Authored By:
Raul Gonzalez, Michael McCutchen, Richard Burke, Nathaniel Eternal,
Ed Laughlin, and Daphne Pappas
Plasmatreat USA, 30695 Huntwood Avenue, Hayward, CA 94544, USA
Summary
Conformal coatings are essential components for the microelectronics packaging industry. These functional coatings aim to protect electronic circuits from environmental factors such as heat and moisture. Typical coating formulations involve the use of epoxy, urethane and acrylic chemistries on polymer printed circuit boards (PCBs) leading to poor adhesion and failure to form void-free uniform coatings that follow the PCB skyline contour.
Plasma treatment is a novel method to increase the adhesion strength of the conformal protective coatings to PCBs through the removal of residual organic contaminants and surface activation. Poor adhesion can be a result of: i) incompatible materials, such as bonding polymers, ii) process residue: contaminants from fluxing, soldering and chemical treatments and iii) handling and storage conditions: fingerprints and dust. Plasma treatment under atmospheric pressure plasma (APP) conditions has emerged as an alternative solution for completely assembled PCBs which does not require a vacuum-based system. APP processes are fast and can be used for the treatment of selective areas of the board. The technology utilizes a dry gaseous medium and does not involve any harsh liquid solvent chemistries. Air-based APPs contain gaseous species that can react and remove organic surface contaminants very rapidly. Furthermore, they can be instrumental in the chemical functionalization and activation of the surface. In this paper, case studies from the application of air-based APPs for the cleaning of PCBs and the improved adhesion of conformal coatings will be presented.
Conclusions
Conformal coating on PCBAs has been a standard application for decades. However, manufacturers of electronic equipment face challenges like bubbles, orange peel, de-wetting, and delamination which are related to the cleanliness of the PCBA surface prior to the coating process. Atmospheric pressure plasmas can be effective in removing the surface contaminants. In Case Study 1, cleaning the PCBA surface with air-based plasma improved the average pull force, in pounds by 20%. Additionally, Case Study 2 showed, that plasma improved coverage resolution in three out of the four conformal coatings that were tested.
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