Effects of pre-bump probing and bumping processes on eutectic solder bump electromigration

Abstract

This paper investigates the electromigration reliability of flip chip packages with and without pre-bump wafer probing via high temperature operation life test (HTOL) using printed and electroplated bumps. Under bump metallization (UBM) of printed and electroplated bumps is a thin film of Al/Ni(V)/Cu and Ti/Cu/Ni, respectively, while the bump material consists of eutectic Sn/Pb solder. Current densities from 7380 to 20 100 A/cm 2 and ambient temperatures at 100, 125 and 150 °C are applied in order to study their impact on electromigration. The results reveal that the bump temperature has a higher influence than the current density when it comes to bump failures. The observed interconnect damage is from bumps with electrical current flowing upward into the UBM/bump interface (cathode). Identified failure sites and modes reveal structural damage at the region of the UBM and UBM/bump interface, in the form of solder voiding and cracking. The effects of current polarity, current crowding, and operation temperature are key factors to electromigration failures of flip chip packaging. The maximum allowable current density of the electroplated bumps is superior to the printed bumps by a factor of 3.0–3.7 times. Besides, the median time to failure (MTTF) of without-underfill packaging is preferred to that of with-underfill packaging by 1.5–2.2 times. Furthermore, the differences in MTTF between pre-bump and without pre-bump probing procedures are 2.0–19.4% and 1.6–10.3% for printed and electroplated bumps, respectively.