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Implementing Quality Control in the Conformal Coating Process
Continued from previous page
formal coatings may not always be tightly controlled depending on the coating type and the appli- cation process. However, espe- cially for solvent-based systems, an incorrect thinning process can lead to bubbles, orange peeling, slow curing, spraying difficulties, and inconsistent coating thick- ness.
The role of thinner is to
reduce the viscosity of conformal coating to fit the desired applica- tion process, whether it is spray- ing, brushing, or dipping. The simplest way to control the thin- ning process is to have controlled
method to control the thinning process of conformal coatings by using the Vision MARK-1. The Vision MARK-1 uses dielectric spectroscopy (DS) to measure the impedance of conformal coating and thinning mixtures. Measured impedances corre-
spond to specific ratios of coating to thinner and can be used to con- trol the thinning process while also providing documentation in SPC charts.
Conformal Coating Thickness
The thickness of a conformal
coating on a PCB is one of the most important aspects to producing consistent, reli- able boards. There is a minimum coating thick - ness that is re quired to achieve full protection as specified by the manufac- turer.
Coatings applied too
Microsection of a coated component showing coating thickness.
mixing on a volume or mass basis followed by viscosity measure- ments using a flow-cup or vis- cometer. Solvent evaporation in
applications such as dipping can affect the viscosity and coating thickness over time, so it is nec- essary to tightly control the thin- ning level in these instances. Insituware provides a new
thick can lead to risks of defects such as CTE mis- match, heat entrapment, and cracking. According to IPC-
CC-830, the recommend- ed thickness for most
standard conformal coatings ranges from 30 to 130µ or 210µ depending on the specific type. The thickness of conformal coat- ing can be measured wet or dry, either immediately after applica- tion or after the full curing process. The technique for measuring
wet film thickness is simple. Using a wet film comb or gauge,
the apparatus is placed on a flat surface of a PCB/test coupon and then withdrawn vertically or rolled along the board. The wet thickness is measured as the range between the last wetted tooth and the first unwetted tooth.
Measuring the dry thickness
of conformal coatings is the most common form of measurement performed for quality control. There are many methods to measure dry thickness, each with their own advantages and disad- vantages. Regardless of the method, implementing routine testing of dry thickness is critical to provide the required level of protection, especially around and on component leads, where capil- lary flow may occur. Taking thickness measure-
ments using a micrometer is a popular choice due to its low-cost and simple operation. Typically, measure-
ments are taken on specif- ic board locations before and after the application of conformal coating and the difference is the thick- ness of the coating layer. This is mainly performed on a representative test coupon to prevent damage to the actual board assem- bly. A few factors that could introduce variability in microme- ter measurements are the com- pressibility of coatings, degree of tightening, angle of measure- ments, and cleanliness. Using eddy current is anoth-
er popular method to measure the thickness of a coating layer on a board. An eddy current probe generates an alternating magnetic field at the probe tip, creating eddy currents on the surface of a conductive layer. The thickness of the coating layer on top of the conductive layer will affect the magnitude of the eddy current sensed by the probe. Ultrasonic testing is another
method of measuring coating thickness that can be performed directly on PCBs without need- ing a previous measurement. Ultrasonic measurements work by sending an ultrasonic pulse through the coating that is reflected off the substrate back into the transducer to produce an echo waveform. The transit time of the ultrasonic pulse is then
October, 2022
Notched wet film gauge.
analyzed to determine the coat- ing thickness.
Optical Methods Optical methods for measur-
ing coating thickness are new, powerful techniques that enable non-contact, non-destructive, and highly accurate coating thickness measurements. Mul - tiple techniques such as chro- matic confocal microscopy, inter- ferometry, and UV fluorescence have matured enough to become viable on the production floor. Insituware has developed
the CC-100 conformal coating thickness tester that uses chro- matic confocal microscopy to pro- vide quick, accurate coating thickness measurements. A spec- trophotometer produces white light that is split into a spectrum of monochromatic light that con- verges at varying distances away from an optical sensor. Light from the top and bottom of a con- formal coating layer is reflected into the spectrophotometer that corresponds to light intensity peaks at individual wavelengths of light. The CC-100 then uses the optical properties of the coat- ing to determine the thickness of a conformal coating layer.
Coating Adhesion Proper adhesion of a confor-
See at electronica, Hall B1 Booth 764
mal coating layer is important to Continued on page 64
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