SMD at home - Page 1
Forum section: electronics / projects

author: mayor
author_rank: Regular Contributor
timestamp: April 14, 2019, 11:32:36 pm
content: Quick question for you guys. Every time I populate a board for SMD using solder paste, I end up having to do a fair bit of manual clean up with a soldering iron. I get solder bridges, parts in the wrong orientation, and general untidiness. I sometimes use ChipQuick low temp paste, and I've tried paste bought off eBay from China. My technique these days is to heat the board from underneath with hot air (doing from above makes it worse, with parts flying off, even at super low air flow!).Is it technique? Solder paste I use?Attached pic is with eBay paste, and I even used a stencil here. Notice solder bridges, misplaced parts, and like I said, general ugliness!

author: jbb
author_rank: Super Contributor
timestamp: April 15, 2019, 12:15:00 am
content: I get this problem a bit too.It looks like you've got too much paste. Have you looked at your stencil thickness and aperture sizing? (I'm no expert, so I can't offer much advice.) The other thing that can get you is if the stencil bows up a little and the paste oozes out between the PCB and stencil. Have you checked for that?In terms of not blowing the components around, I find that the best results come from using a preheater to warm the board up from the underside. This melts the flux which tends to hold the components down. Then you can come down on top with the hot air tool to finish soldering.

author: james_s
author_rank: Super Contributor
timestamp: April 15, 2019, 12:48:00 am
content: I usually end up with a bridge or two on larger ICs but it's very rare that I have any trouble with other parts. Usually it's a matter of having too much paste, it really takes very little. You could *almost* just skip the paste and just add some flux, using the tinning already on the board in a lot of cases.

author: wraper
author_rank: Unknown
timestamp: April 15, 2019, 12:50:47 am
content: How do you apply the paste? You won't get away with simply applying paste from syringe to QFN pads.

author: mayor
author_rank: Regular Contributor
timestamp: April 15, 2019, 01:49:44 am
content: Quote from: wraper on April 15, 2019, 12:50:47 amStencil!

author: wraper
author_rank: Unknown
timestamp: April 15, 2019, 02:11:32 am
content: Quote from: mayor on April 15, 2019, 01:49:44 amWhat stencil, stainless steel or polymer? It should be positioned precisely, and paste on the nearby pads should not merge. To achieve that, it's much better to use stainless steel stencil and apply paste in one pass.

author: jmelson
author_rank: Super Contributor
timestamp: April 15, 2019, 03:49:43 am
content: Quote from: mayor on April 14, 2019, 11:32:36 pmI use stencils I make here with PC board resist and etch technology. My stencils are etched from .003" brass shim stock material. I reduce the size of most stencil apertures to less than the pad size. The finer the lead pitch on the components, the smaller I make the apertures. If you have too much paste, you get bridges, or even have parts float away on a lake of solder during reflow. I have tried a bunch of pastes, and am now using Henkel GC10, sold in the US by Loctite. Works VERY well.I have some boards that come out at nearly 100% perfect, and some that need a little rework.Jon

author: tautech
author_rank: Super Contributor
timestamp: April 15, 2019, 04:27:56 am
content: There's a bit of good info in this thread:https://www.eevblog.com/forum/beginners/so-surface-mount-it-is/This vid from member paulca impressed me:https://youtu.be/rgcupYOnvBo

author: mayor
author_rank: Regular Contributor
timestamp: April 15, 2019, 08:27:21 am
content: Quote from: jmelson on April 15, 2019, 03:49:43 amI'll look into having the apertures reduced (at JLCPCB). That will probably help! Thanks for the hint on GC10.@wraper: Stainless steel.

author: wraper
author_rank: Unknown
timestamp: April 15, 2019, 11:08:37 am
content: Quote from: tautech on April 15, 2019, 04:27:56 amI'm not impressed at all. And this won't work well on fine pitch parts. Most likely you will get uneven amount of solder on pads and many of them shorted.

author: wraper
author_rank: Unknown
timestamp: April 15, 2019, 11:13:56 am
content: Quote from: mayor on April 15, 2019, 08:27:21 amThen either your stencil is too thick, has too large hole aperture, was misaligned or your paste application method suck so it gets smeared. It would be best if you make a photo of PCB with solder paste applied.

