Damage based fatigue criterion for solders in electronic packaging

A fatigue failure criterion based on physical damage mechanism is proposed for solders. A solder structure is defined as fatigued when the portion of its failed grains reaches a critical percolation threshold, since under such state the failed grains may form large cluster of cracks, leading to the structure's mechanical instability as well as electrical degradation. Mechanically the inception of such state corresponds to a turning point of load carry capability in a simple fixed-strain tension-tension test of a solder structure. Experimental data for both lead-rich solders such as 96.5Pb-3.5Sn solders and lead-free solders such as 96.5Sn-3.5Ag showed consistency with the proposed criterion. Experimental data also show that by adoption of this failure criterion a better prediction of mechanical fatigue for a lead rich solder can be achieved. The criterion is derived from a science based fatigue theory, therefore the criterion should be applicable to anisotropic as well as isotropic structures regardless of solder structure sizes, which is useful to the development and design of small-scale (micron scale or smaller) solder joints for current electronic packaging.