Hot air soldering, what am I doing wrong? - Page 1
Forum section: electronics / beginners

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 16, 2019, 12:20:05 pm
content: Hi, so I'm working on a project that has an LQFP48 package. It needed replacement so I de-soldered it and though I'd use the hot air to get it back as well, because it's a pain in the ass to use the iron with all the other stuff in way.So now to the issue. I've been watching a bunch of videos of people soldering QFN QFP etc with hot air and the chip just gets suck in place no problem. But that never happens for me. I add solder to the pads and a bunch of flux, but the flux burns up before the solder even melts so I think that's why it doesn't help me and it doesn't help that I'm shaking like a leaf.But how do other people do it? What am I supposed to do to prevent all the flux from burning to early?I'm truly lost

author: TomS_
author_rank: Frequent Contributor
timestamp: November 16, 2019, 03:21:39 pm
content: Is the part not sticking or blowing out of the way? Maybe too much airflow, or holding the gun to the side instead of top down? Dont get in too close too quick, the component and the surrounding board need to soak up some heat first, so take your time.Ive seen people prod the part with tweezers etc once all of the solder has melted, but I couldnt honestly say I would do that if it has self centered.What sort of flux are you using, is it a "paste" or liquid? How much are you using?You could take a look at Louis Rossmann on youtube. He would seem to have even BGA reflow down to an art, and does not spare the flux. Even with the amount he uses, it almost all disappears before the soldering process is complete.So probably it will all dissolve before the part solders, but that should be fine as it is only needed to clean the surfaces to be soldered, not make the solder melt or form joins (except by ensuring the pads/leads etc are clean of contaminants etc).I havent personally done any hot air reflow, but I have also watched a few videos and other people that do it, and Im picking up bits and pieces I need to give it a go eventually. For now I use drag soldering and that seems to be working just fine for me.

author: madires
author_rank: Super Contributor
timestamp: November 16, 2019, 03:49:07 pm
content: Take some e-junk, practice and figure out which settings of your hot air station work best. Flux is meant to remove oxidation and needs heat to do its job. So it will evaporate during the whole process and leave some residue. The solder's surface tension is what drags components into position.

author: kripton2035
author_rank: Super Contributor
timestamp: November 16, 2019, 04:21:27 pm
content: may be you're using too much air flow ? try half the max value to see if the chip stays in place.

author: xmetal
author_rank: Contributor
timestamp: November 16, 2019, 04:41:17 pm
content: Using leaded solder helps as the chips tend to jump in to to place compared to lead-free. Also it melts at a lower temperature than the latter.

author: nali
author_rank: Frequent Contributor
timestamp: November 16, 2019, 05:28:23 pm
content: The board or rather the pads also need to get heat, are you just directly heating the chip or the surrounding board also?If the board has a lot of copper either as pours or internal power planes then you'll save yourself a lot of hassle by getting a preheater to warm the board, a dead giveaway is a chip with a thermal pad on the underside for example. You can use the air gun to heat the board area but a preheater makes life a lot easier.As mentioned, leaded solder is much easier if you can use it.TBH personally I use a gun to remove, then iron to replace with lots of flux & "drag soldering". Sometimes it's a bit of a pain if there's a connector or something close by but often it's worth the extra few minutes to remove the connector or whatever to make room then faffing around for ages in a confined space.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 16, 2019, 07:27:04 pm
content: Quote from: TomS_ on November 16, 2019, 03:21:39 pmNo it doesn't blow off it just sits there and I have to fiddle with the tweezers to get it in place. I have some chipquick flux pen and I tried getting as much flux as I possibly could, but if that has no part in helping the solder suck the chip in place it's not the problem. I heated the board so that the solder on the pads was molten when I placed the chip on the board so every thing should be hot and toasty.Quote from: nali on November 16, 2019, 05:28:23 pmI use leaded 60/40 solder.I also prefer the iron for soldering but it was such a pain and my patience was running lowBut even when I got it in place I had to touch up some joints that never made proper connection.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 16, 2019, 07:28:51 pm
content: Quote from: madires on November 16, 2019, 03:49:07 pmYeah I'm gonna have to do thatGot some boards from the previous revision and some dead chips.

author: bob91343
author_rank: Super Contributor
timestamp: November 16, 2019, 07:35:08 pm
content: I did a good job when I held the new part in place with a small C clamp. That way I could adjust its position without heat. Once in place, a slight touch of heat finished it. If you don't have a clamp, you can use tweezers with locking tips or even chewing gum. Or an alligator clip or a bench vise.Trying to adjust position while solder is flowing is difficult. In fact, once held in proper position, almost any soldering tool works well. I have used a tiny iron successfully. If you short adjacent pins, heat it again and it should clear up. Use the least amount of solder possible.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 16, 2019, 07:39:45 pm
content: Quote from: bob91343 on November 16, 2019, 07:35:08 pmIt's kinda tedious to place the chip when I have lumps of solder on the pads. I guess if I remove the solder it might work.I've gotten somewhat good at soldering it with just the iron as long as there's nothing in the way.

