Creep-fatigue interactions in solders

Solders in surface-mount-technology (SMT) joints are subjected to high homologous temperatures that cause the solder to deform and accumulate damage by both time-dependent and instantaneous mechanisms. The effects of frequency, stress range, mean stress, waveform (hold times on and off load), and mechanical history are investigated in bulk uniaxial solder in the as-cast condition. Strain is measured on a per-cycle basis as well as cumulatively, to separate the mechanisms of strain storage and damage storage. At hold times less than about 10 s, a significant part of the strain is recoverable, time dependent, and nondamaging. By this strain storage mechanism, life, measured either by cycles to failure or by time on load, can be increased by at least a factor of 5. At lower frequencies the damage is stored as a function of time on load and stress (creep damage). The effective stress for a cyclic creep test is defined in terms of the stress sensitivity of the solder in creep. A technique for measuring damage for life predictions is developed and examined. This technique involves the grain boundary embrittlement of partially fatigued specimens and the statistical analysis of the voids on the surfaces