Terahertz Nondestructive Characterization of Conformal Coatings for Microelectronics Packaging

Measuring the thickness of conformal coatings (CCs) and detecting defects in a nondestructive and noncontact fashion are important to ensure the reliability and performance of packaged electronic components and systems. In this study, we explore the application of terahertz (THz) time-of-flight tomography (TOFT), a noncontact and nondestructive method applied to conformal-coating thickness. We measure the time delay of THz pulses reflected (echoes) from fabricated structures covered by a CC to ascertain coating thickness. Orthogonal matching pursuit (OMP) is applied to deconvolve reflected THz signals from various interfaces, viz., air/CC and CC/substrate. By these means, we map the conformal-coating thickness distribution over significant areas. The CC thickness found over the metal area is <inline-formula> <tex-math notation="LaTeX">$\sim 64.3~\mu \text{m}$ </tex-math></inline-formula>, while it is <inline-formula> <tex-math notation="LaTeX">$\sim 56.2~\mu \text{m}$ </tex-math></inline-formula> over the metal-free area. Moreover, the Gaussian mixture model (GMM), a machine-learning algorithm, is used to detect a possible defect-rich region within the CC itself.