Jack  -

I have used the following for calculating max temp rise allowable:

(Tg-Max Ambient)/safety factor

I.E. Tg = 180 deg C
        Max Ambient = 80 deg C
        Safety Factor = 2.5
       Allowable Temp Rise = 40 deg C

It isn't real scientific and I often fudge the safety factor to get closer
to some nominal value but it gives you some place to start.

Jeffrey A. McGlaughlin
PCB Designer
Battelle Memorial Institute
Columbus Ohio
[log in to unmask]


        -----Original Message-----
        From:   Jack Olson [SMTP:[log in to unmask]]
        Sent:   Friday, 21 May, 1999 12:00
        To:     [log in to unmask]
        Subject:        [TN] Temperature Curves for PCB Traces

        Sorry for posting this here (after already trying the designer
council) but
        if I don't get a logical scientifical semi-provable response I will
be
        forced to use the dreaded "10 degree curve" for the rest of my
designs....
        ouch!


        I can't figure out how to select the appropriate temperature curve
when
        calculating PCB trace widths versus current requirements.
        If you use the chart in IPC-D-275 or IPC-2221, you have to select a
curve to
        get your minimum trace width.
        If you use a calculator, it will solve for the temperature rise, but
you
        still must know if it is acceptable.
        So what is acceptable? Anything that won't make the material hit Tg?
        I'm trying to avoid the dreaded "Just use the worst case"...
        Am I missing something?

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