The influence of room temperature aging on ball shear strength and microstructure of area array solder balls

Solder ball shear strength is determined for ball grid array (BGA) packages in the as-received condition and after solder reflow preconditioning (exposure to multiple solder reflow profiles). Immediately after reflow, the solder shear strength is considerably higher than the solder shear strength measured on packages in the as-received condition. When the shear strength is monitored as a function of the time after solder reflow preconditioning, it is found to decrease significantly with time. Microhardness tests made in parallel with the shear measurements, provide quantitative confirmation of a degradation in mechanical properties of the solder over time. The observed changes in mechanical properties are the result of room temperature annealing, or age-softening of the solder. Cross sectioning and scanning electron microscopy are used to identify and track microstructural changes in the solder balls with room temperature aging. The microstructural changes are correlated to the decrease in shear strength and microhardness and in some cases, to a change in failure mode. These results are discussed in terms of accurate interpretation of shear test data and suggestions are made for test procedures that can minimize data variations induced by solder aging effects.