Stenciling an oddly shaped board (was: small quantities of GC10?) - Page 1
Forum section: electronics / general

author: djacobow
author_rank: Super Contributor
timestamp: August 13, 2019, 12:51:25 am
content: Currently using TS391SNL50 for lead-free home reflow. Lots of people seem to think GC10 is the bee's knees. Would like to try it, but am not thrilled about going in for 500g just to see if it is any better.Is there a way to get smaller quantities in the US?

author: pisoiu
author_rank: Regular Contributor
timestamp: August 13, 2019, 08:11:02 am
content: I made my tests with this one:https://eleshop.eu/loctite-gc10-solder-paste-20g.htmlThey also have galden if you are interested in vapour phase reflow.

author: djacobow
author_rank: Super Contributor
timestamp: August 13, 2019, 02:28:14 pm
content: Thanks! If they ship to the USA, I'll give it a go.

author: djacobow
author_rank: Super Contributor
timestamp: August 13, 2019, 04:01:06 pm
content: Quote from: djacobow on August 13, 2019, 02:28:14 pmOh well, they want 80 euro to ship to California. For that, I might as well get 500g. :-(

author: mrpackethead
author_rank: Super Contributor
timestamp: August 13, 2019, 11:57:21 pm
content: Quote from: djacobow on August 13, 2019, 12:51:25 amAre you intending on using it with a stainless stencil? If you are lookign to use it for hand application ( ie, dispenseing with a needle ) then you will be dissapointed. The attributes that make it good for stenciling make it bad for needles.

author: djacobow
author_rank: Super Contributor
timestamp: August 14, 2019, 02:02:05 am
content: It is for use with a stainless steel stencil, but hand applied using a credit card, not a stencil printer.There board is a little tricky. 15x15 cm, give or take, with many cutouts which can warp the stencil when pressure is applied if I'm not careful.

author: mrpackethead
author_rank: Super Contributor
timestamp: August 14, 2019, 02:49:56 am
content: Quote from: djacobow on August 14, 2019, 02:02:05 amGC10 exhibits very low 'slump' and I'm not sure if its the right word, but i'd almost say its not very 'mallable' when it comes out of the jar. ( this is why its not good for hand dispense ). it is well worth 'working' the paste before you use it, so it 'softens' up a little bit. If you have a stainless stencil, get a hard edge metal squeegee. The results are so much better. they can be purchased for just a few dollars from aliexpress.

author: TimCambridge
author_rank: Regular Contributor
timestamp: August 14, 2019, 09:01:38 am
content: Quote from: mrpackethead on August 14, 2019, 02:49:56 amMetal "wallpaper scrapers" seem to work well. Around $1. I expect the real thing is better, but $1....Do the experts advocate framed stencils for "difficult" boards? A bit more expensive because of shipping, but still fairly cheap.

author: mrpackethead
author_rank: Super Contributor
timestamp: August 14, 2019, 10:27:58 am
content: Quote from: TimCambridge on August 14, 2019, 09:01:38 amI got some on Ali for under $10. They are really rigid and dont' flex, and have a very sharp square edge. thats really important. Somethign like this.https://www.aliexpress.com/item/32971053685.html?spm=a2g0o.productlist.0.0.731c2254atMjzy&algo_pvid=e3977f8d-3b22-49ab-a79f-00dd41c51544&algo_expid=e3977f8d-3b22-49ab-a79f-00dd41c51544-4&btsid=372c2674-ea48-44b2-9d91-abfdd2b500c7&ws_ab_test=searchweb0_0,searchweb201602_10,searchweb201603_53I dont' bother with unframed stencils, becuase the results are just so much better, its quicker setup, with framed stencils. The stencil held in tension.

author: jmelson
author_rank: Super Contributor
timestamp: August 14, 2019, 07:02:06 pm
content: Quote from: djacobow on August 13, 2019, 12:51:25 amWell, the stuff has quite a long shelf life, and just works SO WELL, that I don't mind paying for a 500 g jar. I've had excellent results with it, after having some pretty AWFUL results with what were supposed to be good brands - Indium Corp., Kester, etc.Jon

author: djacobow
author_rank: Super Contributor
timestamp: August 14, 2019, 08:59:29 pm
content: I broke down and ordered some.The biggest heartbreak for me right now is Vcc/GND shorts under decoupling caps. These are almost impossible to debug and I've thrown out too many boards. I have recovered one board by removing each and every decoupling cap and yesterday I had luck by putting a dab of flux over every cap and reflowing again.I am pretty sure that if my printing technique were better, this would not be happening, but I also can't help but notice that I don't get this problem with leaded, even with my lousy paste printing.We'll see if this works any better. In the meantime, I'll work on better paste application.

