QFN Package Soldering

Because QFN packages are leadless, it is probably preferable to put these parts down using solder paste. (OK there are videos on youtube showing this being done with a soldering iron and a lot of gel flux, so I guess it can be done from the 'edge'. In this video the PCB appears to have enormously long pads, probably making the process easier.)

Solder Paste

Solder paste that melts at 138°C is easily available. The good thing about this is that a domestic clothes-iron (secured face-up) can be used to flow the solder. This low temperature subjects the components to less stress, which can be beneficial if you flow the board more than once during assembly. Make sure the paste is either new, or that you have thinned it with liquid flux and/or Iso Propyl Alcohol so that it is the same viscocity that it was when it was new (similar consistency to toothpaste).

Method

I discussed the technique for soldering the QFN with one of my Technician colleagues and his advice was as follows:

  • Do NOT use too much solder paste: Put a TINY amount onto each pad and try not to rely on the solder resist too much
  • (Update: I found that simply tinning each pad with an iron and low temperature solder paste was easier as the solder then stays in place. Clean between the pads prior to placing the IC)
  • Coat the pad side of the chip with a thin layer of flux (preferably gel flux)
  • Drop the chip as accurately as you can onto the board so as to not smudge the paste (not a problem if you tinned the pads)
  • Heat the board from underneath using a hot plate or clothes iron

    • When the board is heated, the chip should be seen to 'snap' down onto the PCB when the solder melts. After the board has cooled you can do a walking pin-pin short test with a DVM. Other than that, just trust the part is soldered correctly until such time that you have reason to doubt the attenuator during board test.

    If anyone has more experience with these parts or has a better method then please drop me an email.

    Solder Paste Flowing with a Clothes-Iron

    A domestic clothes iron can be used to heat the board and flow the solder paste. This should be SECURELY mounted face-up to avoid dropping the board, as this would likely result in all the components falling off, which would be BAD. Similarly, take great care when sliding the board off the iron when it is hot for the same reason.

    Pre-heat the iron to ~150°C (probably near the lowest setting) before placing the board onto the hot plate. Once it is on, observe for solder flow and if necessary, nudge one or two parts with a spike (or gently tap the board) so that the solder surface tension aligns them correctly on the pads (this works less well with larger parts like the tantalum capacitors). Once flowed, CAREFULLY slide the PCB off onto a small board and let it cool down.