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 07:44:24 pm
content: QFN:1. put the solder on the chip's pins, not on the pcb padsQFN, BGA:2. you need a LOT of flux, the chip must swim in the flux, flux must stay there during soldering, so set the airflow such it does not blow the flux off3. put your pcb on a ceramic support, heat the pcb up before soldering, lot of flux4. then put the chip on the pcb and still heat, maintain a lot of flux, change the temperature (heating profile) such you change the air gun distance from the pcb, move the gun slowly down and up5. gently (0.2-0.3mm) push the chip from a side - when the pins are soldered the chip moves back by wetting forces6. the air flow and the temperature have to be adjusted - needs some training.LQFP:1. place the chip on the pads2. bond the chip with solder on two-three places such it holds firm on the pads, shorts are not a problem3. put a LOT of flux on the pads4. solder all the pads with iron tip by moving the iron tip over pins, lot of flux and solder, shorts are no problem5. use the copper wick and a lot of flux - place wick on the pins and heat it up with the iron from top (the wick must be wet of solder), pull the wick over the pins row from left to right (or right to left) slowly at one side6. repeat at all other sides.I did a few LQFP packages with air gun, but had to use the wick afterwards to remove several shorts. So it seems to me it is easier and faster to do it with iron instead.The key is to maintain a LOT of flux all the time - it always works fine when there is a LOT of flux, chip must always swim in the flux.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 16, 2019, 08:18:16 pm
content: Quote from: imo on November 16, 2019, 07:44:24 pmThanksBut in this case it was a LQFP package but still valuable information.

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 08:21:01 pm
content: I added the LQFP too. Btw the easiest soldering is with BGAs

author: TomS_
author_rank: Frequent Contributor
timestamp: November 16, 2019, 09:40:54 pm
content: Quote from: TheHolyHorse on November 16, 2019, 07:27:04 pmHmm. So you didnt have the chip on the board at the same time? Have you tried with the chip on the board while heating it up as well, so its leads also get cleaned by the flux, and also so the parts heat up at the same rate? Placing a cold part in to hot solder doesnt sound like a great idea.I also wonder if maybe you need more solder on your pads? If there isnt enough, perhaps it doesnt have anything to "float" on in order for surface tension to align it.

author: wraper
author_rank: Unknown
timestamp: November 16, 2019, 09:47:32 pm
content: Liquid flux as in flux pen usually is hopeless for hot air. You need tacky (gel) flux. But don't buy Chipquick SMD291, it's junk.

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 10:31:44 pm
content: Even the gel flux liquidize with hot air, therefore you have to find a balance between airflow speed and its temperature such the flux is not blown off the pads/pins. That requires some experimenting. I even had a small table I made with air speed, temperature and distance from the pcb, trying to mimic a profile. The pcb temperature is important as well, you have to avoid cooling it from the bottom side. Tricky.

author: wraper
author_rank: Unknown
timestamp: November 16, 2019, 10:46:10 pm
content: Quote from: imo on November 16, 2019, 10:31:44 pmLiquid flux evaporates, does not stay liquid. Liquid flux is mostly solvent with a little bit of actual flux. Also tacky fluxes have an order of magnitude higher work time.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 16, 2019, 10:58:22 pm
content: Quote from: TomS_ on November 16, 2019, 09:40:54 pmYeah since it didn't align for me and I lost patience I just gave up and left the chip not aligned properly but stuck. I later added more flux and heated it again hoping it would suck it in place.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 16, 2019, 11:01:21 pm
content: Quote from: wraper on November 16, 2019, 10:46:10 pmSo for hot air I should get some better flux, preferably some gel stuff? I'll do some research on that.

author: wraper
author_rank: Unknown
timestamp: November 16, 2019, 11:08:42 pm
content: Quote from: TheHolyHorse on November 16, 2019, 11:01:21 pmAnything other than tacky/gel flux will be spent before solder melts.

author: iMo
author_rank: Super Contributor
timestamp: November 16, 2019, 11:17:45 pm
content: This gel is pretty popular, there are tons of 223 cheapo fakes, I've done with the fakes too and they worked fine as well.You need also a bottle of IPA (>=95%) for cleaning the mess up.https://www.tme.eu/en/details/armalf223tf_35/fluxes/amtech/rma-223-lf-tpf-35g/

author: viperidae
author_rank: Frequent Contributor
timestamp: November 17, 2019, 02:22:22 am
content: QuoteAs Louis Rossman says, the bigger the gob, the better the job.

author: jhpadjustable
author_rank: Frequent Contributor
timestamp: November 17, 2019, 02:23:58 am
content: So 1 rossmann = how many cc of flux?

author: wraper
author_rank: Unknown
timestamp: November 17, 2019, 02:25:53 am
content: Quote from: viperidae on November 17, 2019, 02:22:22 amLouis Rossmann cannot solder properly and he admits it. He uses 50 times more flux than needed. If you have good tacky flux, only a tiny bit of it is completely enough, just enough to flow over the pads. Which it does by itself when heated.

