These flex circuits included trace encapsulation with 
     polyimide coverfilm. Examination of x-sections showed that 
     the dendritic growth was following the field lines between 
     traces in the coverfilm adhesive. For the materials that 
     were failing, the adhesive itself was acting like a solid 
     state electrolyte. The incubation period was related to the 
     time it took for the ionic level of the adhesive to attack 
     the traces and get copper in solution, creating something 
     like a plating cell.
     
     By the way, all of the Rogers R/flex adhesives have been 
     tested and are immune to this effect.
     
     Andy Magee
     [log in to unmask]


______________________________ Reply Separator
_________________________________
Subject: FW: Re[2]: Humidity Testing
Author:  Dill, Norm J::(DILLNJ)  at ~FABRIK
Date:    7/11/96 5:46 AM


From: Dill, Norm J
Date: Thu, Jul 11, 1996 5:46 AM
Subject: FW: Re[2]: Humidity Testing 
To: "'IPC - Technet Input'"
     
Andy,
     Considering your test was run at 100 v with a .006" spacing, is
there
any chance that airborne contaminants contributed to your 300/500
hour 
failures?
     
          Norm Dill
          [log in to unmask]
 ------------------------
Subject: Re[2]: Humidity Testing
Date: Wednesday, July 10, 1996 4:18PM
     
     We've done some testing on flex circuits and found that 
     there was about a 300 hour incubation period before visible 
     defects were evident on the failing circuits. Actual 
     electrical failures (shorting) took up to 500 hours. The 
     insulation resistance falls off early in the test as the 
     moisture reaches equilibrium in the polyimide, and then 
     falls further at about the time of the incubation period.
     
     The continuous bias voltage was 100V. 85C, 85%RH 100 mA 
     current limited, 0.006" spaces between traces. Measurements 
     were taken at 24, 48, 72, 144, 200, 300, 400, 500, 550 and 
     1500 hours.
     
     Andy Magee - Applications Engineer
     Rogers Corp - Circuit Materials Unit 
     Tel: (602) 917-5237
     Fax: (602) 917-5256
     E-Mail: [log in to unmask]
     
     
______________________________ Reply Separator 
_________________________________
     
Subject: Re: Humidity Testing
Author:  SIRGuru::(SIRGUR) at ~FABRIK 
Date:    7/10/96 1:12 PM
     
     
From: [log in to unmask]
Date: Wed, Jul 10, 1996 1:12 PM
Subject: Re: Humidity Testing
To: TechNet
I'll give you my favorite engineering answer - It depends.
     
If you are trying to determine if a fabrication or assembly process 
is
Bellcore-compliant, the answer is no.  Bellcore compliancy is only 
obtained
through the SIR and Electromigration testing in Chapter 13 of TR-78.
     
If you are trying to determine if a vendor has acceptable product, 
and you
don't really care about Bellcore compliance, then I would say the 
answer is
yes.  The seven day SIR test, IPC-TM-650, method 2.6.3.3 rev A, is an

alternative.
     
The Bellcore electromigration test is 85C/85% RH, so is the SIR 
method.
The Bellcore test goes 96 hours before the first measurement in an 
unbiased
state, then a bias of 10 volts is applied.
The SIR test is biased from the start with a 50 volt bias.
The Bellcore test is not tied to an absolute resistance value. 
The SIR test has a minimum of 100 megohms.
     
I'm not really trying to bash the Bellcore electromigration test, but

I
understand the time constraint.  In my opinion (that and 50 cents 
gets you a
Diet Coke), you are best to do both methods concurrently.  The SIR 
test should
be able to allow you to make the decision in 7 days vs. 25 days.
 Alternatively, you could interrupt the electromigration test after
7-10 days
and see if you have any signs of corrosion or metal migration.  It 
has been
my experience that if you have not seen electromigration and 
corrosion after
7 days at 85/85 and a 50 volt bias, you would not see it in 500 
hours.
 Again, my opinion.
     
Hope this was helpful.  If you want to discuss it in greater depth, 
give me a
call at (317) 457-8095.
     
Doug Pauls
CSL, Inc.
[log in to unmask]

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