SMTA International Conference Proceedings

Automated Conformal Coating Inspection & Thickness Measurement

Authors: Owen Sit, Hector L. Fonseca
Company: Nordson Yestech
Date Published: 9/17/2017   Conference: SMTA International

Abstract: Conformal coating is a thin, transparent coating that ‘conforms’ to the contours of the board covering the electronic circuit boards & protecting the electronics components from the corrosive effects of the environment, typically applied at 50-250 µm. The products that are normally subject to harsh environmental conditions can benefit by having conformal coatings and will improve their reliability and endurance, also increasing their operational longevity. In addition to shielding the components, conformal coatings also mechanically strengthens and protects them, making them more able to withstand mechanical stress which is common during the temperature cycle of a product’s operation.

In an electronic product’s built process, conformal coating is most typically applied after functional test is successfully performed on the circuits. At this stage of the process, all the connectors are installed and the circuits are cingulated (de-panelized). Due to this nature, the inline automated application and inspection of conformal coating is most likely done on a line that is separated from the automated assembly line that builds the circuit in the first place. This line will include an automated dispensing machine that dispense the conformal coating, a curing oven that cure the conformal coating; followed by an automated conformal coating inspection machine.

There are areas on the board where it is particularly important to have the protection of the conformal coating. Typically these are the metal leads of the electronics components. On the other hand, it is also important that certain area of the board be completely free of coating. This is called the “keep-out” area. Typical keep out areas are connectors that carry the electrical signals to other circuits. Lastly, there are areas of the board where have or not have the coating is simply optional; we call these areas the “Optional” areas.

Different masking techniques can be used to ensure the keep out area is free of coating; using masking tape is one of them which are labor intensive and very hard to automate. Since conformal coating is done after functional test, board fixture is commonly used to hold the boards. A specially designed fixture can be used to cover the all connectors on the edge to prevent the conformal coating from getting onto them. Such a fixture automatically masked out all edge connectors. Rather than masking, another completely different approach is to use the automated dispensing machine to selectively apply the coating only to the areas that needed the coating.

Similar to the previous production stages, quality of applied coating must be assured. After coating the assembly, proper wetting is mostly visibly checked by manual means in a special darkened area so that the fluorescence stimulated by UV light can be observed by the inspector using a magnifying glass (main method used by > 90% of the electronics manufacturing world), with the usual manual optical inspection disadvantages, among them are that the results are dependent by Inspector experience + Inspector to Inspector variability, it requires higher working space allocation, usually means increased work in progress (WIP), has inadequate product handling with non or reduced traceability and data storage, inspection, hazard & safety awareness training are required, since UV light required for inspection could potentially damage Inspector eyes. Besides this, required magnification for inspection keeps increasing as electronics miniaturization continues moving forward. Per IPC & depending on minimum defect size detection, 5 - 10X magnification usually suffices for current conformal quality standards, however customer and new technology requirements keeps increasing the complexity of the inspection, fortunately magnification of automatic inspection systems typically comply and surpass these requirements.

Key Words: 

AOI, Spectral Interference Laser Displacement.

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