PCB design for handsolder LGA, using via - Page 1
Forum section: electronics / general

author: lordalgar
author_rank: Newbie
timestamp: March 27, 2021, 06:49:47 pm
content: I'm planning on some PCB for a small project, I have a 6-pin LGA component with pads on the edges.I was condiering how to easily solder with a solder station (without hot air, oven, hot plate etc.)I might have seen something a few years ago (not sure), making via's in the pads and solder and forcing solder through the via's for it to stuck. (can pre-tinn the pads). But can't find anything in forums and youtube now.Anyone tried this or think it might work? (This is for a demo, planing 6pcs so far, not for mass production)The only other option I can think about is to make the pads on the PCB stick out (extra long) and you can solder from the outside and hope it will reflow (maybe pre-tinn the pads).

author: gnuarm
author_rank: Super Contributor
timestamp: March 27, 2021, 07:05:26 pm
content: There are different styles of QFN packages. Some have lead that is exposed on the edge off the package so there is something for the solder to wet and form a filet. This should be easily soldered using the same techniques as a QFP.The style that only has exposed lead on the bottom, more like an LGA package, would be harder. I would try starting with a tiny bit of solder on a single pad and placing the package in position reheat that solder. Or it might be better with a tiny bit of solder on two diagonal pads or even three. Some space is required under the part for subsequent wicking of solder to the remaining pads. The trick will be adding just a small amount of solder when soldering the remaining leads, but the usual method of removing excess with lots of flux and solder wick may work to prevent bridges under the package.I just don't know if a via, small enough to fit within a pad would carry the solder. 0.5 mm pitch QFN packages have pads that are only 10 mil (0.25 mm) wide. How do you get a vial hole in that? If it were a via the annular ring would need to be 24 mil (~0.6 mm) for a 10 mil drill or wider than the pad pitch. The 24 mil size is about preventing breakout of the drill from the pad which is not *as* important in this case, but I just not sure it is workable without a much smaller via hole.Then there is the matter of 0.4 mm pitch parts.

author: lordalgar
author_rank: Newbie
timestamp: March 27, 2021, 07:26:24 pm
content: Thanks,Yes, you are probably right, the pads are too small to get a via with enough size to carry solder.(I edit the post as there is no lead exposed on the edge of the package, so it's probably a LGA or behave more like a LGA)

author: PlainName
author_rank: Super Contributor
timestamp: March 27, 2021, 09:50:22 pm
content: I've never managed to manually solder LGA without hot air or oven. The problem is that you need all the pads to be a) at the same height and b) with the same amount of solder and c) the solder fluid, all at the same time.A trick I found helps a lot is to completely clean the PCB pads of solder and load up the LGA pads. Essentially you're reballing them, but without the tricky little ball things. Flip the LGA upside down, wash the pads with flux, then wipe with a loaded soldering iron - the surface tension of the solder should ensure that each pad has exactly the same amount of solder, and they will all have the same height. Place the loaded LGA on the PCB (with more flux) and apply air evenly (or shove in oven). You should see the package suddenly squat onto the pads (and maybe twist a little to line up) and excess flux be ejected. That's when you know it's a good 'un.Trying to do each pad in turn won't work because as soon as you solder one pad that will hold the other pads up off the PCB. If you're lucky you can get capillary action to work on a couple of pads, but rarely all of them. Even then it will be prone to flakiness.