SENSITIZING ACTIVATOR FOR CHEMICAL PLATING

Abstract: Disclosed is a sensitizing activator for chemical plating operations, that uniformly and efficiently deposits a substance capable of catalytically depositing a plating metal from a plating solution, such as colloidal palladium, on the surface of a material to be plated. The activator includes stannous chloride and palladium chloride colloidally dispersed in a solution of sodium chloride and a dilute concentration of an acid which is preferably sulfuric acid. A preferred formulation includes a small amount of vanillin. The activator provides exemplary permeation of and adhesion to a substrate to be plated, and advantageously avoids corrosion of the black oxide that is formed on the inner copper foil of copper-clad laminates.

Description: SENSITIZING ACTIVATOR FOR CHEMICAL PLATING PACKCPOVND The present invention relates to a sensitizing activator for chemical plating. Especially, the present invention relates to a sensitizing activator for use in chemical plating of through holes of a multi-layer printed circuit board. More particularly, the present invention relates to an improved sensitizing activator having an activity of uniformly and efficiently depositing a substance capable of catalytically depositing a plating metal from a plating solution, such as colloidal palladium, on the surface of a material to be plated in advance. It is known that in conventional plating the operation of depositing a catalytic substance such as colloidal palladium on the surface of a resin should be performed before chemical plating. In order to reduce the manufacturing cost of a chemically plated product, how to deposit this colloidal palladium uniformly and efficiently is one of the important problems. The inventor previously found that by using an activating treating agent formed by adding vanillin and an acidic dye to a hydrochloric acid solution of palladium chloride, the amount of palladium used can be reduced and a product excellent in the plating quality over a conventional product can be obtained (see, for example, Japanese Patent Application No. 136473/1974) . The inventor furthered his research and found that when a hydrochloric acid solution of palladium chloride and tin chloride is used instead of the hydrochloric acid solution containing palladium chloride alone as taught in the invention of Japanese Patent Application No. 136473/1974, colloidal palladium can be efficiently deposited and a product improved in the plating quality can be obtained by adding vanillin alone without using any acidic dye, as disclosed in Japanese Patent Publication No. 34220/1980. The sensitizing activator proposed in the Japanese Patent Publication No. 34220/1980 (hereinafter referred to as "conventional technique* 1 ) is effective in the sensitizing step in chemical plating of excellent quality plated-through holes of a single layer printed circuit board. However, when this sensitizing activator is used for plating of through holes of a multi-layer printed circuit board, hydrochloric acid contained in the sensitizing activator causes corrosion of the black oxide that is normally formed on an inner copper foil of a copper-clad resin laminate board in the inner wall portions of the through holes of the copper-clad board, and the sensitizing activator permeates between the copper-clad resin laminate layers and is left therein and, since hydrochloric acid contained in the sensitizing activator has an electrical conductivity, the insulation between adjacent through holes becomes insufficient or the bonding between the copper-clad resin laminate layers is weakened to cause delamination. Accordingly, in the case where the conventional sensitizing activator is used in the step of plating through holes of a multi-layer printed circuit board, the conditions such as dipping time, the concentration and the temperature are restricted and the process control is difficult. Namely, it is difficult to shorten the dipping time or reduce the hydrochloric acid concentration so that the black oxide of the inner copper foil of the resin laminate board is hardly corroded, and the sensitizing treatment is insufficient. Therefore there is a need for a sensitizing activator having an excellent performance, in which the above-mentioned defects are overcome. SUMMARY The present invention meets this need by providing a sensitizing activator obtained by colloidally dispersing stannous chloride and palladium chloride in an aqueous solution containing a predetermined amount of sodium chloride, and adding a dilute acid for preventing oxidation. The acid can be hydrochloric acid, or a polyvalent acid. In particular, it has been discovered that the defects of the conventional sensitizing activator object can be overcome by using a polyvalent acid, which does not corrode the black oxide formed on the inner copper foil of the copper-clad resin laminate board, instead of conventional hydrochloric acid which corrodes this black oxide. Preferably the acid is sulfuric acid, which not only provides exceptionally satisfactory results, but also is most inexpensive and readily available. Preferably the activator includes a small amount of an aromatic aldehyde for promoting uniform dispersion of the dissolved particles, and for enhancing adhesive properties of the activator. The aldehyde can be vanillin. Preferably there is a concentration of 0.015 to 0.03 grams per liter of vanillin, 5 to 10 grams per liter of stannous chloride, 