Per-Erik:
You are very accurate in most of your statements, however I have to disagree
with your original statement that electrochemical migration requires an
applied bias. Electrochemical migration does not require an applied bias.
When two dissimilar metals (Cu circuit/Au pad)are exposed to a corrosive
environment ie MFG and humidity, a conductive medium can form between the
two creating an electrical cell. The less noble metal becomes the anode
which can cause an electrochemical corrosion path to the cathode. This
becomes more dramatic when the less noble metal feature is small in size
compared to the larger more noble metal.
Regards
Michael Barmuta
Staff Engineer
Fluke Corp.
Everett WA
425-446-6076
-----Original Message-----
From: Tegehall Per-Erik [mailto:[log in to unmask]]
Sent: Friday, May 18, 2001 3:38 AM
To: [log in to unmask]
Subject: Re: [TN] ISA-S71.04 G3 "Environmental Conditions for Process
Meas urements"
Craig,
I agree, a bias is not required to get migration of corrosion products and a
MFG test without bias will give you some relevant information, for example
the presence of pores in a conformal coating. However, it will not cause
electrochemical migration which is one of the more important failure
mechanisms that you will have in harsh field conditions. Electrochemical
migration often causes formation of dendrites and is an electrochemical
processs requiring a bias to occur. It may occur beneath a conformal coating
if contaminants (usually flux residues) are present in the interface between
the coating and the board. If you have pores in the coating, i.e. poor
coverage of biased surfaces, the dendrites may grow on the surface of the
coating.
Per-Erik Tegehall
IVF, Sweden
-----Ursprungligt meddelande-----
Från: Craig Hillman [mailto:[log in to unmask]]
Skickat: den 17 maj 2001 22:50
Till: [log in to unmask]
Ämne: Re: [TN] ISA-S71.04 G3 "Environmental Conditions for Process Meas
urements"
The requirement of bias is not necessarily true. CALCE has conducted MFG
tests, most recently on palladium-plated components, in which we have seen
migration of corrosion products without bias. These migration products
eventually travel to adjacent leads and lead to an increase in current
leakage.
Best Regards
Craig
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Tegehall Per-Erik
Sent: Wednesday, May 16, 2001 7:04 AM
To: [log in to unmask]
Subject: Re: [TN] ISA-S71.04 G3 "Environmental Conditions for Process
Meas urements"
To be more specific, the tests were originally designed to accelerate
corrosive degradation processes on contact surfaces. They were developed in
a hugh research programme led by the Battelle Institute in the eighties.
For corrosion of contact surfaces the test methods were proved to be
relavent and the acceleration factors are only valid for that case. For
other types of corrosion mechanisms, the relevance of the test has not been
evaluated to my knowledge. Since contact surfaces are not conformally
coated, conformally coated samples were included in the original work.
Electrochemical migration, which is on of the major failure mechanisms that
you want to prevent by conformally coating, would not be cover by the MFG
tests unless a bias is applied to the assemblies during the test.
Per-Erik Tegehall
IVF
SWEDEN
Phone: +46-31-706 6148
Fax: +46-31-27 61 30
Email: [log in to unmask]
Homepage: http://www.ivf.se
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