Advice on soldering a tiny smd in a small space (QFN-12) - Page 1
Forum section: electronics / general

author: Nexxen
author_rank: Regular Contributor
timestamp: April 17, 2023, 01:24:57 pm
content: Hello!I need some advice on how to properly solder a tiny component, QFN-12 (2mmx2mm).My problem is a pad that came off with the burnt smd, an essential trace to Vcc of the component itself. Obviously not a n/c or gnd, otherwise non funzies I guessI added a pic with the area and the link to the smd (datasheet) for dimensions: 2mm * 2mm (the missing pad is the second from bottom right).My idea is to proceed by removing all adjacent smds that are in the way and resolder them after I'm done.The connection with the missing pad is what I can't really figure out properly.A small copper wire is too much, I don't have micro size to test. Maybe a solder blob under the pad on the SMD I solder?If it protrudes out I can then solder a wire.Thanks in advance!https://pdf1.alldatasheet.com/datasheet-pdf/view/1039486/MPS/MP5077GG.html

author: Psi
author_rank: Super Contributor
timestamp: April 17, 2023, 01:33:01 pm
content: From the datasheet the chip has pads that extend up the sides of the package 0.2mm.So you should be able to attach a wire from the side.Probably need a microscope and a small enough iron tip, but totally doable.

author: Nexxen
author_rank: Regular Contributor
timestamp: April 17, 2023, 09:31:47 pm
content: Thanks for your answer.To avoid issues due to overheating the adjacent components paired with the jump to the Otherworld, I'm going to preheat from the bottom and then use some low speed air.I have an amateur range station but I believe it to be enough to pull through.I didn't check and indeed there's a 0.2mm lead on the side. I'll tin it to create a solder joint.Good catch as it makes a difference. I'll look for these things in the future.Hope it all goes well.Edit: praparing the chip by "buttering" its leads too would be a good idea in this case? I'm thinking about the minute size, blobs and things that could go wrong that I can't see

author: thm_w
author_rank: Super Contributor
timestamp: April 18, 2023, 12:24:44 am
content: Overheating near stuff is not really an issue, if you want you can always use tin foil, copper tape, or kapton tape as shielding. But yeah preheat makes life easier regardless.Pre-tin if you want, but won't make anything easier IMO if you have enough flux, and you evenly tin the PCB pads. Just the sharp pointy tip or knife tip as mentioned by Psi should be enough to hit the side of the IC.

author: Nexxen
author_rank: Regular Contributor
timestamp: April 18, 2023, 01:08:34 am
content: I had a lot of scrap board laying around, I finally had the "ah-ah" moment and stripped a matching trace and rebuilt the missing last bit of the burnt trace.In the picture you can see the copper trace, in green the two terminals of a Cap, in purple where I'll put some solder mask to anchor a bit the trace.With the heat it'll move, or worse, fly away. As it's lilliputian scale I'm satisfied.The copper trace is longer for now, needing more work after the chip is soldered. When it's gonna be easier.I'll try to tin it, otherwise I'll tin the chip.Right way?

author: thm_w
author_rank: Super Contributor
timestamp: April 18, 2023, 01:16:05 am
content: As Psi was saying you attach the wire after the chip is soldered down.Your way could work too but you would likely need to secure the wire with some high temp glue like UV cure soldermask, so it doesn't fly out of the way.

author: Nexxen
author_rank: Regular Contributor
timestamp: April 18, 2023, 01:38:59 am
content: That's what I did.Was long and boring, soldering tomorrow though.Edit: I forgot to thank you all for the support and confirming/pointing me in the right direction.

author: Nexxen
author_rank: Regular Contributor
timestamp: April 18, 2023, 11:06:51 pm
content: I soldered the chip and I believe it went well.The chip moved in place alone and when I touched a corner it went back to the correct position.No shorts around or with any +V.I'll give it a go and see if it revives this baord.Edit: didn't fry again.Edit 2: with cpu it doesn't power on, another component isn't issuing the necessary +1.8V; I'll have to buy that one too - LOLhttps://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MP2147GD-Z/document_id/835/More soldering in the future.Now I know how to procede.Thanks again!!