Implementing Quality Control in the Conformal Coating Process
By Wilson Chen, Insituware
The entire conformal coating process has several key steps that should be regularly inspected and controlled to ensure continued reliability in its application and environmental protection. These processes include:
Implementing process control into all these steps is critical to producing consistent and reliable coated printed circuit boards (PCB). To aid in process control in some of these key steps, Insituware has developed the Vision MARK-1 and the CC-100 Conformal Coating Thickness Tester.
It is advised upon purchasing conformal coating that an incoming inspection test be conducted to ensure that the coating from the manufacturer is consistent and reproducible. Some conformal coating manufacturers have been known to make small adjustments in the formulation without notifying their users. In solvent-based coating systems, an easy and low-cost test involves measuring the viscosity. Two ways that coatingviscosity can be measured are by using flow-cups or a viscometer.
A flow-cup measures the flow time (or efflux time), which is the time required to empty a vessel of known dimensions. The cup is submerged into the coating and as soon as the cup breaks the surface of the coating, the time starts. When the first interruption in flow out of the bottom orifice is observed, the time is stopped. The measured flow time can then be used in a lookup table to determine the coating viscosity. There are various types of flow cups (ISO 2432, DIN 53211, Ford, Zahn, etc.) that can be used depending on the specific application.
A viscometer can also be used to provide a greater degree of precision and reproducibility if required. Various types of viscometers are available, but the most common for measuring the viscosity of coatings are rotational viscometers. Rotationalviscometers use either a rotating spindle or a cup–and–bob system with a rotating bob. The viscometer measures the amount of torque required to rotate a spindle or bob at a specific rate thatis proportional to the viscosity of the coating. Each spindle or cup–and–bob system is rated for a measurable viscosity range, so it is important to choose the correctly sized system for each conformal coating.
The thinning process of conformal coatings may not always be tightly controlled depending on the coating type and the application process. However, especially for solvent-based systems, an incorrect thinning process can lead to bubbles, orange peeling, slow curing, spraying difficulties, inconsistent coating thickness, etc. The role of thinner is to reduce the viscosity of conformal coating to fit the desired application process, whether it is spraying, brushing, or dipping. The simplest way to control the thinning process is to have controlled mixing on a volume or mass basis followed byviscosity measurements using a flow–cup or viscometer. Solvent evaporation in applications such as dipping can affect the viscosity and coating thickness over time, so it is necessary to tightly control the thinning level in these instances.
Insituware provides a new 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 correspond to specific ratios of coating to thinner and can be used to control the thinning process while also providing documentation in SPCcharts. More information about the Vision MARK-1 can be found here.
The thickness of a conformal coating on a PCB is one of the most important aspects to producing consistent, reliable boards. There is a minimum coating thickness that is required to achieve full protection as specified by the manufacturer. Coatings applied too thick can lead to risks of defects such as CTEmismatch, heat entrapment, cracking, etc. According to IPC-CC-830, the recommended thickness for most standard conformal coatings ranges from 30 – 130μm or 210μm depending on the specific type. The thickness of conformal coating can be measured wet or dry, either immediately after application or after the full curing process.
Wet Thickness Measurements
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. Thickness measurements of conformal coating can be used immediately after application to estimate the dry film thickness using the solids content of the coating mixture. However, some initial experimentation is required to correlate the measured wet film thickness to the dry film thickness.
Dry Thickness Measurements
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 disadvantages. 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 capillary flow may occur.
Taking thickness measurements using a micrometer is a popular choice due to its low-cost and simple operation. Typically, measurements are taken on specific board locations before and after the application of conformal coating and the difference is the thickness of the coating layer. This is mainly performed on a representative test coupon to prevent damage to the actual board assembly. A few factors that could introduce variability in micrometer measurements are the compressibility of coatings, degree of tightening, angle of measurements, and cleanliness.
Using eddy current is another 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. The main disadvantage to using an eddy current thickness gauge is the requirement of a conductive layer. This results in the use of a board or test coupon with an internal ground plane or the use of a conductive coupon (e.g., aluminum test panel). When using a board with an internal ground plane, measurements must be taken before and after the coating is applied to calculate the coating thickness. With the use of a conductive test panel, an eddy current measurement may be taken directly on the coating surface. Since this method requires contact with the coating, similar to the micrometer, some factors that could introduce variability are the compressibility of coatings, multilayer PCBs, angle of measurements, and cleanliness.
Ultrasonic testing is another method of measuring coating thickness that can be performed directly on PCBs without needing 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 analyzed to determine the coating thickness. The pulse can also penetrate and reflect off multilayer interfaces, allowing for thickness measurements of multiple layers simultaneously. The disadvantages of using ultrasonic methods are the requirement of a large, unpopulated area due to the size of ultrasonic transducers and the use of a couplant or water to assist in the propagation of the ultrasonic pulse.
Optical methods for measuring coating thickness are new, powerful techniques that enable non-contact, non-destructive, and highly accurate coating thickness measurements. Multiple techniques such as chromatic confocal microscopy, interferometry, 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 chromatic confocal microscopy to provide quick, accurate coating thickness measurements. A spectrophotometer produces white light that is split into a spectrum of monochromatic light that converges at varying distances away from an optical sensor. Light from the top and bottom of a conformal 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 coating to determine the thickness of a conformal coating layer. More information about the CC-100 can be found here.
4. Coating Adhesion
Proper adhesion of a conformal coating layer is important to the final appearance and operation in a PCB. Adhesion failure can result in delamination, cracking, and ultimately the board failure. ASTM D3359 outlines two methods to test the adhesion of conformal coatings using either the X-cut method or a crosshatch method. Both methods use a sharp tool to cut a pattern in the coating layer followed by the application of a pressure–sensitive tape over the cut surface. The tape is then peeled back, staying close to an angle of 180°. The amount of coating removed from the substrate is an indicator of the adhesion level of a coating and is classified into six different ratings based on the ASTM standard.
Quality control throughout the entire conformal coating process is key to ensuring long-term reliability in coated PCBs. Key steps to implement quality control include incoming inspection, coating thinning, coating thickness, and coating adhesion. Various methods and equipment were discussed to provide a comprehensive range of options for quality control. Insituware is dedicated to helping users control materials and ensure a world-class manufacturing process.