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Effects of Mixing Solder Sphere Alloys with Bismuth-Based Pastes



Effects of Mixing Solder Sphere Alloys with Bismuth-Based Pastes
In this paper, the effect of mixing solder sphere alloys with Bi-based solder pastes on the component reliability in accelerated thermal cycling is studied.
Materials Tech

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Authored By:


Francy John Akkara, Cong Zhao, Mohammed Abueed, Sinan Su, Sa’d Hamasha, Jeffrey Suhling, Pradeep Lall, Ph.D.
Department of Industrial and Systems Engineering
Auburn University
AL, USA

Summary


Reliability of the new solder alloys has been a serious concern, especially in the harsh environment. Aging at elevated temperature alters the structural and mechanical properties of the solder joint and leads to the coarsening of the precipitates in the bulk solder and growth of the intermetallic compound layer, which in turn deteriorates the solder joint. Various new elements such as bismuth, antimony, and nickel are micro-alloyed into the SnAgCu based alloys to slow down this deterioration process and improve the reliability. There haven’t been many studies about the effect of solder sphere alloys with bismuth pastes on the harsh thermal reliability. In this paper, the effect of mixing solder sphere alloys (SAC105, SAC305, matched solder sphere - paste) with Bi-based solder pastes on the component reliability in accelerated thermal cycling is studied.

Two surface finishes, namely OSP (Organic Solderability Preserve) and ImAg (Immersion Silver) are considered. The test vehicle consists of 15mm and 6mm Ball Grid Array (BGA) components. The assembled test boards were isothermally aged at 125oC for 12 months, which are then thermally cycled between -40oC to +125oC for 5800 cycles. Two-parameter Weibull analysis is done for all combinations. ANOVA is done to observe the main effects and interaction effects. Representative cross sections are also presented with IMC (intermetallic compound) layer thickness measurement.

Conclusions


The effect of using different solder spheres with different solder pastes and surface finishes were studied in the paper. In most of the cases, the failures were due to recrystallization and precipitate formation. It was observed that solder spheres, indeed had a significant effect on the reliability than the surface finish. This fact was clear from the ANOVA analysis and from the observation that the cracks, which led to failure were on the component side and not much cracks were found on the board side. EDX analysis needs to be done in order to understand more about the precipitates.

Initially Published in the SMTA Proceedings

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