According to the semiconductor industry roadmap, the size of solder balls in future microelectronic packages will continue to shrink progressively to 20 microns size (diameter of human hair) or smaller, while being subjected to ever increasing temperatures. This will subject the solder near the chip (typically Si) interface to severe thermo-mechanical strain. To counteract these severe conditions, which will invariably reduce joint life and hence package life, the program envisions developing adaptive lead-free solders containing nanometer size shape-memory alloy (SMA) particles. The investigators have demonstrated previously that micron size NiTi SMA particles impose reverse shear stress on the solder matrix during thermo-mechanical cycling, and are effective in reducing the inelastic strain in the solder. However, strain concentration problems and the decreasing size of solder balls necessitates the use of nanometer size SMA particles, such that a more diffused strain field is obtained along with the possibility of percolation at low volume fraction. Both solid-state and liquid based processing routes will be investigated. The metric of success will be the effectiveness of nano-SMA reinforcements in reducing inelastic strain range during thermo-mechanical cycling.
The broad outcome of this study will be the ability to design active nanocomposite microstructures with much greater thermomechanical reliability than that feasible at the micron scale. The research will generate new mechanistic insights on the role of nanoscale SMA particles, and practical insights on successful manufacturing with nanosize particles. The work will also contribute to educating civilian students and military officers at two different U.S. institutions in the science and technology of nanomaterial processing and understanding. Throughout the project, the investigators plan to leverage existing relationships with the microelectronics industry to develop processing/testing methodologies that can be eventually transferred to the indust
