Method of electroless deposition of a substrate and sensitizing solution therefor

United States Patent Office

3,674,550

Patented July 4, 1972

3,674,550

METHOD OF ELECTROLESS DEPOSITION OF A SUBSTRATE AND SENSITIZING SOLUTION THEREFOR

Glenn O. Mallory, Inglewood, Calif., assignor to Allied 5 Research Products, Inc., Baltimore, Md.

No Drawing. Filed Mar. 4, 1970, Ser. No. 16,548

Int. Cl. C23c 3/02

U.S. Cl. 117—212 11 Claims

ABSTRACT OF THE DISCLOSURE

Prior to electrolessly depositing a metal on a substrate, the substrate is treated with an aqueous solution of a complex of a metal of the platinum family and thereafter with 15 an aqueous solution of a reducing agent for the platinum family metal. The complex can be a nitrito salt of the platinum metal or a salt of the metal with a keto-carboxylic acid or diglycollic acid.

This invention relates generally to the coating of nonmetallic or non-electrically conductive objects, such as objects made of plastic, glass, ceramic, wood, silicon and the like with a metal, and more particularly to the sensi- 25 tization of such objects prior to coating the same with an electrically conducting film or layer of a metal having low electrical resistance.

In the electrical and electronic industries, the manufacture of acceptable printed circuits has assumed significant 30 importance and efforts heretofore to fabricate suitable printed circuits, while successful in some respects often are attendant with serious disadvantages such as the cost of materials used to produce the desired conductive surface thereon, lack of maximum adhesion characteristics 35 of the conductive surface provided on an essentially nonconductive substrate and a lack of a convenient way or means to effectively and economically plate through holes provided in the circuit board.

Classically, it has been common practice to provide an 40 electroless metal coating on a non-conductive or semiconductive substrate by cleaning the substrate surface to make it wettable or hydrophilic; immersing the cleaned substrate in a bath containing stannous chloride or other stannous salt; immersing the stannous salt treated sub- 45 strate in a bath containing a salt of a metal catalytic to the deposition of the desired ultimate metal coating such as silver nitrate or the chlorides of gold, palladium or platinum whereby these metal ions are reduced to catalytic metal nucleating centers by the stannous ions previously 50 absorbed on the substrate and thereafter electrolessly depositing the desired metal such as copper, nickel or cobalt by treating the thus catalyzed surface with an aqueous solution of a salt of the desired metal in the presence of a reducing agent therefor. Such methods, however, are 55 detrimental especially where they have been employed to catalyze essentially non-conductive surface of through holes in a substrate wherein the non-conductive through hole surface is bounded by a conductive surface, such as a plastic substrate provided with a copper clad on either go or both its planar surfaces. In employing these known sensitizing techniques an immersion deposit of the metal was simultaneously deposited on the metal clad surface and subsequent electroless metal deposits exhibited poor adhesion characteristics especially on the metal clad planar 55 surfaces.

Subsequently, it was proposed to catalyze the substrate by treatment with a bath containing colloidal particles of a catalytic metal and thereafter plating the substrate by treatment with a known electroless metal deposition solu- 70 tion. However, it has been found that the shelf life of such colloidal solutions is relatively short and that favor able and uniform results often required frequent preparation of fresh colloidal solutions. The sensitizing bath of the present invention, however, is particularly advantageous over colloidal solutions in that the bath of this invention can be characterized as an aqueous solution of metal ions having a practically unlimited shelf life. Further, the sensitizing bath of this invevntion is relatively more economical to produce than prior art colloidal solutions.

It has now been found that such detrimental effects can be eliminated by sensitizing an essentially non-conductive surface, especially one bounded by conductive surfaces with an aqueous solution of a metal of the platinum family complexed with an inorganic or organic complexing agent.

According to the present invention an aqueous bath solution of a member of the platinum family and preferably platinum, palladium and rhodium in the form of an inorganic complexing agent selected from the group consisting of a nitrito salt complex such as the diamino dinitrite salts or in the form of a dipotassium or disodium platinum metal tetranitrite has been found particularly effective for use with the present invention.

Further the member of the platinum family can be employed in the form of an organic complexing agent selected from the group consisting of a ketonic carboxylic acid such as pyruvic, levulinic, acetoacetic, a-ketoglutaric, j3-ketoglutaric, oxalosuccinic and oxalacetic acid or an acetylacetonate such as the disodium salt of acetylacetone or an organic acid with an ether-like structure such as diglycollic acid (2,2-oxydiacetic acid).

The amount of metal of the platinum family in the aqueous bath solution whether complexed organically or inorganically is sufficient to sensitize the surface of the substrate and generally the amount will range from about 0.02 to 0.2 gram of metal/liter of solution although it will be recognized that concentrations lower or higher than this preferred range can be utilized depending on such factors as the particular material chosen as the substrate, the particular metal to be subsequently electrolessly deposited thereon as well as the presence on the substrate chosen of a metal clad. Further, the economics of the total sensitizing operation can also dictate the maximum amount of catalytic metal present in the sensitizing bath and thus while the catalytic metal can be effectively present in amounts as high as saturation levels, ordinarily it is not economical to warrant such concentration conditions.

