Issues with soldering QFN components in reflow - Page 1
Forum section: electronics / general

author: richardmartino
author_rank: Newbie
timestamp: May 18, 2021, 04:53:40 am
content: Hi all,I've been having trouble soldering some QFN packages with exposed pads on my boards. I got a stencil and have been using ChipQuik SN63 solder and I'd like to think I have been applying the paste properly? See attached picture, one of my paste-ing jobs.I have been using both a reflow plate and a toaster oven I got from Amazon. Both of them have reflowed the solder nicely, I just can't get the devices to respond to programming (they are ESP32 Pico D4). I'd say it was a design issue, but I've gotten 1 out of 4 to take a program. Not only does it take a program, it works as expected for the most part.Does anyone have advice for a beginner to reflow soldering?

author: ataradov
author_rank: Super Contributor
timestamp: May 18, 2021, 05:21:26 am
content: Your smaller device has more paste than necessary, but generally this does not look too bad. Pads do not need that much paste. The exposed pad itself is typically small, and the excess solder tends to lift the chip.The common solution in a non-production environment is what Louis Rosmann is doing - hot air on the chip until it reflows, press on the chip so that excess solder squeezes out, collect it with soldering iron, reflow again to finally seat the chip. It is labor intensive, so for production you tune the amount of paste.Have you checked for shorts? Do you have pictured of the solder joints? Do they look ok?If you have one reference board, it should be easy to tell what is wrong with the others.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: May 18, 2021, 06:15:41 am
content: It looks like you pushed in the paste instead of holding the squeegee at 45 degree angle and moving across the board resulting in too much of paste on the pads.

author: richardmartino
author_rank: Newbie
timestamp: May 18, 2021, 03:12:44 pm
content: Quote from: ataradov on May 18, 2021, 05:21:26 amOn visual inspection, it is kind of hard to see where the issue is. I think the issue might be too much solder on the EP or other pads. The joints look good, but I do not have an X-ray inspection deviceDo you think the cheap-o 858D would be powerful enough? I tried it before and actually did the press down method, but that was before I bought a Hakko soldering iron with a plethora of fine tips, so I might try that again. Attached is a picture of a not working board.Quote from: 48X24X48X on May 18, 2021, 06:15:41 amI will try getting less paste on there! Thank you!

author: thinkfat
author_rank: Unknown
timestamp: May 18, 2021, 03:53:25 pm
content: It is hard to see, but I think the result after reflow is not OK. It seems the fillet has not formed as it should. C13 for example looks like cold joints. Does your toaster do any temperature profile? Or do you just heat up until the solder melts and then switch off? It looks a little like it took too long to heat up and the flux had evaporated before reaching peak temperature.

author: richardmartino
author_rank: Newbie
timestamp: May 18, 2021, 04:05:05 pm
content: Quote from: thinkfat on May 18, 2021, 03:53:25 pmNo reflow profile, that is exactly what I did. That sounds like that is exactly what happened with the flux, I was watching it happen. When I was using the reflow plate I heated it up to 180 and then increased the temperature to 240 (for the Sn63). I did reflow that one, in an attempt to save it, but when it came out it was shiny.

author: ataradov
author_rank: Super Contributor
timestamp: May 18, 2021, 04:06:12 pm
content: Quote from: richardmartino on May 18, 2021, 03:12:44 pmFrom the top view they don't look too bad, but it is hard to tell without looking up close at an angle. There is definitely more solder than necessary.Quote from: richardmartino on May 18, 2021, 03:12:44 pmYes, sure.

author: ataradov
author_rank: Super Contributor
timestamp: May 18, 2021, 04:08:45 pm
content: I think it looks this way because pads are way bigger than necessary for the component size. Those are the pads for hand soldering. But again, it is hard to tell without good close up pictures.

