Fusing Current--When Traces Me

Many people have contacted me regarding my column
“Trace Currents and Temperatures: How Hard Can We
Drive ‘Em?” in the May, 1998, issue. In that column I talked
about the possibilities and the problems associated with
developing a set of equations for the current/temperature
curves we all have occasion to reference.
But many of you posed a different question. It typically
went something like this: “I need the trace to carry 5 Amps
for only about .5 seconds before…“ something catastrophic
happens. “Then I don’t care if the trace melts or not. What
size trace do I need?”
My first reaction was that this is a fuse question, something
I’ve never seen discussed. When enough of you asked
me about it I began digging for information. What I found
was there isn’t much to be found! But thanks to some
direction from a few experts in the field (Note 1) I discovered
that there is some very interesting theory to draw from.
CAUTION: The information that follows is based on
theory and, to my knowledge, has never been tested on
printed circuit boards. Designs based on this theoretical
discussion should be significantly derated and/or tested before
being committed to production.