Surface Preparation Considerations

Careful cleaning and preparation of the substrate material being plated is required for the effective electrodeposition of copper. Surface oils and greases, buffing compounds, rust, scale, and oxides, especially around weld or solder areas must be thoroughly removed before copper plating to ensure adhesion and to minimize contamination of the plating bath. However, before considering any preparation, it is important to know the type of substrate being used as well as any substrate surface conditions that may be present. This information is important because the preparation cycles used prior to copper plating can vary considerably, depending on the alloy or type of substrate. Also, substrate heat treatment variations can contribute to complications in surface preparation. Because there are also variations in organic and inorganic soil conditions on the work to be plated, preparation cycles should include adequate cleaning, rinsing, and activation steps to ensure quality deposits. Some of the cleaning methods used to prepare substrate surfaces prior to copper plating include soak or electrolytic alkaline cleaning, alkaline derusting, vapor degreasing, and solvent cleaning.

Good rinsing between preparation steps is a very important and often-overlooked step in the preparation cycle. Time, temperature, and concentration considerations should be applied to rinsing techniques as well as to the cleaning processing solutions. Often, rinse times are too short, immersion temperatures are too cold, and the water flow rate is too low to adequately rinse cleaner films from the surfaces.

The activation step is usually carried out with the use of an acid to remove inorganic soils, oxides, or cleaner films from the surfaces. The acid used depends on the type of substrate to be plated. The most commonly used acids in preplate processes are hydrochloric acid and sulfuric acid. More information about the techniques used in these preparation processes is found in the Section "Surface Cleaning" in this Volume. Specifications and practices for copper electroplating are given in Table 5.

Table 5 Specifications and standards for copper electroplating

Specification

Uses

Copper plating

AMS 2418

Copper plating

MIL-C-14550 (Ord)

Copper plating

ASTM B 503

Recommended practice for use of copper and nickel electroplating solution for electroforming

Copper plating in multiplate systems

ASTM B 456

Specification for electrodeposited coatings of copper plus nickel plus chromium and nickel plus chromium

ASTM B 200

Specification for electrodeposited coatings of lead and lead-tin alloys on steel and ferrous alloys

AMS 2412

Plating silver, copper strike, low bake

AMS 2413

Silver and rhodium plating

AMS 2420

Plating, aluminum for solderability, zincate process

AMS 2421

Plating, magnesium for solderability, zincate process

QQ-N-290

Nickel plating (electrodeposited)

Surface preparation

ASTM A 380

Practice for cleaning and descaling stainless steel parts, equipment, and systems

ASTM B 183

Practice for preparation of low-carbon steel for electroplating

ASTM B 242

Practice for preparation of high-carbon steel for electroplating

ASTM B 252

Recommended practice for preparation of zinc alloy die castings for electroplating

ASTM B 253

Practice for preparation of aluminum alloys for electroplating

ASTM B 254

Practice for preparation of and electroplating on stainless steel

ASTM B 281

Practice for preparation of copper and copper-base alloys for electroplating and conversion coatings

ASTM B 319

Guide for preparation of lead and lead alloys for electroplating

ASTM B 322

Practice for cleaning metals prior to electroplating

ASTM B 480

Practice for preparation of magnesium and magnesium alloys for electroplating

ASTM B 481

Practice for preparation of titanium and titanium alloys for electroplating

Cyanide Baths. Although the dilute cyanide and Rochelle cyanide baths exert a significant cleaning action on the surface of the parts during the plating operation, thorough cleaning of parts to be plated in these baths is still necessary.

The high-efficiency sodium cyanide and potassium cyanide electrolytes have virtually no surface-cleaning ability during plating because of the absence of hydrogen evolution. Parts to be plated in these electrolytes must be thoroughly cleaned. Parts also must receive first a dilute cyanide copper strike about 1.3 pm (0.05 mil) thick.

Noncyanide Alkaline Baths. Unlike cyanide baths, noncyanide alkaline baths do not offer any cleaning, and parts plated in these electrolytes must first be thoroughly cleaned, rinsed, and activated. If being used as a strike prior to acid copper or other similar deposit, a minimum thickness of 5.2 pm (0.2 mil) is desired. These systems can be plated directly on properly prepared steel, brass, stainless steel, zincated aluminum, lead-tin, and most high-quality, properly prepared zinc-base die castings (Ref 4, 5). One advantage of the noncyanide electrolyte is the fact that accidental drag-in of acids poses no hazard of the evolution of poisonous cyanide gas, which could occur with cyanide copper electrolytes.

Pyrophosphate Baths. If pyrophosphate electrolytes are to be used, conventional cleaning cycles are generally satisfactory. A preliminary strike should be applied to steel, zinc-base die castings, magnesium, and aluminum. The strike solution may be a dilute cyanide copper, dilute pyrophosphate copper, or nickel. If a cyanide copper strike is used, adequate rinsing or, preferably, a mild acid dip following the strike is recommended before final pyrophosphate copper plating.

Acid Baths. When sulfate or fluoborate copper is to be deposited, steel or zinc must first receive a cyanide or noncyanide alkaline copper or nickel strike. With complete coverage, the strike may be as thin as 2 pm (0.08 mil). After the strike, the parts should be dipped in a dilute solution of sulfuric acid to neutralize solution retained from the alkaline strike bath. The parts should be rinsed thoroughly before acid copper plating. Nickel or nickel alloy parts, when surface activated by reverse-current etching in sulfuric acid, can be plated directly, provided contact is made to the work with the current or power on before immersion into the acid copper solution.

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