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November 2014 • SMT Magazine 59 alloy Development The new high-reliability alloy was devel- oped in conjunction with a consortium of part- ners including academic institutions, end-users and solder manufacturers. The constraints on the project focused on ensuring that the melt- ing point of the new material was equivalent to standard SAC alloys and was capable of operat- ing at temperatures up to 150°C. Furthermore, the alloy also had to be RoHS compliant and contain no toxic or cost-prohibitive elements. The proposed method of developing the al- loy was to focus on the relationship between plastic strain under thermal cycling and num- ber of cycles to failure as defined by the Cof- fin/Manson equation [5] ; as shown in Figure 2. The plastic strain imparted into the solder joint is inversely proportional to creep resistance. Therefore, increasing creep resistance will in- crease the number of cycles to thermal failure. The target for the new alloy development was to match the plastic strain at 120°C peak cy- cle temperature for standard SAC with a new alloy at peak operating temperature of 150°C. Creep resistance improvement in alloys can be achieved using a number of well-established methods, such as solid solution strengthening, precipitation of doped elements for further al- loy hardening or grain refinement. The third option (grain refinement), was discounted be- cause this option is not applicable to high tem- perature applications as the grain structure be- comes more dynamic with increasing tempera- ture. The chemistry of Sn-based solder alloys is well-established, limiting the number of co- alloying elements available. After rejection of a number of candidate materials predominant- ly due to cost, toxicity and excessive melting point adjustment, three candidates of Bi, Sb and Ni were selected. alloy optimization Both Bi and Sb showed improved creep re- sistance, due to solution hardening of the tin- based alloy as shown in Figure 3. The addition of Ni to the alloy does not initially appear to HigH-reLiabiLity, Pb-Free, HaLogen-Free soLDer continues arTiCle Figure 3: Compressive creep analysis of doped SaC387 alloys. Figure 2: Coffin/Manson equation for crack growth in terms of plastic strain.