Hot air tool for small QFNs - Page 1
Forum section: electronics / testgear

author: AndyC_772
author_rank: Super Contributor
timestamp: June 22, 2014, 08:04:00 pm
content: Hi all,I have a couple of jobs coming up which will involve soldering some small QFNs (3x3mm and similar).My hot air gun is a bit big for the job, and tends to blow very small components away, so I'm looking at buying a new one that's better suited for the task. One part in particular is quite sensitive and easily damaged by overheating, so I need a tool which can deliver a gentle breeze that's just hot enough, but not too fierce.One obvious choice is aMetcal HCT-900. I know it'll do a good job, but it's an expensive choice for occasional use. That said, just because it's only needed occasionally doesn't mean I don't need a tool that works well when it is needed.Can anyone please recommend a cheaper option?Also: one key part of the process is seeing when the solder has actually melted, which is tricky with such a tiny component. Has anyone successfully used a USB microscope for this purpose? Are they all fairly similar, or are there some particularly good ones that might be genuinely useful?

author: M. András
author_rank: Super Contributor
timestamp: June 22, 2014, 08:13:13 pm
content: cant give you a cheaper option unless you are willing to go for the cheap chinese route, but take a look at the JBC JT hotair tools, automatic process control and manual mode etc well its 1120eur+vat without accesories but it has an 1000watt heater inside, and a vacuum port for pickup tools,http://www.jbctools.com/jt-hot-air-station-product-13-category-3-menu-2.htmlyou can even ask for a demo unit.

author: georges80
author_rank: Frequent Contributor
timestamp: June 22, 2014, 08:30:40 pm
content: Easy to tell when the solder paste has melted. The component will float on the molten solder. I just use a nice 4x magnifier (the square style) and that works more than adequately with my 1/2 century+ eyeballs.I use a ~US$90 hand held hot air tool (variable speed, variable heat). Easy to do 3mm DFN's (FETs etc). e.g. something like ->http://www.circuitspecialists.com/csihotgun-2.htmlIf your board only has components on the top side, an adjustable hot plate works very nicely since it heats the PCB material as well. Hot plate is my preference when I can use it. I use an ebay adjustable hot plate (without the stirrer option) made by Fisher Scientific (was something in the $60 range) - ceramic top and works a treat.If just using the hot air tool, it pays to preheat the bottom of the board area, especially with thermal connections to the DFN.cheers,george.

author: nctnico
author_rank: Super Contributor
timestamp: June 22, 2014, 08:41:57 pm
content: I solder QFNs with a soldering iron. Just have the pads extend about 0.5mm more beyond the package. What I use to solder the exposed pad depends on the size of the ground plane. I always have blank vias in the exposed pad so soldering from the rear is easy. OTOH it should be doable with an Atten 858D+ hot air station.

author: nanofrog
author_rank: Super Contributor
timestamp: June 22, 2014, 08:58:09 pm
content: The HCT-900 and Hakko FR801 are the cheapest name brand hot air stations I'm aware of (350GBP before VAT & shipping). Cheaper than that, you're looking at used, or a much cheaper Chinese model. If it matters to you, the HCT-900 is also made in China, and the Hakko is still made in Japan.JBC makes two sizes of hot air station, the smaller one meant for things like cell phone rework/repair. That one would be suited IMHO, but it's definitely not cheap.FWIW, tweezers are really handy to both position and keep it from blowing away. When you feel the part drop, let go and move the heat off simultaneously. Surface tension will do the rest. Takes a bit of practice, but it doesn't go flying across the board.But I likegeorges80's suggestion of using a hot plate, and think it would be more cost effective than a new, name brand hot air station.

author: ovnr
author_rank: Frequent Contributor
timestamp: June 22, 2014, 09:03:46 pm
content: I've soldered plenty of .5mm pitch QFNs (and some 2x3mm DFNs) with my Atten 858D+. Works perfectly well, and I very rarely blow components away (and it mostly happens with SOD-123s and SOD-323s - annoying little buggers with annoying leads). I've never bothered to reduce the air flow either - mine is at full blast.You will want a stencil, though. And be sure to segment/reduce the paste mask for the ground pad.

