March 6, 2006
I would never use any liquid masking material that solidifies to a rubber like state on any connector or electrical socket, unless the product used is totally removable via the cleaning process without using any mechanical means (tweezers, etc.). Our experience has been that if the material does not completely peel off the connector socket contacts such that there is residue remaining in the contact cavity, any mechanical method that attempts to grab the residue from within the contact cavity may cause spreading of the contact sockets resulting in poor contact retention and an intermittent electrical circuit. If this type of masking material must be used and one uses mechanical means to remove the material from within the socket, I recommend folks do a contact retention test on any contact that was mechanically probed. We prohibit use of this type of masking material for use on any socket or connector contact.
Control of conformal coating viscosity is critical if the conformal coating is applied by a spray method, such as by using a NORSDON spray machine. If the coating is too thin, when sprayed onto the surface of a PWB, the coating can wick up between the OD of the socket contact and the ID of the socket cavity and find its way into the socket cavity itself. One may be able to detect this condition during visual inspection since a ring of conformal coating may be seen on the top surface of the socket contact. If this ring is seen, chances are that the coating is also on the internal ID surfaces of the contact cavity.
----- Original Message -----
From: Kevin Glidden<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Thursday, April 06, 2006 8:20 AM
Subject: Re: [TN] conformal coat masking
Phil,
Option #2 below is a very good option. A couple more options ...
Use peelable latex mask to fill the connector cavity prior to coat. Make
sure you leave enough material to get a hand or tweezer on it to get it out.
Test a few pieces first to make sure it is actually removable!
Mask this area from coating and then secondarily apply (brush) a thicker
coating. There are some coatings with higher viscosities that are less
prone to wicking. You can also thicken coatings with a filler like
Cab-0-Sil.
Good Luck,
Kevin
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Mikell
Sent: Wednesday, April 05, 2006 5:58 PM
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: [TN] conformal coat masking
Phil,
Assuming that you can change the design at this stage, you have three basic
choices. If you cannot change the design, go directly to the "Null Option"
1. No coat option. Mask the connector and coat the rest of the board.
Depending on environment, the conformal coatings purpose is to keep the
board clean.
If the connector area is not subject to much dirt, not coating around and
under it may be an option.
2. RTV seal. Seal the area around and under the connector with a bead of
moisture cure RTV. The coating can be done before RTV with a mask operation
as above. If performed after RTV seal, the conformal coating might be
changed to Type SR.
3. Change the connector. Many connector series designed for use in high
reliability or harsh environment systems have a version with sealed pins to
prevent wicking of conformal coat.
And there is always the "null option"..... subcontract the assembly to
somebody else to deal with the situation caused by the lack of a proper
concurrent design process.
Steve Mikell
[log in to unmask]<mailto:[log in to unmask]>
> ----- Original Message -----
> From: "Crepeau, Phil (Space Technology)" <[log in to unmask]<mailto:[log in to unmask]>>
> To: [log in to unmask]<mailto:[log in to unmask]>
> Subject: [TN] conformal coat masking
> Date: Wed, 5 Apr 2006 13:41:40 -0700
>
>
> hi,
>
> we are using several qss-050-01-l-d samtec surface mount connectors on
> a printed wiring assembly that is conformal coated with a choice of
> either an ar or a pur material. the connector pin/header interface is
> not sealed, so the conformal coating that must go under this connector
> has an easy path to wick up the leads and contaminate the mating
> surfaces of the connector. what to do?
>
> thanks,
>
> phil
>
--
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