Catalyst solution for electroless metal deposition on a substrate

[57] ABSTRACT

The invention disclosed herein is a catalyst for activating a substrate prior to electroless metal plating and to a process for making the same. The catalyst comprises the product resulting from the admixture of an acid soluble salt of a catalytic metal, a stannous salt, an acid and an extraneous source of halide ions. The extraneous source of halide ions provides an excess of halide ions in the catalyst formulation over that found in prior art formulations. The catalyst differs from prior art catalysts in the excess of halide ions and is an improvement as it may be used at a higher pH to catalyze substrates normally attacked by strong acids, is more stable and is adsorbed onto substrates to a greater extent than prior art catalysts.

Claims, 2 Drawing Figures

<img file="US3904792A_D0001.tif" />

PATENTED SEP 91975

3,904,792

STANNOUS CONCENTRATION (moles) TOTAL CHLORI DE CONCENTRATION (moles)

<img file="US3904792A_D0002.tif" />

FIG. I

<img file="US3904792A_D0003.tif" />

FIG.2

3,904,792

CATALYST SOLUTION FOR ELECTROLESS METAL DEPOSITION ON A SUBSTRATE

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. Pat Application Ser. No. 224,742 filed in the name of Michael Gulla and William A. Conlan on Feb. 9, 1972, now abandoned.

BACKGROUND OF THE INVENTION

1. Introduction

This invention is directed to a formulation for catalyzing a substrate prior to electroless metal deposition.

2. Description of the Prior Art

For electroless plating of substrates, especially for the plating of non-conductive substrates, it has been known for some time that chemically plated metal deposits of suitable thickness and adequate bond strength are commercially practical only if the substrate surface is properly catalyzed prior to metal deposition.

A common method for catalyzing a substrate prior to plating involves contact of the substrate with two solutions known in the art as a two-step catalyst. A process for metallizing utilizing this catalyst comprises contact of a substrate with a first aqueous solution of a reducing agent such as stannous chloride followed by contact with a second solution of a catalytic metal salt such as palladium chloride in hydrochloric acid. The reducing agent reduces the catalytic metal salt in situ on the substrate surface to the catalytic metal thereby providing a catalytic surface receptive to electroless metal deposition thereon. This procedure is employed successfully in many plating-on-plastic applications. However, it is subject to various disadvantages including poor adhesion between the substrate surface and a subsequently applied metal deposit. This is especially true where copper is to be deposited over copper such as in the manufactue of printed circuit boards where copper is deposited over both a plastic substrate and a copper cladding over said plastic substrate. Also, articles in the process of being plated using the aforesaid two-step catalyst must be re-racked subsequent to catalysis before proceeding to additional steps in the plating sequence to avoid contamination of the catalyst through drag-in from preceding steps and rapid deterioration of the plating bath. Metal plate obtained using the twostep catalyst exhibits stardusting—i.e. minor imperfections on the surface of the metal plate.

An alternative method for catalyzing a substrate prior to electroless deposition is also known and is disclosed and claimed in U.S. Pat. No. 3,01 1,920 incorporated herein by reference. In this method, a substrate is contacted with a colloidal catalytic solution formed by the admixture in acid solution of a catalytic metal salt, a stannous salt in molar excess of the catalytic metal salt and a hydrohalide acid. The catalytic metal may be selected from the group of silver, gold and the platinum family of metals. Palladium is the preferred catalytic metal. The excess stannous salt is believed to be responsible for stability of the colloid and prevents it from falling out of the formulation. The catalyst operates as a pH below about 1 and preferably well below 0. The limitation on the pH is due to the fact that the stannous salt hydrolyzes and precipitates at a pH of about 0.9. ..

Though this colloidal catalyst has been widely accepted and preferred for most applications, it is not without some difficulties. One such difficulty is that the highly acidic formulation attacks various substrate materials, especially plastic materials including the plastic racks used to carry the substrate through the plating sequence. Another difficulty is the volatilization of the hydrohalide acid which is undesirable from both a health standpoint and a quality control standpoint. Both of these problems could be overcome if the catalyst formulation could be prepared at a higher pH.

In U.S. Pat. No. 3,672,938, there is disclosed a process catalyzing a substrate prior to electroless metal deposition with a catalyst also formulated by the admixture in acid solution of a catalytic metal salt, a stannous salt in molar excess of the catalytic metal salt and a hydrohalide acid. This catalyst is said to differ from the catalyst of U.S. Pat. No. 3,011,920 in physical form, it being asserted that the catalyst of said patent is a “true solution catalyst” rather than a colloidal catalyst as in the aforesaid U.S. Pat. No. 3,011,920. Regardless of its physical form, it is also highly acidic and suffers the same disadvantages as the catalysts of said U.S. Pat. No. 3,01 1,920.

