ENCAPSULANT FOR NONHERMETIC MULTICHIP PACKAGING APPLICATIONS

Multichip packaging (MCP) is becoming an essential approach of device packaging not only for meeting high operations speed requirements but also for improved system integration. This is true especially when package volume and weight are critical driving factors. As MCP grows to larger sizes to accommodate the ever increasing demand of system integration, hermetic sealing becomes very expensive and technically difficult, if not impossible, to achieve. This prompts us to evaluate various encapsulants for nonhermatic MCP applications. We believe that encapsulation is a critical aspect of the future MCP technologies. The technologies include monolithic and hybrid wafer scale integration as well as three-dimensional MCP's that provide yet an additional level of packaging integration. Encapsulation will provide "reliability without hermeticity" on these advanced MCP's. In addition, it will provide cost, weight, and volume savings with a reliability equivalent to hermetic packages. In this paper, we will present an encapsulant study that identifies a robust silicone material suitable for the MCP applications. In the paper, the properties/characteristics of encapsulant needed for the MCP applications were identified. A literature survey then led us to select six types of encapsulants for a screening test. The encapsulants were tested at 85-degrees-C/85% relative humidity with dc bias and in situ leakage current monitoring. A modified silicone encapsulant (MSE, U.S. Patent 4 888 226) was identified as the most promising material and was subjected to more in-depth studies. Test samples encapsulated with MSE were exposed to five selected chemicals to study their chemical resistance. The samples were then subjected to a temperature-humidity-bias (THB) test for moisture protection studies. In addition, the studies such as material formulation, adhesion, mechanical protection, and temperature cycling were conducted.