Diode Failure Mechanism & Mode

Depends whether you are applying a reverse voltage or not. If the diode is back-biased, the leakage currents doubles every 10C as you raise the temperature, and eventually the current reaches a level where the heat dissipation within the junction is high enough for the junction temperature to run away.

The diode then fails as a short circuit. With a suffiiciently potent power supply, the diode can be vapourised - I did this once and all that was left of the diode was two rounded ends on the bits of wire at either end of where the diode used to be.

With no reverse voltage, you can probably get the diode hot enough for the dopants that create the N- and P-type areas in the diode to diffuse across the junction, wrecking the semi-conducting behaviour that we rely on. Less exciting and much slower.

According to empirically compiled data over many years (decades?), Mil Handbook 338 lists 3 modes of failure for diodes (such as the 1N4148) and the likelyhood of each:

Short Circuit -- 75% Intermittent -- 18% Open circuit -- 6% I suppose "other" would be the remaining 1%, or the numbers were truncated after the decimal place.

These are failures under normal operating circumstances. At higher temperatures, I don't know how the probabilites would change, but I don't believe the percentages would change that much.

As for mechanisms, you would have to get a materials expert to answer that. I have personally seen overheated diodes "go" both ways, most recently being a zener that shorted out.

Tom

Approximately 1 milliampere of forward current. 5 volt supply with 4.7k resistor in series, with the diode far away from the current source.

Jim

All that can be said about this IS: Whoever believes that components fail in "certain" fashion does live in science fiction. The best we have is some statistical numbers because you simple cannot predict what happens in field and Mr. Murphy can always proove you wrong.

Have fun

Stanislaw Slack user from Ulladulla.

Hello Tom,

More than 1% of the bad thru-hole era diodes I found had cracks in the glass. Some of them were missing chunks of glass.

Same as the TO-220 lead bending failures. Somewhere around here in one of my tool boxes I have a red plastic strip that you drop resistors (or diodes) into a slot with the lead spacing you want and bend without stress.

...Jim Thompson

Mine was blue. No idea where it is now.

Graham

Same here. I was going to post a picture, but I couldn't find it :-(

...Jim Thompson

OOOOOOOOOkay. Let me try again. The diode is being operated forward bias at about a milliampere. It has been handled with kid gloves from the day it left the factory until it was carefully installed with no stress on the leads into the circuit. I put the sucker into an oven and start slowly bringing the temperature up.

At what temperature (or AROUND what temperature) would you expect the diode to reach an elastic limit of some sort, either physical or electrical, and not come back to approximately the same forward voltage at room temperature than it had before the thermal stress test? What do you SUSPECT might be the mode of the failure.

Jim

Define "fail". Leakage greater than a specified current at specified voltage? Avalanche breakdown voltage change more than x percent WET room themp value when measuerd at a specified current? Dump the diode into an oven pre-heated to 200C and find that it is still useable 10 seconds later when it is more than thermally stabilized. Note i did not define "useable".

Under those conditions, 200C is just as good as 20C or -75C.

..and so, if you want to decrease collision with Mr Murphy, then revise the design for Justin Case...

And that was due to added stress in the glass-to-metal seal; the extrastress caused by: 1) socketed type testing (proven in MIL reliability tests at FSC in the 80's), 2) assembly (no stress relief when leads were bent for board insertion, and 3) no stress relief added in the shaping of the leads (see NASA and other documents showing a "U" type "hook" on one end).

I also have one of those, and they are very handy. BUT. They do *not* eliminate stress, in fact, one could show that they do little to reduce stress at the lead-body interface, which is critical for glass encased devices.

Excellent start, but better if a gap is left between the part and the pliers. Do not remember the recommended minimum gap.

The picture, the post, or the horse tied to the post?

"The horse may talk" ;-)

...Jim Thompson

In addition to the other info already presented, I suspect it would vary (maybe greatly) with the *brand* of diode used as well.

Dave :)