Silicones have been used for decades in electronics, aerospace and other applications wherein harsh environments with temperature extremes are common. These siloxane-based polymeric systems are unique polymers compared to standard organic-based materials due to their atomic composition. The low modulus of their crosslinked networks allows them to absorb stresses during thermal cycling as well as to resist degradation at continuous operating temperatures up to 250°C and greater. Silicones have low glass transition temperatures (Tg) ranging from approximately -115° C to -60°C which keep their elastomeric systems flexible in cold environments and when experiencing vibration. Thermally conductive silicones provide protection to sensitive electronic components and systems. The silicone matrix is an essential polymer compatible with a variety of fillers due to its unique chemistry, making silicones excellent materials for use as the binder for a variety of thermally conductive fillers where high level loadings can be achieved without dramatically increasing the shear stress.
As electronics are becoming smaller, thinner, vertically stacked and require more power, silicone becomes more desirable to increase reliability. The history and prolific success of silicone speaks to its capacity for reliability. Silicone is typically non-hazardous in its “neat” state once cured and complies with the restricted levels of the regulated chemicals listed in the ROHS and WEEE directives. In medical devices, silicones have proven they can be manufactured to have very high purity for robustness in high-risk applications. Quantifiably, and most relevant for electronics applications, silicones can be processed to have low D4/D5 (< 50 ppm) content as well as to comply with the specifications outlined in NASA SPR- 0022A and ESA PSS-014-702, which require a maximum allowable Total Mass Loss (TML) of 1.0% and Collected Volatile Condensable Material (CVCM) of 0.1%1,2 . This reduces risk of fogging, delamination and other failure-inducing occurrences which volatile species of impure material can cause. Silicones for electronics can also be optimized to exhibit high purity with regard to ionic content < 20 ppm of Na, K and Cl, and their permeability to moisture, a most brutal contaminant, can be adjusted as needed for a given electronic device. Because water is detrimental to many components, extremely low Water Vapor Transmission Rates (WVTR) are often imperative of encapsulating materials. Understanding the opportunities and limitations of silicone allows the formulator or engineer to choose the best options available for maximum performance and protection of components in the harsh environments of electronic applications.
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