Method For Electroless Deposition Of Metal Using Improved Colloidal Catalyzing Solution

Abstract

A colloidal solution of a catalytic metal is prepared by admixing an acid-soluble salt of the metal, hydrochloric acid, a soluble stannous salt present in excess of the amount necessary to reduce the metal salt to colloidal metal, and a quantity of stannic chloride that has been separately aged. A substrate surface upon which metal is to be deposited is treated with the catalytic colloidal solution. The catalyzed surface is then subjected to an electroless plating bath.

Description

BACKGROUND OF THE INVENTION

Various metals can be deposited on certain substrate surfaces by a process known as autocatalytic electroless plating. Autocatalytic electroless plating baths usually contain a salt of the metal being deposited, a reducing agent for the metal salt, a complexing or chelating agent and a pH adjustor.

Some metallic surfaces are inherently catalytic for initiating deposition of certain metals from autocatalytic electroless plating baths. However, non-metallic surfaces must be treated to render them catalytic. One common method of rendering a non-metallic surface catalytic for the autocatalytic electroless deposition of a number of metals is to treat it, first, with a sensitizing solution of stannous chloride. Then the sensitized surface is treated with an activating solution of palladium chloride which deposits an adherent film of palladium nuclei. These nuclei serve as catalyst for initiating the reduction of the metal salt and its deposition on the substrate. After an initial layer of the metal is thus deposited, this initial layer catalyzes further deposition.

The above described method of autocatalytic electroless deposition has been widely used. Another method that has been widely used commercially for depositing a catalytic coating on a surface is the so-called colloidal catalyst solution method, as described, for example, in U.S. Pat. No. 3,011,920. In this method, the initial step of sensitizing the surface with a stannous chloride solution is omitted. Instead the surface is treated with a single solution which comprises an acid soluble salt of a catalytic metal (usually, a noble metal), hydrochloric acid, and a soluble stannous salt which is present in excess of the amount necessary to reduce the catalytic metal salt to colloidal metal. The excess amount of stannous salt reacts to form a protective colloid which inhibits flocculation that would normally occur when stannous ions and noble metal ions are present in the same solution. After treatment of the surface to be plated with the colloidal catalyst solution, the surface is treated with the electroless plating bath in the usual way.

The colloidal catalyst solution method has been found generally satisfactory for deposition of metals such as nickel and copper, on surfaces such as glass and hydrophilic dielectrics, especially where the catalyzed surface is given a treatment with an accelerator such as a solution of sodium hydroxide prior to contact with the electroless plating bath. The accelerator dissolves the protective colloid which is present on the catalyzed surface.

In the case of hydrophobic surfaces, such as Teflon, for example, neither the colloidal catalyst solution method nor the two-step method first described, has been satisfactory. On this type of surface, coverage of the deposited metal has usually ranged between zero and very poor.

The present invention is an improvement in the colloidal catalyst solution method which enables it to be used on hydrophobic surfaces and also provides improved reliability on other substrates. The improvement consists in adding to the colloidal catalyst solution, a solution of a stannic compound that has been separately aged. The quantity of stannic ion added is enough to make the solution at least 0.01 molar in this ingredient.

The addition of the aged stannic chloride introduces a hydrolyzed compound containing stannic ions. The improvement appears to be due to this compound.

DESCRIPTION OF PREFERRED EMBODIMENTS

Examples 1-3

In the examples which follow, the surface being coated was a plate of Teflon plastic. The plating bath was one for autocatalytically electrolessly plating copper and had the composition given below.

Copper Plating Bath Compositions ______________________________________ CuSO.sub.4.5H.sub.2 O 15 g/l Propylenediamine tetraacetic acid, 60 cc/l sodium salt (40% active solution) NaOH 4.0 g/l H.sub.2 CO (37% solution) 40 cc/l NaCN 4 mg/l Temperature 40.degree.C ______________________________________

A plating cycle of 5 minutes was used.

Besides copper (from the above-described bath), nickel, cobalt, gold and alloys of any two or more of these four metals, for example, can be deposited from conventional electroless plating baths using the colloidal catalyst solutions described herein.

The following are colloidal catalyst solutions for catalyzing the surface to be plated. The surface is either dipped in or sprayed with the solution. Treating time can be about 30 seconds or more.