0.1 to 0.2 grams per liter of palladium chloride, 0.05 to 10 volume/volume percent of sulfuric acid, and 50 to 280 grams per liter of sodium chloride. In one preferred formulation, the sodium chloride concentration is about 280 grams per liter, the sulfuric acid concentration being less than about 0.1 percent. In another preferred formulation, the sodium chloride concentration is about 50 grams per liter and the sulfuric acid concentration is about 5 percent. In another aspect of the invention, the activator has the stannous chloride in a weight per unit volume concentration of about 25 to about 100 times a corresponding concentration of palladium chloride. Thus the activator can be supplied in concentrated form for later dilution by an aqueous sodium chloride and sulfuric acid solution, the sodium chloride and sulfuric acid being commonly available from sources local to an end use of the activator. More preferably, the concentration of the stannous chloride is approximately 50 times the concentration of palladium chloride, for facilitating the removal of tin in the plating process, and for limiting particle size in the activator. In particular, the concentrated activator can be diluted to about 1 percent in volume in the sodium chloride sulfuric acid solution. The stannous chloride can be in a concentration of about 500 grams per liter, and the palladium chloride can be in a concentration of from about 10 to about 20 grams per liter. The activator can contain vanillin in a concentration of from about 0.075 to about 0.3 per unit of palladium chloride, preferably about 0.15. The present invention is useful in the sensitizing treatment for facilitating general chemical plating, providing complete permeation and adsorption in fine hole portions of a material to be plated, for excellent uniformity of the plating. More particularly, the invention finds application in the plating of through-holes of a multi-layer printed circuit board. In a further aspect of the invention, there is provided a method for plating a substrate, comprising the steps of: (a) dipping the substrate into a sensitizing activator comprising: (i) stannous chloride; (ii) palladium chloride; (iii) sodium chloride in an aqueous solution, wherein the weight concentration of stannous chloride is from about 25 to about 100 times the weight concentration of palladium chloride; and (iv) sulfuric acid in a volumetric concentration of from about 0.05 percent to about 5 percent; (b) rinsing the substrate; and (c) dipping the substrate into a chemical plating bath comprising the metal to be plated. The method can include the subsequent further step of etching oxide from copper cladding of the substrate, by dipping the board in a suitable etchant. The etchant can be a solution of sodium persulfate. Also, the method can include the further step of dipping the substrate in an approximate 10 percent concentration of sulfuric acid, for accelerating the plating. In yet a further aspect, the invention provides a method for producing a sensitizing activator for chemical plating, comprising the steps of preparing an aqueous solution of sodium chloride, and dissolving stannous chloride and palladium chloride into the solution of sodium chloride. The step of dissolving the stannous chloride and the palladium chloride in the sodium chloride solution preferably comprises the further steps of: (a) dissolving the stannous chloride in a first portion of the sodium chloride solution; (b) dissolving the palladium chloride in a second portion of the sodium chloride solution; and (c) adding the dissolved palladium chloride to the dissolved sodium chloride. DESCRIPTION In accordance with the present invention, there is provided a sensitizing activator for chemical plating, which comprises stannous chloride and palladium chloride colloidally dispersed in an aqueous solution of sodium chloride, which is used in the presence of a polyvalent acid such as sulfuric acid. The polyvalent acid keeps the pH of the activator slightly acidic, acting to prevent oxidation of the other components of the activator. It is to be understood that other sources of halides than chlorine could be used in the practice of the present invention. Thus an activator that is equivalent in at least some respects to that described herein could be based on such other sources of halides as bromine, fluorine, and iodine. Salts of chlorine are used herein because they are inexpensive and readily available, and they provide a plating activator that is exceptionally effective as compared with conventional plating activators. Accordingly, in 1 liter of the sensitizing activator for chemical plating, there are contained 0.015 to 0.03 grams/liter of vanillin, 5 to 10 grams/liter of stannous chloride, 0.1 to 0.2 grams/liter of palladium chloride, 0.5 to 5 volume/volume percent of sulfuric acid and 50 to 280 grams/liter of sodium chloride. The activator can be used, for example, in a saturated aqueous solution of sodium chloride (approximately 280 grams per liter) containing a small amount of sulfuric acid, from about 0.05 to 0.1 volume/volume percent of the sulfuric acid. For this example, 10 to 20 milliliters of a concentrate is added to 1 liter of the dilute sulfuric acid solution, the concentrate containing 0.015 to 0.03 grams of vanillin, 5 to 10 grams of stannous chloride and 0.1 to 0.2 grams of palladium chloride, in a saturated aqueous solution of sodium chloride. The same concentrate can also be used in an aqueous