Effect of reflow conditions on the intermetallic layer in solder joints

The Cu
₆
Sn
₅
intermetallic formed between Pb-free Sn-based solders and Cu substrates plays a critical role in the reliability of solder joints. Cu
₆
Sn
₅
can adopt at least two different crystal structures over the temperature range to which electronics circuitry is exposed during assembly and service. Under equilibrium conditions the change from a hexagonal to monoclinic structure occurs on cooling solder joints below 186°C. However, cooling rates associated with common commercial soldering processes are typically faster than the rate that would permit complete transformation. In this paper the authors report a study on the effect of reflow cooling conditions on Cu
₆
Sn
₅
crystal structure. The findings have important implications for the manufacture of solder joints and their in-service performance.