Experimental investigation and interpretation of the real time, in situ stress measurement during transfer molding using the piezoresistive stress chips

This paper describes a method used for experimental real-time monitoring of thermo-mechanical stress build-up during integrated circuit encapsulation. To detect the stress variations during molding, special stress measuring chips were employed. The working principle of the stress chip is based on the piezoresistive sensors embedded on the surface in a 6 by 6 matrix distribution. [1] The tests were performed at several temperatures and initial conversion ranges. The stages of the molding were analyzed step by step and the shear stress and the normal stress difference distribution was investigated based on sensor locations and the orientation of the chip. The chips are connected to a flexible polyimide board via four wire bonding at one side of the chip. The conductive tracks are extended to the edge of the board where the data acquisition system is soldered to connection pads. The material of the board ensures an elasticity inside the mold cavity and it is proven that it can withstand the weight and clamping pressure applied by the mold wall without causing any damage to the connections.