New generation of lead-free solder alloys with high thermal reliability for applications in harsh environments

Jie Geng, Ph.D. and Hongwen Zhang, Ph.D.

Indium Corporation

Clinton, NY 13413

e-mail: jgeng@indium.com

I. INTRODUCTION

Lead-free solder alloys have been widely adopted by the electronics industry since the Restrictions on Hazardous Substances (RoHS) regulations were implemented in the European Union in July 2006. In the past decades, lead-free SnAgCu (“SAC”) solder alloys such as Sn3.0Ag0.5Cu (SAC305) and Sn3.8Ag0.7Cu (SAC387) have been used extensively in portable, computing, and mobile electronics, which operate in temperatures of 125°C and below. Automotive electronics must operate in temperatures around 150°C for under-the-hood devices and below 125°C for devices in the passenger compartment. These electronics must also be able to function in very low temperatures, requiring an operational range of -40°C to +150°C.

For such harsh environments, the traditional binary or ternary lead-free Sn-rich solder alloys are not reliable enough to survive. Relative to the melting temperature of most Sn-rich solders, the homologous temperature at 150°C equals to 0.876 for SnAgCu-3Bi, 0.863 for SnAgCu, 0.856 for Sn-3.5Ag and 0.846 for Sn-0.7Cu, indicating that atomic diffusion will facilitate microstructural evolution and accelerate joint degradation.

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