I recently had to solder a special MSOP IC with an exposed pad to a PCB. The device was a LT8610 which is a 2.5A synchronous step-down regulator. The exposed pad is needed to lower the thermal resistance and it is internally connected to ground. That’s why you should really solder the exposed pad to the PCB.
It is important to notice that these kind of IC packages do not lie flat on the PCB when they are not soldered yet. The IC stands on its pins and the exposed pad has no contact to the pad on the PCB.
I decided to solder the IC using a cheap hot air soldering station, a flux pen and regular tin-lead solder. These steps worked quite well in my situation:
- Apply (a lot of) solder to the thermal pad on the PCB. Don’t touch the other pads yet. The objective is to apply just enough solder so that the remaining pins of the IC lose contact to the pads on the PCB. That means that the IC only lies with its exposed pad on the thermal pad of the PCB.
- Use flux pen on the PCB and the IC.
- Place the IC on the PCB as exactly as possible. The flux helps by glueing the IC to the PCB a little bit.
- Set the hot air soldering station to ~240 °C.
- Apply hot air around the IC for at least a minute. Don’t point the nozzle to just one location. Use circular motions while heating.
- Set the hot air soldering station to ~350 °C.
- Apply hot air to the IC and around it until the solder on the thermal pad under the IC pulls the IC flat onto the PCB. Then continue for another couple of seconds.
- Lower temperature again and continue heating the PCB for a couple of seconds.
- You are basically done. Solder the remaining pins with a soldering iron as usual.
This should to the trick, at least it worked for me. By using this method you avoid overheating the IC or the PCB because you notice the moment when the solder melts again and pulls down the IC (capillary action). I believe that this method could also be used with QFN or DFN packages. In this case (instead of step 1) you should probably make sure that all pads on the PCB contain roughly the same amount of solder before applying hot air.
Happy soldering! 😉