Solder ball shear strength is determined for ball grid array (BGA)packages in the as-received condition and after solder reflowpreconditioning (exposure to multiple solder reflow profiles).Immediately after reflow, the solder shear strength is considerablyhigher than the solder shear strength measured on packages in theas-received condition. When the shear strength is monitored as afunction of the time after solder reflow preconditioning, it is found todecrease significantly with time. Microhardness tests made in parallelwith the shear measurements, provide quantitative confirmation of adegradation in mechanical properties of the solder over time. Theobserved changes in mechanical properties are the result of roomtemperature annealing, or age-softening of the solder. Cross sectioningand scanning electron microscopy are used to identify and trackmicrostructural changes in the solder balls with room temperature aging.The microstructural changes are correlated to the decrease in shearstrength and microhardness and in some cases, to a change in failuremode. These results are discussed in terms of accurate interpretation ofshear test data and suggestions are made for test procedures that canminimize data variations induced by solder aging effects
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