Steve,
I agree that this is madness, unless the telecom supplier has an end use 
environment of 85/85 where power-on in humid conditions is a reality. 

Assuming that the equipment is not functioning in a steam environment, 
here is the "science" that you are looking for.

Designers, when they have knowledge of the material characteristics, will 
design spacings dependent on the dielectric strength or dielectric 
withstanding voltage of the laminate.  The values in the data sheets for 
the laminates specify these parameters, but only for lab ambient 
conditions, not after 85/85 conditioning.  When you expose a hydrophilic 
material to a long exposure in hot/humid conditions, then you don't have 
that same dielectric strength (dramatically reduced) and you have violated 
the design assumptions.  Polyimide is such a hydrophilic material.  I 
believe Brian Ellis referred to it as blotter paper, in which I would 
concur.   If you allow the circuit to dry out, then you return to the 
expected dielectric strength.   You may already be at an unrealistic 
threshold with 2500 volts/mm gradient, as Brian also pointed out.

Unless the designer had a firm value for dielectric strength of the 
substrate after 85/85 conditioning, which would change as the substrate 
rapidly dried out, then the design would not and could not work.

Doug Pauls




Steve Kelly <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
06/25/2008 10:09 AM
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[TN] Bias voltage test in damp heat






Good Day To All,

Back in April I posted some queries on the tech-net about high voltage
testing after 85C/85RH testing and was basically told this test was
impossible and I agree with that assessment. But some things have a life 
of
their own.

To re-iterate: We are building a 4 layer flex circuit. All layers are 18
micron copper and all lines and spaces assuming perfect etch are 100 
micron
lines and 100 micron spaces. My customer for some reason signed up to pass 
a
250 volt test after 1000 hours of 85C/85RH damp heat test. My premise is
being in this case a polyimide build if they dried the circuit after this
test it should pass. They do not want to dry it and still have it pass. 

The previous threads in summary said this test was "madness" - I agree but
what is the scientific explanation of why this is madness. 

Please note this requirement is being driven by very large telecom 
companies
and the worlds largest provider of internet gear. 

Thanks again for the help.

Regards Steve Kelly

 

Steve Kelly

(416) 750-8433 (work)

(416) 750-0016 (fax)

(416) 577-8433 (cell)

 


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