Comparative study of nondestructive mapping of conformal-coating thickness on microelectronics by terahertz time-of-flight tomography

Conformal coatings are used to protect microelectronic circuitry and increasingly optoelectronics and photonics from detrimental effects of the environment, such as moisture, dust, gasses, and mechanical abrasion. The conventional approach to determine the mean time to failure of conformally coated microelectronic components is usually labor-intensive and time-consuming. We recently showed (Shi et al., 2024) that the quasi-optical approach terahertz (THz) time-of-flight tomography (TOFT) could in principle be used to map conformal-coating thickness over a sample of dimensions on the scale of square centimeters. In this study, we employ THz TOFT to characterize several conformal-coating types on microelectronic test samples in a nondestructive and noncontact manner. This study extends previous work on acrylic conformal coatings. THz TOFT is shown to be effective in the thickness characterization of silicone and acrylic conformal coatings, but not nanometric atomiclayer-deposition metal-oxide coating, which is too thin for the technique.