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BC847 Destryoed

Hello

I have a circuit with a BC857B where the transister gets destroyed.

I am a litle out of ideas. When mounted on the board i have seen arround 150 transistors out of 1800units where destroyed.

The transistor in first place seems shorted between collector and emitter, the ressistance is between 5 to 250 Ohm. The funny part is both "diodes" in the transistor can be measured from base. And the one with 250Ohm i have tried to activete in the circuit and the transistor seems to react and make lower ressistance between collector emitter. (so in fact it looks like a normal working transistor with a build in ressitor of 5-250ohm between collector and emitter)

The product has been produced for several years with out problems, they started in earlier this year..

I have got several transistors decapped, so there seems not to be any burned inside the transistor and the boundings where ok. I have only seen the chip from the top, and collector emitter is in top of each other.

The product is assambled by proofesinel production house and been both reflow and wave soldered (only wave soldered on the bottom side)

Another funny things are there are 2 of the same transistor on the board, we are only seeing one of them with error so far.

I have measred the currents in the circuit it seems ok.

There has been tried to change transistor vendor, and there has been seen the same problem, but only 25 of 1800 with same error.

Does anyone have a auggestion to the error, or seen similar ??

Best Regards

Rapzak

Reply to
Rapzak
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Nearly 10% failure rate indicates a serious problem

Irrelevant.

Did you (or the contractor) change the supplier of the transistor ?

Irrelevant.

Presumably one of thm isn't exposed to a fault condition.

What do you means by 'seems' ?

It sounds to me like the design is marginal (poor) and the 'good' transistors just manage to survive. Without a schematic and application info everything's a guess though.

Graham

Reply to
Eeyore

If you post a URL for your schematic, then we might be able to help. A photo would be good too.

Chris

Reply to
Chris Jones

Notice that the resistance means nothing here, it is just a value your multimeter comes up with depending on current it uses. Use the 'diode' measurement of the meter, it will send a constant current through the diodes and show the resulting voltage. That should be 0.6, and not working in the opposite direction.

So maybe that really is so.

Hi,

I think that your transistor gets partially damaged. As if it starts conducting at some places in the transistor. There can be many causes, but here are two plausible ones:

1) A high voltage peak gives a local breakdown in the chip, and makes a bad conducting channel. This could be caused by for example this transistor controlling the coil of a relay without damping diode or snubber network. Did you change the a relay with biult-in diode to one without? Or changed it to a relay woth other inductance?

2) SOA, Safe Operating Area. The maximum power of a transistor is not only defined by current, voltage and power, but also by SOA. Especially at higher voltages, current must be lower than expected. Even when voltage, current and power are all within specifications, the transistor may fail. What happens is that locally at higher voltages, the p-n layer gets warmer, starts condcuting more there, gets warmer etc etc. This may also cause your problems. Maybe you are using different supply voltages? Or maybe your design was already outside specification, but now it is used in another environment where it is used differently? Notice that as this problems occures at higher voltages, it may happen when you do NOT load the circuits.

Pieter

Reply to
Pieter

If the transistor is being destroyed then:

1/ The voltage across CE is too much. 2/ The BE voltage is getting too high. 3/ Could be a static discharge problem and they are destryoed by handling but unlikely. 4/ The transistor has been put in the circuit the wrong way around.

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Reply to
Marra

And some more: Negative BE voltage (non-conducting direction), can already break down at 5V and actually destroy the transistor depending on the components around it. High temperatures, power must be limited at higher temperatures. Too high collector current. Too high emitter current. Too high base current.

So there can be many more causes, Safe Operating Area can even limit the maximum current of power transistor to half the current you would expect. And this also applies to low power transistors.

Notice thet exchanging C and E will not destroy the transistor but it will give you a transistor with low quality, low gain etc.

Pieter

Reply to
Pieter

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