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Reliability of Isothermally Aged Doped Low Creep Lead-Free Solder Paste
Reliability of Isothermally Aged Doped Low Creep Lead-Free Solder Paste
The primary goal of this study was to find a manufacturable solder paste material that would reduce the effects of aging on the solder joints.
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Authored By:
Sharath Sridhar, Sivasubramanian Thirugnanasambandam, Seth Gordon, Anto Raj, Thomas Sanders, Ph.D., John Evans, Ph.D.
Auburn University, Auburn, AL, USA

Wayne Johnson, Ph.D.
Tennessee Tech University, Cookeville, TN, USA

Summary
In this study, the vibration and drop impact reliability of various doped lead-free solder paste alloys was investigated. The primary goal of this study was to find a manufacturable solder paste material that would reduce the effects of aging on the solder joints. The assemblies were tested at both noaged and isothermally aged conditions. Reliability of 11 different solder paste alloys was studied in comparison with the baseline. CABGA208 package was used to conduct the study, Sn96.5 Ag3.0 Cu0.5 (SAC305) solder bumps and solder paste data was used as the baseline to compare with the doped solder paste alloys. The test vehicle consisted of 16 ball grid array packages (BGA) which were 15mm chip array ball grid array's (CABGA208) with perimeter solder balls on 0.8mm pitch.

Two sets of test boards were manufactured, the first being no aged and the second set of boards were isothermally aged at 1250C for 6 months and then tested. The boards were further categorized into 3 different reflow temperatures and 2 different stencil thicknesses, 4 mil and 6 mil respectively for the testing. JEDEC JESD22 B103B and JEDEC BS111 test standards were followed to conduct the vibration and drop impact tests. Both the no-aged and the aged test boards were subjected to accelerated life testing where the test end state was 20 hours for vibration testing and 300 drops for drop testing respectively. The results discuss the effects of aging, paste material, stencil size and reflow profile on the
reliability of the test assemblies.

Conclusions
The performance of 12 different solder pastes, supplied from 8 different manufacturers was evaluated at no-aged and aged conditions. The test boards, subjected to vibration and drop testing consisted of CABGA208 packages, QFN's and resistors. These boards were built with different temperature profiles and stencil thicknesses to help us choose the best combination for solving the current problem. During the testing, none of the QFN's and resistors failed. Hence we studied the reliability of the CABGA208 packages and compared their characteristic lifetime with the baseline.

Results from the baseline plots show that the decrease in the reliability after aging is higher in drop test when compared to vibration. Solder paste C exhibits better performance than the baseline at the no-aged condition, but the reliability reduces after aging in both the tests. Solder paste K&J performed close to the baseline at no-aged and their reliability drastically decreased after aging. We conclude by saying that the aging effects are alarming in harsh environment electronics applications and are in need of immediate attention.

Initially Published in the SMTA Proceedings

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