Fatigue Life Prediction Model Development for Decoupling Capacitors

There is an increasing need for a low cost solution to increase the speed and functionality of microprocessors. Decoupling capacitors mounted on the organic substrate close to the processor chip serve this purpose. Tin-Silver-Copper based solder materials are typically used to attach the capacitors to the substrate. Fatigue failure of the solder joints during field-use due to CTE mismatch between the capacitor and the substrate and due to proximity of the capacitor to the chip is a potential cause of concern. The solder joint materials exhibit highly rate dependent and microstructure dependent non-linear deformation behavior. The microstructure of the solder joints within a decoupling capacitor is very different from the microstructure of solder joints within a ball grid array package. Projecting fatigue life during field-use, where the temperature excursions experienced are generally less than 50 degrees centigrade, using a predictive model that was developed for a larger ball grid array solder joints or that was developed based on accelerated stress testing alone where the temperature excursions are much larger will lead to inaccurate life prediction. In this paper, we have developed a predictive model specifically for decoupling capacitors that can be used to determine the number of cycles for first fail and the mean number of cycles to failure during any thermal cycling conditions - ranging from harsh to benign - due to solder joint fatigue. Experimental data from test vehicles with 0306 and 0508 decoupling capacitors that were attached to the substrate using either SAC105 or SAC305 solder alloy material was used in conjunction with non-linear finite-element based mechanical modeling for fatigue life predictive model development.