Thermomechanical fatigue testing and analysis of solder alloys

Thermomechanical fatigue (TMF) is one of the most common causes of failure in solder joints. TMF occurs due to the introduction of stresses arising from thermal expansion mismatch during thermal cycling caused by either internal heating from power dissipation, the external environment, or both. Due to its complicated nature including partial compliance of joints, evolution of microstructure, and multiple deformation mechanisms, testing a material's resistance to TMF is not straightforward. A method developed at Rensselaer, based on an apparatus designed at North Carolina State University, allows the direct measurement of each stress-strain cycle during thermal cycling. Through further analysis, using spreadsheet macros, it is possible to consider the energy absorbed by the joint on a per cycle basis. The interpretation of such data as well as the cumulative energy absorption can provide insight into the failure process. The behavior of three alloys, eutectic Sn-Bi, Sn-Pb, and Sn-Ag, will be presented. These data will be compared with that generated by other methods. A comparison of the behavior of these alloys, as well as the apparatus design, test method, interpretation, and possible enhancements is discussed. [S1043-7398(00)01201-9].