Attempts have been made in the prior art to formulate a low acid, higher pH catalyst. Such attempts have been unsuccessful because the low acid catalyst has been formulated by the expedient of reducing the hydrohalide acid content. Such a reduction results in the formation of a precipitate at a pH of about 0.9 for a chloride system. This formation of precipitate is believed to be due to hydrolysis of the stannous ion with the formation of insoluble hydrolysis products. This results in loss of the catalyst. An example of this is shown in the aforesaid U.S. Pat. No. 3,672,938, Example V, where there is disclosed a catalyst having a total acid content of one milliliter of concentrated hydrochloric acid per liter of solution. This formulation is of no commercial value as it is impossible to solubilize the stannous salt and consequently, a stable colloid or catalyst in any other form cannot be prepared.

DEFINITIONS

The following definitions are provided to assist in the understanding of the ensuing text:

“Catalyst formulation” is the product resulting from the admixture of an acid soluble salt of a catalytic metal, a stannous salt in molar excess of the catalytic metal salt, an acid and an extraneous source of halide ions.

“Catalyst component” refers to any one or more of the salts of the catalytic metal, stannous salt or acid used in making the catalyst formulation.

“Actual halide ion concentration” is the concentration of the halide ions in the catalyst formulation if any of the catalyst components are used in the form of a halide. This will be zero if none of the aforesaid components are used in the form of a halide.

“Maximum component halide ion concentration” is the concentration of halide ions that would be in the catalyst formulation if each of the catalyst components were used ,in the form of the halide.

“Total halide ion concentration” is the required amount of halide ions in the catalyst formulation in accordance with this invention.

“Extraneous halide ions” and like terms mean a source of halide ions other than iodide ions in addition to those supplied by the catalyst components. The con3,904,792 centration of the extraneous halide ions is equal to the difference between the total halide ion concentration and the actual halide ion concentration.

“Excess halide ions” are halide ions in the catalyst in excess of the maximum component halide ion concentration and the concentration of the excess halide ions is equal to the difference between the total halide ion concentration and the maximum component halide ion concentration. The concentration of the excess halide ions equals the concentration of the extraneous halide ions when all of the catalyst components used to make the catalyst are in the form of the halide.

“Precipitation point” is the pH at which a precipitate forms in the catalyst formulation rendering the catalyst unsuitable for use. This precipitate is believed to be hydrolysis products of the stannous salt.

SUMMARY OF THE INVENTION

The catalysts described herein are improvements over catalysts such as those described and claimed in the aforesaid U.S. Pats. Nos. 3,01 1,920 and 3,672,938 in that they have greater solution stability, better absorption properties and, if desired, a decreased hydrogen concentration with a correspondingly higher pH.

The invention is predicated in part upon the discovery that the halide ions play a significant role in the functioning of the catalyst and that the catalyst is improved when the concentration of the halide ions is increased beyond that concentration found in prior art catalysts by the addition of an extraneous source of halide ions. The improvements resulting from excess halide ions comprise improved stability and adsorption properties and solubilization of the stannous salt or retardation of the precipitation point. Accordingly, catalysts of increased pH can be formulated thereby providing catalysts suitable for use with materials readily attacked by strong acids.

A catalyst composition in accordance with this invention comprises the product resulting from the admixture of (1) an acid soluble salt of a catalytic metal, (2) a solution soluble stannous salt in molar excess of the catalytic metal salt, (3) an acid and (4) an extraneous source of halide ions in an amount sufficient to provide an excess of halide ions in the formulation. The catalyst formulations of this invention have a PH of less than about 3.5 dependent upon the stannous content as will be explained in greater detail below.

DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 graphically represents precipitation point of a series of catalysts as a function of pH; and

FIG. 2 graphically represents the precipitation point of a series of catalysts as a function of stannous ion concentration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The catalysts of this invention are formulated substantially with materials and in proportions such as those described and claimed in the aforesaid U.S. Pat. Nos. 3,01 1,920 and 3,672,938. The acid soluble salt of the catalytic metal is a salt of any of those metals known to exhibit catalytic properties in chemical plating. Such metals include the precious metals, golld and silver and members of the platinum family. Palladium is generally found to be the most satisfactory of these catalytic metals for the activation of a non-conducting substrate, particularly a plastic substrate, and therefore constitutes the preferred embodiment of this invention, silver, gold and rhodium constitute lesser preferred embodiments of the invention as some difficulty is encountered in the preparation of the catalyst due to limited solubility of the salts of these metals in solution.

The particular salt of the catalytic metal used is not critical and may comprise the halides such as those described in the aforesaid U.S. Pat. No. 3,01 1,920 as well as such other salts such as the nitrate, sulfate and the like. Salts other than halides are suitable as halide ions will be introduced into solution by the extraneous source of halide ions. Preferably, the salt is the halide having an anion common to that of the other catalyst components. It should be noted that when the halide salt is used, some halide is introduced into solutions, but because of the low concentration of the catalytic metal salt used, this amount is generally negligible.

