Effect of elevated current density in lead free solder joints

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Dr. Géczy Attila
Department of Electronics Technology

Nowadays the function density of electric circuits is increasing and their size is decreasing at the same time, therefore trends show that current density is increasing in given electronic designs. Because of the increasing current density lots of new problems appear which deserve attention and scientific investigation, one of them is electromigration which is investigated in this thesis.

In my thesis I present surface mounting technology, electromigration and its effects. I also expound some related scientific examination from the past in connection with electromigration. The aim of my study is to verify a previous work where a finite element analysis was performed for the investiging evolving current density in small scale SMD parts. Based the previous results and other literature the current density can be high enough to cause electromigrationinduced failure in the solder joints.

During my work I designed two different printed circuit boards (one for 0402 and one for 0603 surface mounted jumper resistors) to test the effects of electromigration in practice. The parts types and sizes were chosen to be the same as the simulated values. I used a solder paste printer, I programmed an automatic pickandplace machine and an infrared reflow oven to solder the circuits.

After the circuits were ready for tests I put them into ovens with elevated temperatures and I used a laboratory power supply to apply DC current onto them to induce the effect of electromigration. Prior to applying the DC load I created X-ray images of the circuits and after the current stress test as well to compare the number of voids in the solder joints. During the ongoing stress test from time to time I removed some circuits from the daisy chain and made a cross-section preparation of them.

The results show that the used parameters in my thesis did not made an apparent change caused by electromigration in solder joints, even after more than 650 hours of current stress test.


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