Rate my paste job - Page 1
Forum section: electronics / general

author: liteyear
author_rank: Regular Contributor
timestamp: August 13, 2023, 04:09:33 am
content: After decades developing habits motivated by expediency, I've been taking the opportunity to slow down and take notes.I happen to have a stencil, some paste and lots of spare boards to experiment with some pasting techniques. So I present to you, warts and all, my initial results for criticism. The feedback cycle for good/bad paste is usually so long, and the early signs potentially misleading, that I wonder if you can point out features that would concern or not concern you, right at the point of applying the paste.Already I have some ideas for round two, but I'll keep them to myself to see what you have to say first.Attached are collages of like-for-like shots of the results of pasting 4 boards, one after the other. Also included in each collage is a photo of the stencil and paste prior to (pj1 through pj3) or after (pj4) application. All were pasted with a single pass (plus a small partial pass for pj3), but the pass was in the opposite direction for pj4.Paste is Chipquik SMD291AX250T3 (leaded, T3). Stencil is framed, but notice it has bowed (I've pasted about 60 boards with it). The QFP-32 footprints (U2 and U9) that have caused me a lot of trouble 0.8mm pitch. CN3 and CN4 are 0612 and 0.8mm pitch as well. Passives are 0805. Stencil is 0.15mm by JLCPCB, so hassolder beading treatment.I've scaled the 200MB of originals right down, but they're still 2.2MB each, so be patient and excuse the double post to fit them all.

author: liteyear
author_rank: Regular Contributor
timestamp: August 13, 2023, 04:10:23 am
content: Boards 3 and 4.

author: gnuarm
author_rank: Super Contributor
timestamp: August 13, 2023, 05:23:33 am
content: I'm not sure what you are doing wrong, but looking at U2 only, I see the paste is not always well positioned. It also appears there is a bit too much paste.I'm thinking your stencil is not well positioned. It is crucial to have good positioning to prevent shorts, which you seem to have. It is also important to not have too much paste.The excessive paste can be reduced by reducing the size of the openings for the paste on the pads.What do the boards look like after soldering?

author: fastbike
author_rank: Regular Contributor
timestamp: August 13, 2023, 06:40:48 am
content: I have a jig to position boards and stencils. I've repurposed some parts from a woodworking jig that sets out holes on a 96mm grid. The pins fit into a 3mm hole in an MDF baseplate, with a 6mm hole in the rails of the PCB. The stencil has a 10mm hole. I've not got a diagram, but imagine the PCB place on the base plate, the pins inserted to align the PCB, and then the stencil placed over the top.

author: gnuarm
author_rank: Super Contributor
timestamp: August 13, 2023, 07:26:14 am
content: Quote from: fastbike on August 13, 2023, 06:40:48 amYou've not remotely provided enough information to see what you are doing right or wrong. I don't particularly care how you are doing it. Look at the results and tell me this is working. You have solder paste bridging pads. No? Look at pins 18 and 19 on U2. Pins 17 and 18 on U2 in another image. U9 has shorts all over the place. PWR_IN is very shy of solder paste.Can't you see any of this?

author: ebastler
author_rank: Super Contributor
timestamp: August 13, 2023, 07:44:06 am
content: Quote from: gnuarm on August 13, 2023, 07:26:14 amThat post you replied to was not by the OP. The jig fastbike described was meant as a suggestion to improve stencil positioning, I think.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: August 13, 2023, 08:33:31 am
content: @liteyearthanks for sharing.How thick in millimeters is the stencil? Too thick means too much paste in the well. As@gnuarmsays, smaller holes means less gunk.Anyone here have their own magic numbers for stencil thicknesses?For home stencilling, one hack is to locate the stencil with the mounting holes in the pcb. Bolt the stencil and pcb together - provided this does not obstruct the spatula. And then check location with a magnifier.Also clean every surface thoroughly with isopropyl alcohol first.

