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Thermo-Electromigration in WL-CSP Pb-Free Solder Joints

Idea/Definition Information
Background: 

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.

Problem: 

For a joint of 200 micron in size, a temperature difference of 20°C across it will cause a temperature gradient of 1,000 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.

Approach: 

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.

 
Fig. 1 shows schematic diagram of a daisy chain of flip chip SAC sample of testing. In the chain, only one pair of the solder joints was stressed by a high current density to induce thermo-electromgration. Fig. 2(a) shows a Sn whisker being squeezed out at the upper-right anode corner of a bump in which the electron current went from the bottom to the upper right corner (anode) of the bump. In the other bump in which the electrons flowed down from the upper-left corner, a pancake-type void has been formed along the cathode contact interface, as shown in Fig. 2(b).
 
 
Figure 1
 
Figure 2(a)
 
 
Figure 2(b)