Conformal coatings are applied to most printed circuit boards (PCB's) in use on military programs. They are clear, transparent (to visible light) coatings that help reduce the mechanical effects of vibration and protect the PCB's from moisture and fungus under adverse environmental conditions. It has been a commonly held belief for some time that, from the point of view of thermal imaging, conformal coatings tend to have a high and uniform emissivity in the infrared spectrum and, therefore, provide the PCB's with very close to "blackbody" infrared characteristics. This belief has been challenged recently by investigators who observed indications that MIL specified conformal coatings had far from blackbody characteristics. These characteristics were reported to vary with type, thickness and the wavelength interval (spectral band) measured. In 1985 the US Army Communication Electronics Command (CECOM) initiated the TRAMS (Infrared Automatic Mass Screening) program aimed at the automatic mass screening of PCB's for gross faults at the depot level based on infrared profiles. A conformal coatings investigation was conducted as part of the IRAMS program in order to achieve a better understanding of the effects of conformal coatings on the infrared thermal profiles of PCB's. Predictive investigation was conducted through contact and correspondence with manufacturers and users of conformal coatings. Experimental investigation was conducted by means of measurements using imaging systems operating in different wavelengths and prepared samples with various conformal coating types and thicknesses. This paper describes the work done, the results obtained and recommendations for future measurements.

Date Published: 11 May 1987
PDF: 6 pages
Proc. SPIE 0780, Thermosense IX: Thermal Infrared Sensing for Diagnostics and Control, (11 May 1987); doi: 10.1117/12.940505

Published in SPIE Proceedings Vol. 0780:
Thermosense IX: Thermal Infrared Sensing for Diagnostics and Control
Robert P. Madding, Editor(s)