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I use a REGULAR (for home-use, not for electronics) hot air gun to desolder non-SMD components from PCBs. The components are really hot (naturally) after desoldering and it takes a while until they cool off. Is it better or worse if I cool then off in water?

My issue is related to the fast cooling interval that will appear when I drop the hot component into the water.

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    \$\begingroup\$ Electronics and water usually don't go well together. \$\endgroup\$ – Eugene Sh. May 18 '18 at 18:20
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    \$\begingroup\$ @EugeneSh. - Not really. Water does not enter into components that are sealed (transistors, resistors, ceramic capacitors, ICs, LEDs, etc). My issue was released to the 'too fast' cooling interval that will appear when I drop the component into the water. \$\endgroup\$ – WeGoToMars May 18 '18 at 18:22
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    \$\begingroup\$ @EugeneSh. I use DI water to wash off water soluble flux all the time, it's how I make my proto PCB's \$\endgroup\$ – Voltage Spike May 18 '18 at 18:50
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    \$\begingroup\$ @EugeneSh. Not at all \$\endgroup\$ – PDuarte May 19 '18 at 14:00
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Worse, if you look at the component packaging there is a thermal profile that is to be followed to ensure that the component does not experience thermal shock from expansion and contraction. Thermal shock can disable components or cause them to be intermittent. A thermal profile looks like this:

enter image description here Source: Wikipedia

Water will create thermal shock because of it's low boiling point and high specific heat (capacity to absorb heat), it's proabably one of the fastest ways to cool down a PCB or part. Another way to do it would be to turn a can of dust off upside-down and spray your parts down. But you don't want to do that, the parts will cool down too fast and you could break them.

In fact it's probably a good idea to slowly back away the hot air gun away from the part to let the temperature ramp down. Or turn the temperature down on the heat gun and let the part cool off a little before removing the heat.

For large parts such as BGA's a thermal profile isn't just a good idea, the part will not function correctly if the thermal profile is not followed. Because the pads on a BGA are so small, and the solder connection so small that thermally shocking the solder can introduce discontinuities in the solder connection itself. The nice hot air guns for BGA's can also follow a profile.

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  • \$\begingroup\$ Thanks. Well documented answer. Wow! Cooling interval is super slow (600 seconds)!!!!! \$\endgroup\$ – WeGoToMars May 18 '18 at 18:28
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    \$\begingroup\$ FYI, this is typically how PCB's are made. It varies per part. In a professional SMT assembly environment, ramp rates such as this are the norm, but vary according to recommended parts and/or solder thermal profiles. \$\endgroup\$ – Voltage Spike May 18 '18 at 18:37
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    \$\begingroup\$ Worth pointing out that that is actually a soldering profile, not primarily due to the heat stress on the component. Preheat/soak gets the whole board to solderable temperature and allows the flux to clean the surfaces. A component can have a much faster ramp rate than that profile. Point still stands, though, that plunging into water is a bad move! \$\endgroup\$ – awjlogan May 18 '18 at 19:40
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    \$\begingroup\$ I'm not saying this is a profile you should use, but an example of one. Check your parts and solder before 'flowing' \$\endgroup\$ – Voltage Spike May 18 '18 at 19:41
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It's really hot in a reflow oven and the purpose of the Mfg thermal profile is to avoid thermal shock, ramp slowly then peak quickly above solder liquidus temp for a not to exceed x seconds and then cool slowly with a controlled ramp.

So let it be.

Worse yet components that have absorbed moisture due to the class code of the seal ( e.g. clear epoxy LEDs fit in this category) extended periods of unsealed exposure followed by rapid heating thru 100'C can cause a popcorn failure inside that shears the whisker gold wire bond, that may not be visible.

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  • \$\begingroup\$ Thanks. I was afraid about that. Ok. I will let them on the ground too cool off. A metal plate would be better? \$\endgroup\$ – WeGoToMars May 18 '18 at 18:25
  • \$\begingroup\$ take a look at any SMD profile to get comfortable with what is tolerated. Usually they allow 1 or 2 reflows max for re-use. \$\endgroup\$ – Sunnyskyguy EE75 May 18 '18 at 18:29
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It's probably a bad idea to cool semiconductors (as answered above), however parts such as connectors (i.e. metal and plastic) seem to survive much better if cooled off immediately, in my experience.

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