Nordson MARCH has recently released a white paper showing the results of RF plasma processing on conformal coating adhesion, the conformity of the coating coverage, and the resulting effects on electrical functionality of a fully assembled printed circuit board. The research was done in conjunction with Nordson ASYMTEK, AirBorn Electronics, and SMART Microsystems (formerly known as Desich SMART Center).
The objective of the experiment was to evaluate the effects of RF plasma processing on the conformity of coverage of conformal coating of the knee of individual Nickel-Palladium-Gold (Ni-Pd-Au) leads on electronic assemblies using an acrylic conformal coating and to determine if any change in electrical functionality occurs. "The corrosion of Ni-Pd-Au plated terminals in humid environments is known to be reduced with the application of a conformal coating such as acrylic," explained Dave Selestak, senior business development manager, Nordson MARCH and co-author of the article. "Corrosion has a higher rate of occurrence around the terminal 'knee' of a surface mount component, which may be reduced with the application of conformal coatings. We were looking to see whether plasma processing can enhance the quality of the conformal coating as well as increase thickness of the coating around the knee of a terminal."
The evaluation was done in three stages, with objectives for each below:
Stage 1: To test the effect of argon RF plasma process parameters on discrete components in order to mitigate concern of the effect of vacuum pressure and plasma power on electric functionality.
Stage 2: To evaluate the effect of plasma power and process gasses on a semi-populated PCB using optical metrology and electrical testing on the SOIC20 microcontroller.
Stage 3: To evaluate the effect of plasma time and process gas pressure on both semi-populated and fully-populated and functional PCBs using electrical testing and optical metrology.
The research and results can be found in the paper, The Effects of Plasma Treatment Prior to Conformal Coating, by John D. Vanderford, MSEE and Ann E. Paxton, SMART Microsystems, and Dave Selestak, Nordson MARCH.