SMTA International Conference Proceedings

Overcoming The Challenges Presented With Automated Selective Conformal Coating Of Advanced Electronic Assemblies By Employing Plasma Treatment Technology

Author: David Foote
Company: Nordson March
Date Published: 10/13/2013   Conference: SMTA International

Abstract: The reliability and performance of modern advanced electronic assemblies is steadily increasing to meet market requirements and demands. Contemporaneously, demands are such that material choices become as environmentally friendly as possible. Such constraints have presented the electronics industry with unforeseen challenges caused by certain limitations of legacy materials. For example, RoHS requirements have driven printed circuit board assembly materials to ever higher temperature capabilities. The High Tg materials are designed to withstand the higher reflow temperatures of lead free solders but present their own set of problems down the line in subsequent manufacturing steps. In particular the adhesion of existing conformal coatings to advanced materials has presented significant challenges. Either advanced conformal coatings can be applied to overcome the adhesion issues or an alternative is necessary. In addition to RoHS materials posing challenges for conformal coating adhesion high performance materials also present challenges. System performance is being driven to ever higher levels to take advantage of higher frequency bands for telecommunications and military applications. Higher frequency applications require low dielectric constant (DK) materials to minimize cross talk and signal losses. Typically these materials consist of PTFE which inherently has poor adhesion to most materials. Not only is conformal coating adhesion a challenge but delamination is a fundamental issue during the board manufacturing process.

Additional challenges for conformal coating adhesion are contaminants such as mold release compounds and residual flux. The effects of residual mold release on various surface mount components, quad flat packs, dip packages and the like. Visually these devices appear to be clean and void of contaminants and although the packages have passed various levels of outgoing QC metrics as well as incoming inspection metrics, they often times display the effects of residual mold release when conformal coating is applied via select coating equipment. To a similar extent, manufacturers of printed circuit board assemblies have dealt with excess flux residues, both traditional and no-clean chemistries. With high lead count devices, minimal lead spacing, thus correspondingly high aspect ratios, often time’s residual flux can be left behind even after extensive aqueous cleaning and/or not sufficiently consumed in either the soldering or reflow processes. Given this reality, often selectively applied conformal coatings can be inhibited from sufficiently adhering to the critical junction points at the device body to lead and lead to pad.

Key Words: 

Plasma surface treatment, conformal coating adhesion, plasma surface cleaning

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