The effect of cooling rate on the dendritic spacing and morphology of Ag3Sn intermetallic particles of a SnAg solder alloy

• Published in: Materials in engineering 2011-05, Vol.32 (5), p.3008-3012
• Main Authors: Garcia, Leonardo R., Osório, Wislei R., Garcia, Amauri
• Format: Article
• Language: English
• Subjects: Cooling rate; Dendritic structure; Intermetallics; Microstructure; Morphology; Solders; Solidification; Spheroids
• ISSN: 0261-3069
• DOI: 10.1016/j.matdes.2010.12.046

a-[ordm 3D characterization of the morphology of Ag3Sn by an over-etching technique. a-[ordm Ag3Sn spheroids are associated with fine dendrite spacings, I' 2, of Sn-rich phase. a-[ordm A mixture of Ag3Sn (spheroids+fiber-like) is associated with I' 2 from 10 to 45I14m. a-[ordm A mixture of Ag3Sn (fibers+plate-like) is associated with I' 2 of about 100A plus or minus 15I14m. a-[ordm Pb-free solder manufacturers can control the microstructure by the cooling rate. The size and morphology of intermetallic compounds of Sn-Ag solder alloys can have a significant influence on the mechanical strength of solder joints. The aim of the present study is to characterize the as-cast microstructure of a Sn-2wt.% Ag solder alloy, and to correlate the resulting scale of the dendritic matrix and the morphology of the Ag3Sn intermetallic compound (IMC) with the corresponding solidification cooling rate. Pre-heated low-carbon steel molds and a water-cooled solidification apparatus were used permitting a significant range of solidification cooling rates to be experimentally examined. It is shown that under very slow cooling conditions (0.02A degree C/s) the microstructure of the sample is formed by a coarse dendritic matrix and a mixture of fiber and plate-like Ag3Sn IMC in the interdendritic region with the fibers located along the board line separating the matrix. For cooling rates from 0.15 to 1.15A degree C/s a mixture of spheroid and fiber-like IMC and secondary dendrite arm spacings between 15 and 40I14m, with the spheroids located in the center of the interdendritic region. At higher cooling rates, of about 8A degree C/s only Ag3Sn spheroids (of about 0.5I14m in diameter) prevail in the eutectic mixture.