The amount of the catalytic metal salt is not critical and is primarily governed by cost and functional considerations. Thus, though up to 5 grams per liter or more of the catalytic metal salt is possible, it is desirable to maintain the quantity of the salt as low as possible from a cost consideration without sacrificing the functional properties of the catalytic formulation. Typically, the amount of the catalytic metal salt in a madeup bath does not exceed 2 grams per liter of solution and more preferably ranges between about 0.1 and 1 gram per liter of solution.

The particular stannous salt used to formulate the catalyst is likewise not critical and in addition to a stannous halide, other stannous salts are suitable such as stannous nitrate and stannous acetate. As with the salt of the catalytic metal, the stannous halide having an anion common to that of other catalyst constituents is preferred. When a stannous halide is used, a source of halide ions is introduced into the catalyst formulation though this amount by itself does not provide sufficient halide ions for purposes of this invention.

The amount of stannous salt used is not critical provided stannous ions are present in the catalyst formulation in molar excess of the catalytic metal ions. In this respect, as in the prior art, the molar ratio of the stannous ion to the catalytic metal ion may be as low as 2:1, but preferably varies between 10:1 and 40:1 and may be as high as 100:1.

The hydrohalide acids, other than hydriodic acid, are preferred for purposes of this invention. However, results in terms of stability and catalytic activity with hydroflouric acid are marginal. Hydrobromic acid is better and hydrochloric acid provides the best result. Accordingly, the term hydrohalide acid as used herein is intended to mean principally hydrochloric acid, but also includes hydrohalide acids other than hydriodic acid with the realization that these other acids provide only marginal results. It should be further realized that the term hydrohalide acid means the presence of hydrogen ions and halide ions in solution though the hydrogen ions may be derived from any other acid that does not have an anion detrimental to the catalyst formulation. Thus, sulfuric acid, as an example, may be used as a source of hydrogen ions with all of the halide ions being supplied by the extraneous source of halide ions.

The amount of acid used may be substantially less than in the commercially acceptable formulations of the prior art. In the prior art, the concentration of the acid had to be sufficiently high so as to provide a cata3,904,792 lyst having a pH of less than 1 and typically was so high as to provide a catalyst having a pH below 0. Using hydrochloric acid as an example, as much as 12 moles per liter of solution were used. In accordance with this invention, though such high concentrations of acid can ⁵be used, the acid concentration can be reduced to a level whereby the pH of the catalyst is as high as 3.5. Accordingly, for purposes of this invention, an operative range for the acid is from saturation to that amount that results in a solution pH of 3.5 and in the preferred embodiment of the invention, the acid is used in an amount sufficient to provide a pH ranging between 1 to 2.5. It should be noted that though catalysts can be formulated with a pH as high as 3.5, this is principally accomplished when the stannous ion concentration is relatively low. consequently, the stability of catalysts at this high pH is not entirely satisfactory for storage of catalyst for long periods of time.

From the above description, it can be seen that all of the catalyst components—i.e., the catalytic metal salt, the stannous salt and the acid, may or may not be used in the form of their respective halides though in a preferred embodiment of the invention, they are all halides having a common anion, most preferably chloride. With reference to the definitions set forth above, if all catalyst components were in the form of the halide, the resulting halide concentration, referred to as the maximum component halide ion concentration, would not be sufficiently high to obtain the improvements in the stability and adsorption properties and the retarded precipitation point. Obviously, if one or more of the catalyst components were used in a form other than the halide, then the actual halide ion concentration would be lower than the maximum component halide ion concentration and still insufficient to obtain the improvements noted above.

In accordance with the invention described herein, an excess of halide ions is provided in the catalyst formulation, above the maximum component halide ion concentration, by the addition of an extraneous source of halide ions. The amount of the extraneous halide ions added is equal to at least the difference between the actual halide ion concentration and the required total halide ion concentration.

In determining the required total halide ion concentration, different considerations apply dependent upon whether the pH of the catalyst is below or above the precipitation point, the pH at which a precipitate forms which precipitate is believed to be insoluble hydrolysis products of tin.

