EFFECT OF INERT ATMOSPHERE REFLOW AND SOLDER PASTE VOLUME ON THE MICROSTRUCTURE AND MECHANICAL STRENGTH OF MIXED Sn-Ag-Cu AND Sn-Pb SOLDER JOINTS
Authors: Mulugeta Abtew, Robert Kinyanjui, Ph.D., Narong Nu Company: Sanmina-SCI Corporation Date Published: 9/24/2006
Abstract: One of the critical issues in the transition to lead-free soldering is the uncertainty about the compatibility between the Sn-Ag-Cu lead-free alloy system and Pb. The combination of supply chain constraints on component availability and exemptions to the European Union (EU) Reduction of Hazardous Substances (ROHS) compliance initiative has created a necessity where eutectic Sn-Pb alloy may be used in the soldering of Sn-Ag-Cu terminations or vice versa. Studies have suggested that under given soldering conditions, the solder joints formed as a result of mixing Sn-Ag-Cu and Pb are mechanically sound and reliable while other studies reported that mixing of the Sn-Ag-Cu system with Pb produced mechanically inferior solder joints with significantly reduced fatigue resistance. The metallurgical reaction of Sn-Ag-Cu and Pb is a secondary alloying process that takes place during soldering. Experimental investigation was conducted to evaluate the effects of lead concentration and inert atmosphere reflow conditions on the mechanical strength and fatigue resistance of solder joints formed as a result of mixing Pb with Sn-Ag-Cu. Three levels of Lead concentrations, 0%wt., 5%wt. and 10%wt., in the Sn-Ag-Cu system were evaluated. Reflow soldering was also performed in a Nitrogen environment where with oxygen concentration of 800 measured in part per million (ppm). Post cross-sectioning solder joint evaluation was conducted using optical and scanning electron microscopy to characterize the microstructure as a function of lead concentration in the Sn-Ag-Cu system and oxygen level during reflow. The apparent homogenous mixing of lead and Sn-Ag-Cu appear to vary with amount of lead, peak temperature and soldering environment. Generally, peak reflow temperature of 230oC or higher, presence of 10%wt. of lead and reflow environment with 800 ppm or less of O2 concentration yielded solder joints that exhibit better homogenous morphology. The sheer strength of the mixed solder joints were also found to vary with lead concentration with higher concentration of Pb (>10%wt.) in the solder joints producing better shear strength.