When Traces Melt: Figures 1 and 2 (Note 1) illustrate traces on two different boards. Each figure illustrates the trace at the moment it fuses (melts) due to a significant current overload. The fusing mechanisms are clearly different. So what was different?
Here is the first hint. Assume there is a current that is just large enough to cause the trace to melt (below that, the trace just gets very hot but does not melt.) We call this the fusing current. In one case the trace was subjected to a current fractionally larger than the fusing current. In the other case, the trace was subjected to a current substantially larger than the fusing current.
Here is the second hint. Traces subjected to a current fractionally larger than the fusing current can take a relatively long time to melt (minutes to hours). Traces subjected to very large current overloads melt much more quickly (less than a few seconds).