Stress-induced parametric shift in plastic packaged devices

In order to determine the shift in device characteristics due to stresses the die experiences during assembly, long channel NMOS and PMOS transistors (6.25×6.25 μm) were encapsulated in plastic quad flat packages (PQFPs) and the shifts in device parameters were monitored. Shifts as much as -5.3% in drain current (I_dsat) were recorded for the NMOS devices, and +1.64% for PMOS devices. These shifts indicate a biaxial compressive stress level of some 60-87 MPa after plastic packaging. Die attach alone (prior to plastic encapsulation) results in a biaxial tensile stress in the range 35-55 MPa. Different mold compounds can exert different stress levels and accompanying device parameter shifts. Prolonged high temperature storage, leading to resin shrinkage, also affects device characteristics. Short channel submicron devices, typical of what is used in products, are expected to show little shift. Monitoring device parameter shift is a simple technique and is well suited for quantifying the stresses the die experiences in a plastic package