Rare-earth additions to lead-free electronic solders |
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Authors: | C M L Wu Y W Wong |
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Affiliation: | (1) Department of Physics and Materials Science, City University of Hong Kong, Hong Kong SAR, P.R. China |
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Abstract: | The research in lead(Pb)-free solder alloy has been a popular topic in recent years, and has led to commercially available
Pb-free alloys. Further research in certain properties to improve aspects such as manufacturability and long term reliability
in many Pb-free alloys are currently undertaken. It was found by researchers that popular Pb-free solders such as Sn–Ag, Sn–Cu,
Sn–Zn and Sn–Ag–Cu had improved their properties by doping with trace amounts of rare earth (RE) elements. The improvements
include better wettability, creep strength and tensile strength. In particular, the increase in creep rupture time in Sn–Ag–Cu–RE
was 7 times, when the RE elements were primarily Ce and La. Apart from these studies, other studies have also shown that the
addition of RE elements to existing Pb-free could make it solderable to substrates such as semiconductors and optical materials.
This paper summarizes the effect of RE elements on the microstructure, mechanical properties and wetting behavior of certain
Pb-free solder alloys. It also demonstrates that the addition of RE elements would improve the reliability of the interconnections
in electronic packaging. For example, when Pb-free-RE alloys were used as solder balls in a ball grid array (BGA) package,
the intermetallic compound layer thickness and the amount of interfacial reaction were reduced. |
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