Good Day To All,
I am back with this thread I started in April this year with some more
questions.
After extensive analysis we have found evidence of what IPC 4203 and 4204
calls inclusions in the raw material we use to make the flex circuits. These
inclusions are usually in the 5-10 micron range and they usually come in
groups (few particles in a group). From the SEM/EDX scans they consist of
Si, Mg, Ca and Cl. Carbon and oxygen are also present in the spectra. What
the exact possible formulations are - I am open to suggestions or theories.
Our theory is that whatever these are they absorb moisture and the result
is they form ions and lower the dielectric breakdown between the adjacent
tracks which are at 100 micron space. Under the bias voltage we get a high
resistance short.
Questions: a) does this theory make sense or any comments
b)at the present time my customer believes this test
helps define that the part will work for 25 years and getting them to change
their mind is not likely to happen. On the next generation product they have
moved the spacing to 200 microns - my concern is they may just be postponing
the inevitable- may make it through 500 hours and fail at 750 for example.
Comments on this would be appreciated. Thanks again to the TechNet on this
issue- especially to Mr. Pauls and Mr. Ellis for previous help.
Steve Kelly
PFC Flexible Circuits Limited
PH: (416) 750-8433
Fax: (416) 750-0016
Cell: (416) 577-8433
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Wednesday, June 25, 2008 12:37 PM
To: TechNet E-Mail Forum; Steve Kelly
Subject: Re: [TN] Bias voltage test in damp heat
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
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Steve Kelly <[log in to unmask]>
To
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Subject
[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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