Most electronics companies have transitioned to lead-free processes, both to comply with government legislation and to avoid issues related to mixing of tin-lead and lead-free metallurgies. However, exemptions from lead-free legislation have been granted for certain products, especially those intended for high-reliability applications. One major concern with these exempt products is that, during assembly or rework, lead-free components will have to be used due to the unavailability of tin-lead components. This will result in the mixing of tin-lead and lead-free metallurgies. The mixing of metallurgies can induce new reliability concerns. This study is focused on mixed solder joints formed by attaching lead-free components with tin-lead paste.

Solder interconnect reliability is influenced by the environmental imposed load, solder material properties and the microstructure formed between the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminals, board pad plating and solder materials. These metallurgies react to form the microstructure of a solder joint. Microstructure of a solder joint continuously evolves and affects solder joint properties. A fundamental understanding on the microstructure is required to analyze the changes occurring in a solder joint with time and temperature and make predictions on solder joint reliability under thermal loading conditions.

This dissertation determines key microstructural features present in SnPb, lead-free and mixed solder joints. Changes in the microstructural features were determined for SnPb, lead-free and mixed solder joints exposed to isothermal aging conditions. The effect of microstructural changes on reliability was determined by conducting thermal fatigue reliability tests for SnPb and lead-free solder joints. Whereas, for mixed solder joints, hypotheses has been determined based on microstructural analysis on their thermal fatigue performance compared to SnPb joints. This dissertation doesn't include the effect of microstructural changes on the reliability of mixed solder joints. This dissertation doesn't include the reliability tests for mixed solder joints.

Two microstructural features namely, intermetallic compounds (IMC) and Pb phase were characterized for SnPb, lead-free and mixed solder joints. IMCs are formed at the solder to pad metallization interface and in the bulk solder. It was determined that reaction between Sn3.0Ag0.5Cu solder and Ni/Au component side metallization result in interfacial IMCs consisting of Ni3Sn4 IMC in the as-reflowed stage and IMCs such as (NiCu)3Sn4, (Cu,Ni)6Sn5 and (Au,Ni)Sn4 after thermal aging of 350 hours at 125ºC. With pad metallization of ImAg, ImSn and OSP, IMCs such as Cu6Sn5 are formed after reflow followed by formation of a new Cu3Sn IMC phase after thermal aging of 350 hours at 125ºC. Cu6Sn5 and Ag3Sn IMC were found distributed in bulk solder joints in the as-reflowed and aged (125ºC for 100, 350 and 1000 hrs) solder joint.

This dissertation demonstrated that under thermal cycling, intergranular crack propagates between Sn grains in the bulk solder and Cu6Sn5 IMCs present at Sn grain boundaries in the bulk solder influence crack propagation. It was demonstrated that isothermal aging for 350 hrs at 125ºC causes coarsening of Cu6Sn5 IMC particles in the bulk solder which results in a 50% reduction in number of Cu6Sn5 IMC particles in the bulk solder, thus promoting the crack to propagate faster along the grain boundary. This dissertation determined that isothermal aging for 350 hrs at 125ºC would cause a 25% reduction in characteristic life for lead-free solder joints due to the changes associated with Cu6Sn5 IMC particles.

In conventional SnPb solder joints Pb phase present in the bulk solder coarsens as a function of time and temperature and influences thermal fatigue reliability. Due to the presence of Pb in mixed solder joint, this dissertation determined the extent of coarsening in mixed solder joints compared with SnPb joints. It was determined that mixed solder joints are not prone to Pb phase coarsening under aging for 350 hrs at 125ºC as opposed to SnPb solder joints and therefore would have better thermal fatigue performance compared to SnPb joint under these conditions. This dissertation demonstrated that the presence of Pb in mixed solder results in a 30 to 40% lower IMC thickness compared to Pb-free and SnPb solder joints by being present at the interface as a diffusion barrier between Ni and Sn for IMC formation. Presence of Pb has been known to act as diffusion barrier for SnPb solder joints.