Authored By:
Wilson Chen, Ian Miske, Morgan Miller, Christopher Frederickson, and Dongkai Shangguan
Insituware LLC
NY, USA
Summary
Conformal coating is designed to provide protection and long-term reliability to printed circuit boards (PCB) in harsh environments where high humidity, high temperature, corrosive gases, or salt spray may be present. The coating uniformity and correct thickness is necessary to prevent inadequate coating properties, coating defects, heat entrapment, and long-term reliability issues. Traditional methods for measuring coatings thickness are separated into wet and dry methods. Currently, wet thickness is measured using a wet film gauge, whereas, dry thickness can be measured using micrometer, eddy current, and ultrasonic techniques. However, these methods have disadvantages such as user dependent inconsistencies, requiring a ground plate, and measuring a coupon rather than the board itself. This study aims to highlight the application of chromatic confocal microscopy (CCM), an optical technique used to characterize 2D/3D surfaces, for measuring wet and dry conformal coating thickness.
In this work, conformal coating wet and dry thicknesses were measured using CCM and traditional methods. FR4 boards with varied color solder masks and components were created for realistic use cases. For wet coating thickness, CCM and wet film gauge measurements were taken immediately after application. For dry coating thickness, micrometer and eddy current measurements were taken before application and after full cure, along with CCM measurements. Compared to traditional methods, CCM as a non-contact measurement technique was found to be highly accurate, have lower variability, and have expanded possible thickness measuring locations with a smaller measurement diameter. Therefore, it was concluded that CCM can be used as a powerful alternative to standard methods for measuring conformal coating thickness.
Conclusions
This research provides insight into the capabilities of chromatic confocal microscopy as a powerful alternative to traditional thickness measurement instruments. Micrometer, eddy current, wet film gauge, and CCM methods were used to compare thicknesses of a commercial polyurethane and acrylic coating. CCM also allowed for measurements on top of components and leads that could be compared with cross-sectioning.
The dry thickness of conformal coating measured by micrometer, eddy current, and CCM were mostly comparable with one another. However, micrometer and eddy current measurements are affected by coating topography. Micrometer measurements may result in higher thickness readings because the measurement gauge stops when it touches the highest point of the coating. Eddy current measurements calculate the average thickness which can result in higher or lower thickness readings depending on uniformity. CCM had much lower variations in measurements compared to the other two methods, with most standard deviations measuring below 1μm. This is a significant improvement compared to the micrometer and eddy current, whose standard deviations can vary based on factors such as operator skill, coating compressibility, and measurement location. The color of the solder mask also did not appear to impact CCM measurements.
The wet thickness of conformal coating measured by CCM agreed with that measured by a notched wet film gauge. The variability in wet thickness measured by CCM was higher than in dry thickness measurement due to high volatility of solvents and the coating was susceptible to flow from slight movements. CCM was able to provide an exact value of wet thickness whereas the wet film gauge can only provide a range of thicknesses.
Compared to cross-sectioning, CCM measurements of the top of a 0805 capacitor and the lead of a QFP44 were comparable. CCM was able to take accurate measurements on top of the lead, leg, and top of the QFP44. In addition, the measurements were within a few microns of the cross-sectioning data. On the 0805 capacitor, CCM was within 10μm to the cross-sectioning data. This larger variation was mainly due to slight differences in the planes where CCM measurements were taken and where the component was polished down during cross-sectioning. Using CCM offers the ability to measure coating thickness on components which is critical to ensure proper coverage and thickness.
The results of this study demonstrate the advantages and improvements of chromatic confocal microscopy over traditional thickness measurement tools. This non-contact, optical technique allows for direct measurements on any component on a genuine assembly without the need for a test coupon. Using CCM can offer tighter process controls to ensure long-term reliability of PCBs.
Initially Published in the SMTA Proceedings
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