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14 SMT007 MAGAZINE I FEBRUARY 2019 pressed as melting temperature, and all dosage percentages are expressed in weight percent. Melting Temperature In an SnAgCuBi system, all three elements (Ag, Cu, Bi) affect the melting temperature of the resulting solder alloy. More practically, they can lower the melting temperature of the Sn matrix when their respective dosages in the system are properly constituted. With an objec - tive to lower the required reflow temperature, the identification of the optimal dosage of each element in this quaternary system to lower the melting temperature of the resulting alloy while maintaining the desired level of physical and mechanical properties is an intricate endeavor as well as most scientifically appealing. Within practical dosage ranges, the follow- ing is a capsule view of experimental findings [1] in the relationship between the melting tem- perature and respective dosages (all dosages are expressed in weight percent; percentage bears approximately ±10% variations): • The melting temperature dropped with the addition of Cu and reached a mini- mum at 0.5%. Beyond 0.5% Cu, the melting temperature remained almost constant with further increase of Cu up to 5.0% (Figure 1) • Similarly, the melting temperature de- creased with increasing Ag and reached a minimum of about 3.0%. When Ag content increased from 3% to 4.7%, any further reduction in the alloy melting tem- peratures is negligible. However, when the Cu content is in the range of 0.5–3.0 % and the Ag content is less than 3%, the liquidus temperature of the melting range increases notably with the decreasing Ag content (Figure 2) • Bi in this alloy system plays a major role in further reducing the alloy melting tem- perature. The alloy's melting temperature near-linearly decreased with increasing dosage of Bi, but increasing the dosage of Bi to lower melting temperature is not a panacea, which will be highlighted later (Figure 3) Experimental results are consistent with the indication of binary or ternary phase diagrams where available. Figure 1: Melting temperature vs. Cu dosage wt% [2] . Figure 2: Melting temperature vs. Ag dosage wt% [2] . Figure 3: Melting temperature vs. Bi dosage wt% [2] .

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