An investigation of copper dissolution and microstructural development in lead-free solders

Copper dissolution and intermetallic compound (IMC) formation during reflow of soldered joints are critical issues for joint reliability. Most of studies in the literature aimed at the coarsening and growth of the IMC layer of the soldered joints during service and only limited data is available during soldering process. This is particularly true for lead-free solders, which have attracted the attention of researchers just recently. This paper presents an experimental study of copper dissolution and IMC growth of lead-free solders during the reflow process. Solder buttons of either Sn or Sn-3.5wt%Ag were reflowed over a copper (99.9% pure) substrate for various relfow time periods ranging from 10 seconds to 10 minutes. Four reflow temperatures were selected, 232°C, 250°C, 275°C and 300°C for pure tin and 221°C, 250°C, 275°C and 300°C for Sn-3.5%Ag respectively. The average thickness of the grown IMC layer and the amount of copper dissolved during reflow were determined using the images obtained from metallurgical microscope. The kinetics of IMC growth and Cu dissolution were then quantified and the estimated kinetics parameters can be used to determine the copper dissolution and IMC layer thickness during reflow soldering.