Sensible Design: When Coatings Go Wrong


Reading time ( words)

This month, I consider some of the more common, and often very frustrating, problems that may be encountered when coating electronic circuit boards and components. I also discuss some practical solutions.

As we all know, nothing in life is straightforward. In any engineering discipline, if there is the slightest chance that something might go wrong, you can bet your bottom dollar that it will. The secret is to be prepared for it. For the purposes of this column, I’m going to concentrate on the use of conformal coatings for the protection of electronic assemblies, highlighting some of the potential pitfalls associated with the choice of coating and the method of application. In each case, I will suggest an approach that should mitigate the majority of problems you are likely to encounter.

Problem: The quality and performance of a conformal coating material could be compromised according to the method of application.

This issue is commonly encountered when a product is transferred from one circuit manufacturer to another; for example, a product may be dip-coated in one country but selectively coated in another, with the specification requiring that the same material be used at both sites. The problem that arises here, however, is that using a material formulated for dip-coating in selective coating equipment can result in poor yield due to excessively fast drying and bubble entrapment.

One of my customers spent six months trying to solve a bubble issue internally, without realising that the root cause of this problem lay in the material formulation. After working with the customer, we found that by changing the solvent blend, the bubble entrapment issue could easily be resolved. Moreover, this solution simplified the process and reduced the cycle time. And since the non-volatile formulation remained the same, there was no need to re-qualify.

Problem: Achieving incorrect coating thickness, especially with acrylics.

The IPC specification allows a dry film thickness of between 30 and 130 microns, with the greater thickness being achieved by the application of multiple coating layers. Trying to achieve a 130-micron dry film thickness from a single selective-coating process with a solvent-based acrylic material is a recipe for a disaster, likely to result in excessive bubble formation, film shrinkage, coating delamination and additional stress on components. The result is poorer protection, rather than an improved overall level of circuit protection. Aiming for a uniform 30-50 microns and focusing on achieving perfect coverage at each application is a much better approach to improving the protection of electronic circuits.

Achieving the correct coating thickness is important; bear in mind that if the coating is too thick it can lead to entrapment of solvents in areas where the coating does not fully cure. Similarly, it can cause the coating to crack as it cures or as the result of changes in temperature, or due to mechanical shock and vibration. 

To read this entire article, which appeared in the July 2016 issue of The PCB Design Magazine, click here.

Share

Print


Suggested Items

RTW IPC APEX EXPO 2019: Judy Warner on Altium's Transformative Vision

02/28/2019 | Real Time with...IPC
During IPC APEX EXPO, Guest Editor Kelly Dack sat down for an interview with Judy Warner, Altium's director of community engagement. They discussed Altium's efforts to educate PCB designers, regardless of their CAD tools, and the company's plan to transform the industry by bringing together PCB design and manufacturing.

A Drone's-eye View of the IPC APEX EXPO Show Floor

02/14/2019 | Real Time with...IPC
If you couldn't make it to IPC APEX EXPO in San Diego, don't worry. We have full coverage of this annual must-attend event, from the opening ceremony to the close of the final day. To get a sense of the size of this year's show, check out our drone's-eye view of the show floor. Can we name our drone Air007?

Joe Clark Discusses DownStream’s Updated Tool Lineup and IPC-2581

02/07/2019 | Real Time with...IPC
DownStream Technologies recently revamped their entire product line, from CAM350 through BluePrint-PCB. DownStream co-founder Joseph Clark and Guest Editor Kelly Dack discuss some of these updates, including a new GUI and capabilities such as 3D analysis, as well as news about IPC-2581.



Copyright © 2019 I-Connect007. All rights reserved.