Modelling Indium Interconnects for Ultra Fine-Pitch Focal Plane Arrays

Infrared (IR) detector chips and read-out integrated circuits (ROICs) are assembled most commonly through flip-chip bonding technologies and using indium solder interconnects. The resulting 3D stacked die structure is called Focal Plane Array (FPA). This paper details the development of analytical and finite element models of the FPA assembly for both reflow and thermo-compression bonding processes. The models are used to assess the process requirements and feasibility in relation to the IR sensor characteristics and bonding equipment placement accuracy. The results show that high-density ultra-fine pitch FPAs may be feasible to assemble only with thermo-compression bonding technology, and that this will still require very high precision placement machinery with chip alignment accuracy in the order of 1 μm or better. Quality of the formed indium joints and the resulting stand-off height are found to be very sensitive to the alignment of the IR detector chip onto the ROIC. Reflow bonding process is feasible only if the pitch of the pixel array matrix is above certain size. In this instance, the presented model of the chip motion during reflow can be used to predict if self-alignment of the assembled chips occurs.The thermo-mechanical behavior of the FPA under cryogenic temperature cycling load is also presented in the paper. Thermal fatigue damage of indium joints is found to be lower with higher resolution, smaller pitch size pixel array structures despite the larger land size of the detector chip. With detector chip thickness typically <10 μm, the warpage of the FPA is shown to be insensitive to the size of the pixel array.