations along the designed circuits and to make holes extending through the base board by means of a specific device such as the NC device, and therefore the production cost is increased including the costs of materials and of processing steps, and moreover the production efficiency is low.

In order to improve the conventional method so as to form the electrically conductive circuits or more than two layers on one side of the base board more effectively at a lower cost, it has been required to use an electrically conductive copper paste which is excellent in the electric conductivity and adapted to a metal plating, especiallyto a copper plating and which is available at a lower cost. Generallythecon- ventional electrically conductive copper paste is easily oxidized with a heat for hardening the paste, in contrast to the previous metal such as silver. The oxidization of copper powder in the paste will increase the electrical resistance and decrease the soldering property. These defects have made the conventional electrically conductive paste practically useless.

Further it has been required to activate the surface of the hardened electrically conductive copper paste by means of a catalyst so as to expose the copper pow derfromtheresin paste so that the exposed copper powder may act as the binder, that is, so many nuclei forthe subsequent metal plating. Thus the conventional electrically conductive paste has requires many processing steps.

Japanese Utility Model application serial No. 5542460 discloses a specific method, in which a high dielectric resist of polybutadiene is used as the die lectric coat, an adhesive paste, for example, of 20% of phenol resin, 63% of copper powder and 17% of solvent is used to form designed circuits, the adhes ive paste is thickened to 20 A by means of a non- electrolytic plating, and then the plated adhesive paste is coated with copper so as to form the electrically conductive circuits of more than two layers on one side of the base board. This method has never been industrially reduced to practice.

After many years of research, the applicants are now able to produce for industrial use, electrically conductive copper pastes that avoid the defects associated with the prior art as mentioned above.

The newly developed electrically conductive copper pastes includes the electrically conductive copper paste ACP-020, ACP-030 and ACP-007 P P of Asahi Chemical Research Laboratory Co. Ltd. The electrically conductive copper paste ACP-020 is substantially composed of 80% by weight of copper powder and 20% by weight of synthetic resin, and is extremely excellent in the electric conductivity, but more or less deteriorated in the soldering property.

The electrically conductive copper paste ACP-030 is substantially composed of 85% by weight of copper powder and 15% by weight of synthetic resin, and is slightly lowerthan the ACP-020 as to the electric conductivity, but excellent in the soldering property. In the last place, the electrically conductive copper paste ACP-007P is an improvementoftheACP-030 and may be subjected to a metal plating such as a copper chemical plating without using a catalyst. In otherwords, the copper paste has excellent metal plating properties.

An object of the invention is to eliminate the defects and disadvantages of the prior art. This is achieved by the effective use of the newly developed electrically conductive copper paste having the specially excellent metal plating property to form elelectrically conductive circuits of more than layers on one side of a copper laminated base board, that is, to firstly form a first layer circuit on the copper lamination of the base board, subsequentlyto coat the mentioned electrically conductive copper paste of excellent metal plating property on the parts of the first layer circuits which are to be connected to a second layer circuits to be formed on the first layer circuits, subsequently to heat the electrically conductive paste to harden the same, subsequently to apply a metal plating on the coated electrically conductive copper paste to increase the electric conductivity of the copper paste up to that of copper lamination, to thereby form the second layer circuits on the first layer circuits. In this way, a two-layer circuit board is provided having a property substantially equivalent with the conventional both sided and through hole circuit board with the materials and processing steps being reduced almost to a half of the prior art. In fact, the expensive NC drilling machine is not required and the processing method is remarkably simplified compared with the prior art, and accordinglythefinished product may be provided at a cost almost half of the cost required to produce the conventional product.

