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Part of it will be endcap and "neck" design, to put as
much heat as possible into as little area, if what you
want is minimum power-to-blow.
I've worked on ICs incorporating fuses (as a safety
measure for an ARINC data bus, in fact, which is a
pretty safety-forward bunch) and there's not much
room between "must open" prompt fuse current
and "must not open" long term reliability.
On a PCB you may have other influences such as
cooling airflow, thermal planes etc. that will modify
what you might calculate or measure for fuse blow
particulars on the bench.
A fuse built one-off on PCB or IC needs to have
certain attributes beyond the opening-under-current.
One is, to not compromise the close-in materials'
integrity. On a chip, I would send fused samples
(including over-the-top) to a F/A lab to verify that
the passivation layers were not breached, that the
fuse has cleanly separated (ideally, entrained to the
overglass so it cannot possibly regrow "dendrites"),
than no interconnect damage outside the fuse neck
has occurred and so on. I'm sure there are PCB
equivalents to some of this. I would not expect a
solder mask or con-coat to necessarily tolerate the
melting point of copper, such that a PCB inspector
would bless the board after a fuse event (and you
could safely bet that this would be one of the first
steps a customer with a failed board, might take).
If you have any legal or regulatory exposure I think
you'd be better off to place a blessed, surface-mount
fuse on the board as a designed component. This
will have better behaviors in fusing (like, not making
an observable burn-pit for customer failure analysis
people to point at you, over - or starting a larger
combustion event).
BOM cost delta ought to be small, board area a wash,
and no invention (and its reliability validation) required.
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