Determination of solder bump stand-off height in a flip-chip sub-mount for Micro-Opto-Electro-Mechanical System (MOEMS) packaging applications

Solder is a very common and important material for electronic packaging. The major function of the solder is to provide the electrical connection between different electrical components. In SMT assemblies, solder also provides the mechanical support. Self alignment property of solder joints is one of the advantages of using solder as the interconnection materials in SMT process. It can pull the initially misaligned component back to the desired position by surface tension of the solder during the reflow process. This self alignment property can be also applied to many high-end products such as micro-opto-electro-mechanical systems (MOEMS). The tolerance of the component position is very tight for MOEMS devices. Slight offset in any direction may seriously affect the overall performance of the whole system. In general, the final lateral position of the device after the reflow process can be easily defined by the bond pads locations on the package side and substrate side. However, vertical positioning of the device, which is defined by the standoff height of solder joints, is relatively difficult to predict. In this paper, the effects of the bond pad geometry and the package weight on the final standoff height of solder joints are evaluated. Both the component and the substrate prepared for this study are fabricated on a silicon wafer for the experiment. Chip-on-chip package samples are fabricated by conventional SMT process and tin-lead solder is used in this study. In addition, numerical simulation using Surface Evolver is carried out. The experimental results are used to verify the standoff height obtained from the simulation. The data obtained in this study is useful in predicting the final standoff height of some MOEMS devices at the design stage.