solution containing about 50 grams per liter of the sodium chloride and from 3 to 10 volume/volume percent of sulfuric acid, the concentrate being added to 1 liter of the less dilute sulfuric acid solution. For this example, it is preferred that the sulfuric acid concentration be approximately 5 volume/volume percent, for a high level of activation and minimal effect on the substrate. Hydrochloric acid, which is conventionally used in plating activators, is not used in the formulation of the present invention. The presence of chlorine ions, together with hydrogen ions that are introduced by the addition of sulfuric acid, suggests the presence of hydrochloric acid. However, the concentration of hydrogen ions is quite low in preferred formulations of the present invention, such that hydrochloric acid would be present in insignificant amounts, if at all. Also, hydrochloric acid, as such, cannot practically be recovered from the activator of the present invention. Accordingly, it is believed that the disadvantages associated with the incorporation of hydrochloric acid are not present in the activator described herein. The stannous chloride and the palladium chloride are preferably separately dissolved in an aqueous solution having a high concentration of sodium chloride or other source of halides in the preparation of the activator. The sodium chloride, concentrated near saturation, acts to disperse the particles of the other salts. More preferably, the aqueous solution into which the stannous chloride and the palladium are dissolved also includes vanillin, for enhancing the dispersion of the dissolved components, and also for enhanced adhesion of the activator to the substrate. It is believed that other acids such as hydrochloric acid and other polyvalent acids such as phosphoric acid could be used in place of all or part of the sulfuric acid. However, sulfuric acid is very inexpensive and readily available. Also, since the acid is being used primarily as an anti-oxidizing agent, it is believed that sulfuric acid provides results that are at least as satisfactory as any other cost-effective alternative. The process for plating through holes of a multi-layer printed circuit board is generally conducted in the following order when using the activator of the present invention: a sensitizing treatment step, a soft etching step; an accelerating step; and a chemical plating step. In the sensitizing treatment step, a substrate, which can be a copper-clad resin laminate board having through holes formed therein, is dipped in the sensitizing activator of the present invention and colloidal palladium in the sensitizing activator is deposited on the surface of the substrate, inclusive of the inner walls of the through holes, if any. In the soft etching step, the sensitized copper-clad resin laminate board is dipped in a suitably selected conventional etchant solution to finely coarsen the copper surface and remove the oxide film. The etchant can be a solution of sodium persulfate. In each through hole, the sensitizing activator is left on the resin surface, and the sensitizing activator on the copper surface is removed because copper is slightly dissolved. In the accelerating step, the sensitizing activator deposited on the resin portion in the through holes of the copper-clad resin laminate board is activated to promote reaction of a chemical copper plating solution. The copper-clad resin laminate board__which has passed through the soft etching step is dipped in an aqueous solution containing 10 volume/volume percent of sulfuric acid. The sensitizing activator of the present invention is preferably used for plating of through holes of a multi-layer printed circuit board, but the use of the sensitizing activator is not limited to this plating and the sensitizing activator can be advantageously used in other aspects of circuit board plating, and for other chemical plating processes. The present invention provides a number of advantages in chemical plating processes, for example: (1) Since the sensitizing activator for chemical plating according to the present invention does not contain hydrochloric acid as an ingredient, the sensitizing activator has no adverse effect of corroding black copper oxide formed on an inner copper foil of a multi-layer substrate, and therefore, such conditions as the dipping time, the temperature and the concentration are not restricted and a sufficient sensitizing treatment can be attained. (2) Since the sensitizing activator for chemical plating according to the present invention enables a sufficient sensitizing treatment and can deposit colloidal palladium uniformly on the surface of a material to be plated, even with only a single application of the sensitizing treatment, there can be obtained such a covering capacity that copper can be completely deposited by chemical plating. This is true even if soft etching, accelerating and chemical plating are repeated several times. Therefore, a complete treatment can be accomplished without occurrence of defects that are readily caused in the step of chemical plating of a multi-layer printed circuit board in the prior art, such as formation of voids (blank portions where chemical plating failed) because of a residual oxidizer or the like that was left after the treatment for removal of smear, commonly known as drill smear. The present invention is advantageous over the conventional technique in the above-mentioned points in plating of through holes of a