When the platinum family metal is complexed with a ketonic carboxylic acid or with diglycollic acid the mole ratio of metal to acid to form the complex generally ranges between 1:1 to 1:4 or higher. The minimum amount of such acid is the number of available donor atoms in the acid to essentially fully coordinate the metal.

The aqueous sensitizing bath of the present invention generally has a pH ranging from about neutral to an alkaline pH up to about 12 or even greater, if desired. When the platinum family metal is complexed inorganically, the pH can be as low as about 6, but again, it is preferable to utilize such a sensitizing bath solution in the neutral to alkaline pH range.

To sensitize the surface of the substrate which can be any of the aforementioned materials including such plastic materials as epoxy resins, phenolic resins, polypropylene, polyurethane, acrylonitrile-butadiene-styrene copolymers such as are conventionally employed in the production of printed circuit boards, the substrate is initially treated to render the same hydrophilic in nature, i.e. wettable to the subsequent sensitizing bath operation. This is generally accomplished by slightly roughening the surfaces of the substrate by, for example, etching, to form minute surface

3,674,550 irregularities or pores thereon. Alkaline or acid etchants can be used depending on the composition or nature of the substrate chosen. Alternatively, the surface of the substrate can be freshened by mechanical means such as sand blasting and the like.

It has been found advantageous, particularly in the case where the substrate is polypropylene, to immerse the roughened surfaced substrate in a stannous chloride solution containing for instance 10-25 grams/liter stannous chloride and about 25 ml. HCl/liter. Other conventionally employed stannous chloride solutions can be used when this optional treating operation is employed. The immersion of the substrate in such a stannous chloride solution takes place generally at ambient temperature and lasts from a few seconds to a few minutes. Thereafter the substrate is rinsed with water one or more times.

The substrate, with or without the stannous chloride pretreatment, is then immersed in the aqueous sensitizing bath of this invention for a time ranging from about 30 seconds to 2-3 minutes or longer, if desired, at ambient temperature. The time of immersion, of course, can vary widely, depending on the size and shape of the substrate, the degree to which its surface has been roughened, the nature or composition of the substrate, as well as the particular sensitizing bath and concentration thereof employed.

After immersion of the substrate in the sensitizing bath, the substrate is rinsed with water one or more times, as desired, and is then immersed in an aqueous bath solution containing an alkaline reducing agent to reduce the platinum family metal on the substrate. Conveniently, it has been determined that the preferable reducing agents are alkaline borohydride solutions such as sodium borohydride and amine boranes. Generally the amount of reducing agent in the aqueous reducing bath can vary between about 0.01 to 10.0 g./l., the maximum amount of said reducing agent being limited generally only by economic considerations. Normally, the substrate is contacted with the reducing agent bath solution at ambient temperature for a time sufficient to effectively reduce substantially all the platinum family metal present thereon and it has been found that times ranging from about 10 seconds up to about 4-5 minutes or longer have been sufficient to accomplish effective reduction.

The substrate thus provided with a platinum family metal in reduced form is again rinsed one or more times, as desired, with water and is thus prepared for reception of an electroless metal deposit in accordance with conventional electroless metal plating techniques.

The metal to be electrolessly deposited on the substrate sensitized in accordance with the present invention can be, for instance, nickel, cobalt, coper and the like or any other convenient electrically conductive metal. The electroless plating bath thus comprises an aqueous solution containing ions of the metal to be deposited and a reducing agent therefor. Conveniently, the metal ions can be supplied to the bath from a soluble inorganic or organic salt of said metal such as the chloride, sulfate or nitrate. The reducing agent can be any of those commonly employed in electroless plating operations such as hypophosphites and the like. Complexing agents such as the tartrate and citrate ions can successfully be employed as well as conventional buffer agents to maintain the pH of the electroless plating bath within certain conventional desired limits such as ammonia hydroxide and the like. Other conventional additives to electroless plating baths can also be employed such as sequestering agents, like urea, and the temperature of the bath is conveniently maintained at about 70° C. to 90° C.

This invention may be better understood by reference to the following examples.

EXAMPLE I

0.185 grams of palladium chloride (60 wt. percent Pd) were dissolved in 100 ml. of distilled water. Separately 30 grams of the sodium salt of pyruvic acid were dissolved in 200 ml. of distilled water. These separate solutions were mixed and the pH thereof was adjusted to 11.5 by the addition thereto of a 25 percent solution of NaOH. The volume was then adjusted to one liter by the addition of water thereto to provide a sensitizing bath of the present invention wherein the palladium is complexed with an organic complexing agent comprising a ketonic carboxylic acid.

EXAMPLE II

Example I was repeated except that in place of the pyruvic acid, an equivalent amount of levulinic acid was employed.

EXAMPLE ΙΠ

Example I was again repeated except that instead of the pyruvic acid, α-ketoglutaric acid was used.

EXAMPLES IV-VII

Again Example I was repeated using rather than pyruvic acid, j3-glutaric acid (Example IV), diglycollic acid (Example V), sodium salt of acetylacetonate (Example VI), acetoacetic acid (Example VH) in corresponding equivalent amounts.

