Thermal Fatigue Performance of a Quad Flat No Lead (QFN) Package Assembled with Several Sn-Ag-Cu (SAC), Pb free Solders

At the time the European Union RoHS deadline was enacted in July 2006, the industry practically had reached consensus on the use of near-eutectic Sn-Ag-Cu (SAC) solder alloys. Over the past few years however, there has been a dramatic increase in the number of Pb-free solder alloy choices. research has been driven by the varying reliability performance of near-eutectic SAC alloys under different stimuli, such as impact, vibration, and thermo-mechanical loading, the results of which indicate that there is no one substitute for tin-lead solder and that alloy choice may be application specific. This trend is instigated by the poor mechanical shock performance of near-eutectic SAC alloys and the outcome is the development and implementation of low Ag alloys to improve the mechanical strength of solder joints, especially under dynamic loading conditions. There is however a concern that these low Ag alloys will not have the resistance to thermal fatigue needed of high reliability applications. This paper presents the results of a thermal fatigue study conducted on a 7x7 mm Quad Flat No Lead (QFN) package assembled with SAC105, SAC305, and SAC405 solders. The test matrix also includes SnPb eutectic control cells. The QFN test vehicle provides an expedient and self-consistent method for evaluating the relative fatigue performance of the various alloys. Because the QFN is not a high compliant package, its thermal fatigue resistance with stiffer, Pb free solders is of additional interest. A Study of the "as- received" solder joints was completed to characterize the microstructure of the solder joints with varying silver content. Thermal fatigue was evaluated using an accelerated temperature cycle of 0/100 degress C with dwell times of 10 and 30 minutes, and an accelerated temperature cycle of -40/125 degrees C with a dwell times of 15 minutes. The test results show a direct relationship between characteristic fatigue life and Ag content, with the higher Ag content alloys outperforming those with the lowest Ag content. as might be anticipated, there is also a consistent inverse relationship between fatigue life and dwell time for the Pb free solders. The failure analysis and microstructural evolution is characterized with optical metallography and scanning electron microscopy and the fatigue reliability of the Pb free solders is discussed in terms of the microstructures.