Review on Joint Shear Strength of Nano-Silver Paste and Its Long-Term High Temperature Reliability

Soldering has been the main die attach technology for several decades. Recently, in order to meet the high temperature electronic requirements (high temperature-operating SiC and GaN devices) as well as the health recommendations (replacing the toxic lead present in common solder alloys with lead-free alternatives), several new attach technologies have been developed. Among others, the sintering of nano-silver particles seems to be one of the most interesting choices, and has been extensively investigated during recent years. The emergence of this technology is mainly due to the desired high electrical and high thermal conductivities of the sintered joint, its low elastic modulus offering a good thermo-mechanical reliability, its low process temperature, and its high operating temperature. In this paper, a review of parameters affecting the initial shear strength of the sintered silver joint will be summarized as well as the high temperature long-term reliability issues. The sintering cycle (bonding pressure, bonding temperature, sintering dwell time, heating rate, and the sintering atmosphere), the joint size, and the attached materials properties (nature, roughness), are found to closely affect the initially measured shear strength of the joint. The long-term reliability of the joint has been shown to suffer initially from three phenomena: the silver electro-migration, the decrease of shear strength under harsh thermo-mechanical stresses, and the swelling of the sintered layer. While the latter phenomenon is observed during the storage at temperatures above 350°C, the electro-migration and thermo-mechanical stresses can influence the package reliability at temperatures as low as 250°C. However, some suggested precautions during the module fabrication can lead to the minimizing of the effects of these phenomena and the achievem a more reliable joint.