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NPL/SMART Group Conformal Coating & Cleaning Experience

Day Two

To dip or to spray? The second day’s seminar session opened with a discussion of the relative merits of different conformal coating techniques from Jacques Mycke of Nordson Asymtech.

There were many choices to be made, depending on quantities, sizes and dry film properties: Automatic or manual, total or selective coverage, solvent based, non solvent based or water based, drying or curing, with heat, moisture or UV?

Brush coating was by far the cheapest and simplest method, but slow and heavily dependent on operator skill. Dip coating was a simple mechanical technique, but entry and withdrawal speeds were critical factors and there would always be thickness differences top-to-bottom, and a risk of air entrapment. And dip-coating was not suitable for moisture-cured or UV-cured materials.  Masking was time-consuming and costly, and the subsequent removal and disposal of masking material added to time and cost.

Needle dispensing and jetting were useful techniques for selective coating of small areas, and spray coating, manual or automated, was the preferred method for overall coverage. Selective spraying was flexible and repeatable, but could require masking because of overspray unless additional needle or bead dispensing was used to contain or hide feathering. 

Film coating was a specialised, patented, airless technique for selectively applying lower viscosity materials, which were dispensed as a controlled film rather than an atomized spray. It was a high speed process with high transfer efficiency and single pass coverage, and the need for masking was virtually eliminated. Film coating could be combined with jet coating in a single machine, with the film coater covering the larger areas and the jet covering highly selective areas, and a gel applicator could be added to make dams and plug vias.

The most theatrical presentation came the following hour, from IPC’s Lars Wallin, entitled, “IPC, ESA, and NASA Inspection Requirements for Coating,” with a sub-title: “Is it clean for coating?” Scandinavian eccentricity demanded the attention of the audience. “Do you understand my English?” he asked delegates from Spain and Estonia, in the manner of a stern schoolmaster, but with more than a hint of mischief in his voice. “Are you an expert?”

Continuing with quick-fire questions: “How many IPC standards are there?” “How many refer to cleaning and coating?” and demonstrating their relevance and applicability with the aid of an assembly-shop layout drawing, he arrived at the leading question: “Is it clean for coating?” to which he gave a short answer: “Probably” and a slightly longer answer: “It depends…” backed up by a secure answer: “You can't take the risk of not cleaning” and a cheap answer: “It costs too much to clean!” Then the hard questions came: “Do you have your cleaning process under control?” “Are you sure that your process chemistries are electro-chemically compatible?” “Do you know how clean is clean?” “Does a ROSE or SEC test tell you whether your PCBAs are clean?

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