With regard first to catalyst formulations having a pH below the precipitation point in the absence of the extraneous halide ions, the total halide ion concentration required is not critical, it being understood that the higher the total halide ion concentration, the greater ³³will be the stability and adsorption properties of the catalyst though the improvements in these properties are sometimes difficult to ascertain, especially with those catalysts having a high hydrogen ion concentration—e.g., a concentration such that the pH of the catalyst is below 0. In general, at a pH below the precipitation point, the total halide ion concentration is at least 0.2 moles in excess of the maximum potential halide ion concentration and preferably, at least 0.5 moles in excess. The maximum concentration is not critical and the total halide ion concentration can be at saturation. Accordingly, the range for the total halide ion concentration is from an excess halide ion concentration of at least 0.2 moles to saturation and preferably at least 0.5 moles to saturation. The concentration of the extraneous source of halide ions is that amount necessary to increase the actual concentration of the halide ions to the total concentration of halide ions required as described above. Obviously, a greater amount of extraneous halide ions will be required when one or more of the catalyst components such as the catalytic metal salt, the stannous salt or the acid is not in the form of ¹⁰ the halide.

With regard to catalyst formulations having a pH above the precipitation point (in the absence of an extraneous source of halide ions), the amount of excess halide ion is more difficult to define as it is dependent upon the pH of the catalyst and the concentration of the stannous ions. The relationship between total halide ion concentration, pH and stannous ion content is depicted in FIGS. 1 and 2 of the drawings for the system palladium chloride (1 gram per liter of solution), stannous chloride, hydrochloric acid and lithium chloride as the source of the extraneous ions. It should be understood that other systems are similar to this system though the numerical limitations defining the curves might differ.

In FIG. 1 of the drawings, there is depicted two families of curves. The first family comprises curves A, B, C and D which represent the change in the precipitation point of the catalyst (pH) as a function of total chloride ion concentration for several different stannous ion ³⁰ concentrations. The second family of curves. A', B', C' and D' represent the actual chloride ion concentration derived from the total of the catalyst components —the stannous chloride, palladium chloride and hydrochloric acid, but not the lithium chloride. Curves A and A' are ³⁵ for a stannous ion content of 0.05 moles per liter of solution, B and B' for 0.13 moles per liter of solution, C and C' for 0.26 moles per liter of solution and D and D' for 0.39 moles per liter of solution. The precipitation point for this catalyst system in the absence of any ex⁴⁰ traneous halide ions (lithium chloride) is at a pH of about 0.9. As extraneous chloride ions are introduced into thc system and the total chloride ion concentration is increased, the precipitation point (pH) is also increased, but not as rapidly for formulations having a ⁴⁵ high stannous ion concentration (Curve D) as for formulations having a low stannous ion concentration (Curve A). Thus, it can be seen that the highest pH (about 3.5) is obtainable only with the lowest concentration of stannous ion and the highest total concentration of chloride ion. As the total chloride ion concentration decreases or the stannous ion concentration increases, the highest possible pH decreases.

The curves of FIG. 1 represent precipitation point. Therefore, the area above any given curve represents a stable catalyst while the area below the curve represents a catalyst containing a precipitate that is of no commercial value.

FIG. 1 may be used to determine the amount of extraneous halide ion required for the catalyst formula⁶⁰ tion. This is determined from the concetration difference between curves at any given pH and stannous ion concentration. For example, at a pH of 2 and a stannous ion concentration of 0.26 moles per liter of solution (Curves C and C'), the concentration difference ⁶⁵ between curves C and C' is about 4.5 so that the concentration of extraneous chloride ions required to reach the precipitation point is 4.5 moles per liter of solution. Thus, 4.5 moles of lithium chloride are added to

3,904,792 the formulation to provide a total chloride ion concentration of about 5 moles per liter of solution. However, this chloride ion concentration is only sufficient to reach the precipitation point of the catalyst and the total chloride ion concentration should be in excess of this amount to provide a stable catalyst. In general, for this catalyst system and others within the scope of the invention, the total halide ion concentration should be at least sufficient to prevent formation of a precipitate and this is generally at least about 0.2 moles perliter of solution above the halide ion concentration at the precipitation point of the catalyst and preferably at least above 0.5 moles per liter of solution above that required at the precipitation point, The upper limit is not critical and can be the saturation point of the halide ion in solution. Applying these general guidelines to the specific formulation depicted in FIG. 1, again making reference to the example at a pH of 2 and a stannous ion .concentration of 0.26 moles per liter of solution, the total chloride ion concentration at the precipitation point is 5 moles per liter of solution, but to assure the stability, the total chloride ion concentration should be at least 5.2 moles per liter of solution and preferably at least 5.5 moles per liter of solution. Accordingly, the concentration of the extraneous chloride ions —the lithium chloride, added to the formulation should be more than 4.5 moles per liter of solution, preferably should be at least. 4.7 moles per liter of solution and more preferably, should be at least 5,0 moles per liter of solution.

