Effects of composition and volume on the microstructure of SnAgCu solder balls

In this study the solder alloys SnAg3.5, SnCu0.7, SnAg3.0Cu0.5, SnAg2.7Cu0.4Ni0.005 and SnAg3.8Cu0.7 have been analysed in order to determine variations in microstructure caused by cooling rate, solder composition and ball diameter. These specimens have been melted and solidified on an alumina substrate in order to prevent compositional changes due to interfacial reactions. The ball diameter was varied from approx. Oslash 1100 mum to Oslash 130 mum. Cooling rates of 0.14 K/s, 1.1 K/s and 10.9 K/s have been investigated. Cross sections of these specimens were analysed by optical light microscopy. The results show that the formation of primary intermetallics in these solder alloys depends on cooling rate and not only on composition. The analysis of grain orientation by polarized light microscopy points out the influences of volume and composition on the solder. The results also indicate that the grain structure becomes finer with smaller volume. A special solidification phenomena, which causes an abrupt change of grain size in a solder ball has been investigated for the SnAg3.0Cu0.5 alloy. It will be shown that this effect is connected with a change in the solidification process itself. In determine the influence on the solidification process caused by the interfacial reaction, experiments on Cu/Sn and Ni/Au metallisations have been carried out.