Research of thermal stress between long linear MCT arrays and lead board using FEM

Abstract

For the long wavelength infrared detection, HgCdTe (MCT) photoconductive devices are selected as the core of next-generation meteorological because of its mature fabrication technique and stable performance. During the assembly process, an innovative multilayer ceramic board providing mechanical support is designed as the electrical interconnection between MCT chips and external circuits for cryogenic application. Furthermore, due to its brittleness, long linear MCT device is normally glued to sapphire substrates on the multilayer ceramic board with cryogenic glue. Thus, it can be seen clearly that the assembly structure is a multilayer configuration which comprises various kinds of materials, including ceramic broad, sapphire, MCT and glues. As a result, the difference in Thermal Expansion Coefficient (TEC) between the layers could create the potential to introduce thermal stress at working environmental temperature (approximately 70K), which could result in device performance degradation, even die crack. This article analyzes the thermal stress between long linear MCT devices and a multilayer ceramic board by using Finite Element Method (FEM). According to analysis results, two factors are revealed as the most significant causes for introducing thermal stress: one is the sapphire substrate thickness; the other is the parameters of various materials, for instance Yong's modulus and TEC. Since the structure of MCT detector is determined by system requirements and is under the limitation of manufacture technology, this article reveals two effective approaches to reduce the unavoidable thermal stress: first, choosing the appropriate thickness of ceramic board which is made by Al₂O₃; second, adding another metal cushion Invar. With the above considerations, the distribution of thermal stress is simulated using FEM under different parameter conditions. Based on the results of simulations, an optimal design of package structure which could improve the reliability of linear MCT with minimum thermal stress is demonstrated.