Impact reliability of solder joints

Owing to the impact reliability concern of portable devices, the impact toughness of solder joints was evaluated quantitatively by means of a miniature Charpy test, in which the shear rate was approximately 1 m/s. Four different solder balls (eutectic SnPb, near eutectic SnAgCu, eutectic SnZn and SnZnBi) were bonded to Cu or immersion Au/electroless Ni-P pads, and subsequently some of the samples were aged at 150 degreesC for up to 1000 h. Among the four solders, SnZnBi on the Au/Ni-P showed the highest impact toughness, independent of aging time. Interestingly, the SnZnBi on the Cu showed a significant loss of the toughness with aging time. We compared this miniature Charpy impact test with a conventional shear test, in which a solder bump was pushed off of its bond-pad at a shear rate of 200 mum/s. These two tests are similar in terms of applying shear stress to a bump, but the shear rate of the impact test is three orders of magnitude higher than that of the shear test. Their results were quite different: the impact test revealed the ductile-to-brittle transitions in the joints.