Influence of adhesive on optical fiber-based strain measurements on printed circuit boards

Abstract

The use of optical fiber sensing for strain measurements on printed circuit boards is a recent approach, and there is a lack of f reliable methods to obtain valid strain measurements. This work investigates the influence of different adhesives and their application for strain measurements using optical fiber sensors on PCB, considering room and high temperature tests. Strain developed in PCB during production and quality testing are measured by a single distributed optical fiber sensor, interrogated by Optical Frequency Domain Reflectometry. Different adhesives are evaluated for room and high temperatures strain measurements. Different thicknesses of the adhesive layer are studied under tensile and bending loadings. The mechanical properties of the adhesives and their effectiveness in optical fiber measurements were evaluated. A suitable adhesive selection is required for reliable strain measurements under different testing requirements: cyanoacrylate based for room temperature testing; epoxy or polyimide-based for high temperature measurements. The adhesive thickness should be the lowest and uniformly applied. A linear relationship between adhesive thickness and strain error was found, being necessary the application of a correction factor for valid strain measurements. Optical distributed sensing is highlighted as an alternative approach to conventional strain gauges for PCB strain measurements, reducing the installation complexity, significantly enlarging the number of measured points, without reducing the quality of the strain measurements. Selection of the best adhesive is crucial, as well as its thickness for reliable strain measurements.