I've been looking at this and I really don't
see a problem with the 5 mil spec.

Say we use 1.2 Amps @ 60C all over the place.
I see figure 3-4a reading off at 5 sq. mils.
Dropping down to figure 3-4b for 5 sq. mils,
1 oz reads off a 5 mil width.

I do have a problem with the 3 mil spec even
for 2 seconds. The slope of the 1 oz. line in
3-4b is pretty steep. Extrapolating as best my
eyes can allow puts me back up on 3-4a at less
than 0.75 amps or almost half the 1.2A condition
for 2 seconds. It looks as though you'll get
a possible 120C rise above ambient if you put
double the current through the 3 mil trace.
That means if the ambient is only 20C, your trace
will measure 140C or 284F. That's hot for me.

It also means that the trace resistance will change due
to say a 120C rise above ambient (20C) by a factor of

  +0.51*(resistance of trace at 20C)

or in simpler terms, the resistance will increase
about 50% above room temperature resistance.

This may have minimal effect for 5 mils,
don't know what this would do for the 3 mils.

Just my 2.1 cents worth.

Regards, Doug

----------
From: Orna and Yehuda <[log in to unmask]>
To: [log in to unmask]
Subject: [TN] Conductor width
Date: Thursday, March 12, 1998 6:39 PM

Hi everyone,
We have received a large shipment of boards with
considerable reductions in conductor widths. Conductors
that were supposed to be 0.008" wide became 0.005" or less.
Copper thicness - 1 oz. (35 micrometer).

The duty cycle of the board is as follows:
1.2 Amps for 2 seconds (at startup) and then
50mAmps (at steady state). The maximum temperature
allowed is 60 deg. C abov ambient. As you can see,
the values for conductor width are at the lowest end
of the chart in IPC-D-275.

I am not very considered about the ability to work at
50 mAmps but about the 1.2 Amp. Do you think that an
0.005" conductor will perform well under these conditions?
What if there are local areas with widths of about 0.003" ?

Best regards,
Yehuda Weisz

################################################################
TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c
################################################################
To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body:
To subscribe:   SUBSCRIBE TechNet <your full name>
To unsubscribe:   SIGNOFF TechNet 
################################################################
Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
################################################################