Long-Term Reliability of Solder Joints on Wafer-Level Packaged Accelerometer for Medical Applications

Miniaturization of devices is driving replacement of electronic components with surface mount technology (SMT) equivalent parts, including any embedded sensing devices. In many cases the size of the sensor is restricted by the minimum size of the package rather than by the die. Other solutions to preserve real-estate involve manual mounting of the die onto substrates that have gone through an SMT assembly process. The +/-2g accelerometer presented here is, to our knowledge, the first wafer-level packaged device with solderable terminals that allows the silicon die to be mounted directly onto a substrate in a standard SMT process and without the need for stressisolating interposers. With its small footprint and ceiling requirements (2.1 × 2.9 × 0.8 mm³), and robustness and high performance it is the smallest commercially-available packaged accelerometer suitable for medical applications where these characteristics are critical. The device features terminals that are electrically and mechanically separated but robust enough to withstand large shear forces that may occur during use and board assembly. The device was solder mounted on a variety of substrates without affecting its performance. Most significantly, both device and solder joints were able to withstand extended thermal cycling over a wide temperature range (-55 to 125°C). In this paper, we present the device design, the performance and the long-term reliability test results of this novel and high-performance device on a variety of substrates and solder materials.