What do I need to solder QFN and small BGAs - Page 1
Forum section: electronics / beginners

author: newbie666
author_rank: Regular Contributor
timestamp: January 13, 2019, 03:11:09 pm
content: Hello,more and more ICs I want to design with are not available in friendly packages so I decided to get some gear to be able to solder those small bastards efficiently.I could either get a cheap toaster oven and start modifying it to get a reflow oven or I could invest in a hot air station. Which one do you think would be more practical in a hobbyst lab? If I go hot air route should I get cheap-ass Atten 858 or is spending a bit more for quick 861 sensible in my situation?I'm only doing PCBs in single quantities for my own use so at first I thought hot air would be the way to go, but now after doing some more research I am starting to think I will be able to solder greater variety of packages using the oven. I would probably be cheaper than getting Quick hot air as well.So what should I get? Hot air or toaster oven?

author: Yansi
author_rank: Super Contributor
timestamp: January 13, 2019, 03:25:31 pm
content: Small to decent sized QFNs can be soldered by hot air with a bit of practice. There are numerous tutorials on web. Go read/watch some and make your own opinion, what will suit you best, then try for yourself.I would not dare to solder any BGAs by hand. What do you mean by small? I solder at work some times 144 ball 10x10mm BGAs with a 0.8mm pitch. That I think may be classified as a small. But I wouldn't dare doing it by hand.If you work with SMDs, you should have already had a hot air station. How are you replacing for example SOIC chips? Hot air is a must. Reflow oven is a good to have.What you certainly will need and quickly find useful, is to have a something to preheat the board. There is not much chance to solder any QFN/BGA correctly on a multilayer board, without the preheat.So I would drop the oven, and quickly bought a hot air and a preheating station or how's that called in English.

author: wraper
author_rank: Unknown
timestamp: January 13, 2019, 03:28:42 pm
content: Regardless if you have reflow oven or not, having hot air is a must. It's not OR question. You simply won't be able to fix issues/repair something without hot air. Those are not a replacement for each other.

author: cdev
author_rank: Super Contributor
timestamp: January 13, 2019, 07:09:02 pm
content: One you have some means of desoldering/resoldering a qfn, the easiest way to become familiar with it is - use junk PCBs, every piece of throwaway electronics can help you get better at soldering..To desolder QFNs, heat them up by your preferred means, (hot air with a small nozzle works well, another way is using bismuth solder like chip quik..You remove the chip with tweezers, or some people use a suction grabberclean up the excess solder around where it was, reapply some flux and then practice putting it back.Notice how when everything is properly warmed up and liberal amounts of flux has been applied - if you move the chip a tiny bit off where it should be, the chip just sashays right back into place on its own because of surface tension. Thats what you want to have happen. To do that properly hot air is almost essential. Drag soldering can be done as wellbut if you dont have hot air its easy to have the chip be slightly off the optimal position and the self-correcting aspect wont be available because the chip will be anchored down with the first pin and its position wont be able to be adjusted unless you laboriously desolder it with Chip Quik or similar.Preheating to a temperature well below the melting point of the solder (100 degrees C is maybe even too hot) thats used is helpful to make the process more forgiving but you cannot use a preheater for spot desoldering because once it gets hot enough to melt the (RoHS) solder especially, literallyeverythingon the board will then come off. So use hot air to control what melts and use the preheater to make sure the parts dont undergo the solder melt and thaw too many times because that causes mechanical stress and reduces reliability-You especially don't want to use too much temperature for too long around active components, multilayer boards with vias going through them, and multilayer capacitors. If the FR4 changes color slightly becoming glassy its too much.I have never done a BGA so I can't help you there.

author: Yansi
author_rank: Super Contributor
timestamp: January 13, 2019, 07:13:30 pm
content: With already tinned PCB and chip, this is quite easy. However it is more fun, when the PCB is not tinned yet, the QFN ditto. Or when the QFN comes right from china, where it was improperly stored and the lead surface finish got that craptacular look. Then it may become quite challenging to have a good solder joints.But in every situation, good flux is what helps. Don't fall for those cheap (RMA223 or such) fluxes from China. A lot of good things can be bought from china, including those hot air stations, but cheap flux is not one of them. So get a quality flux, if you want to solder any good.

author: cdev
author_rank: Super Contributor
timestamp: January 13, 2019, 07:18:39 pm
content: I just saw a video where a metalworker was building his own replica of the Antikythera Mechanism.As his flux he used a clear red flux that came quite attractively packaged. It looked gorgeous in that setting, like it was a premium food product or something.Rosin flux is made from two of the constituents of pine sap.

author: bson
author_rank: Unknown
timestamp: January 14, 2019, 01:16:07 am
content: Hot air works well for all QFN with or without a center pad, and small BGAs. For larger BGAs you'll need an oven. Use leaded paste and either a stencil or a toothpick with some practice. (Start with something simpler, like a 0.5/0.65 SSOP.) Without a stencil paste under a 5X microscope. Place parts under a microscope. No need to apply hot air under a microscope, as you can easily see when the paste melts on a QFN and other parts while a BGA will settle. A QFN center pad if you give it holes or a small millout you can solder it from the bottom or just heat until the part settles. You don't need or want flux beyond what's in the paste. Go light on the paste; the #1 beginner mistake is using too much and the pins end up floating in it off the pad. There should be just enough to form fillets around the contact area. Sightly more paste is required for ENIG than HASL, and it's also easier to tell when you have get it right since with ENIG you can tell which portions of the pad has solder vs which parts don't, and you'll get an outline of solder fillet around the pin. And don't worry about cleaning a no-clean flux, it can get frosty but that doesn't matter, it's only a concern if you used too much and got it all over. Something like image below is just fine to leave (0.5mm *SSOP).