Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Typical Plating on Plastics Processing Cycle Outline

STEP

SOLUTION FUNCTION

Alkaline soak cleaning-optional

Removes shop oils, fingerprints, etc

Pre-Etchant

Promotes Etching

Etchant

Promotes Surface for Adhesion

Neutralizer

Removes Residual Etchant

Catalyst

Deposits Tin-Palladium Layer

Accelerator

Activates Catalyst

Electroless plating

For PoP, nickel or copper, for FRI, copper.

Electrolytic plating

For PoP, copper, nickel or chromium

Rack stripping

Prevents plating on the plating racks and fixtures

Most commercial applications, except RFI/EMI shielding, use the initial electroless deposit as a base for subsequent electrolytic plating. The initial copper or nickel layer is coated with successive layers of electrolytic copper, nickel, and chromium. The exact types and thickness of metal used are determined by the end use of the part, e.g., automotive exterior, decorative, plumbing, and others.

Etching. This crucial step is necessary to give best possible adhesion of metal to plastic. For plating of ABS plastic substrates, solutions of strongly oxidizing chromic acid-sulfuric acid-water or chromic acid-water are operated near the point of mutual saturation. The etchant both physically roughens the surface and chemically modifies it to give a very hydrophilic surface. Metal adhesion occurs owing to the combined effects of chemical bonding and mechanical locking to the roughened surface. In the etching of ABS plastic, a polymer consisting of polybutadiene spheroids dispersed in a continuous phase of poly (styrene-acrylonitrile), the chromic acid attacks the polybutadiene at a much higher rate than the continuous phase. This gives an excellent micoroughened surface with superior metal-to-plastic bond strength. A typical recommended formulation consists of 20 vol. % sulfuric acid, 420 g/L chromic acid, and 0.1-1.0% of a fluorocarbon wetting agent. The plastic is treated with this formulation for 6-10 min at 60-65 C.

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