With respect to FIG. 1 described above, lithium chloride was selected as the source of the extraneous.chloride ion because of its very high solubility in solution. Other halide salts are not so soluble. For example, when sodium chloride is selected as a source of extra- ³⁵neous chloride ion, the solution becomes saturated when the total concentration is about 4-5 moles per liter. This puts a practical limitation on the maximum pH obtainable as FIG. 1 indicates that when the formulation contains 0.39 moles per liter of solution of stannous ion, the maximum obtainable pH was 4.5 moles of total chloride ion is about 1.65. when the solution contains only 0.05 moles per liter of solution of stannous ion, the maximum possible pH is about 2.5 with 4.5 total moles of chloride ion.

With regard to the source of the extraneous halide ion, any halide salt having the requisite solubility properties is suitable provided it does not have a cation that would interfere with the functioning of the catalyst. In this respect, illustrative halide salts that are suitable include aluminum chloride, aluminum bromide, magnesium chloride, sodium chloride, sodium bromide, potassium chloride, potassium bromide„calcium chloride, calcium fluoride and the like. Lithium halides are preferred because of their solubility and aluminum halides are least preferred because such salts tend to interfere with the functioning of the catalyst.

In FIG. 2 of the drawings, there is graphically presented a family of curves showing the precipitation point of the aforesaid palladium chloride-stannous chloridehydrochloric acid catalyst system as a function of the stannous ion concentration at different total halide ion concentrations. Again, the source of the extraneous halide concentration necessary to increase the actual halide ion concentration to the total halide ion concen- ⁶⁵tration is lithium chloride. Each curve in the family of curves is numbered and the numbers proceed from 1 through 8. Each number on the curve is the total halide ⁸ ion concentration for that curve. Each curve represents the precipitation point of the catalyst under consideration and it should be understood that the region to the left of any given curve represents a useable catalyst and -³ the region to the right of any given curve represents a catalyst ,iri having a pH in excess of its precipitation point and one wherein a precipitate has formed.

From FIG. 2, it can be seen that as the total chloride ion concentration increases, as one progresses from '9 Curve No. 1 to Curve No. 8, the maximum possible pH also increases. It can also be seen that the concentration of the stannous ion becomes more important at the higher pH levels. For example, where the total chloride ion concentration is 8 moles per liter of solution, the maximum pH obtainable with 0.4 moles per liter of Stannous ion is 2.4 whereas with only 0.5 moles per liter Of stanndus ion, the maximum pH is in excess of 3.5. Since the curves in FIG. 2 represent precipitation points, a slight excess of total chloride ion concentration beyond that represented in the curve is required to make a catalyst free of a precipitate.

The catalyst can be formulated using the procedures of the prior art with the extraneous halide ions dissolved in the acid solution used to dissolve the other catalyst components. A preferred method for formulating a catalyst in accordance with the invention would comprise first preparing a catalyst concentrate and then diluting the concentrate when ready for use. In this way, the concentrate can be made fairly acidic to ensure proper dissolution of the catalyst components and then the pH can be increased to the extent desired by dilution. The concentrate would be prepared by first dissolving the catalytic metal salt in acid solution, then adding the stannous chloride and letting the formulation age. During the ageing process, the catalyst will turn from a dark blue to green to brown coloration. Following ageing, the catalyst can be diluted with a sodium chloride solution. Where a catalyst having a pH above the precipitation point is desired, the same procedure is involved, but as a final step, some of the acid can be neutralized with a suitable neutralizing agent, preferably a weak base sodium bicarbonate.

The following example will serve to illustrate the invention in more detail.

EXAMPLES I TO 4

These examples illustrate the preparation of the catalyst used for the derivation of FIGS. 1 and 2 of the ₃θ drawings. Four stock solutions were prepared and labelled sequentially 1 to 4. The solutions had compositions as follows:

Solution No. ] 2-34

Palladium chloride (gm) 11 ||

Stannous chloride (gm) 1() 25 5075

Hydrochloric acid (37'.»-ml) 80.6 XI).6 X0.6X0.6 ^w^er to 1 liter

The formulations were prepared by dissolving the palladium chloride in the hydrochloric acid and half the water. Stannous chloride was then added in very slowly with stirring and the resulting solution was permitted to age until a dark brown coloration was obtained. The remaining water was then added. The pH of the resulting solution was 0.

Each of the above formulations was divided into 10 equal portions (100 ml each) and lithium chloride

3,904,792 . . ..

added to each to bring the. total concentration to a desired amount. Each of ,the so formed .catalysts were then titrated with, sodium bicarbonate to neutralize the acid to a point where a precipitate formed. This was considered to be .the precipitation point. The chloride ⁵introduced from each, of the hydrochloric acid, the stannous chloride and the lithium chloride as well as total chloride and precipitation point are set forth in the following table, . with reference to the table, it should be understood that the chloride concentrations 19 are set forth in moles per 100 ml of solution though in FIG, 1 of the drawings, this has been converted to moles per liter. Moreover, with regard to FIG. 1, the first point in the curve represents, a known precipitation point for a catalyst having a pH of 0.9 and was derived ¹³from formulation having a higher initial concentration of hydrochloric acid.

