Soldering a QFN package ground from the back of the PCB. - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: August 21, 2024, 02:59:43 am
content: I solder my hobby boards at home using a solder iron (no hot plate or oven) and soldering the ground pad of QFN packages was always a challenge. For my next board, I am thinking of having a 3mm via hole such that I can solder the pad from the back of the PCB. Any thoughts? Am I missing something?EDIT: My thinking is first soldering the regular pads from the top sides and than flipping the board and soldering the ground pad.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: August 21, 2024, 03:06:54 am
content: I've done this, it works fine for small qty hand stuff jobs.OTOH, stencils are now so cheap and you get solder in all the right spots all at once so this is a good option even if you stuff by hand.Just need to make sure you have some palm rest to not smear the paste when you do that.

author: zapta
author_rank: Super Contributor
timestamp: August 21, 2024, 04:10:18 am
content: @KrudyZ, I used to use stencils, solder paste in the fridge, foot switch vacuum pump pick and place, and a desktop IR oven but found it to be more hassle than help and went back to simple hand soldering. If I need more than 1 or 2 boards, I order PCBA service from JLCPCB.

author: ataradov
author_rank: Super Contributor
timestamp: August 21, 2024, 04:19:03 am
content: For hand assembly this works. I used this on Altera MAX10 FPGAs, which have exposed pad in TQFP-144 packages and those are way more annoying to solder by hand without that back hole.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 21, 2024, 11:27:45 am
content: I'd say don't flip the board, but suspend it in some frame, and then poke the soldering iron though the hole from the underside. (Use a fitting tip, big and beveled) This way you can see what you are doing. Have some sharp objects around such as a toothpick and tweezers. Then you can gently poke the QFN while the solder is molten. This helps with making reliable connections. Maybe you have not realized it, but when you solder the center pad, the whole IC gets so hot that the solder on all pads melt. Use plenty of flux.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 21, 2024, 12:11:46 pm
content: There is even a commercial supplier of adapter boards that uses that hole concept:https://schmartboard.com/smt-to-dip-adapters/

author: reboots
author_rank: Regular Contributor
timestamp: August 21, 2024, 04:24:15 pm
content: I have used that technique with TQFP48-EP packages. I placed a single plated via with a 2.2 mm drill inside a land pattern of 3.5x3.5 mm, the size of the pad on the package.I used the same fine-point soldering tip as with the rest of the board, turning up the temperature to 800F or so. I applied flux across the entire footprint, soldered the outside pins, flipped the board and soldered the pad through the via with no-clean flux-core, 63/37 leaded solder. I fed in a fair amount of solder until a bowl-shaped fillet accumulated in the via. This technique seemed to work fine, although I didn't remove any chips to confirm solder coverage across the pad.I didn't have any problem with fried chips, solder bridges to the outer pins, or outer pins desoldering while the pad was being soldered. However, TQFP pins are exposed outside the chip and may not heat as quickly through the package as QFN.Good luck!

author: zapta
author_rank: Super Contributor
timestamp: August 21, 2024, 06:13:37 pm
content: Quote from: Doctorandus_P on August 21, 2024, 11:27:45 amDoes it make sense to use a lower temp solder on the back side? E.g.https://www.amazon.com/Wonderway-Soldering-Electronics-CELLPHONE-Repairing/dp/B0BLSKZ6W1

author: xvr
author_rank: Frequent Contributor
timestamp: August 21, 2024, 08:27:19 pm
content: I used several small holes (around 0.6-0.8 mm) staggered on the bottom pad. The pad itself was duplicated on both layers of the PCB, so the solder mask was open on it. I placed a small amount of solder paste on the bottom side of the chip. Then soldered it with a soldering iron from below (after soldering all the other pins in the usual way). I also added some solder during soldering from below - it is added to the solder paste under the chip (the solder saturates the holes). Several holes made the heat transfer more uniform and helped to increase the conductive thermal coupling between the top and bottom layers of the PCB, effectively reducing the thermal resistance.

author: wraper
author_rank: Unknown
timestamp: August 21, 2024, 08:37:10 pm
content: Sorry, but I find it lame. I see not reason why anyone serious about electronics cannot buy $40 hot air station that is a must if you work with SMD components.

author: wraper
author_rank: Unknown
timestamp: August 21, 2024, 10:43:33 pm
content: Quote from: zapta on August 21, 2024, 06:13:37 pmHow about not bothering with nonsense and buying a suitable tool?This will get job done just finehttps://www.amazon.com/LRT-8858-Desoldering-100-500%E2%84%83-212-932%E2%84%89/dp/B09VZ9HB37YMMV with reliability but should last quite a bit if you don't run it above 750oF.$100 gets a very reliable one suitable for professional amount of workhttps://www.tequipment.net/Quick/957DW+/Desoldering-Equipment/Rework-Stations/

author: zapta
author_rank: Super Contributor
timestamp: August 22, 2024, 02:44:32 am
content: Quote from: wraper on August 21, 2024, 10:43:33 pmI actually have exactly this unit from Tequipent, the hose for too long for my setup so I shorten it. I also have MINIWARE MHP30M which I use to remove ics.My goal here is not saving money on equipment but simplicity and reliability, being able to populate boards with a solder iron, and being able to visually inspect the solder joints.,

author: wraper
author_rank: Unknown
timestamp: August 22, 2024, 09:21:21 am
content: Quote from: zapta on August 22, 2024, 02:44:32 amDoes not makes sense. You actually want to make it hard and unreliable just to avoid using hot air that you already have for some weird reason.QuoteHow making a hole in PCB that will be filled with solder allows you to inspect what happens underneath?

