Agreed.
Contrary to perceived wisdom a large proportion, possibly the majority, of
conformal coating is applied over uncleaned flux. So that's generally
encouraging. Having said that it is not really possible to give a
categorical answer (save a safe no) to your specific application as there
are too many what ifs, maybes and buts to consider along the lines outlined
by Karen. The interaction between individual coatings and fluxes is not
always predictable and can be influenced also by alloy type as well as
workmanship.
Your suppliers may have some data/advise if you dig into them to start you
off, but likely you will need to test specific pair combinations.
When determining a test programme I would advise against tape testing.
Irrespective of any intellectual merit for or against the validity of the
test basis, IMHO it is too technique dependent to have any value.
Regards
Mike Fenner, Technical Manager
Indium Corporation
Europe
Mob: + 44 7810 526 317
Skype: mike.fenner
Tel: + 44 1908 580 400
Fax: + 44 1908 580 411
www.indium.com
---------------------
Indium Corporation
Incorporated in New York, USA
Registered office: (Not for correspondence)
77-97 Harpur Street, Bedford , UK
Registered number: fc16473
* Please think before you print
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Karen Tellefsen
Sent: Thursday, March 04, 2010 8:26 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coat Over No-Clean Flux
You've got to make very sure that the solder wire's core-flux or the liquid
assist flux plays well with the conformal coating and there are definately
combinations of fluxes and coatings that don't get along. This requires
lots of testing. The coating must adhere well to the circuit board with
the flux residues present, so adhesion tests must be done. this is a good
starting point:
http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2.4.1.6.pdf
Then one must test the adhesion, etc. after exposures to adverse
environmental conditions, such as high temperature and humidy and
temperature cycling. Additionally SIR or MIR measurements should be made
for the flux residue and conformal coating combination.
http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2-6_2-6-3-4A.pdf
http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2.6.3.3b.pdf
http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2-6-3-7.pdf
The surface insulation of the flux covered with coating should be as good
or better than the SIR of the flux by itself. Similarly the insulation
resistance of the flux and the coating together should be only. moderately
worse than the coating by itself. After testing, coating should adhere
well to the board with minimal indications of vesication (moisture
bubbles).
I've tested combinations of flux and coatings where the individual
materials have high SIR by themselves, but have very low SIR together.
There are also combinations that work just fine.
Karen Tellefsen - Electrical Testing
[log in to unmask]
908-791-3069
Ken Bloomquist
<Kennyb@TECHNICAL
SERVICES.NET> To
Sent by: TechNet [log in to unmask]
<[log in to unmask]> cc
Subject
03/04/2010 12:47 [TN] Conformal Coat Over No-Clean
PM Flux
Please respond to
TechNet E-Mail
Forum
<[log in to unmask]>
; Please respond
to
Ken Bloomquist
<Kennyb@TECHNICAL
SERVICES.NET>
We have a twelve leaded, through-hole component that has to be hand
soldered
on the circuit board. The assembly gets conformal coated with urethane. We
would like to hand solder these twelve leads using no-clean and then
conformal coat the board. The component would be added as a final step
after
the assembly has first been cleaned.
Since this is not a "total no-clean" process do you see any issues applying
conformal coat over these twelve no-clean solder joints?
Best regards,
KennyB
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