<td> Solution Identification</td><td> ,10 -continued Time to Precipitate (hrs)</td><td> Silver Film</td>

<td> 3-8</td><td> 27</td><td> No</td>

<td> 4-1</td><td> &gt;200</td><td> No</td>

<td> 4-4</td><td> &gt;200</td><td> No</td>

<td> : .4-8</td><td> &gt;200</td><td> No</td>

The catalysts having solution identification numbers beginning with 1 were less stable than the other catalysts because of the low stannous ion concentration. Only one of the catalysts exhibited a silver film characteristic of a catalyst left exposed to air for a long period of time.

To demonstrate the functional properties of the catalysts of this invention, the following plating sequence was used for the plating of an epoxy copper clad circuit

<td> Solution Identification</td><td></td><td> I Cl</td><td> κι-),,,.,'</td><td> (Cl 1_T</td><td> Precipitation Point (pH)</td>

<td> l-l</td><td> .100</td><td> .010</td><td> . ^;0</td><td> .110</td><td> L3</td>

<td></td><td> .100</td><td> .010</td><td> .090</td><td> .200</td><td> 1.7</td>

<td> 1-3</td><td> .100</td><td> .010</td><td> . 190 .</td><td> .300</td><td> 1.9</td>

<td> 1-4</td><td> .100</td><td> .010</td><td> .290</td><td> .400</td><td> 2.3</td>

<td> 1-5</td><td> . 100</td><td> .010</td><td> .390</td><td> .500</td><td> 2.7</td>

<td> 1-6</td><td> .100</td><td> .010</td><td> .490</td><td> . .600</td><td> 3.0</td>

<td> 1-7</td><td> JOO</td><td> .010</td><td> .590</td><td> .700</td><td> 3.3</td>

<td> F-8</td><td> .100</td><td> .010</td><td> .690</td><td> .800</td><td> 3.5</td>

<td> 2-1</td><td> .100</td><td> .026</td><td> 0</td><td> .126</td><td> 1.1</td>

<td> &gt;.&gt;</td><td> .100</td><td> .026</td><td> .074</td><td> .200</td><td> 1.4</td>

<td> 2-3</td><td> .100</td><td> .026</td><td> .174</td><td> .300</td><td> 1.7</td>

<td> 2-4</td><td> .100</td><td> .026</td><td> .274</td><td> .400</td><td> TI</td>

<td> 2-5</td><td> .100</td><td> .026</td><td> .374 '</td><td> .500</td><td> ~&gt;.3</td>

<td> 2-6</td><td> .100</td><td> .026</td><td> .474</td><td> .600</td><td> 5</td>

<td> 2-7</td><td> JOO</td><td> .026</td><td> .574</td><td> .700</td><td> T8</td>

<td> 2-8 .</td><td> JOO</td><td> .026</td><td> .674</td><td> .800</td><td> 3.1</td>

<td> 3-1</td><td> JOO</td><td> .052</td><td> 0</td><td> .152</td><td> 1.1</td>

<td> 3-2</td><td> JOO</td><td> .052</td><td> .048</td><td> .200</td><td> 1.3</td>

<td> 3-3</td><td> JOO</td><td> .052</td><td> .148</td><td> .300</td><td> 1.6</td>

<td> 3-4</td><td> .100</td><td> .052</td><td> .248</td><td> .400</td><td> 1.9</td>

<td> 3-5</td><td> JOO</td><td> .052</td><td> .348</td><td> .500</td><td> TO</td>

<td> 3-6</td><td> .100</td><td> .052</td><td> .448</td><td> .600</td><td> 3</td>

<td> 3-7</td><td> . JOO</td><td> .052</td><td> .548</td><td> .700</td><td> 2 5</td>

<td> 3-8</td><td> JOO</td><td> .052</td><td> • .648</td><td> .800</td><td> 2.7 ·</td>

<td> 4-1</td><td> JOO</td><td> .078</td><td> 0</td><td> .178</td><td> 1.2</td>

<td> 4-2</td><td> .100</td><td> .078</td><td> .022</td><td> .200</td><td> l.&gt;</td>

<td> 4-3 .</td><td> JOO</td><td> .078</td><td> .122</td><td> .300</td><td> 1.4</td>

<td> 4-4</td><td> JOO</td><td> .078</td><td> m</td><td> .400</td><td> 1.6</td>

<td> 4-5</td><td> JOO</td><td> .078</td><td> .322</td><td> .500</td><td> 1.7 ·</td>

<td> 4-6</td><td> JOO</td><td> .078 ,</td><td> , .422</td><td> .600</td><td> 1.9</td>

<td> 4-7</td><td> JOO</td><td> .078</td><td> .522</td><td> .700</td><td> TO</td>

<td> 4-8</td><td> JOO</td><td> .078</td><td> .622 .</td><td> .800</td><td> 2.3</td>

The curves of FIG. 1 are approximations as the precipitation point was observed visually and subject to experimental error. The explanation of the results of this series of experiments is set forth above and will not be repeated here.

