Creep properties of Sn-rich solder joints

Sn-rich solders that have become candidates for use in Ph-free solder joints have anomalous creep hehavior when tested in the form of solder joints. 'Their steady-state creep rates separate into two regimes with different stress exponents (n). The lowstress exponents range from -3-6, while the higlistress exponents are 7-12, unusually hieh values. Strikingly, the high-stress exponent has a strong temperature depeudence near room temperature, increasing significantly as the temperature drops from 95 to 60T The anonidlous creep behavior of the solders seems to be due to the dominant Sn constituent. Pure Sn joints have stress exponents, n, that change with stress and temperature almost exactly like those of the Sti-rich solder joints. These results have the consequence that conventional constitutive relations for steady-state crccp must be used with caution in treating Snrich solder joints, and qualification tests that are intended to verify performance should be carefully designed. In particular, the results suggest a need for rcsearch on the creep properties of Sn, which remain poorly understood.