Dissolution Behavior of Cu and Ag Substrates in Molten Solders PO-YI YEH, 1 JENN-MING SONG, 2 and KWANG-LUNG LIN 1 1.—Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan. 2.—Department of Materials Science and Engineering, National Dong Hwa Univer- sity, Hualien 974, Taiwan; e-mail: samsong@mail.ndhu.edu.tw This study investigated the dissolution behavior of Cu and Ag substrates in molten Sn, Sn-3.5Ag, Sn-4.0Ag-0.5Cu, Sn-8.6Zn and Sn-8.55Zn-0.5Ag-0.1Al- 0.5Ga lead-free solders as well as in Sn-37Pb solder for comparison at 300, 350, and 400°C. Results show that Sn-Zn alloys have a substantially lower dissolution rate of both Cu and Ag substrates than the other solders. Differ- ences in interfacial intermetallic compounds formed during reaction and the morphology of these compounds strongly affected the substrate dissolution behavior. Soldering temperature and the corresponding solubility limit of the substrate elements in the liquid solder also played important roles in the interfacial morphology and dissolution rate of substrate. Key words: Dissolution, Pb-free solder, interface, intermetallic compounds INTRODUCTION Soldering is a well-established technique for elec- tronic interconnects components. During soldering, base metal is dissolved in molten solder and may form interfacial intermetallic compounds (IMCs). Investigation of the interfacial reactions is impor- tant to improve the reliability of solder joints. 1–5 However, the prevention of excessive dissolution of the substrate during soldering, wave soldering, or reflowing should not be neglected. Sn-Pb solders have been widely utilized in electronic packaging, such as flip-chip, ball grid array (BGA), and wave soldering. Several reports have devoted to the study of dissolution behavior of substrate metals in tradi- tional Sn-Pb solder. 6–11 Among these studies, Bader 6 investigated dissolution data of Cu, Au, Ag, Pd, Pt, and Ni in molten Sn-Pb solder. The results showed that the dissolution rate increases in the order of Pt, Ni, Pd, Cu, Ag, to Au. The temperature dependence of the dissolution rates follows Arrhenius behavior, except for Pd. Due to health and environmental concerns, the prohibition of lead will be carried out. Eutectic Sn- 3.5Ag has been regarded as a promising lead-free solder because of its advantages of high strength, acceptable wettability, good fatigue, and creep resis- tance. 12 Alam et al. have reported on the dissolution behavior of Cu metallizations in Sn-3.5Ag and Sn- 3.5Ag-0.5Cu (values in wt.%). 13 They demonstrated that addition of 0.5 wt.% Cu in the solder decreased the concentration gradient of Cu at the interface and thus reduced the driving force of dissolution. However, the melting point of Sn-3.5Ag solder is 221°C, which is 34°C higher than the melting point of Sn-Pb. Sn-Zn eutectic alloy, with a melting point equal to 198°C, has been proposed as another alter- native choice for the eutectic Sn-Pb. The drawbacks of Sn-Zn solders are poor wetting behavior, easy oxidation, and dross formation, which may cause problems in applications. 14,15 In order to increasing the feasibility of Sn-Zn based solders in electronic industry, some improvements have been made which incorporating small amounts of alloying addi- tives with Sn-Zn solders. It was found that eutectic Sn-8.55Zn-0.45Al had a better oxidation resistance than eutectic Sn-Zn solder. 16,17 Additionally, a small amount of Ag also enhances the wettability of Sn-Zn solders on Cu substrate 18 and Ga is capable of increasing the mechanical property of solders. 19 Until now, there is a lack of the dissolution data of substrate metals by Sn-Zn solders. In view of a bet- ter understanding of substrate dissolution behavior will provide further insight of this solder alloy (Received March 14, 2005; accepted December 5, 2005) Journal of ELECTRONIC MATERIALS, Vol. 35, No. 5, 2006 Regular Issue Paper 978