Solder Creep-Fatigue Interactions With Flexible Leaded Parts

Abstract

With flexible leaded parts, the solder-joint failure process involves a complex interplay of creep and fatigue mechanisms. To better understand the role of creep in typical multi-hour cyclic loading conditions, a specialized non-linear finite-element creep simulation computer program has been formulated. The numerical algorithm includes the complete part-lead-solder-PWB system, accounting for strain-rate dependence of creep on applied stress and temperature, and the role of the part-lead dimensions and flexibility that determine the total creep deflection (solder strain range) during stress relaxation. The computer program has been used to explore the effects of various solder creep-fatigue parameters such as lead height and stiffness, thermal-cycle test profile, and part/board differential thermal expansion properties. One of the most interesting findings is the strong presence of unidirectional creep-ratcheting that occurs during thermal cycling due to temperature dominated strain-rate effects. To corroborate the solder fatigue model predictions, a number of carefully controlled thermal-cycle tests have been conducted using special bimetallic test boards.

Finite element simulation of the temperature cycling tests

“…A number of researchers have proposed constitutive models to study damage mechanics of solder alloys. Some of them are [1], [6]- [8], [14], [15], [19], [20], [22], [24], [26], [28], [30], [32]- [34], [36], [38], [39], [41], [42], [46], and others. "A common shortfall of these models is that most are empirical in nature and do not include the microstructural behavior that occurs in Sn/Pb solder during thermomechanical fatigue," Frear et al [14].…”

Computer simulation of solder joint failure

“…Sauber and Seyyedi (1992), used a power law creep model in a finite element simulation to calculate the solder joint creep response on leaded devices. Ross, et al (1992) also modeled leaded devices and included both creep and creep ratcheting in the algorithm to determine stresses and strains in the solder joints. Schmidt (1992) developed a life prediction model of leadless solder joints using incremental damage summation to predict life.…”

Solder Joint Creep and Stress Relaxation Dependence on Construction and Environmental-Stress Parameters

“…These results graphically illustrate the complex interdependency between shear strain and heel-toe tension-compression strain with gull-wing or Jlead type leads. This interdependency reflects the fact that the effective stiffness ratio in the horizontal shear direction drops off much faster with increasing lead height than does the effective stiffness ratio in the normal-to-the-board tension-compression direction, This complex strain interdependency becomes even more important in thermal-cycling environments as discussed by Ross, et al (1992).…”

“…To achieve a quantitative assessment of the complex system level creep-fatigue interactions involved in electronic part solder joints, the authors utilized a unique non-linear finite-element creep simulation computer program especially developed to directly utilize the solder constitutive properties described by Figures 1 and 2 and Equation 2 (Ross, et al, 1991 andRoss, et al, 1992); a similar program has also been developed by Pan (1991). Given stresses following the soldering operation.…”