Possibility of Soldering QFN  package - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 25, 2017, 06:10:07 pm
content: How much success would there be when hand soldering a QFN? I see it would be possible with a microscope but I don’t have one. I could solder the bottom GND pad to keep it from moving but what about the pads ? Maybe lengthen the pads a bit ?Any ideas would be appreciated.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: georges80
author_rank: Frequent Contributor
timestamp: November 25, 2017, 06:34:12 pm
content: It's easy enough - FLUX is your friend. Decent flux (RMA works well) and a chisel tip to 'drag' the solder across the pins/pads. Making the pads a little longer will help for soldering iron method.Solder paste makes it even easier/trivial. Apply solder past, then hot air to reflow. That makes it easy to do the center pad as well.cheers,george.

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 25, 2017, 07:11:24 pm
content: Thanks. I was just lost how to go about it. I will just extend the pads too.Maybe latter I would get a reflow oven to do more boards.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D, Brymen BM869s

author: Ian.M
author_rank: Super Contributor
timestamp: November 25, 2017, 08:19:56 pm
content: How the *#*# do you propose to solder the bottom thermal pad without reflowing it? The only way I can think of with a conventional iron is by customising the board with a large enough PTH in the center pad to get the tip of a fine bit down, and extend the perimeter pads so there's enough length to get the bit on those as well.Then its just a matter of good flux, so the solder will wick under it, solder opposite corners, then each side, flip over and solder the center pad. If you have trouble getting the solder to take, pretinning tha pads on both the board and the chip, and carefully hot-wiping them to level them will let you simply sweat them together with minimal added solder.However if you are reflowing you absolutely don't want a bastardised footprint with a center hole and extended pads, as the hole can rob the center pad of solder, and the extended perimeter pads reduce the self-alignment action due to the solder surface tension, so if you are planning to go to production you'll need a board respin.IMHO improvise some method of reflowing without hacking the footprint. - even if all you've got is a hot-plate or a DIYer's hot air gun. If its excessively crude, you can at least reflow the QFN before hand soldering the rest.

author: KL27x
author_rank: Super Contributor
timestamp: November 25, 2017, 08:27:01 pm
content: Most all the standard footprints for a QFN already extend enough past the part to get a bead on the side pad. If you extend it farther, all you do is open up the possibility that you can solder the part on so far out of whack that the center pad can bridge the other pads.If you do not have to solder the center pad, it is eaiser to leave it bare and to drag solder. Flux the pads, drop the part down, tack one pin, then drag solder while pinning the chip to the board with tweezers. If the chip looks crooked and/or if you can't get one side soldered because there's not enough pad sticking out (i.e., you put the chip on crooked), just solder the 3 sides that work and then hit with hot air; at the exact moment the solder reflows, the chip will spring into place.If the part does not self-align, it's most likely because you got a wee bit of solder on the center pad.* When this happens, the part sticks to the board and isn't going to move. So after X amount of time, if the part isn't springing into place, it's probably not your imagination. It's probably not your hot air station. Try pulling the chip off, tinning and fluxing the pads, and putting it on with hot air.*For this reason, if I'm soldering the center pad (using hot air), I prefer to err on the side of using too much solder than too little. Any excess, you can just squish out by pressing down on the chip after the solder reflows. But too little solder can cause a larger problem.Personally, I prefer a tip with a sharper corner/edge than a chisel tip. Knife or bevel tip have a sharper corner and will get deeper into the 90 degree corner between pad and the very shallow sidepins. The bevel is actually better because it fills the 90 degree angle pretty much perfectly; the angle on the knife is too acute, so you have to hold it at just the right angle to get the joints to form. That said, I prefer the knife when doing batch, because it will hold enough solder to do a lot of chips in a go. And in batch soldering, once you find that specific angle, it is easy to hold it and do multiple chips in a row. For batch soldering, this is another reason you do not necessarily want to extend the pads out. You will have to load the tip more frequently and/or feed solderwire with your off hand.So you will want flux and hot air if you are dealing with QFN much. Even when hand soldering them with an iron, you will want extra flux. And you will want hot air for when things go wrong.

author: Someone
author_rank: Super Contributor
timestamp: November 25, 2017, 08:37:31 pm
content: Quote from: Ian.M on November 25, 2017, 08:19:56 pmThe tip doesn't even need to go "down" the via, you can heat up the pad well enough with a couple of vias exposed through on the other side and adding enough solder paste to compensate for the wicking. If its a thermal pad you're usually running a bunch of vias anyway.

author: Ian.M
author_rank: Super Contributor
timestamp: November 25, 2017, 09:31:11 pm
content: Yes, but that's not inspectable, unless you Kapton tape the chip in place and solder the thermal pad first, which is an alignment PITA.

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2017, 10:26:05 pm
content: Extend the pads to about 0.6mm beyond the package and put several untented vias in the thermal pad. Solder the pads on the top first and then use hot air to solder the thermal pad from the bottom side. Let solder flow into one via and wait until it comes up into another via. You'll need to apply extra flux for both soldering operations.Having the vias in the thermal pad also makes a reflow soldering process much less critical when it comes to the amount of paste that needs to go onto the pad and in many cases this is also recommended to improve heat sinking capability.

author: Ian.M
author_rank: Super Contributor
timestamp: November 25, 2017, 10:50:14 pm
content: .... and we are back to doing it properly with hot air reflow. Hopefully the O.P has a hot air rework station.

author: nctnico
author_rank: Super Contributor
timestamp: November 25, 2017, 11:18:23 pm
content: Quote from: Ian.M on November 25, 2017, 10:50:14 pmOn a two side board it may be possible to use a soldering iron with a big tip but hot air is easier/quicker especially if the vias are in a copper pour without thermal reliefs. An Atten 858d+ hot air station is pretty cheap anyway.

author: amyk
author_rank: Super Contributor
timestamp: November 26, 2017, 03:21:34 am
content: Agree with the others that hot air is the best for surface mount.