Phase Equilibria Studies of Sn-Ag-Cu Eutectic Solder Using Differential Cooling of Sn-3.8Ag-0.7Cu Alloys JAE-YONG PARK, 1 CHOONG-UN KIM, 1,3 TED CARPER, 2 and VISWANADHAM PULIGANDLA 2 1.—Materials Science and Engineering Department, University of Texas, Arlington, TX 76019. 2.—Nokia Mobile Phones, Inc., Research and Technology Access, Irving, TX 75039. 3.—E-mail: choongun@uta.edu This paper is a study of the phase equilibria of the Sn-3.8Ag-0.7Cu alloy investigated by a differential cooling method. The difficulty in assessing phase equilibria of the Sn-Ag-Cu (SAC) system because of the insufficient resolution of conventional characterization techniques is solved by inducing preferential growth of a solid phase in a melt by holding the alloy at the solid-liquid phase- equilibrium field. Application of the technique to Sn-3.8Ag-0.7Cu with varying holding temperatures yielded results that the alloy is slightly off eutectic com- position. The phase-formation sequence of the alloy during solidification was found to be Ag 3 Sn, β-Sn, and finally the ternary eutectic microstructure. Key words: Lead-free solder, Sn-Ag-Cu, differential cooling method, phase equilibria Journal of ELECTRONIC MATERIALS, Vol. 32, No. 11, 2003 Regular Issue Paper 1297 (Received January 20, 2003; accepted July 3, 2003) INTRODUCTION With the growing importance of lead-free solder technology, the eutectic alloy of the Sn-Ag-Cu (SAC) system has gained considerable research interest. The eutectic SAC alloy is considered one of the most promising replacements for the Sn-Pb alloy as it of- fers a number of advantageous properties over other candidate alloys. Some alloys in the SAC system are of commercial use. The key advantages of the SAC eutectic alloy include its relatively low melting tem- perature, 217°C, reasonable wettability, slower growth of the intermetallics at the interface, and superior mechanical properties compared to Pb-Sn and Ag-Sn binary alloys. Further, the constitutional elements in the SAC system are familiar to the sol- der industry, making it easy to apply without major changes to the assembly process. Currently, several SAC alloys with differing compositions are either recommended or used as solder alloys in practice. The Japan Electronic Industry Development Associ- ation recommends Sn-2.5Ag-(0.5–1)Cu(wt.%), and especially prefers Sn-3Ag-0.5Cu because of its low cost and high performance. On the other hand, the National Electronics Manufacturing Initiative recommends Sn-3.9Ag-0.6Cu, while the European IDEALS consortium finds Sn-3.8Ag-0.7Cu recom- mendable as the most adequate. 1 One of the pri- mary reasons for differing recommendations for alloy compositions is the discrepancies existing in the studies of SAC eutectic composition. The first study on the SAC system was performed by Gebahrdt and Petzow 2 and later followed by Miller et al., 3 who carried out compositional and differential thermal analysis of the eutectic mi- crostructure found in Sn-3.6Ag-1.5Cu alloys. Miller et al.’s analysis on the eutectic microstructure suggested that the eutectic composition of the SAC system is Sn-4.7Ag-1.7Cu. However, Loomans and Fine 4 recently reexamined the Sn-4.7Ag-1.7Cu alloy using differential scanning calorimetry (DSC) and concluded that it is an off-eutectic alloy. With an ad- ditional study, they proposed the Sn-3.5Ag-0.9Cu alloy as the eutectic. Moon et al. 5 carried out both theoretical and experimental studies on the phase equilibria of the SAC system and found that the two studies lead to two different eutectic compositions. Their experimental study found that the Sn-3.5Ag- 0.9Cu is close to a eutectic composition, but a theo- retical calculation of phase equilibrium predicted Sn-3.66Ag-0.91Cu to be eutectic. While a micro- structure study led by Lewis et al. 6 supports the