author: jmelson
author_rank: Super Contributor
timestamp: August 15, 2019, 07:47:09 pm
content: Quote from: djacobow on August 14, 2019, 08:59:29 pmOdd, I have NEVER had this specific problem! I have had shorts under some chip scale packages, and those were maddening, indeed. But, never under caps or resistors. I do sometimes get balls of solder on the side of small passives, that is where paste got smushed sideways out from the center of the part. but, it has never caused a short.I have done over 2000 boards on my P&P and homebrew reflow setup. I do manual paste application. But, what I have found is to use a THIN stencil, and then reduce the paste apertures below pad size. This is most important with small-pitch ICs, but helps with the passives, too. I use a 0.003" (.075 mm) brass stencil that I make myself with PC board technology. For the passives, I use an aperture about 50 - 75% of the area of the pad. Oh, and I use either 0805 or 0603 passives, and not anything smaller.Jon

author: mrpackethead
author_rank: Super Contributor
timestamp: August 15, 2019, 09:58:04 pm
content: Quote from: djacobow on August 14, 2019, 08:59:29 pmYour lead free paste probably was 'slumping' more. That is something GC10 doe'snt do nearly as badly.. Slumping is when a 'blob' of solder bulges out sideways, when its still 'wet'. However if your doing 0603's / 0805's then you're probabably got a technique issue, my first pick would be that the stencil is not gasketing well against the pcb, and that your possibly useing a flexible squeegee?

author: thm_w
author_rank: Super Contributor
timestamp: August 15, 2019, 10:15:31 pm
content: Quote from: djacobow on August 14, 2019, 08:59:29 pmI've also never seen this problem on 0603.But its easy to diagnose a shorted cap with a thermal camera. Apply voltage across the shorted rail, then look on the camera which component is heating up. Could also use your finger or freeze spray, but thats a bit harder.

author: SMTech
author_rank: Frequent Contributor
timestamp: August 15, 2019, 10:27:08 pm
content: Quote from: jmelson on August 15, 2019, 07:47:09 pmIf that is what is working for you then there is something freaky going on with either your paste process, your pad size or both. That is a crazy reduction and I would expect to see almost no solder fillet on the device if you went down to 50% with a stencil that thin, with a little bit extra added back on for your untensioned stencil.Quite an effective way to reduce paste to compensate for manual printing is to use shaped apertures such as "homeplate", we're only talking 10% or so even then and this shape reduces the amount of paste under the device. That in turn should reduce the potential for shorts under passives and mid-chip solder balls.If you drop the thickness of the stencil too much then you will find that devices with a centre ground/heat pad such as some QFPs will sit on the PCB above the solder and that pad will not be attached. I've had that happen with a 0.1mm stencil with a 0.5mm pitch QFP, didn't know it until I had to take one off, at which point of course I was quite glad, made removing it much easier...

author: djacobow
author_rank: Super Contributor
timestamp: August 18, 2019, 01:59:37 am
content: A jar of GC10 arrived yesterday. The T3 variant seemed adequate, and the price was a bit less, but it was out of stock, so I got the T4 kind, which has slightly finer metal powder.So far, I've done 10 boards with it, and none of them have had the shorting problem that I was seeing on previous boards. In fact, my pasting is not really much better, but even with the little slop I make, with gloops of paste sometimes spanning pads, the solder is balling up as it should and I'm not getting short. I have not tested the boards yet, but without the Vcc/Gnd fault, at least it should not be painful to clean up what issues that there might be. Overall, I'm quite pleased so far.As for my stencil work, I am sure now that the problem is warping of the stencil when I pull it over parts of the board with cutouts. It causes the stencil to lift a bit in spots, causing the deposition of paste in that area to be too deep, and usually sloppy, too. I think my board might be a worst-case scenario for this. Here's a picture of a scrap board inside my stenciling "jig."The issue is not caused by the cutouts on the inside of the board, but on the big voids on the three "corners" where the board is not in contact with the surrounding scrap holders. I think if I got up some board to tape in those spots, my problem might be mitigated.

