Plated metals recommended adhesives

The use of supplemental plasma etching, called the Etch Back Process, is recommended to achieve high reliability of through-hole multilayer rigid/flexible circuits. The plasma gas provides further etching on both of polyimide films and adhesive layer in the drilled holes. The plasma gas does not etch metal materials and it clears the surface of the copper foils for reliable copper plating as shown in Fig. 65.9. 

Various etchants have been recommended for the remaining metals of engineering practice, but it is doubtful if sufficient work has been reported to differentiate between them or to assess their effect on the durability of the bonds formed with different adhesives. Strong, durable bonds are uncertain with copper because of the ease with which a weak, friable oxide is formed. Even when coated with an adhesive, oxygen can diffuse to the interface and eventually cause failure. Brass has an oxide film almost entirely of zinc oxide and, as with zinc galvanized iron, it can hydrate or form salts with the tackifiers added to some contact adhesives. Cadmium is met with as a plating if a strong, durable adhesive bond is essential, it should be replaced by chromium, the surface of which can be treated as stainless steel. 

These types of adhesives remain fluid in the presence of oxygen but cure to solids in its absence. The simplest anaerobic adhesive consists of dimethyl acrylate ester with the peroxide catalyst system and accelerator. They cure by free-radical polymerization in the absence of oxygen. Anaerobic adhesives have strength characteristics equivalent to those of the structural adhesives. One-part anaerobic adhesives cure at room temperature in several hours, but their cure can be accelerated to minutes by high-temperature cure and by the use of certain primers/ accelerators. Such elevated temperature, primer/accelerator-assisted cure is especially recommended when anaerobic adhesives are used on inactive surfaces, such as unclean metals or plastics, or inhibiting surfaces such as plated surfaces, chromates, oxides, and anodized surfaces. 

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