Correlation Between Solder Joint Fatigue Life and Accumulated Work in Isothermal Cycling

Abstract

The fatigue behavior of solder joints in realistic service applications is still not well understood. Service life prediction based on conducting accelerated tests and extrapolating test results therefore involves a high potential for error. Understanding both the evolution of solder joint properties and the damage accumulation has proved to be critical to reliability modeling. Damage accumulation in isothermal cycling is shown to scale with the accumulated inelastic work even in complex cycling scenarios, so that the life of a solder joint ends upon accumulation of a given amount of work. Individual ball grid array solder joints were cycled in shear fatigue experiments with different load amplitudes and strain rates. The effects of load amplitudes and strain rates on the work accumulation and fatigue life were systematically addressed. The correlation between different loading scenarios and the accumulated work to failure was also discussed. The results showed that the accumulated work until the development of a major crack is constant regardless of the load amplitude. After that the accumulated work to failure is lower for larger load amplitudes. For some reason, a larger fraction of the work appears to be dissipating as heat at lower load amplitude, but only during crack growth. On the other hand, the strain rate affects the fraction of the work going to heat even before the development of a major crack.

Challenges for the prediction of solder joint life in long term vibration

“…We do, as said, ascribe this effect to the build up of an ever more extensive dislocation structure with increasing pathways for atomic diffusion. Whatever the creep mechanism involved, we have shown the average rate of damage in solder to scale with the work per cycle [5,6] so the permanent 'softening' effect leads to amplification of the subsequent damage at lower amplitudes. This explains the breakdown of Miner's rule for solder.…”

“…This scaling does however not account for strong variations between individual solder fatigue life times, a fact that we suggest reflects a strong effect of Sn grain orientation on the fraction of the work dissipated as heat [5]. Anyway, as we shall see available constitutive relations do not apply to solder joints in reversed load cycling.…”

“…The stiffness increases rapidly during the first few cycles as the solder bump flattens somewhat and hardens. After that the stiffness remains essentially constant until the initiation and relatively rapid growth of a major crack [5,16]. This picture changes, however, if the cycling amplitude is varied.…”

“…We have also shown the average rate of damage in solder, and resulting life times, to scale with the work per cycle, in principle allowing for the extrapolation of accelerated test results [5,6]. This scaling does however not account for strong variations between individual solder fatigue life times, a fact that we suggest reflects a strong effect of Sn grain orientation on the fraction of the work dissipated as heat [5].…”

A Mechanistic Thermal Fatigue Model for SnAgCu Solder Joints

“…In the case of isothermal cycling, say in vibration, the resulting accumulation of damage and eventual failure cannot be predicted, even approximately, based on results of fixed amplitude cycling tests alone. [63][64][65] That is a result of longterm effects ('memory') of amplitude on the material's properties. Ignoring these leads to overestimates of life in service.…”