author: jpanhalt
author_rank: Super Contributor
timestamp: August 22, 2024, 09:29:31 am
content: Quote from: wraper on August 22, 2024, 09:21:21 amDoes reflow with solder paste allow one to do that either? I like the idea of adding one or more vias to allow some level of inspection.

author: wraper
author_rank: Unknown
timestamp: August 22, 2024, 09:58:30 am
content: Quote from: jpanhalt on August 22, 2024, 09:29:31 amMy point is that neither allows it. Than why jump through the hoops with vastly inferior soldering process? There is zero inspection possible through the hole, not to say you'll likely get bad center pad solder wetting due to inferior heating. Chances are that hole will be filled with solder but won't even attach to the IC pad or just barely in the middle.

author: Smokey
author_rank: Super Contributor
timestamp: August 22, 2024, 10:00:52 am
content: Quote from: jpanhalt on August 22, 2024, 09:29:31 am... you mean vias that will wick the solder away from the pad and make the joint less reliable?If you are doing a small number of boards, tap the top of the chip with your tweezers as it's reflowing (hot air or hot plate or whatever). You can feel if the big central pad is soldered correctly or not based on how the chip moves.

author: zapta
author_rank: Super Contributor
timestamp: August 22, 2024, 04:43:27 pm
content: Quote from: wraper on August 22, 2024, 09:21:21 amIt's not just avoiding the hot air station but mainly stencil, and solder paste. The hot air station is easily replaced by the Miniware MHP30M.Quote from: wraper on August 22, 2024, 09:21:21 amI will know better once the boards will arrive and I will give it a try but I expect that I will be able to observe the wetting of the bottom ground tab.

author: wraper
author_rank: Unknown
timestamp: August 22, 2024, 06:34:06 pm
content: You don't need stencil to solder it with hot air, not even solder paste. You only need to apply solder by soldering iron and then solder with hot air using tacky flux. Your idea should sound absolutely ridiculous for anyone who is used to soldering QFN.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 22, 2024, 07:34:05 pm
content: @wraperHow about some photos with X-rays after your porcedure w/o paste. What advantage does tacky flux offer?

author: Smokey
author_rank: Super Contributor
timestamp: August 22, 2024, 08:22:18 pm
content: Quote from: jpanhalt on August 22, 2024, 07:34:05 pm> How about some photos with X-rays after your procedure w/o paste.If you are x-raying your boards you shouldn't be hand doing anything. Just get a professional to handle all this. We are talking about prototypes and prototype assembly. For first hand built articles, I do the "apply solder + flux to the pads and reflow" for tiny parts pretty often.> What advantage does tacky flux offer?One thing is it doesn't immediately evaporate. It makes a little puddle that keeps cleaning the solder joints as they reflow.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 22, 2024, 08:27:40 pm
content: @Smokey,Thank you Don Quixote. @wraper can answer for himself.

author: wraper
author_rank: Unknown
timestamp: August 22, 2024, 09:14:54 pm
content: Quote from: jpanhalt on August 22, 2024, 07:34:05 pm"My" procedure is basically what you do for repairs, but as OP does not want to use paste/stencil it seems the most suitable. Tacky flux is a type of flux you must use for reflow soldering if you do not use solder paste and it provides long working time. If you X-ray it, most likely it will be better than with solder paste because there will be little to no voids under center pad (generally ~25% voiding is considered normal). And it certainly will be better than ridiculous nonsense with using soldering iron from opposite side (while having proper tools) that most likely won't even heat a whole pad enough to allow solder wicking into the gap. Not to say QFN actually requires some gap between IC package and PCB filled with solder. If you lay it directly on PCB and then apply solder around and to the center from opposite side, it will be a shit job because there will be basically no gap. Also if you solder around QFN with soldering iron, how do you know solder has actually wicked under the chip?

author: jpanhalt
author_rank: Super Contributor
timestamp: August 22, 2024, 10:30:33 pm
content: Quote from: wraper on August 22, 2024, 09:14:54 pmEveryone else bring data.

author: thm_w
author_rank: Super Contributor
timestamp: August 22, 2024, 10:31:43 pm
content: I wonder how a non-plated hole would compare to the plated, it might let you inspect without wicking away solder?But usually just looking at the IC from the side is enough to tell if its flat enough, once you have some experience.Of course we are not talking about x-rays here as this thread is about hobby level assembly.

author: wraper
author_rank: Unknown
timestamp: August 22, 2024, 10:45:50 pm
content: Quote from: jpanhalt on August 22, 2024, 10:30:33 pmIt's interesting you don't have x-ray inspection issue with hole under QFN approach.

author: wraper
author_rank: Unknown
timestamp: August 22, 2024, 10:50:22 pm
content: Quote from: jpanhalt on August 21, 2024, 12:11:46 pmProbably because those are an oddball product for people who cannot do normal soldering to save their life.

author: dietert1
author_rank: Super Contributor
timestamp: August 23, 2024, 06:23:20 am
content: https://www.eevblog.com/forum/beginners/smd-soldering-for-non-production-(amatuer-one-offs-etc)/msg5589217/#msg5589217For prototypes i often use ready made SMD adapters and i use a soldering iron to wet all pads, filling the vias in the thermal pad and providing enough flux. It's easier with leaded solder. Then i use the heat gun to solder the chip. The advantage of the heat gun over a fine soldering iron tip is a lower temperature limit. If you run the heat gun at 270 °C, the chip won't get damage.I second the recommendation to work looking at the chip and using some tool for a little movement/position adjustment.Regards, Dieter