We propose to form a resistor circuit on the second layer circuits that is, to coat an electrically conductive paste of the excellent electric conductivity property on the parts of the second layer circu its, which are not electrically connected to each other, subsequentlyto heat the electrically conductive paste to harden the same to thereby form a pair of terminals, subsequently to coat the terminals with a resistor paste of a predetermined resistance value, and then to heatthe resistor paste to harden the sametoform a resistor circuit on the second layer circuits. In this way, the conventional operation for securing the resistortothe base board is eliminated, and an ex tremely flat resistor circuit is provided.Moreover, the leading density ofthe base board is increased in addition to the reliability of the finished product which may be of a reduced weight all through the reduced processing steps. Further the finished pro ductmay be provided at a reduced costwithout misarrangement of leads and misinsertion of the resistorwhich may often happen in the conventional method.

In short,the invention comprises the steps of: attaching a copper lamination to one side of a base board; etching the copper lamination to form thereon a plurality of first electrically conductive circuits of a first lamination coating said one side ofthe base board with a plating-resistant resist except the portions which are required to be electrically connected to other circuits to be formed on the first electrically conductive circuits of the first lamination coating the first electrically conductive circuit of the first lamination layerwith an electrically conductive copper paste of being adapted to a metal plating in a manner that the first electrically conductive circuit ofthefirst lamination may be divided into at leasttwo portions to be electrically isolated from each other; heating the base board to harden the base board; cleansing the base board; immersing the base board in a metal plating solution to provide a metal plating layer on the fact of the electrically conductive copper paste so as to form second electrically conductive circuits of a second lamination which are composed ofthemetal plating layer and the electrically conductive copper paste; coating an electrically conductive paste on a part of each of the electrically isolate portions ofthe second electrically conductive circuits of the second lamination heating the base board to harden the electrically conductive paste to form a pair of electric terminals; coating an electrically resistant paste of a predetermined resistance value on a partextending between the two electricterminals; and heating the base board to harden the electrically resistant paste to form a resistor circuit between the two electrically isolated portions ofthe second electrically conduct- ive circuits of the second lamination.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figures 1 to 9 relate to a first embodimentofthe invention, in which: Figure 1 shows a copper laminated base board in vertical section; Figure 2 shows an etching resistant resist coated on the base board in Figure 1; Figure 3 shows first electrically conductive circuits formed by etching on the base board in Figure 2; 3 Figure 4 shows a plating resistant resist coated on the base board in Figure3; Figure 5shows an electrically conductive copper paste coated on the base board in Figure 4; Figure 6shows second electrically conductive circuits formed by non-electrolytic plating on the base board in Figure 5;; Figure 7shows electricterminaisformed with an electrically conductive paste on the base board in Figure 6; Figure8shows a resistorcircuitwith a resistor paste on the base board in Figure 7; and Figure 9 shows an overcoat covered all overthe face of the base board to finish the processing steps of the base board.

Now in reference to Figure 1,a base board 1, which is for example made of a polymer, has a copper lamination 8 attached to one side thereof and is formed as a copper laminated base board 3. An etching resistant resist 7 is coated on the copper lamination 8 excepttheportions3awhere nofirstelectricallyconductive circuits C1 are formed as shown in Figure 3e, and then is heated to be hardened. The etching resistant resist7 is etched to form a plurality offirst electrically conductive circuits C1 of a first lamination layer of the copper lamination 8 which is eliminated at the portions 30a as shown in Figure 3. Then as shows in Figure 4, a plating resistant resist 6, such as the resistCR-2001 developed by Asahi Chemical Research Laboratory Co.Ltd., is coated on the base board 1 except the portions of the first electrically conductive circuits C1 which are required to be elec tricallyconnectedtotheothercircuitssuch as second electrically conductive circuits C2to be formed on the first electrically conductive circuits (shown in Figure 6). Then the base board 3 is heated attemperature of about 1 50"C for about 30 minutes to harden the plating resistant resist 6.

Subsequently an electrically conductive copper paste 9, such as the paste ACP-007P developed by Asahi Chemical Research Laboratory Co. Ltd., which is specifically adapted to a plating is coated by way of screen printing on the first electrically conductive cir cuts C1 in a manner thatthe circuits wil I be divided into at least two portions to be electrically isolated from each other as shown in Figure 5, and then is heated at a temperature of about 1 500C fo r 30 to 60 minutes so as to be hardened.

