Stencil soldering issues

[Rectifire]

Hi All,

I recently tried stencil soldering my projects in the hope that it would save me time compared to hand-soldering, what I am used to doing for years. However, i am running into problems, and I am wondering what I am doing wrong. The biggest issue that I keep having is that the solder paste spreads between pads. This way, after reflowing, all the pads are soldered together, and it takes me more time to rework/troubleshoot the boards than it would have taken if I just soldered by hand. I have added an image of one of my solder paste applications as an attachment. I re-pasted the board a couple of times already before the foto, so therefore some holes are also filled with paste. This is no problem. The problem is more with the SOT23-6's and the QFN in the middle, where all the pins will be one clump after soldering. It is a 0.4 mm pitch, but that should not really be an issue, should it?

My method is as follows:
I clamp the board to be soldered on all four sides with other PCBs, so the stencil can lay flat.
I align the stencil on top of it
I tape the stencil down as well
I apply the paste
I remove the tape carefully
I lift the stenil up on one side, until it is almost vertical and then lift it away

What could I do differently?

[Kean]

Too much paste!

What type/brand of paste are you using?
What kind of stencils are you using, and how thick are they?
Did you (or your stencil supplier) adjust the paste layer or stencil apertures at all to match your stencil thickness, your footprints, and the datasheet recommendations?

[Rectifire]

The paste I have is Chipquick TS391SNL.
The stencil is the default metal stencil from JLCPCB. It is 0.20mm thick.
I have not adjusted the paste layer to match my stencil thickness.

I have used Kicad default footprints and custom footprints ordered the stencil at JLC, not really thinking about stencil thickness or stencil apertures anywhere in the process.

[Kean]

Without seeing a video of your process we can't easily comment on anything else you are doing wrong, and can just make general suggestions based on your photo.  Your description of technique seems basically fine, and what I often do for smaller runs.

Based on your photo, the apertures are just too big - the deposited paste exceeds the bounds of the pads.  A little too much paste will often be fine but is very likely to lead to shorts especially on the QFN, and sometimes parts not sitting properly because they are lifted up by the excess paste which may affect mechanical tolerances ... or just aesthetics

JLC do often seem to adjust the customer provided apertures, particularly if the stencil is added to the cart as part of the PCB order (rather than a separate upload) but you can't/shouldn't rely on that.  In fact people often complain about it as they have already customised the paste layer on their footprints to suit their production process.  e.g. https://www.eevblog.com/forum/manufacture/unwanted-stencil-aperture-reductions-by-manufacturer/

Here are some good tips on using & designing stencils https://www.surfacemountprocess.com/a-guide-to-effective-stencil-design.html

That ChipQuick paste should generally be fine (basically the same as GC10 which is popular), but I also meant to ask about the age/condition of the paste?  It does seem to be slumping especially badly on the lower left area ... SOT23-6s? which I rarely find troublesome, vs smaller pitch parts.  That could be from poor application technique, stencil removal, or badly mixed/aged paste.  I do see my GC10 get like this in hotter weather if I don't mix it before use or leave it sitting open for too long.

[Rectifire]

I could also imagine what happened, especially with those SOT23-6's is that the stencil probably was not flat no the PCBs. I have seen that even with PCBs around the PCB to be pasted, still the stencil is not perfectly flat on it. When pressing on the stencil with my finger, it drops down a little further. But I am not sure how that can be fixed. And when applying the paste, the stencil is pressed firmly against the pcb by the pressure of the card I use to spread the paste. So could that be a problem?

The paste by the way is about six months old and at room temperature which is about 15C (high energy prices). It is indeed very liquidity. Maybe because this is the last bit from the syringe?

Thanks for the stencil design guide, I will have a look at it.

[jmelson]

Your stencil apertures are WAY too large!  Also, you might have a stencil that is too thick.  I use 0.003" (0.075mm) stencils, and give the fabricator a paste Gerber file with the apertures reduced to about 60% area for the non-critical stuff, and maybe down to 40% for the finer-pitch parts.  I also wipe the stencil with the squeegee to remove extra paste after the first squeegee pass, applying the squeegee at the opposite angle to "chisel" off the extra paste.
Some spread of the paste is normal, I use GC10, which really limits the spread of the paste.  If the flux is working right, a little spread of the paste will not cause bridges.
Jon

[Kean]

Try to keep the stencil flat.  Maybe use more surrounding/support PCBs, or even a vacuum stencilling system (see  videos by Unexpected Maker).

Get a better squeegee if possible.  I use a few sizes of stainless ones from AliExpress, with 15cm being my go-to one.  Like these https://www.aliexpress.com/item/32814721939.html

Wipe the bottom of the stencil often.

Using old paste from a syringe can be tricky as it is very difficult to properly mix.  I buy the 500g tubs of GC10 as it makes mixing easy even though I only use about 60-70% of them and that is going 3-6 months past their use-by date.

[SMTech]

The stencils is too thick, should be more like 0.12 for 0.4mm pitch. 
You're pressing too hard and your squeegee angles are probably too acute, this forces paste through and under the stencil rather than simply rolling into the apertures.

This comes up frequently, as I've said before, try and ignore Youtube here, it's full of muppets squishing shitty dry paste  through a stencil using in inappropriate tool you'd normally use for filling gaps your plaster, or worse and then looking pleased with themselves, works fine for SOIC and 1206. Watch the action on a proper printer, that's what you are trying to replicate.

At 0.4mm you are also making life hard for yourself by nor having one in a frame, I know it pumps up the delivery cost, but it certainly helps to have a stencil under tension. Under tension, there's no way its shifting as your squeegee moves & its also easier to lift away without it flexing, twisting or smearing. Making a frame is also fairly doable.

[48X24X48X]

I doubt that it is a 0.2mm thickness stencil. People at JLC usually will select them accordingly to your smallest pitch. With 0.4mm pitch components, it's very likely a 0.1mm pitch stencil. Your paste looks too runny, try stirring it properly before applying them.

[Matt-Brown]

I assume you have a solderpaste layer on uyour cad for the gerbers? The paste layer is usually 2 to 3% smaller than each land pattern per side. Then use a .0127MM stencil (5 mil) and order with further reduced apetures of 3 to 5 % per side and round the corners

[tooki]

It also looks to me as though the topmost support PCB is warped.

Use a hard, flat surface as your pasting board, and make sure EVERYTHING is PERFECTLY flat. Paste needs to be applied so thinly that even just a layer of sticky tape can upend things.

[Psi]

Yep, 0.1mm stencil is what I use for home jobs, and even then I sometimes find it puts too much on.

The consistency of the paste is key.
I find when I take it out of the fridge and use it the first few come out a lot better than the last ones.
As the paste warms up it gets too runny.

I have had inconsistency issue with chipquik brand paste.  So I stopped using them.
Sometimes it's good but other times it just doesn't work right.

You also have to push down with some force to ensure the stencil sits flat and stays flat for the whole operation.
if any gaps form the paste will be pushed under it and ruin it.

And finally, you should really apply the paste in a single pass with the paste squeegee. If you try and do multiple passes it will usually end up pushing the paste that's already in the holes to under the stencil and make a mess. Sometimes you can get away with 2 passes but your goal should be 1 pass if possible.