>> Remember, heating is due to power not current.
>
> That's interesting - this is the first I've heard of this.

It's simple physics, any electrical energy lost inside a component must be 
converted to some other form of energy, usually the majority of which is 
heat.  The electrical energy lost inside a component does not depend solely 
on current, but on power.

Example 1: 10 amps going through a thick cable of resistance .001 ohms. 
Power dissipated = 1 W.  Heating effect: very small

Example 2: 10 amps going through a heating element of resistance 10 ohms. 
Power dissipated = 1000 W.  Heating effect: a warm room.

You can compare those examples to the tracks on your PCB and the IC.

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Invisible wrote:
> Picture the scene:
> 
> I get called to a PC that won't boot any more. I remove one of the PCI 
> cards, and suddenly the PC works perfectly. Hmm.
> 
> On closer inspection, one of the ICs on the PCI card looks... Well let's 
> put it this way. The writing appears to be scortched. The surface of the 
> IC is apparently damp with an oily liquid. There's a bulge in the 
> surface of the IC, together with a few small cracks.
> 
> Here's a question for you: Why would an IC suddenly do this?
> 
> [And more importantly, how in the name of God does a device which only 
> has access to a few milliwatts manage to get warm enough to physically 
> melt??]

IC chips are almost always connected to the power supply (usually 5V) 
and ground.  If there is a shorted component in the IC, then the power 
that briefly flows through the IC will be better measurable in watts, 
and not milliwatts.

Regards,
John

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Here's an additional item of data: This particular IC has been switched 
on 24/7 for approximately 8 years or so. [Except for the occasional shut 
down now and then.]


for a new PCI board, eh? o_O

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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stbenge wrote:
> I guess these are the reasons I unplug my computer from both wall and 
> phone line during a thunderstorm...
> 
> Sam

Thet is a good idea!
I had bought two surge protectors one for my phone one for my PC.
With out me knowing the telephone line lost it's ground and the phone 
surge protector acted as a ground. Luckily I only lost the modem when 
the power surge came.

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Invisible wrote:
> Here's a question for you: Why would an IC suddenly do this?

In addition to what everyone else said, there's the effect of "thermal 
runaway".  Every semiconductor has a tiny leakage current, caused by 
quantum effects, which is why devices use current even when not 
switching. There's more leakage when the device gets hotter, because 
there's more heat jiggling things around, basically. The more leakage, 
the more current, the more heat, the more leakage ....  >POP<

-- 
   Darren New / San Diego, CA, USA (PST)
     "That's pretty. Where's that?"
          "It's the Age of Channelwood."
     "We should go there on vacation some time."

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