Effects of thermal aging and Au addition on the electrical resistance of solder balls in flip chip and ultra-CSP packages

The present study is aimed at investigating the effects of thermal aging and Au addition on the electrical resistance of solder balls (bumps) in flip chip (FC) and UltraCSP packages. The specimens include one FC with eutectic Sn-Ag-Cu solder bumps and two types of UltraCSP, which have 20-mil and 30-mil eutectic Sn-Pb solder balls, respectively. The thermal aging test is performed with thermal aging at 175°C for FC, and 150°C for CSPs for up to 2000 hours, respectively. The electrical resistance of each daisy-chain pair of solder balls is measured using the Four Point Kelvin Method. With the increase in thermal aging time, electrical resistance of CSPs without Au increases gradually. After 1000 hour, the resistance seems to level off. While the resistance of CSPs with Au addition ascends monotonically with prolonged thermal aging. For Sn-Ag-Cu flip chip solder bumps, at 175°C for only 24 hours, the electrical resistance of a daisy-chain pair of solder bumps drops substantially. From 24 to 2000 hours, the electrical resistance increases at first and soon becomes stable. The change in the electrical resistance is related to the microstructural evolution in the solder balls.Copyright © 2003 by ASME