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Seven encapsulants with operating temperature up to 250 °C are surveyed for possible use in high-temperature high-power planar packages. Processability is assessed by studying the flow fronts and the cured properties of the surveyed materials between paralleled plates. Material B failed in the flow test because it dried out in seconds. Materials A, C, and D failed the curability test because A and C showed volume shrinkage during curing, while D cracked after curing owing to its brittle nature. It is found that elastic materials that usually correspond to low glass transition temperatures (Tg) tend to perform better with regard to large-area planar-structure packages. Materials E-G are confirmed to be comparatively stable with respect to temperature, and both dielectric strength and dielectric permittivity decrease by about 40 and 30%, respectively, as the temperature is increased from 25 to 250 °C. The thermal aging test show that the materials harden during the aging process. Meanwhile, cracking starts in the material matrix. The dielectric strength of the sample drops by 60-70% to only around 10 kV/mm once cracking occurs.