Study and characetrization of plastic encapsulated packages for MEMS

Deshpande, Anjali W

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Study and characetrization of plastic encapsulated packages for MEMS

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Technological advancement has thrust MEMS design and fabrication into the forefront of modern technologies. It has become sufficiently self-sustained to allow mass production. The limiting factor which is stalling commercialization of MEMS is the packaging and device reliability. The challenging issues with MEMS packaging are application specific. The function of the package is to give the MEMS device mechanical support, protection from the environment, and electrical connection to other devices in the system. The current state of the art in MEMS packaging transcends the various packaging techniques available in the integrated circuit (IC) industry. At present the packaging of MEMS includes hermetic ceramic packaging and metal packaging with hermetic seals. For example the ADXL202 accelerometer from the Analog Devices. Study of the packaging methods and costs show that both of these methods of packaging are expensive and not needed for majority of MEMS applications. Due to this the cost of current MEMS packaging is relatively high, as much as 90% of the finished product. Reducing the cost is therefore of the prime concern. This Thesis explores the possibility of an inexpensive plastic package for MEMS sensors like accelerometers, optical MEMS, blood pressure sensors etc. Due to their cost effective techniques, plastic packaging already dominates the IC industry. They cost less, weigh less, and their size is small. However, porous nature of molding materials allows penetration of moisture into the package. The Thesis includes an extensive study of the plastic packaging and characterization of three different plastic package samples. Polymeric materials warp upon absorbing moisture, generating hygroscopic stresses. Hygroscopic stresses in the package add to the thermal stress due to high reflow temperature. Despite this, hygroscopic characteristics of the plastic package have been largely ignored. To facilitate understanding of the moisture absorption, an analytical model is presented in this Thesis. Also, an empirical model presents, in this Thesis, the parameters affecting moisture ingress. This information is important to determine the moisture content at a specific time, which would help in assessing reliability of the package. Moisture absorption is modeled using the single phase absorption theory, which assumes that moisture diffusion occurs freely without any bonding with the resin. This theory is based on the Fickâ€™s Law of diffusion, which considers that the driving force of diffusion is the water concentration gradient. A finite difference simulation of one-dimensional moisture diffusion using the Crank-Nicolson implicit formula is presented. Moisture retention causes swelling of compounds which, in turn, leads to warpage. The warpage induces hygroscopic stresses. These stresses can further limit the performance of the MEMS sensors. This Thesis also presents a non invasive methodology to characterize a plastic package. The warpage deformations of the package are measured using Optoelectronic holography (OEH) methodology. The OEH methodology is noninvasive, remote, and provides results in full-field-of-view. Using the quantitative results of OEH measurements of deformations of a plastic package, pressure build up can be calculated and employed to assess the reliability of the package.

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