Sn-Ag-Cu alloys are considered to be one of the best lead-free standard alloy systems. However, the precise composition of the Sn-Ag-Cu ternary eutectic is still unknown. In this study the ternary eutectic composition is placed at Sn-3.7Ag-0.9Cu using a thermodynamic calculation and the microstructures of six alloys, Sn-4.6Ag-0.4Cu, Sn-4.9Ag-1.0Cu, Sn-3.9Ag-1.3Cu , Sn-2.2Ag-1.2Cu , Sn-2.0Ag-0.7Cu , Sn-2.7Ag-0.3Cu, prepared under six different solidification paths of near ternary eutectic composition, are evaluated.
As temperature decreases from the liquid-state, it is predicted that Liquid → L + Primary phase → L + Primary phase + Secondary phase → Ternary eutectic phase + Primary phase + Secondary phase appear under the solidification sequence. Primary, secondary phase have three kinds of solid : Ag_{3}Sn, Cu_{6}Sn_{5}, β -Sn and it is demonstrated that six kinds of microstructure to compound them is obtained. The $Ag_3Sn$ and $Cu_6Sn_5$ phases nucleates with minimal undercooling, but the β -Sn$ phase requires a undercooling of 30 ℃. Because of this undercooling of β -Sn phase for nucleation, for two alloys, Sn-2Ag-0.7Cu, Sn-2.7Ag-0.3Cu, including β -Sn as primary phase, secondary $Cu_6Sn_5$ and $Ag_3-Sn$ are not formed. The ternary eutectic point appears at near 217℃
In order to observe interfacial microstructure of each soldered joint, the alloys are soldered on Cu pad by changing the soldering time (1, 2, 8 min over 250 ℃) and cooling rate (fast cooling rate : 46.67 ℃/sec, medium cooling rate : 11.67 ℃/sec, slow cooling rate : 0.23 ℃/sec, very slow cooling rate : 0.016 ℃/sec). The first forming intermetallic compound (IMC, hereafter) is identified as the $η -Cu_6Sn_5$ phase in all the specimens. This layer is composed of scallop-shaped grains. The IMC layer generally thickens and coarsens with the increased soldering time. When they are soldered at 8 min, the second type of IMC is observed between the preformed $η -Cu_6Sn_5$ phase and Cu pad as the continuous $ε -Cu_3Sn$. The IMC layer of the slow cooled alloys becomes thicker and rougher than that of fast cooled alloys.
Additionally to investigate the effect of cooling rate on joint strength, ball shear test is done. The mechanical behavior is correlated with the observed microstructure. Increasing cooling rate decreases the size of tin-rich dendrites and tin grains. Fast cooled alloys exhibit hardening effect due to the combination of small tin-rich dendrites and fine Sn grains, and shear strength increases by decreasing Sn content and by increasing Ag content.
An numerical method has been developed for calculating the thickness of intermetallic layers formed between Cu substrates and solders during the soldering process. As input, this method requires the temperature-time profile for the soldering process, the isothermal liquid state growth rate parameters for the growth of the intermetallic layer and the experimentally determined Nernst-Brunner parameters for the dissolution of Cu into molten solder. Because dissolution has a significant effect on growth, while precipitation does not, precipitation is not considered. Calculations show that the improved method predicts intermetallic growth between Cu substrates and 96.5Sn-3.0Ag-0.5Cu solder during reflow soldering better than a previous method in which the molten solder was not saturated prior to the start of the formation of the intermetallic layer.
