Authored By:
Cong Zhao, Chaobo Shen, Zhou Hai, Jiawei Zhang, M. J. Bozack, and J. L. Evans
Center for Advanced Vehicle and Extreme Environment Electronics
Auburn University
Auburn, AL, USA
Summary
This paper investigates the impact of isothermal aging on the long-term reliability of lead-free solder joints. The full experimental matrix contains SAC105 and SAC305 solder alloys assembled in ball grid arrays (BGAs) with package sizes ranging from 19mm, 0.8mm pitch to 5mm, 0.4mm pitch, and three surface platings (ImAg, ENIG, and ENEPIG). The test specimens were subjected to isothermal aging at temperatures 25 degrees C, 55 degrees C, 85 degrees C to 125 degrees C with aging times of 0, 6 months, 12 months and 24 months, followed by accelerated thermal cycling from -40 degrees C to 125 degrees C.
A two-parameter Weibull plot shows the reliability degrades up to 70% after two years of aging at elevated temperature. The degradation rate slows with aging time. The degradation is greater with smaller solder joints and perimeter ball alignments when compared to large-diameter solder balls and full array ball alignments. The thickness of the Cu-Sn intermetallic layer (IMC) layer during the testing grows with an approximate ~ t0.5 dependence (diffusion-controlled reaction) during isothermal aging + thermal cycling.
Conclusions
Long-term isothermal aging in elevated temperature has a severe impact on the reliability of SAC solder joints. Two-parameter Weibull plot shows that the reliability of BGA packages degrades up to 70% after two years of aging at elevated temperature (125oC). With increases in aging time and temperature, the reliability of BGA components degrades more. For BGAs with identical ball alignments, packages with large solder balls have better reliability. Further, BGA packages with full array ball alignments performs better than perimeter ball alignments.
BGA packages with ENIG and ENEPIG surface plating yields better reliability after isothermal aging than BGA packages with ImAg surface finish. Microstructural evolution and deformation during thermal aging and cycling causes solder joint failure. For the SAC/ImAg system, the typical IMC layers are Cu6Sn5 and Cu3Sn at the Sn/Cu interface and Ag3Sn inside the solder bulk. For the SAC/ENIG/ENEPIG system, the IMC layer becomes (Cu,Ni)6Sn5 and (Cu,Ni)3Sn. The thickness of the IMC layer grows roughly as a ~ t0.5 dependence during isothermal aging and thermal cycling.
Initially Published in the SMTA Proceedings
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