author: mayor
author_rank: Regular Contributor
timestamp: April 15, 2019, 01:19:43 pm
content: Quote from: wraper on April 15, 2019, 11:13:56 amQuite possible my method sucks! Basically:0. Put a few boards down to keep stencil same height as target board;1. Tape boards together so they form a "solid" board;2. Align stencil3. Tape edge of stencil to one of the boards4. Use a hotel card to smear some paste5. "Rotate" stencil out of the wayI admit, it doesn't produce the cleanest application. But I can't say for sure if it's paste, technique, stencil characteristics...

author: Kjelt
author_rank: Super Contributor
timestamp: April 15, 2019, 01:56:31 pm
content: Aaaaaaah QFN packages,I regognize the horror of qfn reflowingI solved my issues with these packages, by changing the default stencil layout, cut the openings down to max 60% of the pads and a bit smaller than the pads.You can also make the pads a bit longer, whatever suits you but I would not recommend putting paste as broad as the pad. Putting on the ic already is enough to cause bridges.Then I also added soldermasking between the pads, not all cheap boardhouses can or will do this but to prevent bridges it is kind of a must with small pitch.Last but not least , don't use much pressure when putting down the ic, let it barely touch the paste and then release the vacuum of your vacuum tool.If you have a P&P machine you do not have that option.

author: Psi
author_rank: Super Contributor
timestamp: April 15, 2019, 02:14:27 pm
content: yep, few tips from my experience doing paste and reflow at home- Use the thinnest paste stencil you can get- Make sure the stencil is flat on the PCB and stays flat, push down relativity hard with the squeegees.- The paste image should be clean with sharp lines. If it's messy then either your stencil moved around during pasting or your paste is bad- Don't use the cheap solder paste, get something good from a good brand and from a good supplier to you know its legit- If you get any little explosions and parts moving or 'popcorn' during reflow then your paste has got water in it. The water is boiling and causing your problem. Get some better quality paste- Ideally use a toaster oven for reflow. If you have to use hot air use a big nozzle and go much much slower than you would for smt rework. Your goal is to slowly bring up the entire pcb to solder temp.- When you place the parts on on the pasted PCB push them down a little. You dont want them sitting on the paste you want them pushed into it.

author: mayor
author_rank: Regular Contributor
timestamp: April 15, 2019, 02:19:56 pm
content: Excellent advice, thanks. I will see what can be done about the stencil.I did wonder about how much pressure to apply when putting the parts on. So it sounds like just a bit of pressure, then. Maybe I've been pushing them down too hard.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 15, 2019, 02:49:42 pm
content: Quote from: tautech on April 15, 2019, 04:27:56 amYeah, this is cute but it won't work for fine-pitch parts, especially when there is no solder mask between the pads. Of course 1.27mm pitch will be no problem. Many low-cost PCB manufacturers don't have the ability to add soldermask between pads for pitches <= 0.4mm, or sometimes even <= 0.5mm (and quite a few not-so-low-cost ones)... and those that can, this is often a costly option. So just take a look at your PCBs with a microscope and see for yourself. No soldermask between pads is a recipe for solder bridges unless you have managed to dispense just the right amount of solder paste on each pad. Doing that manually is difficult. And even so, you may get occasional bridges.As to hot air... yes due to local turbulences (just consider what happens when you blow air onto a surface), very small/light parts will tend to move around, even at the lowest air flow settings. Pure hell. Pre-heating the board will help, but I really suggest using a reflow oven if you're gonna deal with very small parts.I recently had to hand-solder UDFN-6 parts (0.4mm pitch, 1.2x1.0mm case) and it was no fun, even if there were only 6 pins.Finally managed to do it, but I tried different approaches. I didn't manage to get the hot air approach right. It was atrocious. The parts were either blown away outright, or if not, tended to "surf" onto the flux (and thus get misaligned) before the solder balls had a chance to reflow. I eventually did it with a soldering iron (JBC 210) and a very small chisel tip (one of the smallest I think). And since the pads were designed for reflow assembly, they were not quite long enough to get reasonable access for hand soldering, so that was really no fun. (Had to do it quick and had no access to a reflow oven at the time.)

