Low-Stress Bond Pad Design for Low-Temperature Solder Interconnections on Through-Silicon Vias (TSVs)

Abstract

Low-temperature bonds are thin intermetallic (IMC) bonds that are formed between devices when plated layers of different metals on each side of the component come into contact under relatively low temperature and high pressure. These joints, comprised of completely of IMC compounds, will fail in a sudden unexpected manner as compared to normal solder joints, which fail in a ductile manner, where cracks grow more slowly. This problem of weak interconnects is further exacerbated when these thin interconnections are formed on pads located above through-silicon vias (TSVs). When a change in temperature occurs, the mismatch in coefficient of thermal expansion causes the copper inside the TSV to expand or contract much more than the surrounding silicon. This could result in unexpectedly high tensile stresses in the joints. This additional tensile stress, during post-formation cooling down to room temperature, increases the likelihood of joint failure. This paper presents a novel pad design to overcome the situation of high stress in the joints. The proposed design does not involve any additional fabrication or material cost. Simulation results show that, with the proposed pad design, the maximum tensile stress in the interconnect decreases by 50%. Reliability assessment has also been performed in order to compare the proposed pad design with the conventional design. It is found that the samples with the proposed design have a better drop impact reliability performance than the samples with the conventional full pad design.