Improvements in wire bonding and solderability of surface mount components using plasma cleaning techniques

Examples of plasma-removed residues from printed wiring boards (PWBs) surface mount bonding lands are presented. The land areas are defined in photodefinable conformal coatings by conventional photolithographic techniques and have a nonvisible surface residue which inhibits subsequent bonding to the land. Auger analysis, electroless tin plating, and hot air solder levelling are used as diagnostic tools to detect polymeric residues on the land areas. The three techniques all showed that polymeric residues were present on the copper land surfaces after photodefinition and development. A 10-min plasma chemical etch consistently removed the residue, leaving a clean and bondable copper land surface. A 10-min plasma chemical etch applied to gold-plated copper land areas which had been photodefined in a dry film conformal coating improved the wire bond strength over the untreated samples by a factor of five, to an average value of two times the 5-g minimum bond strength requirement.<
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