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

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From: Won Chang (Won@nvidia.com)
Date: Wed Oct 04 2000 - 15:56:48 PDT


Another reference is MIL-M-38510, Military Specification, General Spec for
Microcircuits. Check section 3.5.5.3 where you can find an equation:

I=kd2/3 (d to the two thirds power)

where: I: maximum allowed current in amperes
                d: bonding wire diameter in inches
                k: a constant (dependening upon wire material and length)

I hope this helps.

Won

-----Original Message-----
From: jrbarnes@lexmark.com [mailto:jrbarnes@lexmark.com]
Sent: Wednesday, October 04, 2000 12:12 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Current flow limit for wire bonding

Brian,
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:
* http://home.earthlink.net/~jimlux/hv/fuses.htm
* http://www.2.ozland.net.au/users/egel/fuses.htm
* http://www2.murray.net.au/users/egel/fuses.htm

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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