[[The]] effect of micro alloying on the microstructure evolution of Sn-Ag-Cu lead-free solder

Abstract

The microelectronics industry is required to obtain alternative Pb-free soldering materials due to legal, environmental, and technological factors. As a joining material, solder provides an electrical and mechanical support in electronic assemblies and therefore, the properties of the solder are crucial to the durability and reliability of the solder joint and the function of the electronic device. One major concern with new Pb-free alternatives is that the microstructure is prone to microstructural coarsening over time which leads to inconsistent properties over the device's lifetime. Power aging the solder is a common method of stabilizing the microstructure for Pb-based alloys, however, it is unclear if this will be an appropriate solution to the microstructural coarsening of Pb-free solders. The goal of this work is to develop a better understanding of the coarsening process in new solder alloys and to suggest methods of stabilizing the solder microstructure. Microalloying is one potential solution to the microstructural coarsening problem.

This experiment consists of a microstructural coarsening study of SAC305 in which each sample has been alloyed with one of three different solutes, directionally solidified at 100μm/s, and then aged at three different temperatures over a total period of 20 days. There are several important conclusions from this experiment. First, the coarsening kinetics of the intermetallics in the ternary eutectic follow the Ostwald ripening model where r3 ∝ t for each alloying constituent. Second, the activation energy for coarsening was found to be 68.1±10.3 kJ/mol for the SAC305 samples, Zn had the most significant increase in the activation energy increasing it to 88.8±34.9 kJ/mol for the SAC+Zn samples, Mn also increased the activation energy to 83.2±20.8 kJ/mol for the SAC+Mn samples, and Sb decreased the activation energy to 48.0±3.59 kJ/mol for the SAC+Sb samples. Finally, it was found that the coarsening kinetics of SAC305, SAC+Zn, SAC+Mn, and SAC+Sb are all much slower than Pb-Sn alloys, therefore, power aging the solder will not be a viable method of stabilizing the microstructure. However, adding small amounts of Zn or Mn may be useful to maintain the original microstructure so that power aging is not required.