Optical inspection for conformal coating

September 03, 2018 // By Francesco Argentiero
One of the main challenges in the production of printed circuit boards (PCB) in the electronic industry is the protection against harsh environments with high humidity, strong temperature changes and dust or chemical contamination. Since the ‘60s, the conformal coating process has been proposed as a solution for these problems.


Fig. 1: Manual application of a protective coating.

The conformal coating technique provides the distribution of a “plastic”, transparent protective lacquer which perfectly adheres to the assembled board, with a variable thickness, also depending on the material used.

Beginning with military and aeronautic applications nowadays there is a widespread demand for protection by conformal coating for diverse applications in most manufacturing environments, where this requirement is becoming an integral part of the industrial processes. The quality control of this protective process is essential to ensure the correct application of lacquer and the compliance with the standards demanded, aiming to prevent failures of the PCBs. The main property to ensure a reliable conformal coating is the thickness of the protective lacquer. Measuring this thickness is a challenge, since these values range from 30 to 300 μm.

 

Coating thickness

The most accurate technology among those available on the market for conformal coating measurement is viewing sections of the PCB under a light microscope; This method is destructive, therefore it is not applicable to series processes.

Most non-destructive methods are based on the inspection of very small surface areas. Cf. for example white light interferometry, pressure probes and eddy current. To verify the whole surface of the PCB, the test must be repeated several times, implying a time consuming process, which is not compatible with production times.


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