Conformal Coating Thicknesses Controlled in Circuit Board Assembly

Conformal coatings are applied to circuit boards in order to protect the board from its operating environment, extending its life and reducing failure rates. However, the choice of coating material is a balancing act that requires weighing protective properties against practical considerations. The type of conformal coating that is chosen will impact the final design, including its thickness.

Coatings can be applied through dipping, spraying, brushing, or chemical vapor deposition. The method that is most appropriate will depend on the board size, component types and placement, production volume and budget, and the desired finish. A parylene coating, for example, offers exceptional resistance to moisture, abrasive particulates, and chemicals; however, the specialized production process makes it difficult to replace in the event of damage.

For low-volume production, where capital equipment is not available or practical, conformal coatings can be applied using aerosol cans and handheld spray guns. These methods can be time-consuming, require masking and tape to prevent accidental coating of unintended surfaces, and quality and consistency is operator-dependent. In higher-volume production runs, automated spray systems can be programmed to move circuit board assembly on a conveyor under a reciprocating spray head for fast, consistent coating. In either case, rigorous quality control must be enforced to ensure that the proper coating is being applied and the coating thickness is consistent from batch to batch.

How Are Conformal Coating Thicknesses Controlled in Circuit Board Assembly?

While encapsulation can provide additional levels of protection for a PCB, it adds weight and cost to the finished product and may not be feasible in all cases. For example, mission critical electronics controlling automobile engines must be able to withstand extreme heat and vibration abuse. Conformal coating is a more affordable alternative that can protect these devices from environmental degradation while offering a high level of reliability.

The thickness of a conformal coating is controlled to ensure that internal and international standards are met. A coating that is too thin provides insufficient protection and insulation, which can lead to failure in the field. Conversely, a coating that is applied too thick interferes with the expected thermal management solutions of the assembly by hindering heat dissipation and causing excessive stress on components and solder joints.

In addition to measuring the actual film thickness, a number of other methods can be used for verification. These include calipers, calibrated thickness gauges (which use magnetic induction or eddy current principles to measure the distance between the probe and the substrate, and correlate it to the coatings thickness), and electrical capacitance methods, which utilize changes in capacitance between electrodes positioned on opposite sides of the PCB to determine the conformal coating thickness.

Ideally, the conformal coating specification is included as an early stage in the PCB design. This will allow the PCB designer to confirm that the thickness specifications are within acceptable ranges and can be shared with the manufacturer during the design review process. The Altium Designer toolset allows for this to be accomplished, enabling an electronic assembly partner to verify that the conformal coating thickness requirements are being met from the start of the manufacturing process.

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