Bill,

Your reported difficulties imply that you may have some ionic residues on the
assembly surface and because a conformal coating breathes, water vapour can
pass through and create osmosis. Osmotic pressure up to 800 bar (zinc salt)
can be evidenced. Result - blisters and circuit failure as the solubalising
residue bridges conductors or component legs.

I assume the coating has been fully polymerised before these tests? That the
coating has been applied to the correct thickness? That you have attained a
suitable cleanliness level???

Ah! Cleanliness --- Please refer to my previous offerings on the subject of
SIR testing or Circuit Reliability Testing. If would like a further copy let
me know and I will send it offline - too many  TechNetters have already
received multiple copies.

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:   Friday, October 17, 1997 4:20 PM
To:     [log in to unmask]
Subject:        [TECHNET] ASSY: Temperature Cycling Test Failures

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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. Has anyone
experienced similar problems?

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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