Subsequently the copper laminated base board 3 is subjected to a treatment in preparation to the next plating process. Namely the base board 3 is cleansed with a water solution including 4-5% by weight of caustic soda (NaOH) for about several minutes, and then the face treatment is made with a water solution including 5-10% byweightof hydrochloric acid (HCI) for above several minutes. As the result, the copper powder particles are exposed from the binder ofthe electrically conductive copper paste 9,thus providing the neucleusesforthe next copper plating treatment. In this case, it is noted that a catalyst is not needed which may be required in the case of nonelectrolytic plating.

Subsequently the copper laminated base board 3 is immersed in a copper chemical plating both to applythe copper chemical plating to the face ofthe electrically conductive copper paste thereby form thereon a copper plating layer 10 as shown in Figure 6. As the result, there are formed second elec trically conductive circuits C2 of a second lamination layer, which are electrically connected to the first el ectricallyconductive circuits C1 of the first lamination layer.The copperchemical plating bath is of pH 11 - 13 and of the temperature 65-75 C, and the thickness of the copper plating layer is more than 5 Fm, and further the plating accumulation speed is about 1.5-3 pm #m per hour.

Thus the second electrically conductive circuits C2 are formed on one side ofthe copper laminated base board 3 bythe electrically conductive copper paste 9 and the copper plating layer 10. Subsequently as shown in Figure 7, an electrically conductive paste 19 such as a silver paste is coated on the electrically isolated portions of the second electrically conductive circuits to form thereon a pair of electric terminals 20, and then is heated to be hardened. Subsequently as shown in Figure 8, an electrically resistant paste 14 of a predetermined resistanmce value is coated on the portion extending between the two terminals 20 of the second electrically conductive circuits C2 and is heated to be hardened.Thus a resistor circuit 13 is formed between the electrically isolated portions of the second electrically conductive circuits C2.

Finally, as shown in Figure 9, an overcoat 11 such as a plating resistant CR-2001 developed by Asahi Chemical Research Laboratory Co. Ltd., is coated all over the so processed side of the copper laminated base board 3, and then is heated to be hardened.

Thus a print circuit base board 12 is finished up.

According to the invention, the metal plating applied to the electrically conductive copper paste 9 may be a precious metal plating such as a silver or gold plating instead of the mentioned copper plating.

Further the first and second electrically conductive circuits C1, C2 may be formed on the overcoat 11 which is coated on one side of the base plate 1 instead ofthe copper lamination 8. In the mentioned way, the circuits of more than 3 layers may be formed on one side of the base board in accordance with the present invention.

Furtheritwould benecessaryto briefly explain the electrically conductive copper paste, an electrically resistant paste and the plating resistant paste which are used in this invention: As to the pasteACP-007P developed byAsahi Chemical Research Laboratory Co. Ltd. bywayofexample for an electrically conductive copper paste which is specifically adapted to a copper plating, it is generally known that copper is easily oxidized, and more especially copper in the condition of powder particles may be more easily oxidized because the exposed outer surface is enlarged. In contrast to the non-oxidizable paste of precious metals, it becomes necessary to provide a paste of such ingredients as to remove the oxidized film of the copper powder particles and also to prevent the reoxidization of the copper particles.In order to provide an electrically conductive copper paste which may be easily used and easily secured to a base material, it is important to properly select and properly mix the ingredients such as copper powder, binder, special addition (for example, anthracene, anthracene carboxylic acid, anthradine, anthranilic acid), dispersant and solvent.

The copper particles are different in the configuration thereof depending upon the production method thereof. In the electrolytic method, the copper particles are deposited in high purity and also in branched shapes. In the reduction method wherein the oxides are reduced by a reducing gas, the copper particles are provided in spongy and porous shapes.

