Overheating components with soldering iron - hierarchy?

I removed some small electolytic capacitors from a modern circuit board to use elswhere which meant that the wires of the electrolytics were
really very short. I then soldered some "extension" leads onto the electrolytics.
Of course, it's possible I may have overheated them.
This mead me think which components are more prone and which less prone to being overheated by the applying a soldering iron for too long or too close?
I guess at the top of the list is some sort of semiconductor. But what is the least prone? Resistors? Tantalum capacitors? Is there a hierachy of common components which can be listed?
And what happens when a type of component overheats? ISTR that resistors can lose up to one-third of their resistence. At least that was in the good old days and maybe it has changed now.
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Don W wrote:

To try to cut down of overheating is one of the reasons I usually try to use a rather hot iron to get in and out as fast as possible. I don't know if this is a valid reasoning or not.
And of course try to put some sort of clamp on the device itself to absorb some of the heat.
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I go along with that. Trying to quantify the various damage thresholds is FAR harder to do than to develop ways to avoid them.
A strong temperature controlled iron is good, and tinning wires beforehand is vital if they're not extremely clean. I bring the solder in with the iron and remove both together to let the residual heat and flux make a clean melt and solidification. For heat shunts I often just leave excess wire to snip off afterwards.
The components most easily damaged I've found are those with thermoplastics in them, like some small capacitors. Peltiers are dodgy too, they use very low temperature solder, so if you break a wire on them, it's screwed unless you can get low temperature solder. Laser diodes are very sensitive too, but not just from heat.
Hand soldering surface mount stuff is dodgy too, I've found that practising various tricks is needed to get those right.
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Germanium semiconductors -- you pretty much had to use a heat shunt. I've overheated LEDs once or twice, resulting in dim operation. I'm not aware that I've ever damaged any silicon semiconductors or other components -- I tend to use a hot iron as quickly as possible.
--
Andrew Gabriel

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Don W wrote:

The key is getting everything right before applying the soldering iron - as with most thing, preparation is everything.
The most common thing damaged by overheating, I would say, is printed circuit boards themselves, with tracks coming off the board.
The most difficult to damage are, perhaps, air-spaced high voltage (eg transmitter) capacitors - which you could probably take a blowlamp to.
I can't, off-hand, remember *ever* damaging anything (other than the afore-mentioned printed circuit tracks) by over-heating.
My standard way of bulk-removing components from scrap printed circuit boards is to hold it component side down and play an electric hot-air gun over the top - tapping the board edge against the bench so the components fall out. The components even seem to take that abuse and still work with no apparent ill-effects...
--
Sue





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a lot of PCB are soldered in an oven its not the heat that kills its the thermal shock of a cold component with a very hot leg
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All of the above posts give valid and good experiences and advice. There is much nonsense and urban myth spoken on this subject in some circles. When thinking about it, just bear in mind that when the boards are produced, they are pre-baked, wave soldered, control-cooled, and then probably baked again, allbeit at a lower temperature, during burn-in soak testing. All of these actions subject the board and its components to far worse stress than you're ever going to give them, with standard hand soldering gear.
As one of the other posters said, static, and failure to take proper handling precautions against it, is the biggest killer of 'delicate' electronic components - LSIs, laser diodes, some MOSFETs, memory chips etc.
Arfa
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Don W wrote:

My opinion:
Damage caused not necessarily by overheating but by lead stress during desoldering is what you'll see more commonly.
Decoupling the two effects is not always possible. In particular, with typical hobbyist desoldering setups, you can either overheat the component OR overstress the component leads.
As to components most sensitive to fairly normal soldering temperatures, polystyrene caps that have their dielectric melt at 85C have to be at the top. Of course these parts have really skinny wires for good reason - to not conduct too much soldering heat into the capacitor body!
Typically carbon composition resistors go up in value (sometimes by a factor of several hundred percent) in response to overheating.
With old electrolytics it's entirely possible they were bad/dried out before you desoldered them!
Tim.
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