Thermomechanical fatigue behavior of Sn-Ag solder joints

Microstructural studies of thermomechanically fatigued actual electronic co mponents consisting of metallized alumina substrate and tinned copper lead, soldered with Sn-Ag or 95.5Ag/4Ag/0.5Cu solder were carried out with an op tical microscope and environmental scanning electron microscope (ESEM). Dam age characterization was made on samples that underwent 250 and 1000 therma l shock cycles between -40 degrees C and 125 degrees C, with a 20 min hold time at each extreme. Surface roughening and grain boundary cracking were e vident even in samples thermally cycled for 250 times. The cracks were foun d to originate on the free surface of the solder joint. With increased ther mal cycles these cracks grew by grain boundary decohesion. The crack that w ill affect the integrity of the solder joint was found to originate from th e free surface of the solder very near the alumina substrate and progress t owards and continue along the solder region adjacent to the Ag,Sn intermeta llic layer formed with the metallized alumina substrate. Re-examination of these thermally fatigued samples that were stored at room temperature after ten months revealed the effects of significant residual stress due to such thermal cycles. Such observations include enhanced surface relief effects delineating the grain boundaries and crack growth in regions inside the joi nt.