Eliability analysis of lead free solder

The reliability of electronic assemblies is highly dependent on the quality of solder joints, and the latter’s response to temperature excursions. Finite element modeling (FEM) has been widely used for the estimation of the lifetime of solder joints subjected to temperature cycling. Thanks to the expertise of decades, a significant number of companies, universities and research institutes were able to have a relatively accurate estimation of life time for SnPb solder. For the lead-free solder materials, first attempts for correlation models show up but there are several problems. First of all, there is a wide range of alloys and alloy compositions, which have a different material behavior (E-modulus, CTE) but also a different resistance to thermal fatigue. Second, it is shown in several papers that lead-free solders have different failure modes compared to SnPb. In particular at low temperatures (-20°C, 50°C), some lead-free materials show brittle behavior and this is not covered by the current simulation models based on creep fatigue at high temperature. Experiments show that the trends in lead-free solder joint reliability are cycling-condition and package dependent. In this paper, the simulation results for commonly used solder alloys are presented and the thermal fatigue reliability of lead-free solder joints has been investigated. An isothermal fatigue test method was used in this study to improve the efficiency of fatigue study, and two different lead-free solder alloys, Sn-Ag-Cu, Sn-Ag were investigated. It was found that the lead-free solder alloy was more reliable compared to the lead alloy and this is package dependent.