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Old 9th Apr 2023, 3:27 pm   #21
agardiner's Avatar
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
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Default Re: Silicon transistor failure

Hi Terry,

So I cannot comment on breakdown voltage as I haven't run any tests. At this stage though, I would not have thought that there is a high risk of complete breakdown in AF stages; yet.

The problem I am regularly coming across, is transistors that have gone low gain, or in some cases, noisy. Low gain transistors cause issues such as RF oscillators to fail, and I have also found AF low gain transistors in audio amps, causing channel mis-match. Checking with a multimeter doesn't show any issue, as the junctions still test just fine. You either need an in circuit test with say a scope, or test the transistor with a transistor tester.

Certainly in the case of radio where the oscillator is not running, my first call now is to replace the transistor and see if it solves the problem.

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Old Yesterday, 10:28 am   #22
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
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Default Re: Silicon transistor failure

Just thinking... if epoxy degradation is affecting transistors in TO92, how much more will op-amps in plastic packages be affected where there's considerable gain after the first stage?
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Old Yesterday, 12:35 pm   #23
Join Date: Aug 2006
Location: Stoke-on-Trent, Staffordshire, UK.
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Default Re: Silicon transistor failure

Oh no I have thousands of old transistors I thought they would last for ever !
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Old Yesterday, 3:15 pm   #24
Join Date: Dec 2007
Location: Haarlem, Netherlands
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Default Re: Silicon transistor failure

They probaby will, mostly.

Even SOT-25 packages (the infamous lockfits) have nowhere near the 100% failure rate (yet) that the leadfree TO-1 and TO-7 packages have. SOT-54 / TO-92 packages could last a 100 years or more, however as others wrote: dependent on the construction (mechanical stress for example).

Maybe the environment is also a factor, but likely not as large as in, for example paper, capacitors.
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Old Yesterday, 10:59 pm   #25
Join Date: Nov 2006
Location: Near Lincoln, UK.
Posts: 434
Default Re: Silicon transistor failure

The big problem is hygroscopy - absorption of water vapour. All plastic encapsulated devices suffered from it. The military would not use them because of the known problem, hence their use of 'proper' ceramic devices. It was exacerbated by cheap materials and cost cutting with some manufacturers and packages worse than others. Up to a point, it didn't matter - the domestic design life of, say, 5 years, would still be met comfortably. Now, 50 years down the road, coupled with bad storage, the problem is a big one.

Plastic encapsulated devices improved over the years and the military will use them now - up to a point. There have been many, many studies over the years and by the time Japanese domestic kit started to appear, the problem was much reduced. An example of the large volume of academic studies on this is here:

You could write a book about it - there's certainly enough material!

Best regards,

Paul M
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