author: mayor
author_rank: Regular Contributor
timestamp: April 15, 2019, 03:35:33 pm
content: OK, so it's not just me :-)I have an IR toaster oven, but never finished a reflow controller project. For home stuff, do any of you eyeball it ? I could stick a thermocouple in there and be the controller!

author: Psi
author_rank: Super Contributor
timestamp: April 15, 2019, 10:59:31 pm
content: Quote from: mayor on April 15, 2019, 02:19:56 pmIt's more important to push them down when you have applied paste with a syringe and have a blob on the pad.If using a stencil the paste coating is usually thin enough that you don't have to push down much.Pushing down a little is good because it makes more contact between the part and the paste and locks it into a depression in the paste. This makes it less likely to tombstone or 90deg rotate during reflow.For getting the stencil flat on the pcb i usually trap unpasted pcbs around the one to be pasted. Like 0-9 on your numeric keypad where 5 is the one to be pasted.If the blank pcbs are the same design and were manufactured at the same time they will be exactly the same thickness and give a perfectly flat surface to use.

author: pix3l
author_rank: Regular Contributor
timestamp: April 16, 2019, 06:34:26 am
content: You could also try using a thinner stainless stencil in order to get less solder paste on the pads. For example PCBWay has four different thicknesses (0.1 , 0.12 , 0.15 and 0.2 mm).

author: Kjelt
author_rank: Super Contributor
timestamp: April 16, 2019, 07:13:14 am
content: Quote from: pix3l on April 16, 2019, 06:34:26 amThe possible problem with that choice is that you can get too little paste on the normal component pads.So in order to facilitate the few "problem" components you take a chance of creating issues with the majority of normal components.

author: tggzzz
author_rank: Super Contributor
timestamp: April 16, 2019, 08:25:45 am
content: Quote from: mayor on April 15, 2019, 03:35:33 pmSaucepan with a glass lid on the gas hob. 5mm of sand in saucepan to spread the heat. Non-contact thermometer is optional.

author: Psi
author_rank: Super Contributor
timestamp: April 16, 2019, 11:58:16 am
content: Quote from: Kjelt on April 16, 2019, 07:13:14 amI always use 0.1mm and have never had a problem of not enough paste.

author: jmelson
author_rank: Super Contributor
timestamp: April 16, 2019, 07:10:20 pm
content: Quote from: Kjelt on April 16, 2019, 07:13:14 amDoesn't seem to be a problem. I do 0603 and 0805 passives, and chips from SOIC down to 0.5 mm lead pitch regularly. I use a .003" stencil. If the apertures are the same size as the pads, I get solder balls near the middle of the passives, and plenty of solder to make good joints. As the lead pitch goes down, I make the apertures smaller, down to about 50% of pad area (which is also kept pretty small) to restrict the amount of solder. This works pretty well.Jon

author: coromonadalix
author_rank: Super Contributor
timestamp: April 16, 2019, 07:15:15 pm
content: At my job we use a paste with very fine solder balls in them, we are able to solder qfn32 ic`s with no problems and very good results. ill try to find the brand and specs, it`s only a green pot with nothing written on it, purchaser secrets lollAnd the paste has to be put in a freezer at 4 degree celsius if we dont use it.

author: JVR
author_rank: Regular Contributor
timestamp: April 17, 2019, 12:13:07 pm
content: Also, pony up for ENIG, HASL can sometimes be so uneven that it lifts the stencil locally, causing massive smudges.

author: mayor
author_rank: Regular Contributor
timestamp: April 19, 2019, 03:43:37 pm
content: Quote from: JVR on April 17, 2019, 12:13:07 pmOuch! If I do any sort of volume, I will consider it. But right now, at JLCPCB, the extra cost of the surface finish is 8x that of the boards themselves :-)@coromonadalixDid you figure out what the green pot brand is?

author: daveyk
author_rank: Frequent Contributor
timestamp: April 19, 2019, 04:03:27 pm
content: Neat video! Me, I would have just soldered that chip via conventional means with my small conical tip Hakko. lol