|Ask the Experts|
August 31, 2020
Conformal Coating Press Fit Connectors
We are having problems withpress-fit connectors. We produce military circuit board assemblies andcurrently use a fast drying, single component, acrylic conformal coating.
We previously coated the assemblies theninserted the press-fit connectors. Now some assemblies need to be tested beforeand after coating. Therefore we need to insert the connectors beforecoating 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 becoated. Is there any exception?
|Expert Panel Responses|
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.
Principal Materials and Process Engineer
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, asi s a pre-heated board surface (flash off solvents quickly). Both help to control wicking and flow after application.
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 terminations 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.
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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