| 摘要 |
A printed circuit board with electronic components wired thereto has a conformal insulating coating consisting of a single component of essentially pure urethane formulation and a minor amount of fumed colloidal silica powder. The insulating coating covers asperities and protrusions as well as the conductors and circuit components while maintaining a transparent characteristic. The coating material is made by mixing of the small proportion of silica with essentially pure urethane formulation under controlled conditions to prevent moisture contamination of the materials. The mixture is thoroughly mixed, deaerated under reduced pressure with a pulsating pressure to break any air bubbles and remove essentially all air. The coating is applied in an environment having at least 30 percent relative humidity and within a confined area defined by an encircling dam masking compound. The coating is allowed to partially cure at room temperature and the confining dam compound removed. For maximum protection, a further period of curing of one week is provided.
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