> > Is it really just the PCB itself initiating the burn? In my experience =
it is
> always a component failure that initiates a burn - but having said that I=
 have had a
> backplane PCB that had an intermittent short in it, on a minicomputer. Bu=
t that
> short caused something else to give up, not the backplane.
>=20
>=20
> I've not seen any cases where the PCB itself failed, it was always initia=
ted by a
> component failure.
> The thing is that surprisingly low energy levels (<0.5W average) can caus=
e the
> component package to degrade, then to arc, and once that happens, the arc=
 easily
> transfers to the PCB.

Yeah, well, the minicomputer family I referred to earlier, we once had a 47=
nF or 100nF capacitor on the 5V rail go low resistance, when we were doing =
a post delivery burn in on a new machine. By the time someone noticed that =
there was smoke coming out the machine, and flipped the power switch, there=
 was a hole about an inch and a half diameter in the PCB. It didn't help th=
at there was a lightly loaded 5V 300A power supply that maintained the 5V w=
hile the cap was doing this - 'how much current would you like, I've got pl=
enty ...'

I have also had 100nF caps short circuit straight out of the box, as a bran=
d new component.

Another time we had a printer delivered from the OEM, and it wouldn't work.=
 One of the techs pulled the PCB assembly out of the back of it, and was le=
aning over it while powered up, only to have a teardrop tantalum whizz past=
 his ear. He was lucky to not lose an eye, and we never determined if the c=
omponent had been fitted reverse polarity, or just died. I do believe that =
one was working at close to its voltage rating.


But as you point out, once the component fails, it doesn't take much wattag=
e to get it hot, after all they are tiny components, so the theta(case-air)=
 is going to be large, which makes dissipation difficult.


--=20
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