author: gnuarm
author_rank: Super Contributor
timestamp: August 13, 2023, 08:37:22 am
content: Quote from: AndyBeez on August 13, 2023, 08:33:31 amAll of this is why I have my boards, including prototypes, made professionally. They handle all the bits that are not at the Gerber level and some that are. So they get responsibility for it working if it's not my design error.I thought JLCPCB or whatever their name is, did assembly? But I guess they only work with the parts they sell.

author: jmelson
author_rank: Super Contributor
timestamp: August 13, 2023, 03:08:12 pm
content: Alignment is fine, but the amount of paste is WAYYY too much! I always reduce the stencil apertures on fine-pitch parts to maybe 60% the area of the pad. The finer the pitch, the more you have to reduce aperture area to prevent solder bridges. The best pastes (Loctite GC10) are a big help. Also, I always specify a .003" *** correction *** (0.075mm) stencil thickness. I just did a batch of boards with similar parts and had excellent reflow results.Jon

author: gnuarm
author_rank: Super Contributor
timestamp: August 13, 2023, 04:08:49 pm
content: Quote from: jmelson on August 13, 2023, 03:08:12 pmI don't think alignmentisfine. Some of the images shows the solder on U2 and U9 so far misaligned, the paste is bridging the gaps between pins. You think this will fix itself and not create bridges? I also see the problem on CN3 and CN4.The registration is off in a different direction in nearly every image.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: August 13, 2023, 05:09:59 pm
content: I just had a look at some other home/DIY manufacturing forums and there seems to be a 'rule of thumb' that hole sizes are reduced by 10% to 15% of the copper pad to allow for lateral slump when the stencil is lifted - which kind of makes sense. Especially when work is carried out in the kitchen sink.Stencil plates range in thickness from 0.150 to 0.120mm depending on the smallest package/pad size. A thickness of 0.125mm is regarded as a 'general purpose' stencil.Quite interesting:https://www.7pcb.com/blog/stencil-thickness-calculationsQuite very interesting:https://www.surfacemountprocess.com/a-guide-to-effective-stencil-design.html

author: jmelson
author_rank: Super Contributor
timestamp: August 13, 2023, 08:55:06 pm
content: Quote from: gnuarm on August 13, 2023, 04:08:49 pmIf the amount of solder paste is correct, and the pads are the right size, then the solder WILL pull together and mostly avoid bridges. Yes, proper alignment of the stencil is helpful, but the right amount of solder is MOST important, in my experience. Also, really good paste like GC10 fixes almost all of these issues.Jon

author: gnuarm
author_rank: Super Contributor
timestamp: August 13, 2023, 09:14:09 pm
content: Quote from: jmelson on August 13, 2023, 08:55:06 pmI don't do hobby work much. I do this professionally. I pay experts to do the manufacturing of my boards. I can assure you, they don't use soldering techniques that "mostly avoid bridges". When they are building 1,000 units a week for me, having a solder bridge on 1 in 10 boards is a production killer.But, the details of assembly are not my domain. I pay people to handle that for me. I know the people I'm paying will lose money if they have too many problems, so they are going to nip problems like this in the bud, so it does not become a production problem.

author: liteyear
author_rank: Regular Contributor
timestamp: August 14, 2023, 12:37:20 am
content: Quote from: jmelson on August 13, 2023, 03:08:12 pmThis is exactly the kind of subjective assessment (and argument!) I was looking for, thank you. Ultimately pasting needs to do two things: put paste in the right spot, and in the right amount. But the world is not black and white, and how to assess these goals have been met is hard to put your finger on.If I can summarise the categories of suggestions and justifications kindly provided so far, I think they are:Better alignment - prevent shortsBetter paste - a variety of things like less slumping and more consistent releaseSmaller apertures - less pasteThinner stencil - less pasteI've discussed the aperture, paste and thickness aspects quite a bit (good example of the firsthereand the last twohere) so will try not to repeat myself too much. Just to say that in my experience, the smudging/alignment has not seemed to cause issues so far, but the excess paste has. Though sometimes it is difficult to separate effects. Interestingly, unlike U2 and U9 (TQFP-32's), CN3 and CN4 (and CN1 and CN2 not shown) haveneverhad an issue in 50+ boards, which surprised me. The unspoken advantage of leadless packages, perhaps.For your reference, here are links to what I used foralignmentandapertures, and the stencil is 0.15mm, electro-polished, stainless steel. Everything is subject to change, but I'm taking a rare opportunity to be careful to try to control for variables.Quote from: gnuarm on August 13, 2023, 07:26:14 amThis comment was misdirected, but a good opportunity to re-iterate my goals here: the results are presented warts-and-all, without comment, to see which features people would consider of concern.Stay tuned for round 2!

