Understanding the design failures or process amended method of exposed copper surfaces at the PCBA used in the High Saulty Environment is important for best strategies.
I would like to discuse from the following Levels:
1) PCB level: Solder Mask Defined ENIG surface finishes are the most corrosion-resistant and stable protection for copper；
1.1) In PCB surface finishes, only Organic solder preservative surface finishes would expose copper after PCBA process, the exposed copper would be test pad/ Top or Bottom ground pad (Thermal Pad) /Fidual Mark/unplugged PTH/untented vias with solder mask after reflow or wave soldering. Exposed Copper could cover soldering or solder mask before PCBA into a system.
2) PCBA Level, using conformal coating isolates PCBA from corrosion environment.
3) System with Enclosure, Waterproof, positive pressure dust cover.
Zhongwei Chen
Process Supervisor, NexPCB Co; Ltd
25 years experience in SMT/COB industry, 10 years in Optical film by Sputtering and Evaporation.
The simple answer is do not have exposed Cu surfaces in those environments. If that is not possible, then the use of a nano coating to prevent moisture impact, if not available, parylene coating.
Gerard O'Brien
President, S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
If copper must be “exposed,” or not ENIG plated or solder masked, than make sure the surface is cleaned (high surface resistance) and then conformal coat, Parylene if possible.
Paul Austen
Senior Project Engineer, Electronic Controls Design Inc
Paul has been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 45 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes as well as moisture sensitive component storage solutions.
Costal regions are not typically sulfur rich, sodium and chloride common. Regions near volcanic activity or areas of high industrial coal burning are rich in sulfur. But creep corrosion is driven by the level of operational cleanliness that allows high sulfur in low air flow areas to react with the exposed copper. removing the sulfur sources or limiting the static air rich in sulfur to collect around these locations.
Creep corrosion (copper sulfide corrosion) and Flowers of Sulfur (silver sulfide corrosion of the silver below the epoxy surface of the resistor) occur when the concentration and air flow are allowing the sulfur to react with the metals and depositing material on the surface of the hardware. The best way to control this is reduce the risk of exposure, coatings have not proven to be as effective and even silicon will act as a collector. Solder mask, no clean flux residues, plugging and tinting vias all are good ways to minimize the copper surfaces.
Terry Munson
President/Senior Technical Consultant, Foresite
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
Not sure this is best directed at PCB manufacturers but my tuppence would be; Conformal coating of the PCB would be essential. Sealing within a suitably rated enclosure would be necessary (IP67 rating minimum)
Grant Main
Technical Sales Manager, Fineline VAR Ltd
40 Years in PCBs. Large and small companies in the UK and the US. Experienced Process Engineer, DFM, Materials, Impedance Control, Surface Finishes, etc. Working with Fineline now across the portfolio of manufacturers covering rigid, flex, rigid flex, ATE, Load Boards, MSAP, and other existing and emerging technologies.
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