Fatigue and Creep in Solder Joints

With the advent of surface mount components, many packaging engineers have been trying to determine the critical performance parameters for the technology. From first principals, the shear strain on a joint will be decreased if the distance between the components is increased. A simple way to do this is to increase the solder volume of the connection then fatigue cycle the parts to failure. The above experiment was performed with four different chip capacitors and six different solder volumes. The tests data confirmed a very consistent and predictable increase in fatigue life with solder joint height. Another item of interest to a package design engineer is how failures are initiated in a surface mount package. The usual assumption is that the only cause for failure is due to fatigue from dissimilar expansion coefficients of the components. Experiments have been performed the show solder cracking will occur in the solder due to the difference in expansion coefficients between the solder and component. No external loading is necessary. The size of the joint may have to be considered to minimize the problem.