author: Smokey
author_rank: Super Contributor
timestamp: August 14, 2023, 02:06:02 am
content: Rate my paste.....4 out of 10.... would not paste...

author: liteyear
author_rank: Regular Contributor
timestamp: August 14, 2023, 04:29:07 am
content: Well round two is a bit more conclusive. The only variable I deliberately changed was to do multiple passes, the last of which was at a steeper angle to scrape the excess off the stencil. I hypothesised this would reduce the thickness of the released paste.See for yourself. Original on the left, new on the right. pj1 is first board, pj4 is last.I was always surprised to find the alignment, although similar, wasnotconsistent between rounds.

author: MR
author_rank: Regular Contributor
timestamp: August 14, 2023, 05:18:25 am
content: Way too much paste...

author: liteyear
author_rank: Regular Contributor
timestamp: August 14, 2023, 09:43:24 am
content: In round 3, I changed two variables: I did a board-by-board alignment with a magnifying glass (finding the apertures were often *bigger* than the pads!) and switched to a plastic squeegee.

author: liteyear
author_rank: Regular Contributor
timestamp: August 14, 2023, 09:53:48 am
content: And lastly, in round 4, I switched to ChipQuik TS391AX (leaded T4) out of a syringe. With the syringe, I pushed out a couple of selectively placed beads, rather than dumping it all at one end. But after the first board it was all smeared out anyway.

author: liteyear
author_rank: Regular Contributor
timestamp: August 14, 2023, 10:30:52 am
content: So after all that, I'm inclined to conclude, given that none of my variations resolved the issue of too much paste, that the stencil is not suitable. Do you think that's a fair conclusion?What big impact variable would you tweak next?Aperture size?Stencil thickness?The GC10 silver bullet?Something else?For some extra data, I've used this stencil to paste about 55 boards that went through population, reflow and test. The first 10 had very frequent bridges, almost always on the TQFP-32 parts. Even this was much better than an earlier spin, where I used the much larger, manufacturer recommended land pattern pads that were a nightmare to rework. With care and finesse (maybe most significantly, only a single pass, and being happy to wipe it off and start again if it doesn't go right), I steadily got this bridge rate down over the next 45 boards to the point where the slight majority of boards didn't need any rework. Still not acceptable to me, but a huuuuge improvement.And FWIW, my opinion on the four next-step variables are:Aperture size feels like a cure-all - I'm not comfortable with how much of the pad (and more!) the stencil reveals, and I've long heard from factories that a % reduction is pretty much mandatory for stencils. Unfortunately, I realise now my PCB fab has beenselectively ignoring my paste layerfor years, so there's a bit of learning to be done to get the best out of them. Meanwhile I've been working on cutting my own stencils using a vinyl cutter, for precisely this ability to fast track the design iteration loop, but it's not ready for prime time yet.My thoughts on thickness arehere(basically, industry standard is well and truly 0.15mm for this pitch, and I'm starting to think it's a furphy).I'm reluctant to go for a super paste, because I'm worried about being reliant on an expensive, single/few source magic fix.Could be anything from warming/cooling paste, to sucking the stencil to the PCB, to sacrificing a chook before sunset. I'm starting to believe (hope!) that I've got a handle on most of variables, and there's not too many chooks to go.But definitely keen to hear your thoughts.

