Plating Anodized Aluminum Substrates

U.S. patent application, Ser. No. 452,993, filed Dec. 27, 1982 concurrently herewith, entitled "Sealing Thick Anodic Coatings on Aluminum Substrates", having I. S. Mahmoud as inventor and commonly assigned.

1. Technical Field

This invention relates to a process for plating copper on hard anodized aluminum substrates.

2. Background Art

Prior art shows that plating of aluminum and its alloys reveals that porous anodizing has been extensively investigated as a surface pre-treatment. For example, U.S. Pat. No. 3,943,039 to Wittrock teaches an anodizing step before plating with nickel. Wernick and Pinner, in Surface Treatment of Aluminum, discuss plating methods in which anodizing is used as a pre-treatment step. In the October 1982 issue of the American Electroplaters Society Journal, Plating and Surface Finishing, David E. Thomas, Page 57, discusses the use of anodizing as pre-treatment to plating, in an article entitled "Adhesion Failure of Electrodeposited Coatings on Anodized Aluminum Alloys."

All of the above-referenced art relates only to thin anodic coatings. Such thin coatings usually use phosphoric acid anodizing. The resulting coating is not a good insulator, and with sufficient current density, electrodeposition is possible.

More recently, anodized aluminum has been investigated as a di-electric material for use in electronic packaging. Metalization of thick anodic coatings is known to be achieved by lamination of copper foils followed by etching. Such a process limits utilization of anodized substrates to one sided circuit board applications.

Many processes are known for plating aluminum and aluminum alloys in the raw oxidized and anodized states. Anodized aluminum plating has been limited to relatively thin coatings of up to 0.051 milimeters (0.002 inch) thickness. These coatings are porous and with proper plating conditions, certain metals may be plated onto the coating without surface conditioning.

Thick coatings, over 0.1 milimeters (0.024 inch and above) are non-conductive and direct plating to these coatings has been heretofore unattainable.

The present invention provides a significantly lower cost packaging alternative with good heat dissipation properties resulting from an improved process for plating thick anodic coatings on aluminum substrates. We provide a novel pre-treatment formulation which is significantly faster. An aluminum work piece is anodized to form a thick anodic coating. The work piece is then sensitized and etched at room temperature for two minutes in a solution comprised of stannous chloride, hydrochloric acid and water. Following a distilled water rinse, the work piece is activated in a solution containing palladium chloride and hydrochloric acid. The activation step is followed by another rinse in distilled water. The result is a substrate which is plated first with an electroless nickel bath then followed by electrolytic plating in a conventional acid copper bath.

Plating over thick anodic coatings, whether electroless or electrolytic solutions are used, is possible after our sensitizing and activating steps .

The following example illustrates the practice of this invention and includes the following steps:

(a) Hard anodize;

(b) seal;

(c) sensitize and etch in a solution consisting of:

stannous chloride (SnCl.sub.2.4H20) 40 g/l

hydrochloric acid (HCl, 36%) 60 ml/l

water 1 liter;

(d) rinse in distilled water;

(e) activate in the solution of 0.25 grams palladium chloride, (PdCl.sub.2); hydrochloric acid, HCl., 36%, 2.5 ml;

(f) rinse in distilled water;

(g) plate in an electroless nickel bath for 5-10 minutes;

(h) plate in acidic copper bath at 1-2 amp/dm.sup.2 current density; voltage 1-2 volt; temperature of 21.degree.-27.degree. C. (70.degree.-80.degree. F.).

It is to be noted that the plating time for copper is a function of the desired coating thickness. The following mathematical relationship relates time and thickness. ##EQU1## where t=thickness in mil;

Sp.Gr.=Specific Gravity-8.95 for copper;

g/amp-hour=1.182 for copper;

C.D.=Current density (1-2 amp.dm.sup.2);

C.Eff.=Current efficiency=95% for acid copper bath.

The preferred sealing step is described in detail in co-pending, commonly assigned, U.S. patent application, Ser. No. 452,992.