author: jmelson
author_rank: Super Contributor
timestamp: May 18, 2021, 04:40:15 pm
content: I think you have too much paste, especially in the thermal pad (center) areas. I always reduce the size of the stencil apertures, and the finer the lead pitch, the more I reduce them. Also, I typically use a .003" thick stencil.For those big thermal pads, the pros usually use a "window sash" grid with bars across the opening to reduce solderquantity and also provide channels for air to escape.Especially for QFN, you need the absolute minimum solder amount on the pads so it doesn't squish out and cause shorts.Take one of your bad boards and check for shorts between adjacent pins and pins to ground pad. I bet you will fine multiple shorts.A trick is to solder sewing needles to probe tip sockets so you can poke the edges of the pads.Jon

author: Feynman
author_rank: Regular Contributor
timestamp: May 18, 2021, 04:57:19 pm
content: As others indicated: Most likely there is too much paste on the EP. Too much paste on the EP connects to one giant solder ball during re-flow. The surface tension of the solder ball might be strong enough to tilt the chip to one direction causing some pins to not connect to their pads. The joints might look fine from above. You should either look at them from an angle or - as a test - manually re-solder all joints with an iron and enough flux.Apart from reducing the size of the stencil openings you can also reduce the thickness of the stencil.

author: richardmartino
author_rank: Newbie
timestamp: May 18, 2021, 06:23:31 pm
content: Quote from: jmelson on May 18, 2021, 04:40:15 pmI did that with some of the broken out pins, like the SWD interface and found nothing actually, but then again I don't have the sharpest tipped meter. I will try the sewing needle trick, I love that. Yeah I think the next revision of this board is going to have some changes to the stencil for sure like you mentioned. Attached is my stencil layer from KiCAD.Quote from: ataradov on May 18, 2021, 04:08:45 pmYes, I used hand solder footprints for the 0603 components, I was originally going to hand solder them, especially with the shortage of components I was going to leave some out and put them in manually with my iron as I got them in. My issue thankfully isn't with the 0603 passives but my poor attempts at the QFNsAt least at the end of this I will have hopefully learned something, like never use QFN on prototypes again!

author: ataradov
author_rank: Super Contributor
timestamp: May 18, 2021, 06:59:08 pm
content: For the amount of solder paste on the pad, in the big one you can safely remove the outside rectangle, just keep the inner 4 dots. This would be plenty. For the smaller device, cut the size of the rectangle in half.You need to adjust the size of the paste window to the size of the actual pad on the device, not the pad on the PCB. Typically exposed pad is smaller in size, and not always square. But if you try to reflow a lot of paste to a small area pad, it will bunch up and bulge.But also, once you get used to them, QFNs are way easier to work with than QFPs. Those boards would be easy to rework with hot air. So I'd rater use that as an opportunity to get some training.

author: richardmartino
author_rank: Newbie
timestamp: May 18, 2021, 07:43:58 pm
content: Quote from: ataradov on May 18, 2021, 06:59:08 pmYou are probably correct. I got another one working! I used less paste, it makes sense given how expensive the stuff is.

author: notsob
author_rank: Frequent Contributor
timestamp: May 18, 2021, 10:55:57 pm
content: Try acupuncture needles

author: tooki
author_rank: Super Contributor
timestamp: May 19, 2021, 04:55:46 pm
content: Quote from: richardmartino on May 18, 2021, 07:43:58 pmCost is not the reason for avoiding excess solder. It’s that excess solder makes for inferior solder joints and can cause significant problems. Plus, excess solder can make it impossible to properly inspect a joint.Whether as wire or paste, the cost of solder is insignificant at the end of the day. It makes up but a tiny part of the product cost, since components and boards cost a lot more.. (Paste just has the big downside of having a very short shelf life. So whereas a hobbyist can buy a big roll of solder and use it over the course of a decade or more, they cannot buy paste in bulk to get the savings of larger containers.)

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: May 20, 2021, 01:56:26 pm
content: Regarding your reflow system, I HIGLY recommend a Controleo conversion kit for a toaster oven. I've been using them in daily production for a couple years, and just decided to make a small army of them instead of buying a tunnel oven.For about $300, you have a great reflow solution.

author: asmi
author_rank: Super Contributor
timestamp: May 20, 2021, 07:41:49 pm
content: Paste is expensive? Really?In addition to what was already said here, try using thinner stencils if possible. I recommend 0.1 to 0.12 mm thickness for small pitch parts (or BGAs).

