Measurement of Temperature Distribution in SnAg3.5 Flip-Chip Solder Joints during Current Stressing Using Infrared Microscopy

Several simulation studies reported that a hot spot exists in flip-chip solder bumps under accelerated electromigration. Yet, there are no experimental data to verify it. In this paper, the temperature distribution during electromigration in flip-chip SnAg3.5 solder bumps is directly inspected using infrared microscopy. Two clear hot spots are observed in the bump. One is located at the region with peak current density, and the other one is at the bump edge under the current-feeding metallization on the chip side. Under a current stress of 1.06 times 10⁴ A/cm², the temperature in the two hot spots is 161.7degC and 167.8degC respectively, which surpass the average bump temperature of 150.5degC. In addition, effect of under-bump-metallization (UBM) thickness on the hot spots is also examined. It indicates that the hot-spot temperature in the solder bump increases for the solder joints with a thinner UBM. Electromigration test indicates that these hot spots have significant influence on the initial failure location.