UV curable coatings for electronic components

Military electronic components demand the highest reliability when applied to extreme environmental conditions. Demands placed upon component manufacturers are equally suitable to the consumer market. The most stringent military test to pass is the component's durability to withstand high temperatures concomitant with high humidities. Before designing a conformal coating for capacitors and resisters, a number of thin films were cast, cured and tested at 85/spl deg/C, 95% RH. The films were heat-cured silicones, epoxies, and ethoxy monomers, and UV-cured acrylated silicones, epoxies, and monomers. Acrylated polymers are free radical curing mechanisms. In this instance, free radicals were generated using UV light. The curing took place in seconds. Vapor transmission across a thin barrier film of these materials was measured to determine the film's integrity in withstanding simultaneously high temperatures and humidities. A number of correlations were discovered in regard to the chemistry needed to enhance a film's resistance to environmental conditions. There are a number of chemical reactions that take place within the film's structure when exposed to water. However, in the absence of chemical reactions, a film's integrity is related to its crosslink density and steric hinderance. The author investigated a number of heat-cured and UV-cured chemistries. In correlating vapor transmission data with a components electrical properties, solid tantalum capacitors were coated with ethoxylated epoxy and acrylated epoxy. Vapor transmission data strongly agreed with the electrical properties of a capacitor when subjected to 85/spl deg/C, 95% RH. Surprisingly the electrical properties were better for the high crosslinked, steric, acrylated epoxy. Rapid cure played a major role in the film's integrity to crosslink uniformly without gas egression or pinholes. The electrical values were consistent with the vapor transmission data.<
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