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Heatsink Induced Thermo-mechanical Strain in QFN Devices
Keywords: Heatsink, FEM, Strain
A Blade Server System (BSS) utilizes Voltage Regulator Modules (VRM) to distribute power to various devices on the system board. These VRMs are typically packaged in a QFN (Quad Flat No-leads) format. Depending on the power requirements of the circuit, the VRMs are mounted as single devices or banked together. In addition, VRMs with high power density require an efficient heat dissipation design through the use of a heat sink. Typically, during field conditions (FC) the BSS are powered on and off once per day with their ambient temperature cycling between 25°C and 80°C. This cyclical temperature gradient drives inelastic strain in the solder joints due to the mismatch of coefficient of thermal expansion (CTE) between the QFN and the circuit card. Also the heat sink, coupled to the QFN and the circuit card, can induce additional inelastic solder joint strain resulting in early solder joint fatigue failure. To understand the effect of the heat sink mounting, a FEM (Finite Element Model) of four QFNs mounted to a BSS circuit card was developed. The model was exercised to calculate the maximum strain energy dissipated in a critical joint due to cyclical straining. Results were compared for a QFN with and without a heat sink. Analysis shows that the presence of the heat sink contributes to higher strain energy and therefore could lead to undesirable early joint failure. Integration of heat sink and VRMs in blade servers therefore requires careful mount design. The mechanical boundary condition associated with head sink to printed circuit board plays a crucial role in determining the excess cyclic stress on a solder joint. Design of the mounting should provide lateral or in-plane slip, essentially, decoupling the heat sink from QFN joint strain. A DIC (Digital Image Correlation) optical measurement system was used to measure the physical behavior of the QFNs mounted on the circuit card with and without the heat sink. A DIC measurement technique was further applied to estimate the slippage at the mounting points. Observations confirmed varying degree of heat sink coupling to the circuit card. Details of the FEM-based modeling results and DIC measurements of heat sink lateral slip are presented in this paper along with a potential design to decouple the heat sink from the QFN joint strain.
Gerard McVicker, Sr. Engneer
IBM Research
Yorktown Heights, NY

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