Additionally, Examples I-VII were repeated using, rather than palladium chloride, the following platinum family metal compounds: platinous potassium chloride, platinous sodium chloride and rhodium potassium chloride. Substantially equally favorable results were achieved with these sensitizing bath solutions.

In addition to the above sensitizing bath solutions containing an organically complexed platinum family metal, the following sensitizing bath solutions containing an inorganically complexed platinum family metal were also also prepared and effiectively used:

EXAMPLE VIII

One gram of palladium diammino dinitrite was dissolved in one liter of distilled water and the pH adjusted to 8.0 with a 10% solution of potassium tetraborate.

EXAMPLE IX

Example VIII was repeated except that instead of palladium diammino dinitrite, platinum diammino dinitrite was used.

Additionally a solution of dipotassium palladium tetranitrite having a pH ranging from about 5-10 wherein the said tetranitrite was present in amounts of about 1 g./l. and a solution of rhodium potassium nitrite (K₃Rh(NO₂)₆) were also used as the sensitizing bath solution with similarly effective results.

EXAMPLE X gram of rhodium potassium chloride (K₃RhCl₅) was dissolved in 900 ml. of distilled water. The pH of the solution was adjusted to about 7.0 by the addition thereto of the disodium salt of diglycollic acid (2,2-oxydiacetic acid) and the final volume was adjusted to one liter by the addition of water thereto. The resulting sensitizing bath solution provided the desired results.

EXAMPLE XI

The above sensitizing baths were employed in the production of printed circuit boards which were fabricated from a polypropylene substrate. The substrate was initially provided on its planar surfaces with a copper clad coating and a plurality of through holes bridging the copper clad surfaces were drilled into the substrate.

The copper clad substrates were then treated in accordance with the present invention to provide a layer of conductive metal on the through hole surfaces in the following manner.

The substrates were thoroughly cleaned by dipping the

3,674,550 same into a hot acid cleaner containing 10% HC1 by volume and then thoroughly rinsed.

The clean substrates were then immersed in a standard stannous chloride solution containing about 10-25 g./l. stannous chloride and 25 ml. HC1/1. for about one minute at ambient temperature.

The substrates were then thoroughly rinsed with water and immersed into a sensitizing bath of this invention for about a minute at ambient temperature. The sensitizing bath had a composition corresponding to the bath formulation set forth in Example I.

On removal from the sensitizing bath, the substrates were again thoroughly rinsed with water and were then immersed in a bath containing an aqueous solution of the reducing agent, sodium borohydride, wherein the borohydride was present in an amount of about 1 gram/1. Immersion time was about 30 seconds which was sufficient to reduce essentially all the palladium on the through hole surface. Essentially no palladium was reduced on the copper clad planar surfaces of the substrate.

After removal of the substrates from the bath containing the reducing agent, the substrates were again thoroughly rinsed and then immersed in an electroless nickel plating bath solution having the following composition and under the following conditions:

Bath composition:G./l.

NiCl₂-6H₂O30

NaH₂PO₂-H₂O 10

Sodium citrate30

Operate at 190° F. and pH 4.5.

The substrates were immersed in the electroless nickel plating bath for a time sufficient to build up a layer of nickel having the desired thickness on not only the copper clad planar surfaces but also on the through hole surfaces. The nickel coating exhibited excellent adhesion characteristics to both types of surfaces.

Example XI was repeated a number of times except that the circuit boards were made of a variety of other plastic materials such as ABS, Kapton, phenolic resins and the like and the electroless metal plating baths were copper and cobalt plating baths having, typically the following compositions:

Copper plating bath:

CuSO₄-5H₂O ------------------------g./l 8

KNaC₄H₄O₆-4H₂O____________________g./L_ 90

HCOH (37%)----------------------ml./l10 pH (NaOH) = 12-12.7 ambient temperature.

Cobalt plating bath:G./l.

CoCl₂-6H₂O 30

NaH₂PO₂-H₂O20

Sodium citrate 35

Ammonium chloride 50

Water to make 1 liter temperature 194° F.;

pH, 9-10.

In addition to using the sensitizing bath of Example I, the sensitizing baths of Examples II-X were also used in a method essentially the same as that outlined in Example XI with equally favorable results.

Further, rather than using a metal clad substrate, plastic substrates without a metal coating on the planar surfaces were also employed and favorable results were also achieved.

In another embodiment of the present invention, the printed circuit boards described above were also sensitized, prior to immersion in conventional electroless metal plating baths, by immersing the same in an aqueous solution of a gold salt of an inorganic or organic acid. A convenient inorganic gold solution is gold sodium thiosulfate while gold thiocyanate and gold thiomalate solutions were found to be acceptable organic media.

A gold sodium thiosulfate sensitizing bath was pre6 pared as follows: 0.5 gram of gold sodium thiosulfate was dissolved in 1 liter of distilled water and the pH thereof was about 7.8.

One gram each of gold thiocyanate and gold thiomalate were separately dissolved into one liter, each, of distilled water and the pH of each one liter solution was adjusted to about 8.0 with a solution of potassium tetraborate. Each of these solutions was then used as a sensitizing bath in the manner described above.