Abnormal growth of tin whiskers in a Sn3Ag0.5Cu0.5Ce solder ball grid array package

Abstract

It has been discovered for the first time that Sn whiskers appeared in Sn3Ag0.5Cu0.5Ce solder joints of ball grid array (BGA) packages after storage at room temperature (natural aging) for less than 3 days and they grew at a high rate of 2.9 Å/sec. In one particular case, whiskers even formed after 1 day of storage at an extremely high growth rate of 8.6 Å/sec. Experimental investigations showed that a number of CeSn₃ clusters existed in the Sn3Ag0.5Cu0.5Ce solder matrix after the reflow process. Further natural aging in air for several days caused the CeSn₃ phases to oxidize rapidly, from which many Sn whiskers sprouted and grew to a length of hundreds of micrometers. The most commonly observed whiskers have been long fiber-shaped ones of 0.1 µm to 0.3 µm in diameter (type I), while short whiskers larger than 1 µm in diameter can also be found (type II). Here in our case, the surface oxide of the CeSn₃ phase possessed a higher content of Ce than of Sn, which implied that a Ce-depleted region (nearly of pure Sn) was left beneath the oxide layer. The abnormal whisker growth was attributed to the compressive stress squeezing the Sn atoms in the Ce-depleted region of CeSn₃ phase out of the oxide layer.