Effects of Dwell Time and Ramp Rate on Lead-Free Solder Joints in FCBGA Packages

Many studies on eutectic solder by Amagai in 1999, Sharma and Dasgupta in 2003 and Zhai, Sidharth and Blish in 2003 have shown that the dwell time beyond certain limit has a minimal effect on the MTTF. Additional dwell time will not produce additional damage beyond a limit or boundary. However, our experiments consistently showed that the fatigue life of the lead-free solder balls decreases significantly when the dwell time increases from 15 minutes, to 30 minutes and until 90 minutes. Further failure analysis confirms that the failure mode and failure location is same when dwell time changes. The longer dwell time is, the more accumulated creep damage is. The results imply that it takes long time to entirely achieve the relaxation for the lead-free solder material. In addition, results also showed that the lead-free solder joint during thermal shock fails faster than thermal cycling. The faster ramp rate does impose more damage on solder joint than a slow ramp rate. It is concluded that the ramp time and dwell time have conflicting effects on solder joint reliability. Finite element analysis is conducted to have a fundamental understanding of the effects of ramp rate and dwell time on lead-free alloys. A remarkable agreement on the correlation between the finite element analysis and experimental results was achieved. The numerical results revealed the failure mechanism of solder joint associated with the ramp rate and dwell time. Thermal shock has a much faster ramp rate, thus imposing much more damage to the solder joints than thermal cycling. In that sense, the fatigue life decreased when the frequency increased. However, a longer dwell time causes more creep in the solder joint, thereby lowering the fatigue life significantly. This means that fatigue life decreases when frequency decreases. It can be concluded that frequency as a single parameter for a reliability model doesn't account for the conflicting effects of ramp rate and dwell time. The finite element results also show that the majority of damage occurs during the ramp period. The dwell time at high temperature is predicted to have a negligible contribution to the total inelastic strain energy density