Various of the above formulations were again prepared though the total chloride ion concentration was increased by 0.1 moles per 100 milliliters so that the total chloride ion concentration was in excess of the chloride ion concentration and the precipitation point. Stability of these formulations was determined by pouring a portion of the formulation into a beaker and leaving the beaker exposed to air for a prolonged period of time. In this way, the catalyst formulation had a relatively large exposed surface area. The catalyst was left exposed to air in this manner until such time as a precipitate formed. The results obtained are set forth in the following table:

<td> Solution</td><td> Time to</td><td> Silver</td>

<td> Identification</td><td> Precipitate (hrs)</td><td> Film</td>

board base material provided with a random array of through-holes. The catalysts used were those described in the immediately preceding table.

I. Pre-clean the copper substrate.

a. Clean the substrate by immersion in hot alkaline cleaner and rinse in clean water.

b. Pickle in an acid bath with an etchant for copper, for example, a cupric chloridehydrochloric acid bath, and rinse.

c. Dip in a 10 per cent by volume hydrochloric acid to remove residues, and rinse.

2. Catalysis:

Immerse the clean substrate for 30 seconds or more in the catalyst described above to catalyze both the copper surface and the plastic surface both on the back side of the circuit board base material and in the through-holes.

3. Accelerate:

1-1 154 Nr.

<td> 1-4</td><td> 60</td><td> No</td>

<td> 1-8</td><td> 3</td><td> Yes</td>

<td> 2-1</td><td> &gt;200</td><td> No</td>

<td> 2-4</td><td> 1 65</td><td> No·</td>

<td> 2-8</td><td> 11</td><td> No</td>

<td> 3-1</td><td> &gt;200</td><td> - No</td>

<td> 3-4</td><td> 148</td><td> No</td>

3,904,792

Immerse in an acidic accelerating solution, for example. a 10 per cent by weight perchloric acid solution, for one minute or more, and rinse.

4. Metal Deposition:

Immerse the catalyzed surface in the desired metal ⁵deposition solution, for example, a copper bath such as that of Example 1 Of U.S. Pat. No.

3,329,5 12 included herein by reference, for a sufficient time to build up the desired thickness of the metallic coating. Rinse thoroughly and dry. ¹⁰

5. Electroplate:

Immerse the metal coated substrate in a 1() per cent solution of hydrochloric acid to assure a clean copper coating, rinse and electroplate copper over the electroless copper coating until a desired thickness ^{* * * 15 * * * * * * * * * *}is obtained.

With the above process, each of the tested catalyst formulations provided a strong uniform coating of conductive metal on the plastic surface exposed in the through-holes and on the back side of the circuit board 20 base material as well as on the copper clad. There is no necessity for removal of the metal coating over the copper clad material prior to electroplating as the bond between the copper clad and the electroless copper deposit is quite strong. 25

EXAMPLES 5 THROUGH 8

<td> Solution No.</td><td> 5</td><td> 6</td><td> 7</td><td> 8</td>

<td> Palladium chloride (gm)</td><td> 3</td><td> 3</td><td> 3</td><td> 3 ³«</td>

<td> Stannous chloride (gm)</td><td> 30</td><td> 75</td><td> 150</td><td> 225</td>

<td> Hydrochloric Acid I2N (ml)</td><td> 24.2</td><td> 24.2</td><td> 24.2</td><td> 24.2</td>

<td> Sodium chloride (gm)</td><td> 174</td><td> 174</td><td> 174</td><td> 174</td>

<td> Water</td><td></td><td> to 3 liters</td><td></td><td></td>

The above formulations were prepared by dissolving the palladium chloride and sodium chloride in one half of the volume of water. The stannous chloride was then added in an initial amount such that there is an excess and the balance added slowly with stirring. The solu- 40 tions were then permitted to age until a brown coloration was obtained. The solutions were then split into three equal 1 liter portions and additional sodium chloride in a given amount was then added to each of the separate solutions. The catalysts were then titrated with 45 sodium bicarbonate to increase the pH and determine the precipitation point of the catalyst. The following table sets forth solution identification, chloride content from each of the hydrochloric acid, stannous chloride and sodium chloride, the total chloride content and the 50 precipitation points of the catalyst.

through 4. Moreover, each of the catalysts set forth in the immediately preceding table were highly effective in catalyzing a substrate in the metallization process described above.

