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20 SMT007 MAGAZINE I JULY 2019 The Model A solder ball and the solder in the deposit of solder paste on which it is mounted can be considered as an isolated system in that all the materials that will form the final solder joint are already present. In that regard, it is differ - ent from a joint being wave soldered in which there is a virtually unlimited supply of solder on which the joint substrates can draw to form a fillet. To the extent that during the soldering pro- cess, there can be some reaction between the copper or nickel substrates, those substrates should also be considered as part of the sys- tem; whether those reactions need to be taken into account depends on the size of the sol- der ball. In some very small joints, the Sn from the solder and the solder ball consumed in the reaction with the substrates to form the inter- metallic compounds Cu 6 Sn 5 or Ni 3 Sn 4 could be a significant fraction of the total amount of Sn available in the solder ball/solder paste sys- tem. However, for the purposes of explaining the method of calculating the extent to which the solder ball is consumed in the process of reflow with a low-temperature solder, that effect will be neglected. Where reactions with substrates are significant, they can be factored into the calculations. For the purpose of explaining the method, the system will be simplified to a solder ball and the solder in the solder paste. In a sol- der paste, the solder itself accounts for about 50% of the paste volume. The remainder is the flux medium that determines the printing characteristics and tackiness of the paste, pro- vides the fluxing action required to facilitate coalescence of the powder particles into a sin- gle mass of solder and the wetting of the joint substrates, and controls the surface tension of the liquid solder that determines the joint pro- file. However, once the solder ball has been wetted, the flux medium plays no role in the process that determines how much of the BGA ball is lost to the mixed alloy. To further simplify the explanation of the method, it will be presumed that the solder ball is pure Sn and the low-melting-point solder a simple binary Sn-Bi alloy. For the low-Ag SAC alloys often preferred for BGA spheres because their greater compliance reduces the incidence of pad cratering in drop impact, the alloy is already about 98.5wt% Sn (with the remain- der typically being 1wt% Ag and 0.5wt% Cu). Even SAC305 is 96.5wt% Sn (with the remain- der 3wt% Sn and 0.5wt% Cu). The process that determines the extent of penetration of the LMP alloy into the ball involves only the Sn. The Ag is present only as the Ag 3 Sn intermetallic compound and the Cu as the intermetallic compound Cu 6 Sn 5 — both of which remain in fairly stable equilib- rium with the Sn phase over the likely temper- ature range of the mixed alloy reflow process. Therefore, they would play no role in the inter- actions that occur in the reflow process. With the likely tolerances on the quantities of the materials in the system, the difference in the outcome with a pure Sn ball should not be very different from that with SAC alloy balls. However, with the basic principle established, allowance can be made for the presence of Ag and Cu in the BGA ball alloy. Commercial Bi-Sn alloys usually contain a small alloying addition to improve their prop- erties—typically 0.5wt% of Ag or Sb. Over the temperature range of the mixed alloy reflow process, the Ag has no solubility in Bi and would be expected to be present in the low- melting-point solder as the Ag 3 Sn intermetallic compound. Sb is completely soluble in Bi and Sn and could play a role in the determination of the final equilibrium, but its effect would be small and will not be taken into account in the proposed model. Again, allowance could be made for its effect once the basic model is recognized. If the interaction between the solder ball and solder from the solder paste is allowed to pro- ceed to equilibrium, the factors that ultimately determine the extent to which the solder ball alloy is replaced by mixed alloy are: • The location of the solidus line on the Sn-rich side of the Sn-Bi phase diagram • The composition of the low-melting-point alloy • The peak reflow temperature