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Drop/Shock Behavior of Low Temperature Solder BGA
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Authored By:Sufyan Tahat, Sa’d Hamasha, Ph.D. Auburn University Al, USA Michael Meilunas Universal Instruments Corporation NY, USA SummaryAs electronic devices continue to expand into all aspects of modern life, the demand for reliable soldering materials has become more critical. The transition to lead-free alternatives has led to the emergence of high-temperature Tin-Silver-Copper alloys and low-temperature Tin-Bismuth alloys, each adapted to specific applications. This study examines challenges posed by aging, a critical factor influencing the long-term performance of solder joints. Aging, often associated with gradual material changes, can impact mechanical and thermal properties, potentially compromising the reliability of electronic assemblies. To address these challenges, this research utilized controlled aging conditions to simulate real-world scenarios, with a particular focus on room temperature aging for two alloys: SAC305 and L29. Samples were evaluated using drop/shock testing to assess the durability of BGA solder joints under mechanical stress shortly after assembly and up to 50 days later when stored at room temperature. Results showed that aging at room temperature enhanced the reliability of L29 alloy but had negligible impact on the SAC305 alloy. ConclusionsThis research investigated the impact of room-condition aging up through 50 days on SAC305 and L29 solder alloys using drop/shock testing as a means to evaluate solder joint reliability. The study found that L29 solder joints were inherently more brittle than SAC305 solder joint owing to their bismuth content. This increased brittleness necessitated testing the L29 solder joints at a lower shock input pulse than the SAC305 solder joints. Due to the different test conditions, the experimental results for SAC305 and L29 are not meant to be directly compared, but instead the SAC305 results should be used to establish normal expectations for solder behavior over time. It was found that SAC305 performance was not impacted by room-temperature aging with samples tested shortly after reflow soldering performing similarly to those tested 50 days later. Normal variability from part to part and within the drop test procedure were likely larger factors affecting the test results than the time after reflow. Notably, the L29 alloy exhibited an improved drop/shock performance due to aging with samples tested shortly after reflow demonstrating 33 to 80% lower lifetime when compared to the various aged samples. These results indicate that the L29 samples are quite weak coming out of reflow and require perhaps 7 days or longer to mechanically stabilize. Failure analysis revealed damage modes typically encountered during drop testing. In both cases, for SAC305 and L29, IMC to nickel layer failures were frequently observed at the corner solder joint positions. The IMC to nickel layer failures were associated with earlier failures for any given aging condition, and were found to be most common with the After Reflow test condition for the L29 solder. Most failures were observed to occur near the PCB pad side of the joint. Both SAC305 and L29 samples produced failure due to separation between the copper PCB pads and IMC formation but only L29 was observed to result in bulk solder failure near the PCB pad. L29 Bulk solder failure was more likely to occur with aging indicating that the intermetallic strength of the L29 alloy was improving with time. Cracking through and around the IMC layers was also encountered with SAC305 and L29, but to a lesser extent than the other modes. Initially Published in the SMTA Proceedings |
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