Re: [SI-LIST] : Current flow limit for wire bonding

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From: [email protected]
Date: Wed Oct 04 2000 - 12:11:51 PDT

In researching the ampacity (current-carrying capacity) of wires and printed
circuit board (PCB) traces last year, I ran across the Onderdonk equation for
the fusing (opening) current of wires as a function of time. I found three
sites on the Internet with the identical information on designing fuses using
the Preece Equation and the Onderdonk Equation:

Gold isn't listed in the table, but its melting point of 1064C is very close to
that of copper (1083C). The Onderdonk equation suggests that both metals thus
should behave about the same, letting us use the Preece Equation

I = 80.0 * (0.025^1.5) = 316mA

I would not feel comfortable running more than 200mA or so continuous through
each wire, myself.

A. J. Rainal's paper "Current-Carrying Capacity of Fine-Line Printed Conductors"
 (Bell System Technical Journal,
Vol. 60 no. 7, September 1981, p. 1375-1388) pointed out that for printed
circuit board (PCB) traces there is a
"runaway" current:
* Below which the trace's temperature rise will stabilize.
* But above which, if the current is held constant, the trace's temperature
will continue to rise until it melts open
    or sets the board on fire.

This is due to the increase in copper's electrical resistance with temperature,
versus conduction/convection/
radiation carrying the dissipated power away. Since the electrical resistance
of almost all metals increases with
temperature, I would expect to see a similar behavior for your gold wires.

                                              John Barnes Advisory Engineer
                                              Lexmark International

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