Thermo-mechanical reliability of lead-free solder interconnects

Lead-free solder for electronic assemblies and systems is fast becoming a reality primarily because of market driven forces. While the industry has identified possible alternatives to SnPb solder, much work still needs to be done, especially in the following areas: solder materials characterization (temperature and stress dependent inelastic behavior creep and stress relaxation, bulk versus joint behavior), failure mechanisms related to the solder joints of the new alloys (will creep deformation still play a dominant role for e.g. thermally induced low cycle fatigue?), temperature cycle data, for instance, on real components (acceleration factors may depend on accelerated test conditions and solder alloys, and field conditions may be much more benign than accelerated test conditions), life prediction models (models have to incorporate time and temperature dependent behavior of solders, implementation of constitutive equation in FEA software is one of the keys, isothermal fatigue data should not be useful for life prediction model development), solder-surface interactions (solder reacts with metallization to form interfacial intermetallics, intermetallics grow with time and temperature, metallization consumption by intermetallic growth, intermetallics within the solder, thermo-mechanical properties of the intermetallics), and assembly process development. The impact of some of these issues on the reliability assessment of lead-free solder interconnects through experiments and FE calculations is discussed in the paper, shown on examples like SMD-components and Flip-Chips.