Removing QFN44 chips and re-soldering them? - Page 1
Forum section: electronics / general

author: Seytonic
author_rank: Newbie
timestamp: December 27, 2016, 12:36:32 pm
content: Hi, I'm designing my first pcb based project around arduino and want to use the same chip as in the arduino pro micro (atmega32u4). I'm planning to buy a load of arduino pro micros from aliexpress, remove the chips and solder them onto my board. This way is infact cheaper than buying the chips by themselves here in the UK, and they'll come preloaded with the arduino bootloader. My question is how hard will it be to resolder QFN44 chips, given they'll already have a bit of solder on the pins and I know they're very small. Apologies if this is a silly question, but I've never done any pcb rework before so I'm a bit new to all this. Thanks for your help

author: cowana
author_rank: Frequent Contributor
timestamp: December 27, 2016, 12:44:55 pm
content: If you've got a hot air station, it's a trivial task, as long as you've got some additional flux. I've done that plenty of times.- Start by applying solder to your board (ideally solder paste, although you could tin the pads manually if that's the only option)- Add additional flux to the board- Heat up the donor board, pull off the IC with tweezers- Place the IC on your new board, and apply hot air until it reflows

author: Seytonic
author_rank: Newbie
timestamp: December 27, 2016, 01:06:41 pm
content: Quote from: cowana on December 27, 2016, 12:44:55 pmAwesome, I was just worried that it wasn't practical or something like that. Thanks for the help.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: December 27, 2016, 01:06:54 pm
content: You need a hot air station, flux, some heat proof tweezers and either good eyesight or a magnifier or wide-field microscope. Once you have those it should be easy.Are you really sure you couldn't find a source of atmega32u4?

author: Seytonic
author_rank: Newbie
timestamp: December 27, 2016, 02:17:41 pm
content: Quote from: NivagSwerdna on December 27, 2016, 01:06:54 pmI've found quite a few sources however it would cost me almost twice as much for one chip in the UK as compared to china, and in china buying an arduino pro micro is only a few cents more than buying the chip by itself. With a whole arduino I can also salvage other components like the crystal and the chip itself will come with the bootloader, so that's one less headache.

author: hydrogen maser
author_rank: Regular Contributor
timestamp: December 27, 2016, 03:08:53 pm
content: My little cheapo Chinese 858D $40 hot air tool works fine for this kind of stuff. If you have never done it before I recommend practicing on a random board that has packages near the size and pitch you plan on doing. It does take a little practice to get the technique down, but it is easy once you get the hang of it. You will need magnification unless you have better than perfect eyesight. I prefer to use a binocular microscope at 7x to 10x for that size package. You can use solder paste or just regular solder (.3mm is good for that pitch), whatever is easier for you, I typically don't use solder paste for rework, just my preference.

author: doobedoobedo
author_rank: Regular Contributor
timestamp: December 28, 2016, 12:54:12 pm
content: I've done this many times myself.Best to check which bootloader is on pro-micros from aliexpress. The ones I got came with the Leonardo bootloader and needed to be re-flashed with the correct one.

author: DTJ
author_rank: Super Contributor
timestamp: December 28, 2016, 01:01:25 pm
content: I've found using an electric frypan on a low setting to preheat the board to 100C or 150C really helps in removal / rework jobs and you don't have to be as aggressive with the hot air.

author: elecman14
author_rank: Unknown
timestamp: December 28, 2016, 06:17:04 pm
content: Quote from: DTJ on December 28, 2016, 01:01:25 pmThe poor mans solder pre-heater...My advice for this would be that some QFN packages have a metal lead exposed on the outside of their body. If you design your PCB with a longer pad on the QFN you may be able to hand solder them if you run into problems.

author: harry4516
author_rank: Regular Contributor
timestamp: December 29, 2016, 10:53:58 pm
content: Quote from: Seytonic on December 27, 2016, 12:36:32 pmthis is your first PCB based project, sono, it is not cheaper if you calculate the price + your hard work + the destroyed parts (and its sure you will destroy a couple of them during removing and resoldering)Do yourself a favor and buy new QFN.

author: Jay1011
author_rank: Contributor
timestamp: December 30, 2016, 01:27:45 am
content: I'd recommend using a hot plate rather than hot air. That way you heat up the entire board, and when it reaches the point at which the solder has melted, use tweezers to pull off all the parts you want to salvage. Only takes a few seconds and I think it's a lot easier than hot air, especially if you want to salvage several parts. I don't even bother using flux most of the time I do this (though flux does help).That said... getting the parts off the board is the easy part. Now you need to prep the parts so they can be resoldered. You'll need to clean the pads/leads with IPA, inspect to see which ones are in fact reusable (beware damaged/bent leads or QFN lands than have detached). Then you'll need to possibly remove excess solder on some of the QFN lands so that you have half a chance of getting good planarity when you go to reattach it to your own board. Etc., etc.In the end, you'll likely be wishing you spent the extra money to just buy new (and guaranteed non-counterfeit) parts to save yourself the time and hassle. Just my opinion.

author: hydrogen maser
author_rank: Regular Contributor
timestamp: December 30, 2016, 04:19:15 am
content: I have had a different experience than Jay1011, but I don't use a board preheater/hotplate, I just use hot air to remove parts and place them on the new board that already has the pads pre-tinned with solder and then flux on them, hold the package down in place with tweesers, apply hot air - done. 99% of the time this works fine and you minimize the amount of heat applied to the package. I don't need to prep the parts after they are removed from the old board, very little of the old lead-free solder sticks to the part leads and most of it stays on the pads of the doner board. YMMV.