Bill, Further to my message yesterday, I have been reviewing your problem with
my colleagues and have the following for your consideration:

"If the polyurethane product you are using is a two-part material it may be
that residual moisture trapped beneath the component is causing the problem
directly by reaction with the isocyanates. The gas produced form this is
carbon dioxide and can form bubbles.

R-NCO + H20 ( R-NH2 + CO2

I suspect it is more likely that the conformal coating is not penetrating
beneath the SMT devices, and the air bubble produced is subsequently expanding
when the units are cured. The air pressure then forces the bubbles out around
the edge of the device, between the legs."

I would only add that the majority of Polyurethane coatings contain
isocyanates. This includes single component materials.

Let me know if we can be of more assistance.

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

This message is in MIME format. Since your mail reader does not understand
this format, some or all of this message may not be legible.
------ =_NextPart_000_01BCE86B.0AFBF380
Content-Type: text/plain

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]
>

------ =_NextPart_000_01BCE86B.0AFBF380
Content-Type: application/ms-tnef
Content-Transfer-Encoding: base64


##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following
text in the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or
847-509-9700 ext.311
##############################################################

##############################################################
TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c
##############################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TECHNET <your full name>
To unsubscribe:   SIGNOFF TECHNET
##############################################################
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
##############################################################