Design and characterization of wirebonds for use in high shock environments

MEMS inertial sensors, packaged in hermetic chip carriers, utilize free standing wirebonds to connect to the package I/O pads. Considerable care is taken in design of these wirebonds to balance impedances and minimize cross talk between excitation and readout channels. Many applications of MEMS inertial sensors require that they survive or operate in high acceleration or vibration environments. Any displacement of the wirebonds in these environments could adversely affect sensor bias and scale factor, or in extreme cases, cause sensor failure. Newer generations of high performance, navigation grade, inertial sensors are considerably larger than their predecessors and nearly fill the internal cavity of the chip carrier. Consequently, wirebond geometries can be highly constrained within these packages. Validating the wirebond design in an inertial sensor package, which is subject to a high-G environment, requires extensive testing on appropriate rail gun and shock table equipment. These tests are costly and equipment availability may be limited. It is also difficult to assess the sensitivity to variations in bond geometry or various bond defects using only physical testing. We are developing parameterized finite element models of wire bonds for use as tools to aid in design of sensor packages, and to guide the implementation of quality monitoring test and inspection requirements. Validation of the models is being done by subjecting well characterized wirebond configurations to air gun and drop table shock loads as well as conventional wire pull tests. We are also using a sensitive force gauge to measure the load required to displace a wire normal to the plane of its loop. Analytical expressions have also been developed for simple configurations, which serve as a check on both the finite element model and the experimental measurements.