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November 21, 2018

Conformal Coating Press Fit Connectors

We are having problems with press-fit connectors. We produce military circuit board assemblies and currently use a fast drying, single component, acrylic conformal coating.

We previously coated the assemblies then inserted the press-fit connectors. Now some assemblies need to be tested before and after coating. Therefore we need to insert the connectors before coating process.

Wicking of the coating is a huge problem. If we mask the connectors coating will not be applied to the plated hole pads. IPC-610-E indicates that every connective and soldered point needed to be coated. Is there any exception?  

S.C.

Experts Comments

First, IPC-A-610 is not a requirements document. It is a visual acceptance document. Requirements would come from J-STD-001 or the engineering drawing.

But, with that being said, we always conformal coat after compliant pin insertion. You can try spray coating with very light sprays of your acrylic coating, the light passes will cover over the through-holes without flooding.

Failing that, ask your coating vendor if they make a gel form of your coating, one that will coat the area but is too thick to flow down the holes. Failing that, you can coat your boards inverted, so that gravity counters the capillary force.
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Doug Pauls
Principal Materials and Process Engineer
Rockwell Collins
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
After review of the pertinent section of IPC-A-610, I actually don't see that there is any requirement for all soldered surfaces to be covered. In fact, if your boards have any bottom-terminated components (including BGAs and the now almost-ubiquitous QFNs) there will be un-coated areas with any spray process.

The areas under multi-row through-hole connectors will also normally be uncoated. The IPC document simply states that all areas that are designed to be covered need to be covered, and those that are not intended to be covered shall not be.

I do believe that in most cases the intent is to coat the lands of the plated through-holes on the non-component side of the press-fit connectors. It is normally possible to do so without wicking coating completely through the board. The fact that the coating covers the contact between the press-fit connector and the hole wall is of no consequence unless rework is required, in which case coating removal becomes necessary.

Control of coating thickness and flow after application is the key. Multiple thinner coats are an advantage, as is a pre-heated board surface (flash off solvents quickly). Both help to control wicking and flow after application.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
There is always a compromise on coating. The key to this is the pain  in the process and its cost versus the practical process.  

If you cant get sealed connectors then one practical way we use is to  fill the holes in the connector body individually from the back. Very  time consuming and expensive but it works. This leaves the  termninations free to be coated through capillary. We use a gel  version of the coating material. Then you mask the PCB from the front  as standard.
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Lee Hitchens
Owner
SCH Technologies
Lee has worked within the conformal coating and electronics industry for over 18 years. His work includes scientific research into long term reliability of electronics, technical sales of conformal coating materials and equipment, owner of SCH Technologies, a conformal coating service in the UK, a member of the Diamond Coating Solutions Group, a global liquid conformal coating and Parylene coating service solutions provider, a founding member of Nexus3c, Conformal Coating Centre and a partner of Thin Film Partners.
Firstly there are two schools of thought regarding press fit connectors and conformal coating. One theory says coat the complete assembly and then insert the connector throughout the coating, I personally think that this is the best option for complete protection. The other theory says completely avoid any coating near to the press fit connector and do not coat it.

As to the problem you are experiencing I suggest that you ask your coating supplier for the gel version of the acrylic coating that you are using now. Apply the gel around the base of the connector before coating, job done. As the gel is the same composition as the coating it is 100% compatible and does not need to be removed.  

To fully comply with the IPC-610-E I would suggest putting the gel onto the base of connector or board when inserting it. This will cover all connective points under the connector and also prevent any coating entering the connector during the coating process.
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Chris Palin
European Manager
HumiSeal
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
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