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Thermal Fatigue Reliability of SAC305 due to Mixed Metallurgy Assembly
Analysis Lab |
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Authored By:Richard Coyle, Richard Popowich Nokia Bell Laboratories, Murray Hill, NJ, USA Faramarz Hadian, Eric Cotts SUNY-Binghamton, Binghamton, NY, USA Babak Arfaei Juniper Networks, Sunnyvale, CA, USA Julie Silk Keysight Technologies, Santa Rosa, CA, USA Keith Howell Nihon Superior Co., Ltd., Osaka, Japan Hongwen Zhang, Jie Geng Indium Corp., Clinton, NY, USA Derek Daily Senju Comtek Corp., Santa Clara, CA, USA Craig Portz Craig Portz, former Celestica, Garland, TX, USA Xinzhi Feng Rochester Institute of Technology, Rochester, NY, USA John Davignon HDP User Group, Spokane, WA, USA SummaryHigh-performance Pb-free solder alloys are being developed to address requirements for increasingly aggressive use environments. Most of these new alloys have a SAC (Sn-Ag-Cu) base composition with major alloy additions of antimony (Sb), bismuth (Bi), Indium (In) or combinations of these elements. High reliability end users understand the need to develop thermal fatigue reliability data for these new solders, and investigations are underway to evaluate their performance with ball grid array (BGA) as well as other components. This study presents thermal fatigue data generated in thermal cycling for the case of BGA mixed metallurgy assembly. In this study, mixed metallurgy is defined specifically as solder assembly of a SAC305 BGA with a high-performance solder alloy paste containing 6 wt. % Bi. There is interest in this solder paste because Bi additions to Sn-based, Pb-free solder alloys promote solid solution strengthening and are known to increase resistance to thermal solder fatigue. However, in contrast, Bi has been associated with a non-fatigue, interfacial failure mode that could limit resistance to thermal fatigue. Mixed metallurgy performance was compared to the SAC305 performance baseline alloy using Weibull statistics, microstructural characterization, energy dispersive X-Ray spectroscopy (EDS), and failure mode analysis. The thermal cycling results show that in situ alloying of Bi from the solder paste into the SAC305 BGA balls during surface mount assembly improves thermal fatigue performance by as much as 40% relative to that of SAC305. The reliability improvement is slightly better for the 84CTBGA than the 192CABGA. This is attributed to the higher paste to ball ratio for the 84CTBGA, which subsequently results in higher Bi content in the solder ball. There were no negative effects observed related to Bi content or Bi segregating to soldered interfaces. ConclusionsThe thermal fatigue performance of mixed metallurgy ball grid array assemblies was assessed using a test vehicle comprised of two daisy chained SAC305 components, a 192-pin chip array ball grid array (192CABGA) and 84-pin thin core BGA (84CTBGA), with a matching daisy chained printed circuit board. Mixed metallurgy assembly is defined as a surface mount assembly process in which the composition of the solder ball alloy does not match the composition of the solder paste alloy. In this investigation, a Pb-free solder paste containing 6 wt. % bismuth (Bi) was used to assemble the SAC305 BGA components. After surface mount assembly, the Bi content in the larger 192CABGA was measured using SEM-EDS as 0.77 wt. % and measured as 1.51 wt. % in the smaller 84CTBGA. Thermal cycling was performed with a 0 to 100 °C profile. The thermal cycling results show that in situ alloying of Bi from the solder paste into the SAC305 BGA balls during surface mount assembly results in a significant improvement in thermal fatigue performance compared to the SAC305 performance baseline that contains no Bi. For the 192CABGA, assembly with the 6 wt. % Bi paste increases the characteristic lifetime by a factor of 1.7 and the 1% cumulative failure by a factor of 2.8. For the 84CTBGA, assembly with 6 wt. % Bi paste increases the characteristic lifetime by a factor of 2.1 and the 1% cumulative failure by a factor of 1.8. Scanning electron microscope analysis with backscattered electron imaging and energy dispersive X-ray analysis of failed thermal cycling samples indicates that Bi dissolved into the SAC305 solder ball from the solder paste remains in solid solution during cycling. There is no evidence of any negative effects observed such as interfacial cracking caused by Bi segregating to soldered interfaces. In summary, mixed metallurgy assembly of the two SAC305 BGA components used in this study with a solder paste containing 6 wt. % Bi results in a substantial improvement in thermal cycling performance compared to the assembly of the same components with SAC305 solder paste. During thermal cycling, Bi that alloyed into the SAC305 during surface mount assembly remains in solid solution. There is no accumulation and stratification of Bi at soldered interfaces or boundaries, and the characteristics of the fatigue cracking are unaltered from those of SAC305. Although there is no indication of a quality or reliability risk for the mixed metallurgy assembly of the BGA components and assembly parameters used in the investigation, there are caveats to consider. These results should be used with discretion because the final bismuth content in mixed metallurgy solder joints is expected to vary with BGA sphere size and stencil printing parameters. The current results also apply to the specific 0/100 °C thermal cycling profile and the two BGA packages used in the study. The Bi behavior and thermal cycling results could be different with a more aggressive thermal cycling profile that has a higher upper temperature extreme and larger temperature range (ΔT). The results for the two BGA packages showed measurable differences, so results could be expected to vary for larger, smaller, and more complex packages. Printed circuit board final finish is another factor that affects solder joint reliability, and this was not addressed in the current study. The selection of component types and testing parameters will be dictated by the end users and their design requirements, operating conditions, and service environment. Initially Published in the SMTA Proceedings |
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