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Old 20th January 2007, 06:28 PM   #1
reins is offline reins  Germany
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Default Aging solder joins of power transistors

Hi,

I observed degrading solder joins of the power transistors of my Accuphase amp. The Amp is thirty years old.

When I re-soldered them, they even crackled.

Has anybody experienced this phenomenon?

I think it's due to mechanical stress between the heat sink and the PCB. It could be caused by warming and cooling of the transistor.

What do you thing?
Stephan
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Old 20th January 2007, 06:36 PM   #2
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funny you mention that just now. I noticed the same thing on an old amp I'm redoing. Not really sure what's up with that. It stinks too,..might being the wiring insulation though.
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Old 20th January 2007, 07:41 PM   #3
poynton is offline poynton  United Kingdom
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Some of the old solder I used in the late 60's did have a funny smell and I too have noticed it when repairing old stuff.
I put it down to the type of flux used rather that old wiring.

I think the problem, which i too have seen in some old Armstrong amps, is due to thermal stress.

A similar phenomenon is found in old computers or any old equipment with socketed chips. In old IBM PCs, the memory chips would get so loose they would fall out !!!!

Andy
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Old 20th January 2007, 08:09 PM   #4
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there are hundreds of solder flux "recipes" used around the world.... some are pretty bad, smelling like cat urine in some cases...... especially bad are the water soluble fluxes..... kester fluxes aren't bad, most of them smell like pine. i suppose also, like vegetable oils do, that some fluxes could become rancid after a long time. organic chemistry isn't my strong suit, but a lot of organic oils and resins slowly oxidize over time, usually making them smell very bad.

yes, the thermal cycling of transistors causes stress failures in solder connections. you will notice most of the time, it's the collector leads that have stress rings in their solder joints, since it's the collector lead that's got a direct thermal connection to the transistor die, and expands/contracts the most.
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Old 20th January 2007, 08:12 PM   #5
Nordic is offline Nordic  South Africa
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The worst melling flux is that crap in old valve gear...
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Old 21st January 2007, 10:27 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
When I re-soldered them, they even crackled
gas escaping from within the solder?

Remelting the solder with a tiny smear of paste or liquid flux over the joint should renew the connection and take out ALL the fatigue stress/strain.
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Old 21st January 2007, 11:31 AM   #7
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Default Re: Aging solder joins of power transistors

Quote:
Originally posted by reins
Hi,

I observed degrading solder joins of the power transistors of my Accuphase amp. The Amp is thirty years old.

When I re-soldered them, they even crackled.

Has anybody experienced this phenomenon?

I think it's due to mechanical stress between the heat sink and the PCB. It could be caused by warming and cooling of the transistor.

What do you thing?
Stephan
A part from thermal stressing, the main cause for this occurrence is poor quality of solder which require higher melting temperature (and consequent higher stress of PCB when the equipment was build) and it's more stiff than good solder, with
subsequents cracks when equipment go in thermal cycle for years. Usually this defect come up after some or many, when warranty is abundantly gone for good...

Hi
Piercarlo
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Old 21st January 2007, 02:22 PM   #8
reins is offline reins  Germany
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Quote:
Originally posted by AndrewT
Hi, gas escaping from within the solder?
No, gas hasn't escaped. The crackling was mechanical.

Stephan
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Old 21st January 2007, 03:45 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by reins


No, gas hasn't escaped. The crackling was mechanical.

Stephan
that implies strain, followed by release as the area resisting became less as the melt proceeded.
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Old 21st January 2007, 08:24 PM   #10
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I may be wrong but from my experience I tink they have trouble to set wave or automated soldering process for all types of component at same time if they set process for light pins or wire it will not work on heavy component ( many times I saw connectors pins or relay with poor solders and cracks ).
I think they have not be heated to correct temperature in automated process, this is why they dont last.
If they set process for heavy component it may be too hot for light components, this is why you may have trouble with heavy components when they are soldered on automated machines.
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