Hi Peter:
> Not quite. I^2·t will tell you the let through current of
> the copper trace, but will not necessarily tell you if the
> construction will be compliant. The compliance criteria for
> this test include:
>
> * no damage to the trace (no lifting, probably no
> discoloration)
> * no damage to the PWB (no delamination, burning; I don't
> know if this includes burning off of solder mask)
> * before and after earthing impedance must comply with the
> 0.1 Ohm maximum impedance
> * no change in earthing impedance greater than 10% of the
> before and after earthing impedance results
This test implies a near 0-ohm fault to the
PE, where the PE circuit includes a PE trace
on the PWB.
If there is a zero-ohm fault, an over-current
device, somewhere, will operate. (Indeed, this
is the function of the PE circuit!) A zero-ohm
fault implies a large-area contact with a fair
amount of contact pressure for at least the
period of time to operate the overcurrent device.
(A point-contact fault would blow a hole in the
copper trace due to very high current density
at the point of contact.)
Consequently, the product must be removed from
service and repaired before being returned to
service.
If the 0-ohm fault is on the PWB, then the PWB
will need to be replaced. It is difficult to
imagine a fault of 0-ohm proportions that could
be repaired without replacing the PWB assembly.
Indeed, if the PWB PE circuit carries the high
transient current, it may very well be that the
supply conductors on the PWB may be blown off
the PWB. So, I question whether the compliance
criteria need be applied.
> There is also the much more variable solder in the earthing
> path. While manufacturing techniques have come a long way
> in terms of consistency, the amount of solder in a joint and
> the quality of the joint itself can play a significant role.
> It should be expected that a lower melting point solder will
> perform less well than a higher melting point solder.
> Appropriate process controls will have a positive effect.
An ideal solder joint involves an amalgam at
the joint with the conductors. The properties
of the amalgam are typically "greater" than
the property of either material alone. As in
copper plumbing joints, an idea joint has very
little solder between the two components being
joined.
My guess would be that the current path will be
that of least resistance, which will minimize
the current through the solder around the joint.
So, I would doubt that the solder (of a good
joint) would be much affected by the current
pulse.
Best regards,
Rich
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