Heat Sink Induced Thermo-Mechanical Joint Strain in QFN Devices

A Blade Server System (BSS) utilizes Voltage Regulator Modules (VRM), in the form of Quad Flat No-Lead (QFN) devices, to provide power distribution to various components on the system board. Depending on the power requirements of the circuit, these VRM's can be mounted as single devices or banked together. In addition, the power density of the VRM can be high enough to warrant heat dissipation through the use of a heat sink. Typically, during field conditions (FC) the BSS are powered on and off up to four times 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 coefficient of thermal expansion (CTE) mismatch between the QFN and the circuit card. In addition, 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 QFN's mounted to a BSS circuit card was developed. The model was exercised to calculate the maximum strain energy in a critical joint, due to the cyclical straining, and the results were compared for a QFN with and without a heat sink. It was determined that the presence of the heat sink did contribute to higher strain energy and therefore could lead to earlier joint failure. While the presence of the heat sink is required, careful design of the mounting should be employed to provide lateral slip, essentially decoupling the heat sink from the QFN joint strain. Details of the modeling and results, along with DIC (Digital Image Correlation) measurements of heat sink lateral slip, are presented.