D. Morgan Tench

Solderability assessment via sequential electrochemical reduction analysis

A recently-developed sequential electrochemical reduction analysis method permits nondestructive assessment of solderability loss associated with surface oxides on metals and can be applied to almost any part geometry, including printed wiring board through-holes and surface pads. Preliminary results indicate that metal sulphides can also be detected, at least in some cases. The type and amount of oxides detected for tin coatings are shown to correlate with solderability determined by the wetting balance method. The data indicate that formation of excessive amounts of Sn02 in the surface oxide is a primary cause of Sn solderability degradation.

Oxidation of Solder Coatings

Solder coatings are widely used to protect copper printed wiring board (PWB) circuitry and electronic component leads from oxidation that can lead to loss of solderability. The coating is usually eutectic Sn-Pb that is either applied directly by hot dipping or electroplated and then densified by reflowing (melting). Numerous studies and ample practical experience have shown that, when properly applied and sufficiently thick, such coatings remain solderable even after several years of normal storage. In practice, however, the coating composition/morphology and storage conditions may not be optimum, allowing solderability degradation to occur. In addition, molten solder tends to flow away from curved areas in the substrate, resulting in localized thinning during solder dipping or reflow. Exposure of the underlying Cu-Sn intermetallic layer in the thinned region can lead to loss of solderability. Thinned solder is a particular problem at PWB through-hole rims and is commonly referred to as “weak knees.”