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JULY 2019 I SMT007 MAGAZINE 21 If the time that the system is held at reflow temperature is limited, then the system might not reach thermodynamic equilibrium. In that case, another factor determining the extent the solder ball is replaced by the mixed alloy will be the time at reflow temperature. The Process The process that occurs during reflow can be described as follows: 1. The solder powder in the paste melts and coalesces into a single mass of molten solder 2. The molten solder wets the substrate pad and the lower part of the solder ball 3. The flux medium—having done its job of facilitating reflow and wetting—is displaced from the solder mass but largely remains as a coating on the molten solder, contributing to heat transfer and protecting the molten solder and the lower part of the solder ball from oxidation 4. Sn from the BGA ball starts to dissolve in the molten LMP alloy, increasing the Sn content of the molten solder 5. Dissolution of Sn from the solder ball continues until the Sn content of the molten solder moves into the two-phase region of the phase diagram when a solid phase—an Sn-Bi solid solution—starts to freeze out of the melt 6. Dissolution of Sn from the BGA ball into the molten solder continues until the composition reaches the solidus composition at the reflow temperature at which point dissolution of Sn stops 7. With the system then completely solid, the only mechanism by which further intermixing of the LMP alloy and the BGA ball can continue is by solid-state diffusion; during the time available in a commercial reflow, the profile would be negligible If at this point the temperature is increased, some of the mixed alloy will melt, and there will be an opportunity for more Sn to dis- solve from the BGA ball until the composi- tion reaches the solidus at that higher temper- ature. This process is illustrated schematically in Figure 4 for a low-melting-point alloy that is 50wt% Bi/50wt% Sn and a reflow tempera- ture of 165°C. At the end of Stage 3, the Sn-Bi alloy is fully molten (Figure 4, Point A). If the flux has done its job, the solder will have fully wetted the solder ball as well as the substrate. Sn from the ball would be starting to dissolve in the molten solder, and the Sn content of the molten increases. At Point B, the Sn content of the molten Sn-Bi alloy has reached saturation (approxi- mately 58wt%Sn). As more Sn dissolves in the remaining liquid, a solid phase starts to freeze out. That phase is a solid solution of Bi in Sn with a composition of approximately 86% Sn. By the time the system reaches Point C (approximately 70wt% Sn), the wt% solid Sn-Bi in the semi-molten solder mix can be cal- culated by the lever rule as: Sn from the solder ball continues to dissolve until it reaches a level of approximately 86wt% (Point D) when the mix of low-melting-point alloy and Sn is completely solid. While the temperature remains no higher than 165°C, the system remains solid with solid-state diffu- sion the only mechanism as the Bi can migrate further into the solder ball. Figure 4: The process of a pure Sn BGA ball dissolving into Sn-50Bi solder at 165°C.