Today, more than 40 sensors, which control the safety systems, comfort functions and the energy budget, are already integrated into compact class vehicles. For protection against environmental impacts, the sensitive sensor element is closely wrapped with a die-cast housing of plastics. This encapsulation may, however, damage the sensor, as the silicon of the sensor and the plastic material of the encapsulation expand differently at different temperatures. The mechanical stresses caused by this in the sensor may lead to a premature failure of the electronic system. For a reliable and thus safe automobile electronics, the encapsulation process must be improved with the aid of exact stress measurements.
For this purpose, an X-ray diffractometer, which can measure the stresses in a silicon sensor in a non-destructive way, has been developed at PTB. With the aid of high-energy X-radiation of more than 15 keV, the die cast packing of the sensors can be penetrated. In this way, stress values of up to – 200 MPa were determined on the encapsulated chip which indicates an enormous pressure load by the encapsulation. A special metrological challenge was the amplification of the weak measurement signal. It was met by combining a molybdenum fine focus source and an especially adapted glass capillary optics.
A parallel development of the company Bosch, in the case of which a force measuring chip passes through the encapsulation process instead of a car sensor and determines the mechanical stresses inside the encapsulation via an electric measurement, could be validated with the aid of PTB‘s X-ray diffractometer.
