Electromigration has been regarded as a serious reliability problem in microelectronics. With the increased demands and consequent miniaturization of electronic devices, electromigration is posing a challenge not only to the reliability of integrated circuits but also to the electronic packages.

Real-time viewing of electromigration in flip chip solder joints could be useful to learn more about how to mitigate electromigration. Previously, some methods involved SEM monitoring which require continuous interruptions to the electromigration test. Previous electromigration imaging techniques require cross sectioning of multiple flip chip samples to capture changes at different time intervals. This process is time consuming and introduces sample-to-sample variation when comparing micrographs from different samples. In this paper, a methodology is reported that allows for quasi-continuous in-situ optical microscopic imaging of the same sample.

A flip chip sample was coated in epoxy and ground at the SAC305 solder joint location, serving as the plane for viewing. Current stressing was applied to the solder joint to produce current crowding to facilitate electromigration, with a theoretical current density of  for 5A of current. In-situ optical microscopic imaging and resistance measurements were performed simultaneously until an open circuit was observed. At 92 h, a crack started to form and continued growing until the end of current stressing. At 159 h, a void started to form and grew to a diameter of 20 μm. After 287 h, current was stopped as an open circuit occurred. Microstructural changes can also be monitored after current stressing is stopped, if they exist due to the thermal stress occurring once the sample cools down. These changes highlight the importance of real-time imaging to trace the source of specific events, which otherwise would not have been captured.

The method presented in this paper provides a technique to observe morphological changes in a solder joint due to current crowding, which has great potential carry-over to understanding and comparing electromigration in test structures. This method can be used to aid in the monitoring of void nucleation and growth. Furthermore, this method helps illustrate the sequence of events occurring due to electromigration. Failure mechanisms involving void formation as well as factors such as IMC formation and grain rotation could be better understood through in-situ imaging. Changes in the micrographs can be traced to their respective time periods and resistance measurements. Lastly, the use of this method can distinguish between defects caused by electromigration to those caused by manufacturing processes.


Real-Time Viewing of Electromigration in Cross-Sectioned Flip Chip Solder Joints

Division / Sponsors: 
Electronic & Photonic Packaging Division (EPPD)

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2020/volume/84041

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