SMTA International Conference Proceedings

Investigation of the Factors Influencing the Performance of Low Temperature Solder

Authors: Rohit Valooran, Martin Anselm, Ph.D.
Company: Department of Manufacturing and Mechanical Engineering Technology, College of Applied Science and Technology at Rochester Institute of Technology
Date Published: 9/17/2017   Conference: SMTA International

Abstract: This paper intends to validate the hypothesis that bismuth (Bi) dissolution increases as peak reflow temperature is increased. Increased mixing reduces Bi precipitate concentration and increases strength. Reducing Bi in solder paste (40 vs 58%) should provide low Bi precipitate concentration at lower temperatures and increase strength (Maalekian, Xu, & Seelig, 2015). 12mil SAC305 spheres were populated on a 4-layer OSP finish board with 6mm clearance on all edges with 12mil and 10mil SMD pads on a 0.5” grid printed with eutectic Sn-58Bi and Sn-40Bi-Cu-Ni, the boards were then sent through a reflow oven for the developed profiles. Shear force and cross- sectional analysis were conducted to determine the shear force and the mixing percentages of the joints. (Aspandiar, Byrd, Tang, Campbell, & Mokler, 2015)

A two level three factor experiment was designed as follows: Reflow profile (High, Low), pad size (12mil, 10mil) and solder paste (Sn-58Bi and Sn-40Bi-Cu-Ni). The shear force data and the mixing percentages were compared across all factors and their interactions to determine the factors that were significant. The correlation between joint strength and mixing percentage was also analyzed.

The results show that reflow profile and pad size have a significant effect on the solder joint strength, whereas the solder paste does not have a significant effect. Also, there was a direct relationship between mixing percentage and solder joint strength.

This study demonstrates that greater solder paste volume and better solder paste volume to solder ball volume ratio, higher peak temperature and higher time above liquidus (TAL) helps in improving joint strength and mixing percentages for low temperature solder. It was also found that better solder paste mixing percentages result in higher joint shear strength.

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