author: zapta
author_rank: Super Contributor
timestamp: June 22, 2014, 10:00:47 pm
content: Quote from: AndyC_772 on June 22, 2014, 08:04:00 pmWhat kind of hot air gun do you have? If it's like these oneshttp://www.tequipment.net/soldering-equipment/hot-air-guns/then it's not a good fit.Try one like this onehttp://www.eevblog.com/2011/04/25/eevblog-167-atten-858d-hot-air-rework-review/, you can find them on ebay for ~$70. Then get a set of nozzles like this onehttp://www.ebay.com/itm/6pcs-BGA-Circular-Nozzles-850-Hot-Air-Rework-Reflow-Soldering-Station-/200934757038?pt=LH_DefaultDomain_0&hash=item2ec8a512ae. You will be able to control the heat, air flow and heated area size.I have done numerous DFN 3x3mm with a similar setup and had not problems.

author: Psi
author_rank: Super Contributor
timestamp: June 22, 2014, 10:21:23 pm
content: I solder 3x3mm accelerometer DFN's with hot air all the time.- I dab a tiny bit of solder paste on all the pcb pads (less is better than more)- Sit the chip on top and get the alignment right- Turn the air flow down to about 40%- Gently heat the area from about 4cm away. (Temp on rework station is about 350C but i'm not sure how accurate that is.)- Once i see the chip reflow i remove the heat and check alignment.- I then clean my iron tip and run it down both sides of the chip with a little fresh solder (using the magnifier lamp to see what im doing).The DFN side pads seems to take just enough solder from the iron to make nice joints without bridging.I guess you could say that i use hot air to get the chip attached to the pcb with correct alignment, then finish it with the iron.

author: Sebastian
author_rank: Regular Contributor
timestamp: June 22, 2014, 10:36:49 pm
content: I have an AOYUE Chinese hot-air rework station (http://www.aoyue.com/en/ArticleShow.asp?ArticleID=380).It is now a few years old and still works perfectly fine. (I don't use it very often) The build quality is pretty good, especially for the price of about $100 on ebay.I don't have any comparison, but I think the fact that the fan is inside the base unit makes it easy to use because the hand piece is small and lightweight.I would recommend it if you don't use it a lot. If you use it daily, I would say you should get one from a decent brand like Weller or JBC etc..

author: Psi
author_rank: Super Contributor
timestamp: June 22, 2014, 10:48:38 pm
content: yeah, mine is a AOYUE too.It has a few software bugs but nothing major.Seems to be some weirdness not knowing the physical position of the rotary encoder at startup, so you have to move it to both extremes before you get all the knob range back.And it seems a bit susceptible to mains noise. Any large spike on the mains causes the MCU to reboot.

author: Sebastian
author_rank: Regular Contributor
timestamp: June 22, 2014, 11:11:14 pm
content: Interesting, I don't think mine has a micro controller. It only has two knobs and they are potentiometers and not encoders. Which model do you have?

author: nixfu
author_rank: Unknown
timestamp: June 22, 2014, 11:49:06 pm
content: I have been looking at buying one of these ATTEN AT858 clones. I have been doing all my SMD with a cheapo hot air gun from Harbor Freight, and an old toaster oven(reflow).http://www.ebay.com/itm/US-STOCK-858D-SMD-Digital-LED-Display-Hot-Air-Gun-Rework-Station-/261398444124$49 + Free Shipping, its hard to beat for a pretty good station with some different size attachments and for the price I can afford to get some other more specialized shape/size attachments such as the square ones for BGP etc.

author: zapta
author_rank: Super Contributor
timestamp: June 23, 2014, 04:48:08 am
content: Quote from: nixfu on June 22, 2014, 11:49:06 pmThese cheap hot air station do a very good job in reflowing boards or rework. Workmanship is not always great so you may want to open it upon arrival and do a quick inspection.Personally I prefer the analog ones because it's easier to raise the temperature between the soaking and reflowing steps but the digital ones should be just fine.

author: Psi
author_rank: Super Contributor
timestamp: June 23, 2014, 05:40:02 am
content: Quote from: Sebastian on June 22, 2014, 11:11:14 pmIts an Aoyue 968, duel iron/hair station.hm.. yeah, your right, it's a pot.It's the pot that controls the airflow.If you set the knob to say 20% then turn off the unit when you turn back on the airflow is at ~80% and if you turn the knob down it hits the stop almost instantly (since its was at 20% physical position).You have to turn the knob all the way up before you can then turn it down to get the lower levels back.Definitely got a mcu, the hot air temp display is a text LED display that scrolls AOYUE when you turn on.

