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"Soldering" with a heat gun

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anvoice:
I need to solder on two QFN components on a board. Worse is, they each have a big tab in the center that it would be nice to solder on too, which is impossible by hand. However, a reflow oven is not in the budget for this prototype project.

Therefore I was wondering if I can use a heat gun to carefully heat the board from the back or front until the solder paste melts, at which point I'd immediately stop and solder on the rest of the components by hand. It's just these two that are a problem. So my question is:

Is this a safe substitute to a reflow oven for a one time job?
Do I risk damaging the FR-4 by heating from the back side? I've read about some people getting bubbling in their boards by overheating them with a heat gun.
Thank you.

JustMeHere:

--- Quote from: anvoice on August 22, 2018, 11:12:36 pm ---I need to solder on two QFN components on a board. Worse is, they each have a big tab in the center that it would be nice to solder on too, which is impossible by hand. However, a reflow oven is not in the budget for this prototype project.

Therefore I was wondering if I can use a heat gun to carefully heat the board from the back or front until the solder paste melts, at which point I'd immediately stop and solder on the rest of the components by hand. It's just these two that are a problem. So my question is:

Is this a safe substitute to a reflow oven for a one time job?
Do I risk damaging the FR-4 by heating from the back side? I've read about some people getting bubbling in their boards by overheating them with a heat gun.
Thank you.

--- End quote ---

I've seen videos of people doing it.  Yes, you do risk burning everything.  Get a hot air soldering station.  Temperature control will save headaches. 

IanB:
This video Mike did may be helpful:

https://youtu.be/d-f-SBC0GrU

KL27x:

--- Quote ---Do I risk damaging the FR-4 by heating from the back side?
--- End quote ---
It's certainly possible to heat it from the backside. It's going to require higher temps to the board to do this, practically speaking. Copper is a good thermal conductor, but that is only a small part of the board. The FR-4 is a good insulator, and the bottom will reach a higher temperature if you heat from the back. The typical way to heat from the bottom is with a hotplate to spread the heat evenly and to bring the board up to temp gradually and carefully. This would be rather tricky to do with a heat gun, IMO.

Hot air and paste is viable, yeah. You should want some extra flux however you do it, for post flow cleanup if necessary. Note, if you are using a paint stripping gun from the hardware store, the nozzle and potentially the air flow might be rather large. Heating a larger area of the board is not a deal breaker, but it's going to make it harder to get tweezers on the part without burning your hand.  If the part skitters around due to flux boiling off, it may snap to the wrong pads. It won't stay still on the paste unless you ramp up very gradually and keep air flow low.

Pretinning the pads with solderwire and adding flux is another way to get the part on there. But you will need to manipulate the chip to keep it on the pads, even more so than with the solder paste. The smooth pretinned  beads and boiling flux turn the part into a hovercraft. So you'll have to figure out if you can do this without burning your hand. What I do is just pin the chip to the pcb with tweezers using a proper hot air station.

https://youtu.be/mr1UVPsExiE?t=377

In either case, you might need to go around the edges of the chip with a soldering iron, after. And you may need to reapply flux, if necessary. In the video, he uses BGA flux and a lot of it. So he can install that chip twice and then some without adding any more flux.

anvoice:
I did think about a hot air station, but was hoping to get away with what I have. It's only these 2 components that will be on the board at the time so the risk of damaging other ICs at least is removed.

Thank you, that's a helpful video, but I don't want to scrape off soldermask from the opposite side as that will expose signal traces I believe (plus they might lift if I understand correctly). Also I was thinking about the following video where the guy heats it from the top:


Another helpful video! I was just hoping to avoid getting a hot air station for what might essentially turn out to be a single job. Perhaps I'll try using the gun from the top then, to avoid the insulating issue of the FR-4, and get a second pair of hands to hold the part in place with tweezers.

Don't get me wrong, I will get the right tools if necessary, but I don't know when I'll be using the hot air station again.

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