multi-layer printed circuit board. Also, in the sensitizing activator of the present invention, colloidal particles are more finely dispersed than in the conventional technique, and therefore, when the sensitizing activator of the present invention is used at the sensitizing treatment widely in general chemical plating, the sensitizing activator completely permeates and is absorbed in fine hole portions of the material to be plated. Thus excellent penetration and absorption as well as uniform chemical plating can be attained. An exemplary formulation of the sensitizing activator of the present invention was prepared for use in chemical plating of through holes of a printed circuit board as follows: In 1 liter of an aqueous solution of sodium chloride having a saturation concentration and containing 1.5 grams of vanillin was dissolved under heating 500 grams of stannous chloride and 10 grams of powdery palladium chloride to form a starting solution. Then, about 10 ml of the starting solution was added to 1 liter of an aqueous solution containing 50 grams of sodium chloride and having a concentration of 5 to 10 volume/volume percent of sulfuric acid. In a test of this formulation, a copper-clad glass-epoxy resin laminate board having through holes formed therein, which had been subjected to a customary degreasing treatment, was dipped in this sensitizing activator for 2 minutes to effect a sensitizing treatment. Then, the laminate board was passed through the washing step, the soft etching step, and the accelerating step and was then subjected to chemical plating of copper. A chemical plating layer free of voids was formed in a state closely adhering to the copper foil without causing corrosion of the black oxide of the inner copper foil of the copper-clad resin laminate board, and a chemical copper plating layer excellent in the reliability of conduction of electricity was uniformly formed in the entire plated through holes. In a convenient process for producing the activator, 1 Kg of sodium chloride is added to 3 liters of water in a suitable container, the combination being heated to approximately 60*C to form a supersaturated solution. Next, two liters of saturated salt solution are poured off into another container, in which is operated a magnetic stirrer, and 3 grams of lognin vanillin (in crystalline form) are added, the temperature being maintained at approximately 60*C during stirring for from about 30 to about 40 minutes, or until the vanillin is completely dissolved. The resulting vanillin salt solution is then cooled, and can be stored indefinitely. Next, 1 Kg of stannous chloride crystals are put into a 3000 ml beaker, the beaker being filled with part of the vanillin salt solution to a total volume of 1.9 liters. A slight amount of heat is applied for maintaining the temperature at from about 20'C to about 30*C, stirring occasionally until the crystals are dissolved. Also, 200 ml of the vanillin salt solution is put into a separate glass container, and 20 g of powdered palladium is slowly added while stirring with a magnetic stirrer, at from about 40*C to about 60*C. The addition of the palladium can normally be completed within about one minute. The stirring is continued for about one hour, or until the palladium is dissolved, as indicated by the absence of ■■ grinding'* sounds from the stirrer. The dissolved palladium is next added to the stannous chloride salt solution in the 3000 ml beaker, while stirring, heat being applied for raising the temperature to about 80*C, preferably from about 78*C to about 82*C. Once the temperature reaches approximately 80*C, it is maintained at that level for one hour, then the resulting concentrate is cooled. It is intended that from 5% to about 10% of the contents of the beaker be lost to evaporation. Thus approximately two liters of the concentrate are produced as described above. If necessary, the beaker can be selected to provide a more desirable form factor for increasing or decreasing the rate of evaporation. Dilution of the concentration for use can be performed as in the examples given above, without the use of special techniques. Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein.

Claims: What is claimed is: 1. A sensitizing activator for chemical plating, which comprises stannous chloride and palladium chloride, colloidally dispersed in an aqueous solution of sodium chloride, and a dilute concentration of acid selected from the group consisting of hydrochloric acid and a polyvalent acid. 2. The apparatus of claim 1 wherein the acid is a polyvalent acid. 3. The apparatus of claim 2 wherein the acid is sulfuric acid. 4. The activator of claim 1 further comprising a concentration of an aromatic aldehyde. 5. The apparatus of claim 4 wherein the aromatic aldehyde comprises vanillin. 6. The activator of claim 1 wherein the stannous chloride is in a concentration of from about 5 to about 10 grams per liter. 7. The activator of claim 1 wherein the palladium chloride is in a concentration of from about 0.1 to about 0.2 grams per liter. 8. The activator of claim 1 wherein the sodium chloride is in a concentration of from about 50 to about 280 grams per liter. 9. The activator of claim 1 wherein the acid is sulfuric acid in a volume concentration of from about 0.5 to about 10 percent. 10. The activator of claim 9 wherein the sodium chloride is in a concentration of about 280 grams per liter and the concentration of sulfuric acid is less than about 0.1 percent. 