World’s Smallest, Membrane-Based Capacitive Differential Pressure Sensor-Package Structure, Material Selection, Assembly Challenges and Solutions

The MEMS pressure sensor market is growing rapidly with many potential applications. Micromachined Differential Pressure Sensors (DPS) are widely used in monitoring HVAC filters, exhaust vents/blowers in biotech/wafer fabrication plants, pharmaceutical industries, air quality monitoring in clean rooms and data centers, etc. In the era of the Internet of Things (IoT) and smart manufacturing, robust materials, miniaturized package sizes, and low-cost packaging solutions are required. Numerous packaging challenges emerge from the stress-sensitive MEMS assembly, substrate design, environmental exposure of wires and metals, package encapsulation, etc., which drive various package structure options. However, there have been limited studies regarding the optimal, cost-effective, reliable, and more importantly miniaturized DPS packaging solutions for IoT applications.In this paper, the authors have developed the world's smallest commercially available membrane-based capacitive differential pressure sensor in an LGA package. An extensive test matrix has been considered to develop the most optimal solution including but not limited to designing the overall package, various assembly materials performance characterization, substrate design optimization for required performance, and running reliability stresses for accelerated life testing. The overall package structure consists of a 20-pin LGA substrate, pre-molded application-specific integrated circuit (ASIC) die, capacitive MEMS placed in the molded cavity on top of the substrate, various wire bond connections, and a plastic lid made of liquid crystal polymer (LCP) with 2 ports on the top side. The differential pressure MEMS has a cavity from the bottom side and a membrane on the top side. For differential pressure measurement, it is required to access both the top and bottom sides of the MEMS membrane. This has been enabled by a unique embedded air cavity/channel design within the substrate. The top side pressure is accessed directly through one of the ports while the bottom side pressure is accessed through a hole in the substrate right underneath the MEMS which leads to the embedded air cavity/channel and up to the other port. Because of the sensitivity of the membrane in the MEMS to external stresses due to CTE mismatch, a very low modulus die attach material with a relatively thick bond line thickness (BLT) has been used. The die attach served two purposes, firstly, creating a continuous conduit to access pressure from the substrate channel without any leakage, and secondly, stress decoupling between the MEMS and the rest of the package. An extensive DOE has been carried out to find optimal BLT that ensures adequate stress decoupling, robust package sealing against leakage, as well as good bleed control to prevent blockage to the substrate and MEMS die backside holes. The substrate channel dimensions have been optimized to prevent condensation inside the channel during field application. The MEMS wire bonds, bond pads, and substrate bond fingers have been protected by special coating due to condensation risk on exposed metallic surfaces. Three different coating materials with two different thicknesses have been investigated. The ASIC is connected to the MEMS by wire bonding through the substrate. The substrate design has been optimized for parasitic capacitance/leakage control using S-parameter modeling.The performance of the developed package has been characterized by measuring the burst pressure, orientation sensitivity, RH sensitivity, offset, and normalized span change of the pressure reading. Results also included MEMS coating bleed control on the pre-mold surface, UV-cure vs. heat-cured coating material performances, LCP shear test, and MEMS die attach results for various BLT to control performance and port hole blockage. The package has been qualified for various accelerated life testing including MSL3 pre-conditioning, -55 to 125°C, 700 temperature cycling (TC), 130 °C/85% RH/96 hours u-HAST, and 125°C, 1000 hours high temperature storage (HTS). The developed package also passed a burst pressure of more than 5 bars which is about 2X higher than existing commercial differential pressure sensor packages in the market.