SMTA International Conference Proceedings

Conformal Coating Over No Clean Flux Residues Part II

Author: Timothy O’Neill
Company: AIM
Date Published: 9/28/2014   Conference: SMTA International

Seika Machinery, Inc.

Abstract: Building on the work completed in early 2013, AIM’s Technical Team has advanced their understanding of the process through continued research and development.

AIM’s earlier study focused on the overall feasibility of applying conformal coating over No Clean flux residues. The first task was to establish what methods would be the best to characterize ‘compatibility’ and what materials would be best suited to perform the task. What was discovered was that, coating over no clean is feasible, ‘compatibility’ is a fuzzy term and the mission profile of the assembly determines the best material set for a specific application.

The primary objectives for the second phase of testing are,to better understand the performance of the materials in combination, and to assess the impact of advancements in both coating and flux technology.

It was important include all of the readily available coating technologies. The three major types are acrylic, urethane and silicone. Subsets of these three categories are differentiated by the curing method; air dried, moisture cured and ultraviolet light (UV) cure with secondary moisture cure. To these ends, all of the major coating manufacturers in North America and Europe were engaged. The focus was on companies that have predominant market share both in terms of volume and market penetration. The effort had to be collaborative as AIM does not possess the equipment and expertise to ensure the coatings are applied in the best possible way. It is significant that, applying coating over residues violates the first rule of the coating manufacturer’s instruction:

Cleanliness of the substrate is extremely important to the successful application of a conformal coating. Surfaces should be free of moisture, dirt, wax, grease and all other contaminants. Otherwise, ionic or organic residues on the substrate could be trapped under the coating and cause problems with adhesion or electrical properties. The highest long term reliability for a coated printed circuit assembly will be when the conformal coating is applied over a clean, dry substrate.

Key Words: 

Conformal Coating, No Clean, Compatibility, SIR

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