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Effect of Bi Content on Properties of Low Silver SAC Solders



Effect of Bi Content on Properties of Low Silver SAC Solders
A systematic study is presented to address the effect of Bi content on soldering and mechanical performance of low silver SAC alloys.
Materials Tech

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Authored By:


Mehran Maalekian, Ph.D., Yuan Xu
AIM Metals & Alloys
Montreal, Canada

Karl Seelig
AIM Metals & Alloys
Cranston, RI, USA

Summary


Since the adoption of RoHS and REACH regulations for electronics manufacturers, Sn-Ag-Cu (SAC) alloy systems have largely replaced the Sn-Pb solder alloys with Sn-3Ag-0.5Cu (wt%) (SAC305) the most widely accepted. This high silver content SAC alloy (greater than X percent) has a major deficiency however, the relative fragility of the solder joint. This is due to the formation of primary Ag3Sn platelets crystals in the solidified structure leading to poor drop-shock performance causing major concerns for portable devices.

Lowering or eliminating Ag content of SAC has been proposed as a solution but with limited success. In this work, a systematic study is presented to address the effect of Bi content on soldering and mechanical performance of low silver SAC alloys. Thermal behavior (melting and solidification), wetting and spreading performance, and tensile and hardness properties of alloys are compared and a promising lead-free SAC-Bi solder alloy that also demonstrates tin whisker mitigation properties is recommended.

Conclusions


A systematic study on low silver SAC0607-Bi solder alloys was carried out in order to propose an alloy with optimum soldering and mechaincal properties. Properties such as thermal behavior, wetting performance, hardness and tensile strength, as well as tin whisker growth were investigated and compared with reference SAC305.
The following conclusions can be summarized:

Alloys with Bi contents higher than 6% form a detectable low melting temperature phase

Melting temperature of SAC-Bi alloy decreases linearly with increasing Bi content up to 3%.

Pasty range which the difference between solidus and liquidus temperatures increases drastically with Bi content above 3%.

Tensile strength and hardness both increase linearly with Bi content in the range of 0-6 wt%.

Tensile elongation decreases with Bi addition. The trend is more rapid in the alloy with high concentration of Bi, i.e. 6%. This is attributed to additional strengthening mechanism; precipitation hardening that is activated at high Bi contents (Bi>3%).

Wetting time decreases with Bi additions. Optimum wetting performance (short wetting time and high wetting force) is obtained for the alloy with 3% Bi.

Tin whisker growth is mitigated in SAC-Bi alloy as compared with SAC305.

Based on the soldering and mechanical data in this study, it is concluded that SAC0607-3Bi is the best choice in terms of melting, wetting, mechanical and reliability performance as compared to SAC305. Future work will include the manufacturability, process capability and product reliability of SAC0607-3Bi for electronics assembly.

Initially Published in the SMTA Proceedings

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