Combined thermal and thermomechanical modelling for a multi-chip QFN package with metal-core printed circuit board

Multi-chip quad flat no-lead (QFN) (also known as micro-leadframe (MLF)) package with split leadframe design allows the control FET, synchronous FET, driver and PWM control chips into a single package. When power density increases, thermally sensitive substrates with metal core, such as insulated metal substrate (IMS) or direct-bond copper (DBC) substrate, are required to meet the thermal requirement. However, it has not been clear how a metal-core substrate affects solder joint reliability. This paper combines the CFD-based thermal modeling with visco-plastic finite element modeling to predict both thermal behavior and mechanical performance under temperature cycling. First, the detailed thermal model is established and validated against the experiment. A two-resistor compact model, on the basis of the validated detailed model, is applied to system-level thermal analysis. It is found that the use of IMS board can reduce the board area by half compared to FR-4 board. Second, a full 3-D finite element nonlinear model is built with the temperature distributions from the thermal modeling in an accelerated cycling condition. We found that the fatigue life of solder joint decreases by a factor of 4 when IMS board is used. A comprehensive model study is performed to investigate the full 3-D model versus a 3-D slice model, as well as the impact of the linearization of center solder pad. A trade-off design in thermal performance, chip layout, leadframe design, and the selection of board material is discussed to meet both thermal and mechanical specifications.