Solder joint perfection

Singularities. At multimaterial interfaces, as well as crack tips, elastic analysis will report singular stresses (Ref 46,47). The outer surface of the solder-to-pad interface where crack initiation is most likely, is an example of such a multimaterial interface. Except in the case of a perfectly plastic material, derived stresses, strains, and strain energies will he mesh dependent. Einer elements will return higher stress values than will larger elements. Since these derived quantities are most likely used to predict solder joint fatigue life, predictions become inextricably tied to the chosen mesh. 

Soldering is used not only to make a mechanical hond between surfaces but also to provide a leak-proof seal when Uqulds have to he contained. It can also be used to provide a permanent electrical connection. To provide a secure joint, it is essential that the joint surfaces are perfectly clean and free from rust, grease or any other substance likely to prevent good metal-to-metal union. The most likely cause of a bad joint, assuming the surface is clean, is the thin oxide film which is present on all metals. Oxide films can be removed using emery clotb or a flux. 

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