The electrically conductive copper pasteto be used in connection with this invention is required to have the following properties: 1. To be easily coated by way of screen printing in formation offine patterns.

2. Fixedly secured to the base board.

3. To be resistantagainsta high temperature alkali bath of copper chemical plating.

4. Fixedly secured to the copper plating.

5. Having an invariable viscosity in the elapse of time to maintain a stabilized printability.

In order to satisfythe above mentioned require mentstheelectricallyconductive copper paste is required to contain the copper particles of high purity in the branch shapes as deposited by the electrolysis and/or the copper particles of porous spongy hapes as reduced from the metal oxides. The copper particles may be processed into flakes.

Further in order to highten the content rate ofthe copper particles in the paste, it is required to fill the copper particles of different sizes and shapes to a maximum density.

As to the binder of the electrically conductive copper paste, the binder is required to act as a vehiclefor so much copper particles and as an effective adhesive to the base board. Further the binder must resist against the alkali bath of a copper chemical plating.

It was found that the electrically conductive copper paste was best when the copper paste contained the epoxy resin which has a larger content rate of copper particles and hightensthe deposition rate of the plating, and further increases the adhesive property of the plating film.

With respecttothe property of the copper plating deposited on the electrically conductive copper paste ACP-007P, the copper plating is reddish brown and paste-like and has a viscosity of 300-500 ps atthe temperature of 25 C. The adhesive property to a copper laminated base board and to a resin base board has been confirmed by a taping test. Further the adhesive property to the electrically conductive paste has been confirmed bythetaping test. Thesoldering property is more than 96% as to the extension rate and is more than 3.0 kg as to the tensile force (3 x 3 mm2).

The components of the electrically conductive copper paste and the conductivitythereof are men tioned in detail inthesameapplicant'sJapanese Patent applications 55-6609 (laid open: 56-103260) (corresponding U.S. Patent No.4353816) and 60216041 (corresponding U.S. Patent No.4353816) and 60-216041 (corresponding to U.S. Patent application of Serial No.06/895716), and therefore the description thereof is omitted herein.

With respect to the electrically resistant paste, the paste contains a refined powder of carbon or graphite orthe like of high purity as an electrically conductive element and a heat hardened resin such as epoxy resin, phenol resin, melamine resin, acrylic resin orthe like as a binder, and further contains, as a viscosity modifier, a solvent which is evaporated slowly at a high temperature.

The components ofthe electrically resistant paste are each required to have a specific property. For example, as the functional powder, the particles must be fine and uniform and further of high purity as well as high quality. Further the particles must be of little difference in the electric resistance value and must befamiliarwith the resin to be mixed therewith.

Astothe property of polymer it is preferablethat the paste is easily dissolved with the particles and will notbefilmy ifplacedfora long time in a normal temperature. The paste further is required to be not hardened ate'normal temperature and to be quickly hardened when heated. The hardened paste must not be vary in volume and must be slightlyflexible and further easily adhesive to the base board.

Furtherthe paste must be resistant against heat and humidity, and also must be easily adhesive to the un dercoat as well as the overcoat.

Asto the property of solvent, is paste is required to be stabilized in the successive printing operations, that is, not to fill the prints and not be deteriorate the emulsion film. The paste is further required to be slow in the evaporation speed at a normal temperature and reluctantto absorb water, notto abruptive change the viscosity thereof atthe temperature +10 Candto have no poison and/orirritating smell at a normal temperature and in the vapor at the time of heating.

The electrically resistant paste such as the paste TU-1 K has been developed by Asahi Chemical Research Laboratory Co. Ltd. so as to fully satisfythe requirements as mentioned above. The electrically resistant paste maintains very stabilized resistance, that is, the resistntvariation rate is only about 0.5% at the soldering temperature at 2400C and 260 C.

Furtherthe paste will not abruptly absorb the heat and will not respond to the heat until the soldering temperature is reached as actually indicated by a heat difference analysis curve, and therefore the volume variation ofthe resistor is extremely small.