author: DIYGUY Chris
author_rank: Contributor
timestamp: May 22, 2021, 10:16:17 pm
content: Hummm are you sure you've used stencil for this! because if I consider the image that you've dropped on the main post it is a bit messy out there (especially the pads of the MCU).As some users said, next time get a thinner Stencil and get a low temperature solder paste (suitable for hot plate soldering) and also after finishing the assembly it is recommended to inspect the pads if there are any joints or shrted pads you can remove it using a solder iron some flux and twisted copper wire.Never forget to clean the board with some flux removal solvent after finishing the assembly.Good luck.

author: mon2
author_rank: Frequent Contributor
timestamp: May 22, 2021, 11:28:12 pm
content: Are you using a stencil holder and manual (hand) printer? If not, it is recommended. You should be able to pick up one on Aliexpress or similar.Suggestions are:1) watch the thickness of the stencil - too thick = too much solder paste on the pads and is not good and will lead to shorts during the heating process2) watch the pressure of the squeegee blades - for the same reason as above. Once the paste is mixed, only should require 1-2 passes of the blade to deposit.3) in general, you want to be able to see clearly between each pad that you have printed to prevent shorts in the oven4) consider the paste you are using - highly highly recommend Loctite GC10 paste that we use for our SMT production. It is a magic paste with a no clean chemistry. This is a halogen free; ROHS / Pb-free solder paste. We use it on our Ekra SMT printer but have also used the same paste with a manual print process. We used the squeegee blade and the SMT stencil (jlcpcb.com) for some funky shaped PCBs we could not shuttle into the machine via the rails.5) a good stencil shop will know what are the best aperture openings / reductions for each SMD component.JLCPCB.COMis good for low cost stencils.Better shops are (we have used each vendor listed here many times):Stone LiTopbest Technology Limitedwww.topbest-cn.comAdd: 18# Lingbei 5th Road,The first industrial park,Fenghuang,Fuyong Street,Bao'an District ,Shenzhen 518103, ChinaCell phone: +86 -17819006069Email: stone@smtsite.comShenzhen Power Stencil Co., Ltdstencil@powerstencil.com<< Loctite datasheet is attached >>You can buy this solder paste from Adtool in Quebec who sells their product line - or perhaps your local Loctite distributor.

author: MarkR42
author_rank: Regular Contributor
timestamp: May 24, 2021, 10:05:54 pm
content: I have put down the esp32-pico-d4 chips with paste and hotplate by hand,My first design was a total fail, using the standard-ish land pattern on the pcb. I put down three boards and only one even slightly worked, and even that had some problems then failed completely.So I respun the board witha) Much longer pads for the qfn pads. This is super handy as it means it is much more tolerant of too much paste - the extra paste has somewhere to go. I made these something like 0.6mm longer than standard, which doesn't sound like a lot but really helps.b) Smaller centre padThis helps because it dramatically reduces the probability of an invisible, unfixable short from the centre pad to one of the other pins. It also maybe means there's enough space between the centre pad and the outer pads, for a bit of solder mask to appear, which I think also raises the chip up by a tiny amount, perhaps enough that it helps the excess solder to not create bridges.Now with the modified footprint, it works much, much better. I didn't even use a stencil and I've been able to put down a few working ones.

author: Matt-Brown
author_rank: Regular Contributor
timestamp: June 07, 2021, 04:06:37 pm
content: Board should really be ENIG for QFN packages (much flatter for paste and reflow). Then y,ou can use a 0.005 mil (thou) stencils, round all corners on the stencil and ap reduction of 5% per side on power pads anbd windowpane the power pads.

author: Smallsmt
author_rank: Frequent Contributor
timestamp: June 08, 2021, 07:25:38 am
content: There are a lot of good advices shown above.1. Center pad paste should only cover 30 to 40% of area using 0.1mm stencils because too much tin will push the IC up during reflow.2. Keep QFN IC dry and check if you unsolder the non working parts if pads are covered by tin or maybe ther is a thin white corrosion preventing pins from soldering.3. Your paste print is not clean and paste on outside pins need to be reduced too.4. Extending outside pin pads is a good solution if there are no RF relevant things to keep them short.https://www.surfacemountprocess.com/a-guide-to-effective-stencil-design.html