author: MR
author_rank: Regular Contributor
timestamp: August 14, 2023, 10:45:44 am
content: you have a few options, make the stencil thinner or the aperture smaller

author: Psi
author_rank: Super Contributor
timestamp: August 14, 2023, 10:53:32 am
content: Seems like1) Too much paste2) Paste is too wetSometimes the 'paste too wet' thing happens because the paste tub has been sitting for ages and the flux part migrates to the top of the tub leaving the bottom paste too dry and the top paste too wet.Like the oil in a jar of peanut butter.Other times it can be bad paste, crap brand/quality etc..Well done with the stencil alignment through, that can be tricky to get right.I use metal pins through my stencil and PCB.

author: SMTech
author_rank: Frequent Contributor
timestamp: August 14, 2023, 12:06:59 pm
content: This may or may not help for comparison. This is a manually printed board using a basic clamshell printer with XYY adjustment allowing for alignment through the apertures with the board beneath. The stencil is 0.12mm framed from PCBWay and is 1:1 with the pads on the board. The board is fairly low tech 0805 and SOIC so its not exactly challenging, the reflowed board has decent fillets and no signs of excess paste or solder balling. The print itself looks fine to the naked eye, crisp and well defined but once you zoom in on these photos you can see a small amount of "seepage" on some pads, indicating poor gasketing, this was the last PCB of about 40 prints, it probably needed a wipe on the underside more frequently than it was getting.Squeegee blade is stainless steel and the board is printed in one smooth fairly rapid pass at about 60degrees, the stencil is basically shiny behind the blade pass.

author: liteyear
author_rank: Regular Contributor
timestamp: August 14, 2023, 02:14:41 pm
content: Very interesting. You've inspired me to have another go! This time I stirred the paste tub right from the bottom, wiped everything down with a paper towel after cleaning with IPA, and used firm even pressure with the stainless squeegee to more or less clear the stencil as I made the pass.I've attached a bit of a highlight, but even still the slumping is evident. D1 in particular looks like a bit of a smoking gun - when the middle collapses like that the paste has gotta go somewhere right? Could explain why they all look a little pregnant.On the other hand, your paste looks to have held its shape very firmly, at least at that zoom level. The extra 0.03mm height of my stencil could make the tower a little more unstable (it's 25% taller after all), but it seems hard to believe it's the dominant cause.Of course, I'm not sure exactly what effect it has at the end of the day, and I suspect small/shorter apertures will gloss over most ill-effects anyway. And to be fair, apart from the infernal TQFP's I haven't really had too many ill effects, but it does look ugly.What paste is that?

author: jmelson
author_rank: Super Contributor
timestamp: August 14, 2023, 03:11:27 pm
content: Quote from: gnuarm on August 13, 2023, 09:14:09 pmMany people would think I'm insane, but I have a $150K P&P machine in my home basement. (I bought it at auction for $500, but then that was the smallest outlay to get it set up.) I'm sort of bridging the gap between hobby and small scale production. I just did a batch of 32 small boards with 67 parts/board, and very similar part types to the OP's boards. TQFP44, 0805 passives and other stuff. I had ZERO solder bridges. And, I used a home-made unframed stencil visually aligned with a hinge of masking tape. But, I do reduce aperture size with a .003" thick stencil, and swear by Loctite GC10. I HAVE had TONS of soldering problems with some other name-brand solder pastes in the past. The only thing that good was from Warton Metals in the UK, that I paid an arm and a leg to import to the US.Now that PCBWay will make stencils for $25 each, I just let them do that job for me.My old P&P machine was not as accurate as my new one, and that did cause some issues back then.Jon

author: gnuarm
author_rank: Super Contributor
timestamp: August 14, 2023, 03:24:10 pm
content: Quote from: jmelson on August 14, 2023, 03:11:27 pmIf you can get that to work affordably, great. My previous boards used 0603 passives and 0.5 mm TQFPs. Now I will be working with 0402 passives and BGAs.I do the parts I'm good at, and pay others to do the parts they are good at. Besides, there's no way I can produce 1,000 units a week with a home setup.The design I'm presently working on will have some $ 100 profit on each board. Once I'm done with design and test, and have completed the test fixture, I will be able to receive purchase orders, write a purchase order and do none of the real work myself. The ideal company!