With respect to the plating resistant resist such as the resistCR-2001 developed by Asahi Chemical Research Laboratory Co. Ltd., so as to be used in the present invention,this resist is coated on a first circuit which is not electrically connected to a second circuit which is to be formed on the first circuit.

Therefore the resist is required to have an isolating property and atthe same time an alkali resistant property. Actually the resist has been developed to maintain the acidity more than 4 hours in the alkali bath of700C and of pH 12 just likethecopperchemical plating bath.

Similarto the electrically conductive copper paste ACP-007P,the resist contains as a main component an epoxy resin and is printed through an 180-mesh polyester screen and is then heated for 30 minutes at thetemperature 150 Cso as to be hardened. The printing film is preferably 15-30 FLm so as to resist chemicals and voltages. The main features are as follows: the resist is easily adhered to the base on which the resist is coated, and to a copper lamination and further is not deteriorated is immersed in the alkali bath of pH 12 for a long time. The resist is quite safe in the practical use becausethehardenertobe used is alkali having little poison.The resist is coated byway of screen printing and has a hardener 10 g mixed with the principal component 1 00g thereof, and is hardened in a set time 15-30 minutes atthe temperature 1 50-2000C.

The plating resistant resist is green in the condition of ink and has an adhesion (cross-cut) 100/100 on a copper lamination, a surface hardness of more than 8H when measured by pencil, a solvent resistant property (in trichloroethylene) by more than 15 sec., a soldering heat (260 ) resistant property of more than 5 cycles, a surface isolation resistance value of more than 5 x 1 013fl, a volume resistance value of 1 x10Q14 -cm, avoltage (15 Fm) resistant property of more than 3.5 kV and a dielectrictangent (1 MHz) of less than 0.03.

Claims (7)

1. A method for forming electric circuits on a base board comprising the steps of: a) attaching a copper lamination to one side of the base board; b) etching copper lamination to form thereon a plurality of first electrically conductive circuits of a first lamination; c) coating the said one side of the base board with a plating-resistant resist except the portions which are required to be electrically connected to other circuits to be formed on the first electrically conductive circuits ofthe first laminations; d) coating an electrically conductive copper paste of being adapted to a metal plating on the said one side of the base board such thatthe first electrically conductive circuits of the first lamination may be divided into at leasttwo portionsto be electrically isolated from each other; e) heating the base board to harden the so processed base board; f) cleaning the base board; g) immersing the base board in a metal plating solution to provide a metal plating layer on the face of the electrically conductive copper paste so as to form second electrically conductive circuits of at leasttwo electrically isolated portions of a second lamination which comprise the metal plating layer and the electrically conductive copper paste; h) coating an electrically conductive paste on a part of each of the electrically isolated portions of the second electrically conductive circuits of the second lamination; i) heating the base board to harden the electrically conductive paste to form a pair ofelectricterminals; j) coating an electrically resistant paste of a predetermined resistance value on a part extending between the two electric terminals; and k) heating the base board to harden the electrically resistant paste to form a resistor circuit between the two electrically isolated portions of the second elec trically conductive circuits of the second lamination.

2. A method according to claim 1, wherein the base board is made of polymer.

3. A method according to claim 1 or claim 2, wherein metal plating is a chemical copper plating.

4. A method according to any one of claims 1 to 3, wherein one electrically isolated portion of the first electrically conductive circuits includes at least two circuits electrically connected to each other, and another of the electrically isolated portions of the first electrically conductive circuits includes at least one circuit.

5. A method according to claim 1 ,wherein one of the electrically isolated portions of the second electrically conductive circuits includes at leasttwo circuits electrically connected to each other, and the other of the two electrically isolated portions of the second electrically conductive circuits includes at least one circuit

6. A method of forming electric circuits on a base board, substantially as herein described with refer ence to the accompanying drawings.

7. Electrical circuits formed on a base board, substantially as herein described and shown in the accompanying drawings.