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Microstructural Evolution During Electromigration in Eutectic Tin-Bismuth BTC Solder Joints
The subject of this follow-up paper is the microstructural evolution of BTC eutectic Sn-Bi solder joints during electromigration. The growth of intermetallic phases at the solder-substrate interfaces and the extent of bismuth electromigration were measured as a function of temperature, current density and time in a bid.
Technical Paper
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Authored By:
P.Singh, L. Palmer, T. Wassick, C. Ravenelle, IBM Corporation
R. F. Aspandiar, B. Franco, L. A. Swaminathan, Intel Corporation
H. Fu, iNEMI
V. Vasudevan, Dell Technologies
A. Allen, HP Inc.
K. Howell, Nihon Superior Co
K. Murayama, Shinko Electric Industries
H. Zhang, Indium Corporation
A. Lifton, MacDermid Alpha Electronics
T. Munson, S. Middleton, Foresite
R. Coyle, Nokia; S. Murali, Heraeus Materials
Summary
Eutectic tin-bismuth alloy and its variations have been identified as the solders of choice for applications that require low-temperature assembly, even though they pose serious metallurgical challenges. Their high homologous temperature results in the solder microstructure constantly changing and the high electromigration effective valence of bismuth results in a thick, relatively brittle bismuth layer accumulating on the anode side of the solder under current stressing. In addition, as in any tin-based alloy joining to copper surfaces, the intermetallic growth too needs to be considered.
Two iNEMI teams have been researching Sn-Bi alloys. One has focused on the assembly process and solder joint mechanical reliability aspects and the other on electromigration. Now, in the next phase, the two are being combined to study the effect of electromigration on the mechanical and electrical properties of the solder joints. We have recently completed and published work on the electrical effects of electromigration over a range of temperature that when plotted on an Arrhenius plot can be used to predict the electromigration life of bottom-terminated-component (BTC) eutectic Sn-Bi solder joints under any application condition.
In this paper, we report on the microstructural evolution of BTC eutectic Sn-Bi solder joints during electromigration in terms of bismuth redistribution and accumulation at the anode end, and the intermetallic growth on copper and nickel substrates. These findings will greatly aid in the design of experiments to research the effects of electromigration on the mechanical properties of Sn-Bi solder joints.
Conclusions
The growth rate of Ni3Sn4 intermetallic, between the solder and the ENIG finished component did not grow much with time even at temperatures as high as 100 oC. The thickness of the Ni3Sn4 intermetallic remained about 1 μm for the duration of the test. The growth rate of the combined thickness of Cu3Sn and Cu6Sn5 intermetallic decreased in a near parabolic fashion. Its rate of growth was slower on the anode side compared to the cathode side as a result of bismuth accumulation on the anode side acting as a diffusion barrier between the solder and the PCB copper. Electromigration did not have a noticeable effect on the combined Cu3Sn and Cu6Sn5 intermetallic growth rate. The scatter in the data did not result in a linear plot when plotted with the abscissa as the square root of time.
The bismuth electromigration measured in terms of the drift of the center of mass of the bismuth atoms had a near parabolic behavior with time as did the solder joint resistance data reported in an earlier work by the same authors [1]. The scatter is the data was high, most probably due to the anisotropic behavior of the Sn-rich body centered tetragonal phase.
Initially Published in the SMTA Proceedings
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