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FEA Modeling of Accumulated Creep Strain for Short Leaded MOSFET Solder Joints
Keywords: Solder, Creep, Modeling
High-volume manufacturing of semiconductor devices conventionally utilizes RF power supplies to couple RF energy to a plasma reactor for surface treatment on a Silicon wafer. High-power Silicon MOSFET devices are critical components in the generation of this RF power. In these industrial applications, these devices dissipate power in excess of 300W, and the corresponding magnitude of heat flux contributes to thermal management challenges commensurate with mechanical stress related to mounting. Conventionally, high-power RF devices with high dissipation require direct mounting to liquid cold plates. The leaded parts of these devices are generally soldered to FR4 printed circuit boards (PCB). The device leaded frame materials, for example, Alloy 42, have a relatively low coefficient of thermal expansion (CTE) and are often bridged to the FR4 board with a high CTE in the thru-board direction. This results in a global CTE mismatch, and along with high temperatures, produces bending stresses on the device leads and solder that adheres the device to the PCB. Additionally, there is a local CTE mismatch in the lead-solder–solder pad joint causing thermal-related stresses in the solder. At high solder temperatures and slow loading, creep accumulation has been well documented to play a major role in SnAgCu and SnAg solder life, now commonly used in lead-free electronics. We describe the application of a two- step hybrid CFD/FEA analysis approach. First, the board and device level deflections and temperatures are determined, including the effects of board/device warpage, RF heating, liquid and air flow. In the second step, an FEA lead/solder sub-model is developed from the board level temperature and displacement data. This sub-model solves for creep accumulation over three thermal cycles using the Garofalo constitutive creep model. We use this model for the preferential solder strain analysis that is a function of the temperature and stress loading profile. Solder shape/size, temperature cycle, and strain relief approaches are evaluated and presented on creep strain versus time curves.
Stephan Fatschel, Principal Mechanical Engineer
MKS Instruments, Inc
Rochester, NY
United States

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