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Finite Element Analysis of Tin-Bismuth Electromigration of Solder Joints
This paper is based on the planar solder joint approach which affords non-destructive real-time monitoring of the solder joint microstructure in a scanning electron microscope.
Technical Paper
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Authored By:
Mehdi Hamid, Prabjit Singh, Tom Wassick
IBM Corporation
MN, USA
Haley Fu
iNEMI
Shanghai, China
Raiyo Aspandiar
Intel Corporation
OR, USA
Summary
Electromigration is a mass transport phenomenon involving diffusion, electric current, mechanical stress and temperature gradients. Atomic movement from cathode to anode in metal lines subjected to high current density causes voiding on the cathode side and hillocks on the anode side of solder joints. The movement of atoms towards the anode causes back stress that counters electromigration to the extent that it may stop electromigration in very short solder joints.
The need for a low melting point solder, lower printed circuit board and component warpage, eco benefits (green technology) and lowering reflow temperature to accommodate components which can’t withstand tin, silver and copper alloy solders reflow temperatures have been leading the microelectronic industry towards Sn-Bi alloys. In this work a finite element model was developed to predict the resistance of a Sn-Bi solder joint to electromigration and to determine the temperature, current density, and mechanical stress distribution in a solder joint. A novel empirical electromigration test method is introduced that can estimate the temperature of the solder joint before and after the electromigration test. The modeling and test results are compared and discussed.
Conclusions
It is known that three factors that play major role in electromigration in Sn-Bi solder joints are current crowding, Joule heating and crystallographic orientation of the grains. In this study the effect of all three were analyzed. The effect of crystallographic orientation of Bi grains were implemented by calculating the activation energy of Bi grains in Sn-Bi alloy through experimental testing. Current crowding and Joule heating effects were implemented by employing governing equations for these physical phenomena. The predicted solder temperature due to electromigration and Joule heating was in a close agreement with measured solder joint temperature.
Additionally, the solder resistance for the given ambient temperature and applied current was predicted to with 10 % margin of the measured resistance value. The reasons for the observed discrepancy between the measured and predicted resistance values may be due to solder-conductor cross section mismatch as well measurement errors during testing and solder shape mismatch. To improve modeling prediction results in subsequent studies, geometry modification as well as acquiring more detailed Sn-Bi material properties related to physics of electromigration are recommended. Also, more testing data points are required to verify modeling results for a range of current density and temperatures of Sn-Bi solder joints.
Initially Published in the SMTA Proceedings
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