11. The activator of claim 9 wherein the sodium chloride is in a concentration of about 50 grams per liter and the concentration of sulfuric acid is about 5 percent. 12. The activator of claim 1 wherein the stannous chloride is in a concentration of from about 5 to about 10 grams per liter, the palladium chloride is in a concentration of from about 0.1 to about 0.2 grams per liter, the sodium chloride is in a concentration of from about 50 to about 280 grams per liter, and the acid is sulfuric acid in a volume concentration of from about 0.05 to about 5 percent. 13. The activator of claim 12 further comprising vanillin in a concentration of from about 0.015 to about 0.03 grams per liter. 14. A sensitizing activator for chemical plating, comprising stannous chloride and palladium chloride colloidally dispersed in an aqueous solution of sodium chloride, the stannous chloride and the palladium chloride having weight per unit volume concentrations in a ratio of from about 25 to about 100 for stannous chloride per unit of palladium chloride. 15. The activator of claim 14 in a volumetric concentration of about 1 percent in an aqueous solution of sodium chloride comprising sulfuric acid. 16. The activator of claim 14 wherein the stannous chloride is in a concentration of about 500 grams per liter. 17. The activator of claim 14 wherein the palladium chloride is in a concentration of from about 10 to about 20 grams per liter. 18. The activator of claim 14 wherein the sodium chloride is in a concentration of from about 50 to about 280 grams per liter. 19. The activator of claim 14 further comprising vanillin in a weight concentration of from about 0.075 to about 0.3 per unit of palladium chloride. 20. The activator of claim 19 wherein the concentration of vanillin is about 0.15 per unit of palladium chloride. 21. The activator of claim 20 wherein the sodium chloride is in a concentration of about 280 grams per liter and the sulfuric acid is in a volumetric concentration of between about 0.05 and about 0.1 percent. 22. The activator of claim 20 wherein the sodium chloride is in a concentration of about 50 grams per liter and the sulfuric acid is in a volumetric concentration of about 5 percent. 23. A sensitizing activator for chemical plating, which comprises stannous chloride in a concentration of about 5 grams per liter and palladium chloride in a concentration of about 0.1 grams per liter, colloidally dispersed in an aqueous solution of sodium chloride in a concentration of about 50 to about 280 grams per liter and vanillin in a concentration of about 0.015 grams per liter, and sulfuric acid having a volume concentration of from about 0.05 to about 5 percent. 24. A method for plating a substrate, comprising the steps of: (a) dipping the substrate into a sensitizing activator comprising: (i) stannous chloride; (ii) palladium chloride; (iϋ) sodium chloride in an aqueous solution, wherein the weight concentration of stannous chloride is from about 25 to about 100 times the weight concentration of palladium chloride; and (iv) sulfuric acid in a volumetric concentration of from about 0.05 percent to about 5 percent; (b) rinsing the substrate; and (c) dipping the substrate into a chemical plating bath comprising the metal to be plated. 25. The method of claim 24 wherein the weight concentration of stannous chloride is approximately 50 times the weight concentration of palladium chloride. 26. The method of claim 24 wherein the board comprises a substrate having copper cladding, the method further comprising, following the step of dipping the board into the sensitizing activator, the further step of dipping the substrate in an etching solution for etching an oxide film from the cladding. 27. The method of claim 26 wherein the etching solution comprises an aqueous solution of sodium persulfate in a volumetric concentration of approximately 10 percent 28. The method of claim 26 further comprising, following the step of dipping the board into the sensitizing activator, the further step of dipping the substrate in an aqueous solution of sulfuric acid in a volumetric concentration of approximately 10 percent for accelerating the plating. 29. A method for producing a sensitizing activator for chemical plating, comprising the steps of: (a) preparing an aqueous solution of sodium chloride; and (b) dissolving stannous chloride and palladium chloride into the solution of sodium chloride. 30. The method of claim 29 wherein the step of dissolving the stannous chloride and the palladium chloride in the sodium chloride solution comprises the further steps of: (a) dissolving the stannous chloride in a first portion of the sodium chloride solution; (b) dissolving the palladium chloride in a second portion of the sodium chloride solution; and (c) adding the dissolved palladium chloride to the dissolved sodium chloride. 31. The method of claim 30 comprising the further step of heating the combination of the dissolved stannous chloride and the palladium chloride to a temperature of approximately 80 β C for a duration of approximately one hour. 32. The method of claim 29 wherein the solution of sodium chloride is a saturated solution. 33. The method of claim 29 further comprising the step of adding a concentration of vanillin to the solution of sodium chloride.

Inventors: KIYOSHI OKABAYASHI, KIYOSHI, OKABAYASHI
Applicants: SOLUTION TECH SYSTEMS, SOLUTION TECHNOLOGY SYSTEMS
Application year: 1989
Main classification: C23C18
IPC codes: C23C18, H05K3