A methodology to establish baseline metrics for assessing the isothermally aging of Sn‐Pb solder interconnects

A methodology was developed to establish baseline metrics for assessing the isothermal aging of 63Sn‐37Pb (or 60Sn‐40Pb) solder joints in circuit board assemblies. Those metrics were the intermetallic compound layer thickness at the Sn‐Pb solder/Cu interface and the Pb‐rich phase particle size distribution in the solder. The baseline, or as‐fabricated, values for these metrics were 0.71±0.27 μm and 3.2±6.5×10⁻⁶ mm², respectively. The rate kinetics were determined for growth of the intermetallic compound layer and coarsening of the Pb‐rich phase particles by isothermal aging experiments. The/were: (1) intermetallic compound layer thickness (μm)=0.714+ 3.265×10³ t^0.58 exp(−52200/RT); and (2) Pb‐rich phase particle size (mm²)=3.2×10⁻⁶+1.47× 10⁻³ t^0.32 exp(−31000/RT).