COOLING RATE OPTIMIZATION AND RELIABILITY OF Pb-FREE Sn-Ag-Cu STACKED BGA MODULES

摘要

The reflow profile parameters are critical in determining the reliability of the Sn-rich Sn-Ag-Cu based Pb-free solder joints. For instance, the cooling rate from peak temperature determines the microstructure of the Sn-Ag-Cu based solder joints. Depending on the Sn-Ag-Cu solder alloy composition, a high cooling rate may result in fine-grained microstructure. While a slow cooling rate on the other hand, results with increased time in the liquidus region (as well as increased pasty range), that corresponds to the growth of large intermetallic compounds (IMCs) which compromise the integrity of the Pb-free solder joint. In some instances, large Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) are found to grow in the solder joint. The growth of large IMCs depletes the Pb-free solder of Cu and Ag. Consequently, this alters the mechanical characteristics of the solder joint such as the coefficient of thermal expansion (CTE) and thus impacting the overall solder joint reliability. Optimized cooling rates are necessary to achieve the desired Pb-free solder joint reliability. This paper presents the results of an experimental study on the effects of cooling rate and multiple-reflows on the reliability of ball grid array (BGA) solder joints. The studies were conducted on the solder joints, both on conventional BGAs attached to printed circuit boards (PCB) assemblies and on Pb-free BGA stacked modules. The assemblies were subjected to isothermal aging and accelerated thermal cycling conditions as well as mechanical shock. The evolution of the solder joints microstructures were examined by cross-sectioning the assemblies followed by observing them using optical and scanning electron microscopy. The characterization of various phases in the solder joint was performed by the use of an energy dispersive spectroscopy (EDS).