Microstructure evolution of SnPb and SnAg/Cu BGA solder joints during thermal aging

The microstructures and performances of eutectic SnPb and near eutectic SnAg/Cu solder joints on the Au/Ni/Cu substrates of ball-grid-array (BGA) socket were evaluated and compared after reflow and thermal aging treatments at 150 and 170°C The growth rate of intermetallic (IM) layer was characterized and analyzed using SEM and EDS line scan techniques. Ball shear tests were used to determine the failure shear stresses of the joints. It is found that the lead-free SnAg/Cu alloy has a faster IM growth rate during reflow, compared to the eutectic SnPb. In solid-state aging, however, a faster growth rate is found for the interfacial IM layer with SnPb solder than for the lead-free alloy of which no clear IM growth after aging at 150 and 170°C for 735 hours. In addition, significant phase coalescence was found in the aged SnPb solder joints. Spheroidization and coalescence of Ag-rich phases in the eutectic regions were also observed for the aged lead-free SnAg/Cu solder joints. From the ball shear test results, the failure shear stresses decrease as aging time increases for the SnPb and SnAg/Cu joints aged at both 150 and 170°C excepting an initial increase of the failure shear stresses for the SnAg/Cu solder joints aged at 4-6 hours. Also, the decrease of the failure shear stresses is leveled around aging time of 20 hours up to the maximum aging time in the study for SnPb aged at 150°C and SnAg/Cu aged at 150 and 170°C. However, generally, better high temperature performances were observed for the lead-free SnAg/Cu solder joints compared to the eutectic SnPb joints