Novel Test Device for Non-destructive Experimental Characterization of Void Evolution in Microscale Solder Joints subjected to Thermal Aging

There is a trend towards reduction in micro-bump size and pitch in 2.5D/3D packaging driven by increased bandwidth and power efficiency requirements. One of the reliability challenges that arises is the void evolution in the solder joint volume of the micro-bump accompanied by the growth of Cu-Sn IMCs in the Cu pillar after thermal aging for long periods of time. In this study, we describe a methodology to fabricate test devices with multiple sub-30 µm pitch Cu-Sn-Cu junctions, with the capability to observe the growth of voids in the micro-bumps in a non-destructive manner. Typically used material and void characterization methods for microbump interconnects are destructive, requiring cross-sectioning and polishing of the samples before observation of voids in solder joints. The cross-sectioning is cumbersome, results in loss of data, and once cross-sectioned, the samples cannot be used for further thermal aging. A non-destructive method of characterization assures continuity in the experiments during the void evolution.The fabricated test device consists of multiple sub-30 µm pitch Cu-Sn-Cu junctions that are directly observable under a Scanning Electron Microscope(SEM). This setup can be used to do experiments ex-situ (by periodically pausing experiments, to measure void growth under SEM) or in-situ (with measurements of void growth in real-time under SEM). The test devices can also be subjected to current via a 4-wire Kelvin sensing probe setup attached to 3-axis piezo stages with a resolution of 50 nm. Multiple devices with varying sizes of solder bumps are fabricated on a single die. The dies are aged in an environmental chamber at a temperature of 175 ◦C for hundreds of hours. They were periodically taken out of the environmental chamber and the void evolution in the solder joint on the devices measured under both optical microscopes and under an SEM. Finally, the effect of different bump sizes on the reliability of the solder joints is reported in this study.