Another common misconception (at least with adhesive materials) is the concept of Tg (glass transition temperature). Most of the adhesives used in the electronics industry are thermosetting materials. As such they do not “melt” but have two physical properties that are related. The Tg which is the temperature at which the material softens and the decomposition temperature – the temperature at which the material breaks down. Materials can “function” above their Tg, but their physical properties will be altered. For example, silicone materials (such as bathroom caulk, or conformal coatings) often have Tgs that are below room temperature. Yet they still adhere, protect from the atmosphere (moisture), etc. Since most adhesives used in electronics (FR-4, underfill, SMA) are epoxies, it is useful to understand how these materials are affected above the Tg. The most significant effect on epoxies is that the CTE increases significantly (typically 2-3 times). That doesn’t necessarily mean that a device encapsulated/underfilled/coated with one of these materials will fail. There are other physical properties that come into play as well (especially when it comes to reliability) such as modulus, and adhesion. The best way to confirm that a material is suitable for the application is to test it using known established test methods.
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