I am trying to solder a PIC18F2520 28-pin QFN package (6mm x 6mm) onto a PCB. I read that it can be done with solder on each pad and a lot of flux, but do I have to get rid of the flux afterwards, i.e. will it short out the pads?
If anyone has done something similar, is it very difficult? I made my pads a little longer than the contacts on the chip so that they stick out a little outside the chip (just over a millimeter).
It’s best to remove the flux as it can cause problems, especially with high impedance inputs like those for an ADC, or a reset pin.
Leon
will dumping isopropyl alcohol on the pcb work fine? how long should i let it dry for?
Isopropyl alcohol works well. I put some on the solder side of the PCB and use an old toothbrush on it and then rinse again.
Mike
No, flux is not conductive but it is a good idea to remove it, even the flux that is non-corrosive. It remains sticky and will collect dust. The dust can be conductive plus it just looks nasty.
Phil
upand_at_them:
Isopropyl alcohol works well. I put some on the solder side of the PCB and use an old toothbrush on it and then rinse again.Mike
Cool. I’ve never actually done this before so let me get something clear…
We can just put isopropyl alcohol on top of the chips/etc. that are already mounted on the PCB and use a toothbrush to wash it out. Once it’s dry everything’s fine?
Also, sparkfun’s skillet method works wonderfully, I went out to Target and bought a $20 hot plate and I got reflow and a good bond with just solder on the pads and solder flux.
Yep.
One caviat, some components (such as open frame switches) can be sensitive to washes. Be sure to use distilled water (de-ionized water if you can find it) to do the final wash and let the board fully dry before powering up.
good point. check the datasheet - it might say “washable” or give criteria for washing. I’ve seen it with relays and switches mostly.
Well, when it comes to Isopropyl, or any other alcohol, I’d be careful around big mechanical bits - I’m not sure about the flash point of it, nor how easy it is to ignite, but getting it into a relay and still having vapor there when the relay hits (think large current load and a little spark)… well.
Also, it’d probably dissolve any lubricants used. Other than that, it shouldn’t be any problem at all.
In any case, just to reaffirm it: Flux isn’t conductive in any low-voltage application. If you deal with very high voltages (which I doubt, seeing as it’s a PIC and all), it can potentially cause problems.