2
\$\begingroup\$

I was practicing desoldering components with a desoldering heat gun (Quick 861DW) of some boards I salvaged from a broken Sony Bravia HD-Ready monitor.

I applied 400C air flow and used some flux on the pads of the SMD capacitors.

On the first board I practiced with, they all came off nicely without discoloration or anything (in some the plastic did get a bit burnt,) but on a second board I tried with (using the same procedure,) they all started to inflate before the solder melting point and being able to detach them.

Why would that be? Would it be because of the substrate board material? The capacitors seem to be the same type.

Here are some photos of the first board and capacitors that came out well:

enter image description here

enter image description here

Here are photos of the second board and capacitors, they are turned as I had to twist them to release the air pressure before they exploded:

enter image description here

enter image description here

After that I tried with another different board and again they were all removed without problems.

\$\endgroup\$
3
  • 2
    \$\begingroup\$ Welcome, Practicing first is smart. I would suggest backing down to about 350, you will be less likely to damage the components. Also it depends if the solder is lead free or not, that has an effect on the solder melting point. The board type and thickness will also change things, layers have a big impact and if there is a lot of copper on the board that will cause it to take longer. \$\endgroup\$
    – Gil
    Jul 20 at 19:46
  • \$\begingroup\$ Thank you for your comment Gil. I’ll be using lower temperatures from now on, as you and rdtsc have suggested 400C it’s to high. I’ll also try preheating the components before applying the final temperature on next practice boards. \$\endgroup\$
    – Elias4l
    Jul 21 at 8:40
  • \$\begingroup\$ Aluminium electrolytes don't like heat. If these are used parts then I'd try to remove them with a solder iron first, so you don't head the body of the cap unnecessarily. Figure out which pad that's the ground pad, as that will be the difficult one to heat up. Also, it isn't a very good idea to salvage such used caps since they have a limited life cycle and has already seen some use before you start removing them. \$\endgroup\$
    – Lundin
    Aug 10 at 8:06
3
\$\begingroup\$

It is likely three things:

  1. 400°C is very hot. I've had 370°C de-laminate boards before. If you can get away with using a lower temperature, do it. Hot-air desoldering can take longer than other means; be patient.
  2. The exact formulation of solder used. There are many types, and some melt at a significantly higher temperature than others. Generally, the higher the temperature, the stronger the joint.
  3. Less "thermal relief" on the second board. When desoldering components mounted directly to a solid plane of copper (copper "pour"), the entire area must be heated before the solder will melt, because the copper conducts heat away from the solder joint faster than it is applied. For this reason, thermal reliefs [were] used to ease this issue. These are simply several small connections from the copper plane to the component pad(s), which electrically conduct well, but allow the pad to thermally conduct worse, allowing easier soldering/desoldering. In the early days of surface-mount components, everything had thermal reliefs. But as switch-mode power supplies became the norm (and so too the need for very low-ESR capacitors) and also high-performance in small volume (exacerbating thermal issues), it has become common to directly tie component pads to large areas of copper - either for electrical (low parasitics) or thermal (heat-sinking) reasons. In any case, lack of thermal reliefs make rework more challenging. (It doesn't matter for assembly, as the whole board is reflow-soldered at once.)

To mitigate these, you can touch each solder joint with an iron and "contaminate" them via a number of special low-temperature solders specifically designed for rework. These can be found which melt at ~200°C. This makes it easy to remove stubborn parts, even those using very high melting point solders. Down-side is, now you've contaminated those joints with a very soft metal, which will have to be removed and cleaned thoroughly if being resoldered.

You can also use a "hot-plate" or "heat-bed" to pre-heat the entire board to some lower temperature (say 200°C) prior to removing individual components. This makes it much easier, but you do have to take care not to melt any plastic bits.

\$\endgroup\$
1
  • 2
    \$\begingroup\$ Thank you very much for your thorough answer. I had no idea about the thermal relief evolution on boards, I will take it into account from now onwards. I used 400 degrees because I saw videos on YouTube in which they recommended to use the maximum temperature for the least time posible, but it wasn’t the right choice for this case. Thanks again \$\endgroup\$
    – Elias4l
    Jul 20 at 13:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.