Thermomigration: An experimental damage mechanics study on nanoelectronic lead free solder alloys
Abdul Hamid, Mohd Foad
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This dissertation focuses on experimental study of thermomigration in lead-free solder alloys. Thermomigration in microelectronic solder joints was not a concern until significant miniaturization of electronics devices required to run high current densities with smaller solder joint sizes. High current density induces electromigration and Joule heating at the same time. The imbalance of Joule heating generated at top and bottom of solder joint causes a temperature gradient which is large enough to induce thermomigration damage. In the literature, most studies report electromigration induced damage without considering the influence of thermomigration, thus the effect of electromigration and thermomigration can not be individually identified. This dissertation studies the experimental damage mechanics of thermomigration without electromigration by studying formation and destruction of intermetallic compound, and vacancy migration due to diffusion driving forces. Microstructural degradation and hardness testing were used to quantify thermomigration induced damage. After studying material science and physics behind the thermomigration process, using test vehicles, the combined effects of electromigration and thermomigration were studied experimentally. The studies were repeated at a subzero temperatures to see the effect of low temperature on thermomigration and electromigration, and reliability of nanoelectronic solder joints. A new time to failure equation is proposed to show a threshold temperature below which diffusion slows down significantly. By ensuring the solder operating temperature is well kept below the threshold value by proper thermal management, the solder joint life can be extended.