Effect of thermal cycling on the growth of intermetallic compounds at the Sn-Zn-Bi-In-P lead-free solder/Cu interface |
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Authors: | Guohai Chen Xiaoyan Li Jusheng Ma |
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Affiliation: | (1) Department of Materials Science and Engineering, Tsinghua University, 100084 Beijing, China;(2) Datang Mobile Communications Equipment Co., Ltd., Beijing, China |
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Abstract: | The low-temperature Sn-9Zn-1.5Bi-0.5In-0.01P lead-free solder alloy is used to investigate the intermetallic compounds (IMCs)
formed between solder and Cu substrates during thermal cycling. Metallographic observation, scanning electron microscopy,
transmission electron microscopy, and electron diffraction analysis are used to study the IMCs. The γ-Cu5Zn8 IMC is found at the Sn-9Zn-1.5Bi-0.5In-0.01P/Cu interface. The IMC grows slowly during thermal cycling. The fatigue life
of the Sn-9Zn-1.5Bi-0.5In-0.01P solder joint is longer than that of Pb-Sn eutectic solder joint because the IMC thickness
of the latter is much greater than that of the former. Thermodynamic and diffusivity calculations can explain the formation
of γ-Cu5Zn8 instead of Cu-Sn IMCs. The growth of IMC layer is caused by the diffusion of Cu and Zn elements. The diffusion coefficient
of Zn in the Cu5Zn8 layer is determined to be 1.10×10−12 cm2/sec. A Zn-rich layer is found at the interface, which can prevent the formation of the more brittle Cu-Sn IMCs, slow down
the growth of the IMC layer, and consequently enhance the fatigue life of the solder joint. |
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Keywords: | Lead-free solder thermal cycling intermetallic compounds (IMC) diffusion |
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