SMTA International Conference Proceedings


Author: Kent Larson
Company: Dow Corning Corporation
Date Published: 10/11/2007   Conference: SMTA International

Abstract: As electronics move into ever increasing service temperatures, materials must meet higher thermal stability demands. Likewise, the adoption of lead-free solders has often come at significantly higher reflow temperatures which also stress electronic materials. As an assembly can only be as thermally stable as its most sensitive individual component, the material properties of every item come into greater scrutiny. Supplier product literature offers a first step in understanding a material’s thermal stability, but many claims are made with few specifics on stability or how it is determined. Upper temperature use limits for materials tend to be ill defined and may be based on somewhat arbitrary exposure durations until some chosen property degrades to some chosen lower limit of usefulness.

Silicones are a common protective material used in high temperature applications. Very high temperature stability is often claimed, but usually without substantiation or details. A study was undertaken to evaluate the high temperature performance and reliability of a variety of silicone adhesives, encapsulants and coatings. Short, mid and long term duration stabilities were determined according to procedures outlined in UL Standard 746B used to generate 10 year Relative Temperature Index (RTI) type of values. The silicone adhesives and encapsulants obtained 10 year stability temperatures of 130-200C, while coatings achieved values of 200C. Short term exposures of up to 325C were evaluated. Arrhenius plots were prepared of property loss vs exposure temperature and duration to provide predictive models of thermal stability over very broad ranges of time and temperature.

Key words: silicone, thermal stability, high temperature, Arrhenius plot

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