Steve,
It is the glue that is hydroscopic. I have seen this on two products
that we currently produce. One unit is a flex circuit with heat sinks glued
to the back of the assembly. The other has a kapton stiffener under a fine
pitch surface mount connector. Both the stiffener and heat sinks are applied
to the flex circuit by the board house. We had issues with components draw
bridging and pillowing on both assemblies and isolated them to components
over the areas that had some type of adhesive on the secondary side. I was
able to take some samples and place them on a hot plate and to my surprise
they would blister and all the blisters would form in the areas of the heat
sinks and stiffeners. If I let the unit cool and retested it the same day
the blisters wouldn't reform. If the assembly was left out for a week it
would blister when retested. I then baked some sample boards out and placed
them in a N2 chamber for a week, retested them and no blisters were found.
The boards that tested out for no blisters was left out for a week and
retested, and the blisters would form again. We now bake all of our flex
circuits prior to reflow.
Regards,
Ted T
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Kelly
Sent: Tuesday, September 16, 2008 12:15 PM
To: [log in to unmask]
Subject: Re: [TN] Bias voltage test in damp heat
Hi Arnaud,
This is a flex circuit- basically kapton and polyimide glue- I
do not see where the glass would come from. Regards Steve Kelly
Sent from my BlackBerry device on the Rogers Wireless Network
-----Original Message-----
From: arnaud grivon <[log in to unmask]>
Date: Tue, 16 Sep 2008 18:15:20
To: <[log in to unmask]>
Subject: Re: [TN] Bias voltage test in damp heat
Steve,
From the description you gave, the particles you detected may
correspond to E-glass fibers (assuming Al instead of Cl in the elements
). You can refer to IPC-4412A to have a detailed description of a glass
fiber composition.
Dimensions you indicated fits also well with glass fibers and glass
fibers do absorb moisture.
Difficult to explain however how to include glass fibers into a
polyimide film...
Just a thought.
Best regards,
Arnaud Grivon
Steve Kelly a écrit :
> 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
>
> [log in to unmask]
>
>
> cc
>
>
>
> 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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