Evaluation of the creep of the Sn62Pb36Ag2 solder alloy by the measurement of the stresses in a silicon die

The Sn₆₂Pb₃₆Ag₂ solder alloy is widely used in electronic applications. To predict the lifetime and to control the reliability of power devices, solder creep law must be taken into account in the thermomechanical finite element simulations. The creep law used in the simulations must be the most representative of the real behaviour of the solder joint in its operating environment. To determine a mathematical model of the solder creep, silicon stress chips with piezoresistive sensors are used. These chips are mounted on a substrate so that the solder joints have the same microstructure as that in an operating standard power hybrid assembly. Nevertheless the measurements are made on the top of the silicon chip where are located the stress sensors, while the solder joint is on the bottom of the chip. This is why finite element calculations are necessary to fit the creep within the solder alloy. From these experimental data, the equivalent stresses at the surface of the silicon have been calculated. As a result, we have obtained the evolution in time of the equivalent stresses at the die surface. In this paper, we show how this experimental curve has been used in thermomechanical finite element simulations to find a mathematical model of the creep in the solder. Then this formula allows for an extrapolation of the behaviour in time of the solder and then of the hybrid assembly in simulations