Effect of Electromigration and Isothermal Aging 
on the Formation of Metal Whiskers and Hillocks in Eutectic Sn-Bi Solder Joints and Reaction Films
Numerous electronic system failures have been attributed to short circuits caused by metal whiskers that bridged closely spaced circuit elements when they were maintained in a high-current-density environment. Typically, in single-phase interconnect materials, atoms are driven from the cathode to the anode and a compressive stress can build up at the anode end of the stripe, which can induce the formation of whiskers and hillocks. However, the electronic solders used in interconnects are multiphase materials where primary and secondary diffusion atoms/ions exist. In order to accelerate the growth of whiskers and hillocks, a high current density (104 A/cm2) combined with high ambient temperatures (80°C) was applied to eutectic Sn-Bi solder joints. Metal whiskers and hillocks were observed on the overflowed solder film above the Cu substrate. However, in the absence of the electric field, metal whiskers and hillocks could also be squeezed out from the solder reaction films after several days of isothermal aging (125°C), demonstrating that the chemical reaction between Sn atoms and Cu atoms can provide the driving force for the formation and growth of metal whiskers and hillocks.