author: true
author_rank: Frequent Contributor
timestamp: June 23, 2014, 05:51:20 am
content: Quote from: Psi on June 23, 2014, 05:40:02 amI have a 968a+ and it doesn't do this. Airflow is dialed in with the pot but can be overridden depending on if smoke absorber is on or if the hot air rework was turned off. Always flows the amount set to at power on, though.I only use the hot air on this unit so I couldn't tell you if firmware bugs exist elsewhere, but hot air always seems to work OK.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 23, 2014, 09:14:21 am
content: Quote from: M. András on June 22, 2014, 08:13:13 pmVery nice! But probably overkill in my case; I'd be better off subcontracting out the occasional job rather than buying something quite so high end.Quote from: nctnico on June 22, 2014, 08:41:57 pmNot really an option in this case, sadly. The PCBs already exist, and they use the device manufacturers' recommended footprints which, for some reason, never seem to include the extra exposed pad area outside the package outline that makes this method do-able. It's a frustrating decision; do you go with the recommended footprint which (presumably) gives the best yield in an automated process, or a modified one to make hand soldering easier? I guess perhaps I got it wrong this time.Quote from: georges80 on June 22, 2014, 08:30:40 pmNice idea that I hadn't considered, thanks. I do tend to work on double sided boards more often than not, though, so even though one would probably work for this particular job, it's less future proof than I'd like.Thanks for all the suggestions about different hot air stations. TBH I'm wary of anything that arrives from China via Ebay, or which may arrive with 'quirks' for want of a better (polite!) term. Life is too short, and good quality new equipment is a tax deductible business expense.I think I'll go with the Metcal.

author: jc101
author_rank: Frequent Contributor
timestamp: June 23, 2014, 10:35:02 am
content: I bought a Xytronic LF-852D from Rapid recently (http://www.rapidonline.com/Tools-Equipment/Xytronic-SMD-Hot-Air-Rework-Station-600W-LF-852D-85-1225), came with a number of tips and a spare heater element too.Been very happy with it thus far.

author: PlainName
author_rank: Super Contributor
timestamp: June 23, 2014, 01:48:30 pm
content: I solder 3x3mm accelerometers and the like with a 100% success rate (now - it used to be in the range 0-30%). My tool of choice is an Aoyue 852 hot air rework station, pretty much the same as 90% of the other Chinese ones. Nowadays I use an IR camera to determine actual temperature, but that's just frills and not strictly necessary.My tip for a successful job is:1. Get the PCB pads dead flat. Wipe 'em with desoldering braid to ensure they have no sticky up solder bits.2. Invert the IC and slop flux on the pads. I use a jelly, but that kind of preference isn't important. Can't have too much.3. Solder the IC pads. You want enough solder that some will come off with the iron, so you can either apply iron then dab on solder, or load solder on the iron and dab that on the pad (hence the generous flux usage). This is the critical step since the idea is to brim the pads with solder so they won't take any more. The pads are all the same size, thus they will all take the same amount of solder, and the surface tension will cause a blob that is exactly the same height on each pad.4. Clean the excess flux of, flux PCB pads, place chip, heat until soldered.This method is pretty much the same as using solder balls but without the balls or rig.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 23, 2014, 02:41:57 pm
content: That's pretty much the technique I've been using, though the tricky bit seems to be getting an equal ball of solder on each pad.I've successfully fitted a couple of devices today using my Steinel HG2310, so maybe I can put off buying a smaller heat gun for a while after all. The Steinel works just fine provided it's set to minimum air flow, a slightly higher working temperature (I used 370 deg C), and crucially, not getting it too close to the part. A bit of extra distance helps reduce the air speed around the device being soldered, and the higher temperature compensates for the heat loss.Not perfect, but free!

author: PlainName
author_rank: Super Contributor
timestamp: June 23, 2014, 05:19:28 pm
content: QuoteAnd not too far away. The bump to step over is getting used to it taking some time. With an iron you dab and it's done, but with air you need some patience - like baking food, you can end up with a crisp shell and cold insides.

author: zapta
author_rank: Super Contributor
timestamp: June 23, 2014, 06:40:59 pm
content: Quote from: dunkemhigh on June 23, 2014, 05:19:28 pmThe Pace video calls it WPI (work piece indicators), you look closely and figure out what's happening.