Bill, It would b extremely helpful to know which type of coating you use. Let
me know offline if it will spare the blushes of the supplier.
As to the problem:
Bubbles under the coating could be caused by ionic residues at the surface and
you are actually seeing blisters rather than bubbles.....OR
The coating is being applied too thick. This may have caused solvent
entrapment. In other words the solvent at the point where the coating is
thickest, may not have been released in due time. The surface skinned over and
the solvent ecame trapped but, unable to defy the laws of physics, the solvent
wants out, converts to gas and.... Bubbles!!!
ALSO: You know that a coating to MIL-I-46058 should be applied to 0.001 to
0.005 mils. If it is a Urethane (UR) or Acrylic (AR) this can be 0.002 +/-
0.001 if it is Epoxy (ER) or Silicone (SR) it may be 0.003 +/-0.002 INCHES!!
WHY the difference??? Because if you look at the coating at molecular level
you will find that the molecules of UR or AR resemble tennis balls whereas ER
and SR resemble basketballs... what am I talking about?...the fact that all
coatings breathe and permit air, which may contain moisture to migrate in and
out of the film. The vapour transmission rate of AR and UR will be TYPICALLY,
lower than ER and SR.
Also, I remember a similar problem with your colleagues in the UK. They
resolved it by applying a thin coat (material viscosity at <30 CPS via
dipping) then a "Finish" coat (viscosity around 200 CPS via dipping). Talk to
your colleague Andy Glover in Birmingham, he MAY be able to help.
Let me know please, I do have more!
Graham Naisbitt
Concoat Ltd Email: [log in to unmask]
Alasan House, Albany Park Tel: +44 (0)1276 691100
Camberley, Surrey GU15 2PL UK Fax: +44 (0)1276 691227
-----Original Message-----
From: TechNet On Behalf Of BROMLEY, Bill
Sent: Monday, November 03, 1997 8:13 PM
To: [log in to unmask]
Subject: [TN] ASSY: SMT/Temp. Cycling Test Failures revisited
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Fellow technetters:
I would appreciate any help in the following subject....
We are experiencing temperature cycle (-37C to 55C) test failures on mixed
technology (thru-hole one side, SMT other side) PWB assemblies coated with
Polyurethane per Mil-I-46058. The same assemblies pass static temperature
testing at the low, ambient and high points of the temperature range. Uncoated
assemblies will pass the temperature cycle test. It is suspected that small
bubbles in the conformal coating (within allowable size per specification
criteria) are accumulating condensation during temperature transitions,
causing bridging between conductive surfaces on the SMT components. The
surface mount components that we are using are attached to the board with a
small dab of adhesive in the middle of the component. Then, after the
components are wave soldered to the board, two very small gaps exist under the
components between the adhesive and the solder connections. The air bubbles
that we see in the conformal coat are next to these gaps, leading us to
believe that the air under the components escapes during the coating cure
thereby forming the air bubbles. Some of these bubbles are large enough to
bridge between the two solder connections. It is suspected that these bubbles
are accumulating moisture which causes leakage currents between the conductive
surfaces on the SM components.
How do we get the air out from under the components without causing bubbles in
the conformal coat? Is it acceptable to have conformal coat flow under the SM
components?
> Bill
>
> Bill Bromley
> Lead Designer
> Lucas Aerospace PSA
> MailStop D960
> 777 Lena Dr.
> Aurora OH 44202-8025 USA
> voice: 330-995-1000 ex 3092
> email: [log in to unmask]
>
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