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

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From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Tue Oct 10 2000 - 12:41:12 PDT


I found the MIL spec based on a web search. What I quoted was directly from an
on-line copy of MIL-M-38510. I can't vouch for it's accuracy. The document I
saw looked like a scanned-in copy.

The constant applied is dependent on the length of the wire. The 30,000 value
was for a 40 mil length between attachment points, and the 20,500 value was for
length greater than 40 mils. I don't have any other information than that.

I found the same 10,244 value for copper in my old "Reference Data for Radio
Engineers" under 'Fusing currents of wires'. The last sentence says: "Owing to
the wide variety of factors that influence the rate of heat loss, these figures
must be considered as only approximations."

---Bob Lewandowski

Brian Seol wrote:

> Bob, Doug, John, and Won,
>
> Thank you for your reply. These are very helpful for my work.
> Bob, I have a question regarding the constant term you gave.
> You mentioned k = 30,000 or 20,500 for copper. This is quite different
> than that from Doug Brooks' papers and Standard Handbook for
> Electrical Engineers (http://home.earthlink.net/~jimlux/hv/fuses.htm).
> They suggested k = 10,244 (d in inches) or 80 (d in mm) for copper.
> Could you please confirm this?
>
> Brian Seol
> Tessera Inc.
>
> -----Original Message-----
> From: Bob Lewandowski [mailto:Bob.Lewandowski@Vixel.com]
> Sent: Wednesday, October 04, 2000 6:35 PM
> To: Won Chang
> Cc: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Current flow limit for wire bonding
>
> Minor problem, the equation from MIL-M-38510 is I = k*d^(3/2), {three
> halves
> power, not two thirds}
>
> k = 30000 for gold or copper with a bond-to-bond length <= 0.040 in. (0.1
> cm).
> k = 20500 for gold or copper with a bond-to-bond length >0.040 in. (0.1cm).
> (For Al wire k = 22000, and 15200, respectively)
> d = wire diameter in inches.
> I = DC or rms current.
>
> For a 1 mil gold wire < 0.04" long, I < 0.95A; > 0.04" long I < .65A.
>
> ---Bob Lewandowski
> Vixel Corp.
>
> Won Chang wrote:
>
> > 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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