author: KL27x
author_rank: Super Contributor
timestamp: November 17, 2019, 03:17:07 am
content: QuoteI have used MG Chemicals liquid rosin flux to hot air reflow hundreds of QFN, using lead solder to pretin the pads. I find1. You need enough flux that it bubbles and the chip wanders away on its own, so this is annoying2. I would pin the chip down with tweezers while heating it, and this solves the problem. So it's not really that annoying.3. I have not "burned" the flux? I mean, I don't know what kind of flux is in your pen, but I agree with others that a pen doesn't usually lay down much flux. I would put a bead of rosin flux down, with a prominent meniscus, using a syringe.I think Luis Rossman (slash AvE; why am I not surprised he watches AvE) is mostly right re "the bigger the gob." I have gotten feedback before about an issue where excess undried flux caused failure in production testing. It is a problem that goes away by itself in about 24 more hours. If the PCB is made on Monday and goes into testing on Tuesday, it might still be too wet. Now, if you used too much gel flux and you don't clean the residue, maybe it takes another week to dry out on its own? You can also fix this issue faster by touching an iron to the flux for a bit to dry it out.Whether you need gel flux or not might depend. I've heard that gel flux is supposedly designed for longer reflow/working time. I don't know if this is true in all (or even any) cases, though. It might be especially desirable to have longer working time if the board has a very high thermal mass/conductivity. This might be something to suspect is your case, but you/OP desoldered the chip in the first place, presumably without a notable problem.

author: Jwillis
author_rank: Super Contributor
timestamp: November 17, 2019, 04:49:49 am
content: Have you tried Preheating the board. I've only done a couple because like you I shake a lot . But I did find that heating the entire board in the oven to around 200 F help me. Or, if there's a lot of plastic that could get damaged , preheat locally where the chip is to be welded .Check out PCB preheating for a proper procedure since I'm a real novice at SMD soldering.

author: agehall
author_rank: Frequent Contributor
timestamp: November 17, 2019, 09:52:20 am
content: Just use better flux. I use the Amtech stuff that Louis Rossmann uses and it is just awesome. Kind of hard to get hold of in the EU though so I usually stock up whenever I pass by NYC.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: November 18, 2019, 11:51:41 am
content: Seems like getting some good quality flux and pre hearing the board is the way to go.Thanks for all the support

author: hermitengineer
author_rank: Regular Contributor
timestamp: November 18, 2019, 06:50:33 pm
content: Just a beginner with QFP myeslf, and reflecting on my many failures before giving up and getting TSSOP versions, but I wonder if one of the major problems when hand-soldering them is that you don't get an even amount of solder on each pad? Some will have plenty of solder and others... not so much. So when you heat the chip up and even get it to stick in place after the solder melts, you still might end up with unconnected pins. Or at least, I almost always didThe tweezers probably do help in that area though, as they will squish some of the (hopefully not egregious) excess solder out and make sure that all pins get at least some solder bonding the chip to the pad when it cools. I wasn't trying the tweezers at the time other than to hold it in place, so that's just my speculation.The other issue is when it has the middle pad, because it requires "just enough" solder. Not enough, and the pad can't act as a good heat sink. Too much, and the chip just floats on the middle pad.

author: iMo
author_rank: Super Contributor
timestamp: November 18, 2019, 07:35:04 pm
content: You do not bother with soldering individual pins (smd flatpacks). You put a lot of solder flux (the amtech 223 gel for example) - such the pins are fully covered by it, put a bit of solder on the iron tip, and run with the tip over the pins (while the package is positioned at the pcb pads properly - you must fix the package to the pcb with a solder blob put somewhere on the pins at the opposite package side).Many pins will be soldered together (sometimes all). Then put again a lot of flux at the pins, take the "copper wick - desoldering braid" and place the wick at the pins (wick must swim in the flux), heat up the braid from top with iron tip such the wick starts to suck in all the excess solder - you move the wick (together with iron tip) slowly over the pins.In one single move you get perfectly soldered pins, even they are 0.5mm pitch. Use a LOT of solder flux (all must swim in the flux), otherwise the pins stay shorted or not soldered properly.

author: Jwillis
author_rank: Super Contributor
timestamp: November 18, 2019, 07:48:15 pm
content: Interesting discussion all around .I found this method but haven't tried it myself and was wondering if anyone here would have any input .

author: KL27x
author_rank: Super Contributor
timestamp: November 19, 2019, 12:37:02 am
content: hermitengineer:QuoteImo it sounds like hermitengineer is talking about hot air reflow.hermitengineer:QuoteYes. I like to err on too much solder than too little. The excess just squishes out and stays out. If this squeezeout is too ugly or causes a bridge, then I would have to go over it with a soldering iron. But if you start with too little, then the chip suctions to the center pad and doesn't align itself. It will take a hard nudge with tweezers to move it, and where it stops will be random.If you don't have to solder the center pad and the part has "side pad," I personally find it to be faster/easier to use an iron to solder the outer pads vs hot air reflow.