author: mrpackethead
author_rank: Super Contributor
timestamp: August 18, 2019, 06:47:43 am
content: wow. your up against it on that, with a unframed stencil. absolutely with nothing under teh stencil, thats going to be a challenge.yes, definately pack up some more pcbs in against those edges..

author: spongle
author_rank: Regular Contributor
timestamp: August 18, 2019, 02:30:33 pm
content: For complicated PCB outlines like that, I like to laser-cut a template to provide alignment and support the stencil. 1/16" acrylic is very close to normal 1.6mm PCB thickness..If you don't have a makerspace close by, DirtyPCBs provides a reasonably priced service.

author: djacobow
author_rank: Super Contributor
timestamp: August 18, 2019, 06:04:30 pm
content: Hmm, I'll check into this. Can they do it from the gerbers, or do I have to make a separate dxf for them? It would be super cool to have some kind of think that exactly matched the board outline and thickness.this morning I slightly improved my jig, and it's a bit better. I think I only get a smooshed bit on maybe one or two footprints on one edge now.

author: NorthGuy
author_rank: Super Contributor
timestamp: August 18, 2019, 08:41:55 pm
content: I would order complementary PCBs which would fill all the voids, and I would glue them down.

author: mrpackethead
author_rank: Super Contributor
timestamp: August 18, 2019, 10:19:17 pm
content: Quote from: NorthGuy on August 18, 2019, 08:41:55 pmYeah, thats likely to be the best solution and would be cheap as well.

author: djacobow
author_rank: Super Contributor
timestamp: August 18, 2019, 10:26:14 pm
content: Quote from: NorthGuy on August 18, 2019, 08:41:55 pmInteresting. How much would you enlarge the outline relative to the real board? Zero? 10 mil? Presumably tight is good but interference fit is perhaps going to be too much.

author: jmelson
author_rank: Super Contributor
timestamp: August 19, 2019, 01:53:41 am
content: Quote from: SMTech on August 15, 2019, 10:27:08 pmAh, well, I make these stencils myself, with general PC board etching technology, and I DO get some "growth" of the apertures during the etch. So, at least part of this reduction is to compensate for the over-etching of the stencil.But, I now have the process pretty well dialed in, and it is working quite reliably. I do everything from lots of 0805 passives and some really large regulators and capacitors to 0.5 mm lead pitch quad flat packs, so I have to find a compromise between the fine pitch and the bigger passives.And, I do get a nice solder fillet on the passives.Jon

author: jmelson
author_rank: Super Contributor
timestamp: August 19, 2019, 01:57:18 am
content: Quote from: djacobow on August 18, 2019, 01:59:37 amYes, the GC10 has better wetting and rheology before the reflow than a bunch of other major brand leadfree solders that I have tried.Really makes soldering much easier.Jon

author: jmelson
author_rank: Super Contributor
timestamp: August 19, 2019, 02:02:57 am
content: Quote from: NorthGuy on August 18, 2019, 08:41:55 pmI made a frame with a plate with a 1" grid of tapped 10-32 holes and stepped bars with slots. So, two of the bars are left aligned at 90 degrees, and the other 2 bars are moved to completely surround the board. (One of the bars is pretty short.)This indexes the boards to a consistent position. The top of the bars has a 0.062" step, so that the top of the bar is even with the top of the board. I tape the stencil to the bar with masking tape as a hinge, and manually align with a magnifier to line up the apertures.It all sounds really jury-rigged, but I have done over 2000 boards with this rig.These bars could be re-aligned to odd angles to fit an odd-shape board.Jon

author: NorthGuy
author_rank: Super Contributor
timestamp: August 19, 2019, 03:28:14 am
content: Quote from: djacobow on August 18, 2019, 10:26:14 pmI would probably go for 6 mil. Tighter fit will help stencil alignment. But too tight will make it difficult to take the board out. It always can be corrected with Dremmel if needed.

author: mrpackethead
author_rank: Super Contributor
timestamp: August 19, 2019, 03:46:06 am
content: Tab routing is not 100% accurate. you'll need to allow for the variation in accuracy of both 'PCB's

author: Koen
author_rank: Frequent Contributor
timestamp: August 19, 2019, 11:03:15 am
content: Add two small holes in the PCB. Add them to the stencil. Order dowels 0.05mm smaller. Stack three PCBs, insert dowels, add stencil, print. No machine, no jig, no frame.

author: Harjit
author_rank: Regular Contributor
timestamp: September 10, 2019, 11:13:20 pm
content: @Koen - on the shape @djacobow showed, how would you do that?