Thermo-Electromigration in WL-CSP Pb-Free Solder Joints
Owing to the ban of Pb-based solder in consumer electronic products and the trend of miniaturization in wireless and portable devices, the reliability of Pb-free solder interconnection is one of the most challenging problems in electronic manufacturing industry, especially the failure caused by thermo-electromigration. Joule heating due to the on-chip Al interconnect has generated thermo-migration to accompany electromigration in solder joints.
For a joint of 200 micron in size, a temperature difference of 20°C across it will cause a temperature gradient of 1,000 ï°C/cm, which can lead to thermo-migration. Electrical gradients caused by current flow and the different characteristics of metals used in die-to-package and package-to-board connections can lead to electro-migration. We define the combined effect as thermo-electromigration. These effects can be cumulative in certain circumstances and lead to early joint failure.
To determine mean-time-to-failure (MTTF) of thermo-electromigration in SnAgCu solders of low Ag with different dopants at current density from 1.5 x 103 to 1.5 x 104 A/cm2, test methods and equipment designed by Professor King-Ning Tu of the University of California Los Angeles (UCLA) will be used.