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Finding the Cause of Cold Solder Joints
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New-Generation, Low-Temperature Lead-Free Solder



New-Generation, Low-Temperature Lead-Free Solder
In this paper, the roles of additive and bismuth content will be discussed. Eutectic SnBi and three newly designed SnBi-based alloys were experimented upon.
Materials Tech

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Authored By:


Boon-Ho Lee, Chun-Yu Chang, Chih-Hsiang Li, Kuo-Shu Lin, Shih-Lo Yueh, Seiji Kobayashi
SHENMAO Technology Inc.
Hsinchu County, Taiwan

Watson Tseng
SHENMAO America, Inc.
San Jose, CA, USA

Summary


Sn3Ag0.5Cu (SAC305) is the major solder alloy after RoHS was adopted by the European Union. Since its melting temperature is relatively higher than eutectic SnPb alloy, the peak reflow temperature increases. This transformation in the assembly industry impacts the component requirement, where the deformation probability (warpage) of a flat component is increased, which impacts the production yield. A lead-free, low-temperature SMT solder is needed to resolve this dilemma.

Low-temperature SMT assembly refers to the reflow process with a peak temperature less than 200°C. The new process provides a few advantages like reducing energy consumption, reducing BGA component warpage during reflow and diminishing non-wetting open (NWO) and head-on-pillow (HoP) defects. The SnBi alloy is one of the candidates used in low-temperature SMT assembly. However, the brittle mechanical property of conventional SnBi alloys will degrade the reliability of the assembly. The SnBi alloy properties can be altered via several means.

In this paper, the roles of additive and bismuth content will be discussed. Eutectic SnBi and three newly designed SnBi-based alloys (Sn57Bi1AgX, Sn48Bi1AgX and Sn40Bi1AgX, X represents <0.5wt.% of additive element) were experimented upon. Solder pastes that were blended with the aforementioned alloys and flux were used to assemble on the PCB with BGA components that have SAC305 solder spheres pre-mounted. The same reflow profile was used for all pastes. Cross-sectional analysis, shear testing, drop testing and thermal cycling testing were conducted to determine the microstructure, shear force, drop reliability and thermal reliability. The results show that the microstructure, especially the bismuth-rich phase, became finer and the shear force was elevated when the additive was added. On the other hand, the drop reliability improved with decreasing bismuth content, and the thermal reliability improved with increasing bismuth content.

Conclusions


The result of this study showed that the Bi phase became finer with Ag and X element additions in the SnBi-based alloy, and hence altered the mechanical performance and reliability of the SnBi-based alloy. It was also found that the three newly designed alloys have the following features:

  1. The solidus and final melting temperature of Sn57Bi1AgX is close to the eutectic SnBi alloy. The Bi phase becomes non-continuous and needle-like with Ag and X element additions, significantly improving mechanical properties and thermal cycle reliability.
  2. Sn48Bi1AgX has less bismuth content than the eutectic SnBi, hence the final melting temperature is 12˚C greater than the eutectic SnBi. The thermal cycle performance and drop reliability are better with Ag and X element additions.
  3. Sn40Bi1AgX has better performance in the thermal cycle and drop tests than the eutectic SnBi; however, the final melting temperature is 30˚C greater than the eutectic SnBi, showing that a higher peak temperature is needed.


Initially Published in the SMTA Proceedings

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