High Reliability Lead-free Alloys for Performance-Critical Applications

Increased complexity of interconnection metallurgies, and additional demand for higher functionality and performance have been driving novel designs and electronics miniaturization. Consequently, higher I/O's density, finer pitches and smaller package sizes are also changing the requirements of Pb-free solder alloys. Hence, there is a need for solder alloys with thermal and mechanical reliability better than SAC305, but with lower, similar or higher melting temperatures, depending on the application. In this paper, we characterize various high reliability solder alloys using uniaxial tensile tests (at different temperatures and strain rates) and creep tests. Alloying additions are used for controlling the growth of intermetallic compounds and microstructure strengthening. Major additions impact the melting behavior and the bulk mechanical properties, whereas minor alloying additions influence the diffusion kinetics and have significant impact on their thermal reliability. The uniform distribution of intermetallics minimizes dislocation motion and deformation, resulting in alloy strengthening.Compared to SAC305, the high and ultra-high reliability alloys presented here show superior mechanical properties. The effect of temperature and strain rate on the mechanical behavior of these alloys are investigated by uniaxial tensile tests at room temperature and 150°C, and strain rates from 10^-4 to 5/s. Deformation during thermal cycling up to 150°C is expected to be controlled by creep, due to the high homologous temperature. Thus, high temperature creep test is used for estimating thermomechanical properties and longer reliability of these alloys in actual usage. As the melting behavior of these alloys cover a wide range of melting temperatures, they can be used in various applications, such as assembly of heat sensitive packages, automotive under-the-hood, semiconductors, LEDs and power electronics.