"Soldering" with a heat gun - Page 1
Forum section: electronics / beginners

author: anvoice
author_rank: Frequent Contributor
timestamp: August 22, 2018, 11:12:36 pm
content: 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.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: August 22, 2018, 11:42:40 pm
content: Quote from: anvoice on August 22, 2018, 11:12:36 pmI've seen videos of people doing it. Yes, you do risk burning everything. Get a hot air soldering station. Temperature control will save headaches.

author: IanB
author_rank: Super Contributor
timestamp: August 22, 2018, 11:54:13 pm
content: This video Mike did may be helpful:https://youtu.be/d-f-SBC0GrU

author: KL27x
author_rank: Super Contributor
timestamp: August 23, 2018, 12:05:34 am
content: QuoteIt'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=377In 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.

author: anvoice
author_rank: Frequent Contributor
timestamp: August 23, 2018, 12:16:28 am
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: August 23, 2018, 12:21:10 am
content: Note that in Mike's video, he specially designed the board to be soldered that way. If you have a board that you did not design yourself then Mike's approach won't work.

author: anvoice
author_rank: Frequent Contributor
timestamp: August 23, 2018, 12:30:51 am
content: No, I did design this board. Moreover I just went back to it and noticed that there are very few signal traces running across the center pads of the ICs from the back. So I could remove some of that solder mask and solder from the back with an iron. Do you think that would be preferable to a heat gun?

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2018, 12:31:34 am
content: At least preheat from underneath with the likes of a frying pan partially filled with sand.An ordinary paint stripping gun has the disadvantage of no real flow control and the nozzles are generally way too big. You might be lucky enough to get it down but don't try until you've practiced a bit on a junk PCB.Airflow rate will be the enemy here and you'll need to back away some so to not blow the IC all around the place.

author: anvoice
author_rank: Frequent Contributor
timestamp: August 23, 2018, 12:36:17 am
content: Can do. The heat gun I plan to use actually has continuously variable temperature which I could ascertain with a thermocouple before taking it to the board. I can try to rig up a vent with some sheet metal that would divert most of the air away from the PCB too,

author: KL27x
author_rank: Super Contributor
timestamp: August 23, 2018, 12:36:22 am
content: QuoteIn case it helps, the 858D hot air station is a fairly popular and "known quantity" on the forum. It has enough power for most any 2 layer board, I think. And it is something like $30.00 on eBay. You can do a lot better, if you think you will need more power/features in the future. But you could do a lot worse.QuoteThat second hand should perhaps be your own for the more immediate feedback! If both parts are near the edge of the board or on a small board, it does look to be doable with a regular heat gun. If it were in the middle of a large board, you'd have a hard time flowing the part first, without burning your hand on the one side or the board on the other. Being at the edge should be a huge help in successfully using a huge heat gun without a reducer. You should want to put the board to the edge of the bench, I imagine, so most of the hot air would not be deflected over to your other hand!

author: IanB
author_rank: Super Contributor
timestamp: August 23, 2018, 12:36:51 am
content: Quote from: anvoice on August 23, 2018, 12:30:51 amIn Mike's video, he specifically designed a hole through the board under the central pad of the IC so that the iron was in direct contact with the IC and did not have the FR4 in the way. That way the heat path was iron -> solder -> IC. The conclusion to draw is that the board should be designed from the outset with assembly in mind. If that is not the case for you, maybe you can borrow a hot air station from someone, or take your board to someone with a hot air station?

author: anvoice
author_rank: Frequent Contributor
timestamp: August 23, 2018, 12:46:59 am
content: Quote from: KL27x on August 23, 2018, 12:36:22 amGood call! I'd never let someone get burned over my board. Well, these components are pretty close to the edge so I should be good that way.I'll look into the hot air station, if it's cheap enough maybe I could get it instead of the heat gun.Quote from: IanB on August 23, 2018, 12:36:51 amI can still make a hole there to be honest. Even if it's not large enough for the iron to pass through, it would get filled with solder and I would probably be able to solder those ICs on. I'll definitely give it some thought.No, I don't have anyone I know with a hot air station. I'm a chem major so all my acquaintances, few as they are, are chemists. All alone in this vast sea of electronics.

author: Brumby
author_rank: Unknown
timestamp: August 23, 2018, 01:00:39 am
content: A hot air gun is a bit like using a sledge hammer to crack a walnut. A variable temperature one just means you can have different weight in the head. Is is possible? - Yes. Is it advisable? - I would have to say "No", but I will qualify that with a big emphasis on this:Quote from: tautech on August 23, 2018, 12:31:34 amEven then, I would not be doing it, personally.Quote from: anvoice on August 23, 2018, 12:16:28 amDo you do any heat shrink? Absolutely magic with a hot air station and, if set properly, it is really hard to do damage to wiring. Also, with adjustable temperature and flow, you can use it for all manner of things where gentle, controlled heating is required. I've used mine several times for warming up labels so that they can be more easily peeled off.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: August 23, 2018, 01:05:55 am
content: FWIW I have soldered center gnd pads from the top with ease. I was using my hot air station. I was successful with a central ground plane.I tap them down gently when the solder is reflowing*. A very gentle nudge will confirm if the part is down. If it moves back after you nudge it, you got it.You will want a jewler's loupe or one of those USB microscopes for pin inspection.* not sure if "reflowing" is the right word.

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2018, 01:11:33 am
content: Quote from: Brumby on August 23, 2018, 01:00:39 amYep, those of us with rework stations quickly forget the learning curve and as a result we'd probably have a go with an ordinary hot air gun.Quite right Brumby, not a good idea if you haven't done hot air reflow before.

author: anvoice
author_rank: Frequent Contributor
timestamp: August 23, 2018, 01:37:51 am
content: Ok, you guys have pretty much convinced me. I think I'll either get a hot air station instead of a heat gun, or use Mike's approach that Ian mentioned and make a hole in the back of the board for direct contact with the ICs.Thanks for the great advice!

author: nanofrog
author_rank: Super Contributor
timestamp: August 23, 2018, 01:49:49 am
content: Quote from: tautech on August 23, 2018, 12:31:34 amI was thinking of an electric skillet, but this would do just as well.OP, definitely practice with some junk boards first to get the technique down. Preheating with an electric skillet or similar would also be advisable with the large pad under the chip (no hole).