In the aforesaid Examples 5 through 8, the maximum concentration of extraneous chloride ion is about 4.5 moles per liter of solution because of the limited solubility of sodium chloride. Accordingly, using sodium chloride, the maximum possible pH obtainable with 10 grams of stannous chloride per liter of solution is about 2.5.

The following are examples of formulations within the scope of the invention. All are capable of catalyzing a substrate following the procedure described above.

Example 9

Palladium nitrate (gm)

Stannous nitrate (gm)

Nitric acid (ml)

Calcium chloride (gm)

Water

Example 10

Palladium sulfate (gm)

Stannous Sulfate (gm)

Sulfuric acid (ml)

Calcium bromide (gm) Water

Example 1 1

Palladium chloride (gm) Stannous fluoborate (gm) Fluoroboric acid (ml) Calcium chloride (gm) Water

Example 12

Palladium bromide (gm) .

Stannous bromide (gm) Hydrobromic acid (48#-ml) Sodium bromide (gm) Water

Example 13

Palladium bromide (gm) Stannous sulphate (gm) Nitric acid (ml) Magnesium bromide Water

Example 14

Gold chloride (gm)

Stannous chloride (gm) Hydrochloric (37#-ml) Sodium chloride (gm) Water

Example 15

Platinum chloride (gm)

Stannous chloride (gm) Hydrochloric acid (37#-ml) Sodium chloride (gm) Water

Example 16

Rhodium sulfate (gm) Stannous sulfate (gm) Sulfuric acid (96# ml) Sodium chloride (gm) Water

Example 17

100 to 1 liter

100 to 1 liter

0,5

200 to 1 liter

0.50

100

150 to 1 liter

0.50

250 to 1 liter

I

200 to 1 liter

200 to 1 liter

I

200 to 1 liter

<td> Solution Identification</td><td> l Cl 1,,,,</td><td> |C1 l.s„&lt;7</td><td> |C1 |.„„₇</td><td> icri,</td><td> Precipitation Point (pH)</td>

<td> 5-1</td><td> .1</td><td> .10</td><td> 1.00</td><td> 1.20</td><td> 1.3</td>

<td> 5-2</td><td> .1</td><td> . 10</td><td> 1.80</td><td> 2.00</td><td> 1.7</td>

<td> 5-3</td><td> .1</td><td> .10</td><td> 3.80</td><td> 4.00</td><td> 2.3</td>

<td> 6-1</td><td> .1</td><td> .26</td><td> 1.00</td><td> 1.36</td><td> 1.2</td>

<td> 6-2</td><td> .1</td><td> .26</td><td> 1.64</td><td> 2.00</td><td> 1.5</td>

<td> 6-3</td><td> . 1</td><td> .26</td><td> 3.64</td><td> 4.00</td><td> 2.0</td>

<td> 7-1</td><td> .1</td><td> .52</td><td> 1.00</td><td> 1.62</td><td> 1.2</td>

<td> 7-2</td><td> .1</td><td> .52</td><td> 1.38</td><td> 2.00</td><td> 1.3</td>

<td> 7-3</td><td> .1</td><td> .52</td><td> 3.38</td><td> 4.00</td><td> 1.8</td>

<td> 8-1</td><td> . 1</td><td> .78</td><td> 1.00</td><td> 1.88</td><td> 1.2</td>

<td> 8-2</td><td> .1</td><td> .78</td><td> 1.12</td><td> 2.00</td><td> 1.2</td>

<td> 8-3</td><td> .1</td><td> .78</td><td> 3.12</td><td> 4.00</td><td> 1.5</td>

From the above results, it can be seen that the results obtained are similar to those obtained in Examples 1

Palladium chloride (gm) Platinum chloride (gm) Stannous chloride (gm)

0.5

0.5

3,904,792

-continued

Example 9

Hydrochloric acid (37%-ml) 25

Sodium chloride (gm) 100

Water lo 1 liter

EXAMPLES 18 TO 20

The following examples illustrate a catalyst with a very low stannous ion concentration:

<td></td><td> 18</td><td> 19</td><td> 20</td>

<td> Palladium chloride (gm)</td><td> 0.25</td><td> 0.25</td><td> 0.25</td>

<td> Stannous chloride</td><td> 3.2</td><td> 3.2</td><td> 3.2</td>

<td> Hydrochloric acid (37% ml)</td><td> 2.0</td><td> 10</td><td> 80</td>

<td> Sodium chloride (gm)</td><td> 200</td><td> 200</td><td> 200</td>

<td> Water</td><td> to 1 liter</td><td></td><td></td>

All of the above were made according to the process of Example 1. The formulation of Example 18 was stable in a petri dish exposed to air for a period of 87 hours while the formulations of Examples 19 and 20 were stable for a period of 87 hours.