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Low-temp, Tin-bismuth-silver Solder Pastes
Low-temp, Tin-bismuth-silver Solder Pastes
Staff from Koki Company and Christopher Associates Inc. collaborate on a study on bismuth usage in lead-free solder.
Materials Tech

Authored By:
Jasbir Bath, Manabu Itoh, Gordon Clark, Hajime Takahashi, Kyosuke Yokota, Kentaro Asai, Atsushi Irisawa, Kimiaki Mori
Koki Company Limited, Japan

David Rund, Roberto Garcia
Christopher Associates Inc., Santya Ana, CA USA
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Summary
The electronics industry has mainly adopted the higher melting point Sn3Ag0.5Cu solder alloys for lead-free reflow soldering applications. For applications where temperature sensitive components and boards are used this has created a need to develop low melting point lead-free alloy solder pastes. Tin-bismuth and tin-bismuth-silver containing alloys were used to address the temperature issue with development done on Sn58Bi, Sn57.6Bi0.4Ag, Sn57Bi1Ag lead-free solder alloy pastes.

Investigations included paste printing studies, reflow and wetting analysis on different substrates and board surface finishes and head-in-pillow paste performance in addition to paste-in-hole reflow tests. Voiding was also investigated on tin-bismuth and tin-bismuth-silver versus Sn3Ag0.5Cu soldered QFN/MLF/BTC components. Mechanical bond strength testing was also done comparing Sn58Bi, Sn37Pb and Sn3Ag0.5Cu soldered components. The results of the work are reported
Conclusions
Based on the tests conducted during the evaluation the following was determined:

1. Sn58Bi, Sn57.6Bi0.4Ag and Sn57Bi1Ag solder pastes show good printing and reflow performance over the variety of components tested.

2. The tin-bismuth solder pastes were found to have good head-in-pillow performance and were acceptable for pin-in-paste soldering.

3. Voiding studies on power transistor components showed low voiding with the developed tin-bismuth pastes with minimal effect on voiding from silver additions to the tin-bismuth pastes.

4. Paste durability studies showed good results over the 5 day print and reflow testing for the developed tin-bismuth paste.

5. Pull and shear testing data for tin-bismuth soldered components were equivalent or better than Sn3Ag0.5Cu and Sn37Pb soldered components.

6. There were minimal differences in pull and shear testing of Sn58Bi, Sn57.6Bi0.4Ag and Sn57Bi1Ag soldered components.

7. Cross-sectional analysis of Sn58Bi, Sn57.6Bi0.4Ag and Sn57Bi1Ag soldered components showed good bonding to the board and component interfaces
Initially Published in the IPC Proceedings
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