Effect of Post-Mold Curing of Epoxy Molding Compound on Wirebond Reliability in a MOSFET

Automotive microelectronic packages experience severe thermal stresses that occasionally cause components and electrical interconnection damage. In tins field of application high reliability is of decisive importance, especially for aluminum wire bonds with diameters of the order of 125 mum, connecting the silicon device with the output pins. Silica filled epoxy resins as encapsulation in chip packages play a major influence on reliability and functionality of microelectronic devices. The paper intends to help understand the reliability of Aluminum wirebonds inside a leaded electronic package like Power MOSFET (metal oxide silicon field effect transistor) under the influence of post-mold curing (PMC) on thermo-mechanical properties of the mold material, like glass transition temperature (T_g), coefficients of thermal expansion (CTE) and flexural moduli. Experimental validation indicates an electrical open gate in the specimen caused due to delamination of gate bond wire (SAT and SEM images) near the leadframe bonding post under power thermal cycling from -40C to 125C. Finite element modeling is used to simulate the stresses the package goes through during reliability tests. The FE model consists of the entire MOSFET package including the lead frame, over mold, chip, substrate. Al wire loops and corresponding bond pads. The simulation considering thermal mismatch predicts high in-plane shear forces at the heel of the wire bond supported by the empirical test results. Optimum values of thermo-mechanical properties having a combination of low CTE and high E value at a higher T_g (obtained by increased PMC duration), recommended to ensure reliability of wire bond. Further, results show evidence of higher variation of in-plane shear force on the wire bond with change in CTE as compared to E. Results derived from numerical simulation and experimental data is used to serve as a guideline in the selection of suitable molding compound.