Evolution of Ag3Sn during reflow soldering

Ag₃Sn is an intermetallic compound that always find in SnAgCu lead free solder. It presents as large volumetric size in the bulk solder and at the interface. The size of the Ag₃Sn is about 3∼10x larger compared to Cu₆Sn₅ under the similar reflow conditions. Since intermetallic compound has lower coefficient thermal expansion as compared to its solder. The solder will crack along this interface during thermal cycling. Thus, it is of interests to understand the phenomenal growth of Ag₃Sn compound during reflow. To do that, Pb-Free solder ball with compositions of SnAgCu is first deposited on electroless Ni immersion Au (ENIG) at the board level, and Organic Solderable Preservatives (OSP) surface finishes substrate pads. The substrate is then loaded into reflow oven. Once the reflow oven is fully loaded with the substrates, the oven is shut down, and substrates are retrieved and quenched into the water. Heat index (temperature and time) are recorded with respect to the substrate positions, and the reflowed solder is then cross-sectioned and etched to reveal the Ag₃Sn structures and thickness. Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Transmission Electron Microscopy are used in these studies. The intermetallic thickness and the morphologies of the Ag₃Sn during reflow soldering are discussed. One of the interesting structures to reveal in this paper is 2D and 3D structures of Ag₃Sn. This paper would able to provide insights into the physical phenomenology of Pb-free solder joint formation during reflow and the mechanisms behind the solder joint formation.