I'm trying to understand why static discharge (or just plain high voltage) damages semiconductors, in particular integrated circuits. What happens when a chip gets hit by a voltage spike that it can't handle?
Summary for the average lazy forum user:
yungman, thanks! That's pretty incredible. Have you ever looked at chips with built-in ESD protection, like DragonPetter was talking about? (capacitors and diodes to prevent overvoltage) Have you ever seen a failure despite ESD protection? How did that happen?
DragonPetter: That makes sense. What about when all exposed I/O was protected, and the failure occurred despite ESD coming in on a protected line?
DragonPetter: Fair enough, but it sounds like you still had some failures. What caused the ESD protection to fail?
(Anyone else have experiences with ESD protection failures? yungman?)
Don't count on the input protection inside the IC. We can't even count on the diodes outside the IC!!! I always use cheap long body carbon comp resistor in series with the input before the protection. As I said before, the instantaneous current is easily hundreds of A, even a few hundred ohms resistor will limit the current. But you cannot use any film or short body resistor, they jump over the body, they burn open.......It's the current.
DragonPetter: I'm trying to figure out where some failures are coming from despite the chips all having fairly good internal ESD protection on every line that could even remotely be getting hit. The problem is the device has to be too small to do protection off the IC.
Yungman: That's more or less what I was afraid of. Have you found that embedding the chip in a bubble of epoxy (ASIC style) keeps voltage from jumping around internally and escaping the protection? Does the protection inside the IC work better against certain kinds of transients, and fail more quickly against others? (fast risetime low current vs slower high current, for example)
That is a good point. I have seen naive designs that I had to help review and the engineer told us the IC had internal ESD protection. Some senior engineers informed him how this is a naive approach to compatibility requirements.
I usually consider that the internal ESD protection is there more for the manufacturing/shipping of the IC before it is even soldered to a board than it is there for its operating lifetime. Its best not to take ESD for granted, there are figures that say annual ESD damage results in billions of dollars of lost money. What's worse is that ESD damage can occur and the device sometimes will still operate for a certain amount of time before it eventually fails, so that a failure is hard to predict or know its source. This is really serious when electronics are used in human safety applications.