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September 2007

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Subject:
From:
Steve Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory <[log in to unmask]>
Date:
Wed, 19 Sep 2007 12:14:06 -0500
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Hi Inge and Everyone!

I have your diagrams posted, they're here:

http://stevezeva.homestead.com/files/Inge_s_diagrams.doc

Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Wednesday, September 19, 2007 1:52 AM
To: [log in to unmask]
Subject: Re: [TN] Trace Peel on FR4

Robert,

this design area is very complex. Don't you have a guidance book?
Anyway, you need sit down and make a analysis of the situation. There
are numerous factors to count upon. E.g. copper resistivity is not
linear, it increases with temperature. Via resistivity and losses are
often forgotten, maybe they get hotter than the traces. And the cooling
of the traces is dependent of many things, like air velocity, heat
radiation, lateral heat transportation, number of layers, copper
thickness, board thickness etc. My reaction is, that something is very
wrong when the temp diff between the board (approx room) and the traces
is (105-25)= 80 Centigrades. Copper traces should not be used as 'heater
elements' for making a cup of coffee (we took small pieces of PWB,
etched a meander loop, dipped the board in a cup with water, ran a lot
of current through, and got hot water!). So, my opinion is, that current
losses of copper traces should be kept to as low as possible level.
Yes, conservative. 

I'll send two diagrams to Steve for his famous wall. These diagrams will
show you trace cross section vs. current vs. temperature vs copper oz
etc....may give you a helping hand. And somewhere I have a very good
paper from TI that describes every step and all maths you need. If I
find it, I'll send it to you.

Inge
 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Croslin, Robert
Sent: den 18 september 2007 19:59
To: [log in to unmask]
Subject: [TN] Trace Peel on FR4


Dear 'Netters,

We have a product used to measure on/off status on flat screen TV's that
was designed a few years ago.  Over time, with the newer, bigger TV's,
there is a much higher current draw than we'd envisioned way back when.
So, as the devices cycle back through our repair depot, we're making
mods to upgrade them to handle these new TV's.  A problem we've seen in
testing is that the traces that handle the AC pass-through are now
getting quite warm.  We've measured the traces at 105*C at room
temperature.  The base is plain old FR4.  We haven't been able to get
any to fail in testing, but are concerned about the differential under
load possibly causing the trace to peel away from the FR4.  Our concern
is thermal cycling over time; could it cause the trace to lift?  If the
trace is hot but the board cool in a steady state condition, is there
any risk there?

Does anyone know of any specifications that would address this
situation?

Bob Croslin
Nielsen Media Research



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