15.1 COMPATIBILITY OF LEAD-FREE SOLDERS WITH SMT REFLOW PROCESS

Due to the toxicity of lead, there is a tremendous amount of effort to eliminate lead from the solders used in the electronics industry. The move toward lead-free solder alternatives in North America and Europe accelerated significantly1 since the Japanese industry announced its aggressive lead-free road map.2 For instance, Toshiba, Matsushita, and Hitachi have announced plans to eliminate all lead interconnects in their products by 2001, 2004, and 2004, respectively. However, the preferred solution for lead-free alternatives varies from region to region, and there are a number of alloys considered promising. The most favorable Pb-free solder systems identified by the industry3 ^,4 comprise primarily alloys of Sn with Ag, Bi, Cu, Sb, or Zn, such as 99.3Sn0.7Cu, 96.5Sn3.5Ag, 95.5Sn3.8Ag0.7Cu, 93.6Sn4.7Ag1.7Cu, 96.2Sn2.5Ag0.8Cu0.5Sb, 91.7Sn3.5Ag4.8Bi, 90.5Sn7.5Bi2Ag, 89Sn8Zn3Bi, 95Sn5Sb, and 58Bi42Sn. Unfortunately, although some reliability data have been generated in the past,3 most of the promising alloys were evaluated under a single flux system. The compatibility between flux and alloy often dictates the performance of reflow soldering, such as solder balling, wetting, processing window, and stability. Since the flux chemistry varies from supplier to supplier, and since the use of more than one supplier is considered crucial for ensuring a steady process, an alloy compatible with a wider range of flux systems obviously will have greater prospects of being accepted by the surface-mount technology (SMT) industry. In this study, a group of the most promising Pb-free alloys reported are tested against a broad range...