Next generation low stress plastic cavity package for sensor applications

MEMS inertial sensors are widely used in automotive safety systems. For example, accelerometers are used to detect crashes and gyroscopes are used to detect roll over events. One of the principal performance issues for these inertial sensors is the stability of the device output over temperature. The origin of the offset drift with temperature is the deformation of the MEMS structure that results from thermal strains. The sensors signal conditioning electronics process the thermal deformations just as they would deformations due to acceleration. Because the thermal strains are temperature dependent, the device output has temperature dependence. Several approaches have been developed to improve the device performance over temperature. These include modifying the design of the MEMS structure to reduce its susceptibility to thermal strains, the use of silicone gels to decouple the MEMS structure from the package stress, modifying the package design to reduce the package stress, and the use of electronic temperature dependent correction of the output. Many commercially available sensors use a combination of several of these approaches. One of the most straightforward approaches to improving sensor performance is to use a cavity package rather than an overmolded package. In an overmolded package, the entire sensor is encapsulated in the epoxy mold compound. In this case, there is significant coupling between the package and the sensor, so the thermal strains are readily transmitted to the senor. In a cavity package, they only mechanical connection between the sensor and the package is through the die attach. Thus, the package thermal strains cannot readily act on the device. Several cavity package technologies are available commercially. In the past, both ceramic and metal have been used for inertial sensor. They offered a very low stress package and excellent reliability. But these packages are no longer viable for high volume sensor manufacturing because their costs are too high. Another barrier to their use is that the vast majority of the semiconductor assembly infrastructure is used for transfer molded plastic packaging. What is needed is a low-cost cavity package for sensor applications. Pre-molded plastic cavity packages are ideal solution for application for stress sensitive inertial sensors. These packages have several features that make them ideal for the application: first, they are much lower cost than a ceramic or metal package of the same size; second, the packages can be molded onto a lead-frame in an array format that is compatible with standard transfer molded assembly lines. Thus, the cost of assembly for the packages can be very low. Finally, the packages are lead-free, which is an important environmental concern.