The bonding of surface mounted components to printed wiring boards (PWB's) is critical to the high-yield assembly of components to the PWB. This process is one of the last steps performed in a complicated manufacturing and assembly sequence. Poor bondability at this late stage of assembly produces costly scrap. Aggressive wet chemical processes are often used to improve bondability by cleaning the bonding lands in an attempt to salvage PWB's with poor bonding yields. The aggressive wet chemical processes may succeed in cleaning the residues from the metal bonding lands, but the process may cause the polymeric materials that surround the land areas to lift away from the lands or become porous, or may even destroy surface markings and nomenclature. Even with careful process control during the final formation of land areas in the conformal coating, a thin residue, often invisible to the eye, can partially or fully cover the bonding land area. The residue may be extremely thin, but it inhibits bonding and is very resistant to conventional wet chemical cleaning methods. Plasma chemical etching is the one chemical process that can remove the residue from the metal lands and restore bondability without damaging other surfaces of the ready-to-assemble PWB. This dry, environmentally benign vacuum process completely removes organic residues by breaking them up into oxygen, hydrogen, and carbon species. These species then recombine to form volatile compounds that are desorbed from the surface and removed by the vacuum system. The surface that is left behind is chemically clean of organic materials. In this paper we report on examples of plasma-removed residues from PWB surface mount bonding lands. 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 leveling 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. Aggressive wet chemical techniques used to remove the polymeric residues were either ineffective or inconsistent in their ability to remove the residues. In contrast, 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. The results showed conclusively that a plasma chemical etch process can consistently remove polymeric residues and promote bondability to ready-to-assemble PWB lands. At the same time, the visual clarity and mechanical integrity of the surrounding conformal coating surface remained unchanged.
