how do pins on ICs typically die ?

Maybe it was killed by a fault in the VFD ?

Graham

INside each chip there is "black magic smoke". Each chip smoke enable the function of the chip. If you mis handle the chip, in any way, the smoke is release from the chip's body. After that the chip never works the same again.

IN a perfectly, good and functioning unit, should it suddenly stops working or perhaps stop over night (when you would not be present to see the smoke!) :

here are some reasons:

over voltage in transient state(s) over current during transient states ESD electrostatic discharge from handling and creating a "spark" over voltage results. over heating, over time

some times, power does not come up sequentially, and timing of power transients must be considered.

Power supply reversal, not tolerate well !

Mechanical degradation of chip lead to substrate connection.

Marc

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thanks for smoke joke :} and especially thanks for reasons

so, what sorts of tests can / should i be doing to find out if there is a problem in circuit board that may have led to IC pin/line failures ?

chip was in circuit and presumably not handled prior to failure. the only event that **may** have preceeded failure was is either spilled liquid on circuit board (cofee or coke) or an alkaline battery leak on main board (after sitting for a long period) and then it was powered on .

did i mention this is a hobby and so i do not have experience to know all the sundry conditions that may lead to such failures, and how to look for the hose conditions.... which is why i am asking ?

thanks again for any useful help youi can offer. rob

Probably none. How expensive is the replacement part?

25 MOhm (presuming M = Meg) is not usually considered a short. IIRC, VF display voltages aren't high enough to care about that high a resistance.

The other thing you might check for is a short _between_ the non- functioning outputs. Compare those readings to measurements between functioning outputs, and between non-functioning and functioning. If you observe any anomalies, remove the IC from the board and repeat. Or follow the PCB traces from the questionable pins and look for a short. If you don't find any apparent external cause for blowing up those outputs (and you've cleaned up the battery contamination from the PCB), try the new part.

TM

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Not so expensive (

I've tested and serviced several thousand of these, and never have I seen a VFD kill a driver. A short on the other hand...

Hi!

Sometimes. It depends upon the value and rarity of said chip! Expensive ones tend to go "bang" very quickly, let lots of smoke out and don't work at all afterwards.

More common and less expensive chips can usually take a lot of attempted smoke releases before giving up. I had a 387 math-co and motherboard with the pin one portion of the socket installed improperly. I couldn't see the correct silk-screened orientation under the socket. The co-processor took three incorrect insertions and power-ons before it "died". :-(

William

My favorite datasheet of all, the Signetics WOM, has a graph showing "number of pins remaining" vs "number of socket insertions".

It's not directly applicable here, of course, but I'd extrapolate:

If your VFD display is a module in a larger system, I would look at power sequencing for the logic vs the drivers and display voltage. If there's anything pluggable/unpluggable that's an obvious way to sequence things wrong (for example, having ground become floating before the signal lines is a bad thing!).

Tim.

thanks for help Tim, its possible a previous epairer unpugged something as you state. The circuit board had obvious solder work done, it wasn't pretty and that is coming from a mere occasional amateur solderer.

thanks again , robb