author: ebastler
author_rank: Super Contributor
timestamp: August 14, 2023, 05:15:49 pm
content: Quote from: liteyear on August 13, 2023, 04:09:33 amA bit late, but I did not see the following mentioned yet: In your original set of pictures you also showed what the stencil looks like after you had applied the paste. I was surprised to see what appears to be a rather thick layer of paste all across the top of the stencil, at least for PCBs 2, 3, and 4.This suggests that you did not really push the squeegee down enough. If you leave such an extra layer of paste on the stencil, the same extra paste thickness will be present on all your PCB pads -- i.e. the stencil thickness does not actually define how much paste you apply. After you have pulled the squeegee across, the stencil surface should be pretty clean.The 150 µm stencil you have is on the thick side, but should be usable if you take care not to leave extra paste.

author: NorthGuy
author_rank: Super Contributor
timestamp: August 14, 2023, 09:13:44 pm
content: Quote from: ebastler on August 14, 2023, 05:15:49 pmExactly. This is the problem that causes you TQFP bridging.Position the stencil so that there's no gap between the stencil and the PCB, then apply the paste so that it fills in the openings in the stencil, but there's nothing above or below the stencil (as in yoir picture for board 1). The easiest way to accomplish this is with two passes. The first pass at about 20 degrees squidgy to stencil, with liberal amount of paste under the squidgy, pushing down hard to make sure the paste fills in all the openings. The second pass at about 80 degrees squidgy to stencil, pushing down hard, removing the excess paste from the top of the stencil. If there's any paste left, repeat the second pass. This will make sure the paste is applied evenly and there's no difference between edges of the pads and their middle. Then remove stencil vertically maintaining the alignment until the stencil is off - for example use a hinge.

author: liteyear
author_rank: Regular Contributor
timestamp: August 15, 2023, 01:45:03 am
content: Quote from: NorthGuy on August 14, 2023, 09:13:44 pmThis makes sense (and was my big high hope variable change between rounds 1 and 2). I think maybe the empirical evidence lead me in the wrong direction though because the stencil developed a bow. So the experiments I ran where I did as you describe had poorer results (smeared deposits) than ones where I only did a single pass and left some solder on the stencil.By the way, I have a lot more experience with frameless stencils. This time I used a frame to try to leverage the benefits of longevity and of a stretched stencil, but to my surprise the best feature is definitely the hinge! Much less nerve racking to lift the stencil in one movement than trying to peel a floppy one up. On the other hand, the stretching has sort of back-fired now that it is bowed and there's not much I can do about it.

author: Psi
author_rank: Super Contributor
timestamp: August 15, 2023, 05:55:50 am
content: Quote from: jmelson on August 14, 2023, 03:11:27 pmAgreed 100%I've also used loctite paste and always worked well.Kester also worked well for me.Chipquik was very hit or miss. Usually miss.

author: SMTech
author_rank: Frequent Contributor
timestamp: August 15, 2023, 07:12:02 am
content: Quote from: NorthGuy on August 14, 2023, 09:13:44 pmStrongly disagree, this first pass is causing you problems, you want one clean pass. If the paste is exercised/stirred properly (and not straight of of the fridge) and rolling in front of the squeegee, it will fill the holes in one go, your variables are speed and pressure. The pressure is largely determined by the size of the paste roll.

author: jmelson
author_rank: Super Contributor
timestamp: August 15, 2023, 04:42:55 pm
content: Quote from: NorthGuy on August 14, 2023, 09:13:44 pmYes, this almost exactly describes my system.Jon