This is a new one for me.
The Er of FR4 has some frequency dependence, even when properly cured. I
can see how undercured material could have modified dielectric properties.
The decreased cross link density would leave the polymeric chains more
mobile, thus more able to orient to the field at lower frequencies. I
suppose you could also have unconsumed curing agents or catalysts that could
alter electrical properties.
And I can see how advancing the cure by baking could help this. But if you
had water absorbed in the dielectric, it would also alter the electrical
properties and would also be fixed by baking.
I'd recommend different parameters for each problem though. For
undercuring, the bake should be done at the curing conditions recommended
for the material. You should be able to get this from either the board fab
or the laminate supplier. For water removal, I'd use a lower temperature,
longer bake.
If you really have undercuring, it can be verified by thermal analysis. If
you do two scans of the material and see a large increase in the Tg between
the first and second sweep, it is undercured. Again, your fab or laminate
supplier should be able to assist in this testing.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kaye Clayo
Sent: Wednesday, April 27, 2005 12:53 PM
To: [log in to unmask]
Subject: Re: [TN] How to Bake FR4 Boards
Judy
From what I understand, and that is not a lot, hook is when
Er changes with frequency at low frequencies, which changes
the capacitance, which results in distortion of output signals.
If anyone has any other information about this phenomenon,
please enlighten us.
Kaye
At 03:36 PM 4/27/2005, Judy Brown wrote:
>What is a "hook problem"?
>
> Judy
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Kaye Clayo
>Sent: Wednesday, April 27, 2005 12:10 PM
>To: [log in to unmask]
>Subject: [TN] How to Bake FR4 Boards
>
>Dear Technet:
>
>Can someone tell me what temperature and duration
>we should bake some unpopulated FR4 (HASL) boards?
>
>We are attempting an experiment to see if we can "bake" a
>potential problem out of certain boards, but we don't know
>what the baking temperature / time should be. The potential
>problem we are looking to bake out is under cured laminate
>which could be causing a hook problem.
>
>Please help.
>
>Kaye
>
>
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