Investigation of Solder Fatigue Acceleration Factors

Solder fatigue was investigated experimentally for effects of the strain loading waveform in isothermal mechanical fatigue, and frequency and strain range in true thermal cycling. Rapid strain loading followed by a long hold period at constant strain was found to be very damaging compared to constant rate cycling at the same peak amplitude and period. Thermal cycle fatigue data showed a stronger dependence on cycle period and weaker dependence on strain range than reported results in isothermal mechanical fatigue. Implications of the experimental findings are discussed with regard to solder joint fatigue and accelerated life testing of leaded chip carriers. Isothermal mechanical fatigue testing was carried out at room temperature with a custom torsional apparatus having an optical servo-loop for strain control. Mean joint compliance change per cycle was used as a fatigue metric. Experiments were carried out on 5/95 and 63/37 Sn/Pb solder joints. Mean stress in the solder joints was measured at 23°C and found to relax significantly after several minutes. Thermal cycle experiments were carried out with an air impingement apparatus capable of cycling the test samples between 30110°C in periods ranging from 2 to 120 min. Ceramic chip carriers were soldered to aluminum, zinc, and stainless steel substrates to permit fatigue life data to be taken from 0.3-14 percent strain range. A conventional crack criterion was used as the fatigue metric in the thermal cycle experiments. A simple finite element model of a 68 post molded leaded chip carrier (PLCC) was analyzed to examine expected stress loading of the solder joints under typical operating conditions. Transient thermal stress loading and low stress creep of the solder joint are discussed in light of this model and known creep properties of solder. It is shown that during typical operating cycles, the total creep strain can account for the majority of plastic or nonrecoverable strain.