In-situ investigation of EMC relaxation behavior using piezoresistive stress sensor

• Viscoelastic behavior of molding compound was investigated by piezoresistive stress sensor.
• Measurements were conducted on component and system levels.
• Humidity intake of molding compound was measured.

The relaxation behavior of an epoxy molding compound (EMC) subjected to a constant strain can cause new reliability challenges in automotive electronics. This problem will be exacerbated due to the ever-increasing demand in modern electronics systems for miniaturization with more functionality, yet it has not been studied extensively to mitigate its effect on reliability. In this study, a piezoresistive silicon-based stress sensor is used to understand the stress state in an electronic control unit (ECU), more specifically the relaxation behavior of EMC caused by the storage time of an ECU (i.e., duration between production and actual usage). Mechanical stresses are measured by the piezoresistive stress sensor that is encapsulated in a standard microelectronic 3 × 3 mm land grid array (LGA) package. The relaxation behavior is observed at three different temperatures for 1 week: 75 °C, 100 °C and 125 °C. The relaxation behavior is measured continuously for one more week after cooling the package to room temperature (at 25 °C). An additional test is conducted at 85 °C with 85% relative humidity to investigate the effect of moisture diffusion on the package. The experimental results clearly indicate that the proposed approach can be used for better understanding of the evolution of stresses in molded packages during their lifetime, especially during storage, which in turn can lead to more optimal designs in the future.