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FEBRUARY 2024 I DESIGN007 MAGAZINE 39 finishes shown are standard ENEPIG, thin nickel ENEPIG, and ultra-thin nickel ENEPIG. Interestingly, it can be observed that as the nickel interlayer thickness decreases, the drop shock performance improves, primarily because of an increasingly higher proportion of copper-tin solder joint. Initially, it was believed that the complete elimination of nickel, as seen in EPIG or modi- fied EPIG finishes, would further enhance the drop shock performance. However, this is not the case. e reason lies in another deposition mechanism utilized during the plating process for EPIG finishes, which results in microvoid formation 5 . Silver-gold, immersion silver, and OSP fin- ishes all provide robust drop shock perfor- mance, which is linked to the preferred cop- per-tin solder joint formation in those finishes, compared to more brittle nickel-tin solder joint in nickel-containing finishes. Solder Joint Electromigration Test As described in the experimental method- ology section, the solder joint electromigra- tion tests were done at constant current (5A) and temperature (100°C). e resistance was monitored throughout the testing period (400 hours). If there was a 20% increase over initial resistance, it was considered a failure. Figure 3 presents the electromigration results for the solder joints of all the surface finishes tested, displaying the resistance plotted against the testing time for up to 400 hours. With the exception of standard EPIG and modi- fied EPIG, consistent resistance is observed throughout the entire test duration for all sur- face finishes. However, both standard EPIG and modified EPIG exhibit a sharp increase in resistance aer approximately 50 to 100 hours of testing, indicating that the solder joints have been compromised during the high current/ high temperature test. X-ray imaging was employed to examine the alterations in solder joints before and aer the electromigration test, indicated by the red arrow in Figure 4a. As the X-ray images clearly display, it is evident that the solder joints in standard EPIG have collapsed following the test. To conduct a detailed examination of the solder joints, a cross-section sample was made based on the X-ray imaging results. Figure 4b displays a scanning electron microscope (SEM) image of a standard EPIG solder joint (cathodic side) aer undergoing the electromigration test. e inset provides an overview of the cross-sec- tion. e red circles indicate voids that appear in the solder joint when subjected to high cur- rent density, leading to collisions between elec- trons and metal atoms. ese collisions cause the metal atoms to move in the direction of electron flow. Over the course of the electromi- gration testing, this movement of metal atoms Figure 4: X-ray imaging of standard EPIG solder joints before and after electromigration test.