PERFORMANCE ASSESSMENT OF MEMS GYROSCOPE AND SHOCK DURABILITY EVALUATION OF SAC305-X SOLDERS FOR HIGH TEMPERATURE APPLICATIONS
Patel, Chandradip Pravinbhai
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Recent advances in MEMS technology have resulted in relatively low cost MEMS gyroscopes. Their unique features compared to macro-scale devices, such as lighter weight, smaller size, and less power consumption, have made them popular in many applications with environmental conditions ranging from mild to harsh. This dissertation aims to address a gap in the literature on MEMS gyroscopes by investigating the effects of elevated temperatures on the performance of MEMS gyroscopes. MEMS gyroscopes are characterized at room and elevated temperatures for both stationary and rotary conditions. During the test, MEMS gyroscopes are subjected to five thermal cycles at each of four temperature ranges (viz. 25degC to 85degC, 25degC to 125degC, 25degC to 150degC and 25degC to 175degC). A simulation model is developed in MATLAB Simulink to simulate the temperature effect on the MEMS gyroscope. Simulation results show good agreement with experimental results and confirm that Young's modulus and damping coefficient are the dominant factors responsible for temperature-dependent bias at elevated temperatures. Solder interconnects are one of the weakest elements in MEMS devices. Thus, the reliability of solder interconnects is separately studied in this dissertation. Though, SAC305 (96.5%Sn3.0%Ag0.5%Cu) is the industry preferred solder in combined thermal cycling and shock/drop environments, it exhibits better thermal cycling reliability than drop/shock reliability. One of the ways to improve drop/shock reliability of SnAgCu solder is by microalloy addition of various dopants such as Mn, Ce, Ti, Y, Ge, Bi, Zn, In, Ni, Co etc. Thus, the second part of this dissertation aims to evaluate the shock durability of SAC305 and SAC305-X (where X refers to two different concentrations of Mn and Ce dopants). High temperature isothermal aging tests are conducted on selected solders using QFN44, QFN32 and R2512 package types at 185degC and 200degC up to 1000 hours. Isothermal aging test results showed that interfacial IMC growth reduction can be achieved by microalloy addition of selected dopants in SAC305 on both copper and nickel leaded package types. Shock durability of selected solders is examined on as-reflowed and thermally aged test boards. Mechanical shock is performed using a custom shock machine that utilizes a shock pulse of 500G with a 1.3 millisecond duration. The shock test results showed that the mechanical shock reliability of SAC305 was significantly improved on both as-reflowed and thermally aged test boards by microalloy addition of one of the selected dopant in SAC305.