Debra Anne Fleming

The influence of the Pb-free solder alloy composition and processing parameters on thermal fatigue performance of a ceramic chip resistor

This paper presents the results of a thermal fatigue study of a 2512 ceramic chip resistor assembled with various Pb free solders including SnCu, SAC105, SAC205, SAC305, and SAC405. The test matrix also includes some limited evaluations with other variables such as cooling rate (solidification rate), thermal preconditioning and nitrogen (inert) reflow atmosphere. The matrix also includes a SnPb eutectic control cells. The resistor test vehicle provides an expedient and self-consistent method for evaluating the relative fatigue performance of the various alloys. A study of the as-assembled solder joints was conducted to characterize the microstructure of the solder joints with varying silver content. Thermal fatigue was evaluated using an accelerated temperature cycle of 0/100 °C with dwell times of 10 and 60 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 also was 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.

Phenomenological Study of the Effect of Microstructural Evolution on the Thermal Fatigue Resistance of Pb-Free Solder Joints

Unlike SnPb solders, the thermal fatigue reliability of the Sn-Ag-Cu (SAC) solders is believed to be influenced significantly by both the initial and evolving microstructures. This paper presents a phenomenological study of the relationship between the initial SAC solder joint microstructure, the evolving microstructure, and the thermal fatigue performance measured by accelerated temperature cycling (ATC). To reflect the board assemblies that are in field use, commercial surface mount components with multiple geometries and materials and from different package assemblers were joined to the board with different lead free SAC alloys. The initial microstructures of the board level solder joints were altered in a variety of ways including: 1) varying the solder joint cooling rate; 2) varying the number of solder reflow exposures; and 3) exposure to different isothermal temperature exposures. In all cases the solder joint microstructure was exposed to one or more of these treatments prior to exposure to temperature cycling. In addition, some of the test boards were exposed to different cycling dwell times to determine if the microstructural evolution that occurred during ATC testing effected the respective characteristic lifetimes of the joints. The microstructural evolution was tracked and characterized with optical metallography and scanning electron microscopy. These results could have practical implications in terms of limiting the ability to develop acceleration factors and effective strain-based models for predicting Pb-free solder joint life.