Colloidal Catalyst Solutions ______________________________________ Example 1 Example 2 Example 3 ______________________________________ H.sub.2 O 600 cc/l 600 cc/l 600 cc/l HCl (Conc.) 300 cc/l 300 cc/l 300 cc/l SnCl.sub.2 50 g/l 37.5 g/l 37.5 g/l (Anhydrous) Aged SnCl.sub.4 3.2 .times. 10.sup.-.sup.2 1.6 .times. 10.sup.-.sup.2 2.4 .times. 10.sup.-.sup.2 (molar conc.) Na.sub.2 SnO.sub.3.3H.sub.2 0 None None 1.5 g/l PdCl.sub.2 1 g/l None None HAuCl.sub.4.H.sub.2 O None 1 g/l None H.sub.2 PtCl.sub.6 None None 1 g/l ______________________________________

The pH of these solutions should be maintained below about 1.

The aged SnCl.sub.4 may be prepared as an aqueous 0.5 molar solution aged for one week at room temperature. In the catalyst solution, its concentration is preferably 0.01 to 0.5 molar. Aging time varies with conditions. At elevated temperatures it can be as little as a few hours. At ordinary room temperature it is preferably at least 4 days to 1 week.

After treating the Teflon surface with the catalyst solution, the surface is rinsed with water and then preferably treated with an accelerator as described in U.S. Pat. No. 3,011,921. This may be a 5 percent solution of sodium hydroxide, for example, and treating time is a few minutes.

It will be noted that the above described examples of colloidal catalyst solutions may contain Na.sub.2 SnO.sub.3.sup.. 3H.sub.2 O as an optional ingredient. If present, this ingredient functions as a means of generating additional protective colloids for the catalytic metal.

The surface is again rinsed with water after treatment with the accelerator and it is then treated with the plating bath.

Platings on Teflon using the above-described materials and procedures have produced about 95 percent plating coverage with all three examples. For comparison purposes, another three Teflon plates were treated with similar colloidal catalyst solutions except that the aged SnCl.sub. 4 was omitted. The plating baths and plating cycles were the same. However, in these comparison examples, plating coverage was only about 5 percent on each plate.

An example of a nickel autocatalytic electroless plating bath that can be used in the present process is:

NiSO.sub.4.6H.sub.2 O 25 g/l Na.sub.4 P.sub.2 O.sub.7.10H.sub.2 O 50 g/l Dimethylamine borane 1.5 g/l NH.sub.4 OH (conc.) 50 cc/l Temperature 40.degree.C

In the present method, the colloidal catalytic metal can be gold or any one of the platinum group of metals. The platinum group of metals consists of platinum, ruthenium, rhodium, palladium, osmium and iridium.

Claims

We claim:

1. A colloidal catalytic solution for catalyzing a substrate prior to electroless metal deposition thereon, said catalyst comprising the solution resulting from the admixture of an acid soluble salt of a catalytic metal selected from the group consisting of gold, and the platinum family of metals, hydrochloric acid, a stannous salt soluble in aqueous solution, said stannous salt being in excess of the amount necessary to reduce said metal salt to colloidal metal, and a quantity of a solution of stannic chloride which has been separately aged for the equivalent of at least 4 days at room temperature prior to admixture, said stannic compound being present in a concentration of about 0.01 to 0.5 molar in said catalytic solution.

2. A solution according to claim 1 in which said catalytic metal is palladium.

3. A solution according to claim 1 in which said catalytic metal is gold.

4. A solution according to claim 1 in which said catalytic metal is platinum.

5. The method of making a colloidal catalyst solution for application to a substrate surface prior to electroless metal deposition thereon, comprising admixing an acid soluble salt of a catalytic metal selected from the group consisting of gold, and the platinum family of metals, hydrochloric acid, a stannous salt soluble in aqueous solution, said stannous salt being in excess of the amount necessary to reduce said metal salt to colloidal metal, and a quantity of stannic chloride solution which has been separately aged for a period which is the equivalent of at least 4 days at room temperature and said stannic compound being present in a concentration of about 0.01 to 0.5 molar in said catalyst solution.

6. The method of claim 1 in which said catalytic metal is palladium.

7. The method of claim 5 in which said catalytic metal is gold.

8. The method of claim 5 in which said catalytic metal is platinum.