[SOLVED] Print Solder Paste Consistently without Bumps or Bridging - Page 1
Forum section: electronics / general

author: niconiconi
author_rank: Frequent Contributor
timestamp: July 16, 2020, 11:09:26 pm
content: Recently I need to prototype some PCBs with fine-pitched 0.4 mm QFP and QFN chips, and I decided to try stencil printing and reflow soldering at home. I used a 30 cm x 40 cm framed stencil from JLCPCB, and the PCB was 5 cm x 10 cm in size.The first board was extremely successful, but I got repeated bridging around the QFN chips in later revisions.I discovered the quality of paste printing is crucial. If the solder paste is printed on the QFN footprint cleanly, in a perfectly-flat manner without bumps or bridging, the reflow will be a success, otherwise bridging is ensured (on the other hand, 0603 resistors are more forgiving). But it is difficult to get consistent results by hand.So far I've tried two methods:* Tape the board on the back of the stencil after alignment, apply pressure on the stencil, apply solder paste, and remove the tape and board. This method minimizes stencil warping, but alignment and removal is a challenge, it's difficult not to disturb the solder paste while removing the stencil.* Tape the board on the workbench, align the stencil, clamp it in place (yeah, I got one of those cheapest silkprinting bench, since then I learned their usefulness is very limited), print, and lift the stencil. But due to the small size of the board, it stays at the center and warps the stencil outwards. There can be a significant gap between the stencil and my board due to the size mismatch of the small board and the larger stencil, leading to excess solder paste and imprecise printing.How much the stencil warps depends on board size and chip location. It seemed that my first board was just in the "right" size and the chip was placed at a lucky location where the stencil warps the least, leading to initial success. But after the layout has changed, the problem worsened.What are some tips and tricks to overcome this problem without purchasing an expensive X-Y-Z table or an expensive stencil?

author: SilverSolder
author_rank: Super Contributor
timestamp: July 16, 2020, 11:51:06 pm
content: Maybe it has something to do with the solder paste you are using? (is it stiff / soft, old, cold, etc.)I would try thinning it slightly with some liquid flux just to make it easier and more consistent to work.

author: niconiconi
author_rank: Frequent Contributor
timestamp: July 17, 2020, 12:29:49 am
content: Quote from: SilverSolder on July 16, 2020, 11:51:06 pmIt's a tube of generic Sn63Pb37 solder paste, soft, only 2 months old and responsible for my initial successful board. I always store the paste in a small Peltier beverage cooler. Before each use, I always wait 3-4 hours for the paste to warm up. The temperature inside the cooler may be non-ideal due to its limited cooling. To rule out degraded paste, I purchased another tube of new paste immediately after the first failure for comparison. I'm not sure, but as far as I can see there's no significant difference and equally unsuccessful on the second prototype (on the other hand, there's no date code on the new paste and it was shipped unrefrigerated, an alternative explanation could be that both tubes of paste are equally degraded by the hot summer and made them less forgiving).My main suspect is still the paste printing accuracy and consistency, but perhaps I should try different solder paste manufacturers as well.

author: fourfathom
author_rank: Super Contributor
timestamp: July 17, 2020, 01:22:33 am
content: I may not be understanding how you are securing your circuit board, but you really ought to have the entire surface -- board and surroundings -- in the same plane. I used to do this by building up an area surrounding the circuit board, using other circuit boards (or raw PCB stock). The surrounding stock is taped or screwed down to hold the circuit board you are pasting secure. I use masking tape to hold the stainless stencil (no frame) in place.Now that I've got a CNC mill, I mill out a recess (in MDF board) that snugly holds my circuit board. The recess is just deep enough so that the PCB sits flush with the surrounding surface. I mill deeper finger-holes at the board corners so I can retrieve the board after screening.I've been able to hand-screen 0.5mm -pitch pads with no trouble.

author: niconiconi
author_rank: Frequent Contributor
timestamp: July 17, 2020, 02:58:17 am
content: Quote from: fourfathom on July 17, 2020, 01:22:33 amThanks for the tip. I do have a row of discarded FR-4 copper boards. It should work for this purpose. I'll try again tomorrow and report back.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: July 17, 2020, 03:30:24 am
content: I always get the framed stencil from JLC with the polished option (useful for smaller pitch components). It should not be a problem because I even used it with 0.4 mm pitch components with no issues on a standard SAC305 paste. A lot of solder paste application went wrong because of the holding the squeegee angle itself. People always push in the paste into the stencil openings instead of passing through at an angle at 1 pass. I always do a 1 pass with 45 angle degree. Done this for more than 20K of boards without any stencil printer.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: July 17, 2020, 06:59:28 am
content: I'm using T4 20-38 um grain size.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 17, 2020, 10:37:27 am
content: Have you tried an experiment like e.g. applying solder paste through your stencil onto a plain, un-etched PCB board, just to see what the results look like? That might be a good way to perfect your manual technique...

author: niconiconi
author_rank: Frequent Contributor
timestamp: July 17, 2020, 07:13:01 pm
content: Quote from: blueskull on July 17, 2020, 03:43:55 amYou found my problem, I was using Type 3 solder paste. I'll try the "junk PCB template" method with Type 4 paste and see whether the result improves once I received it.

author: niconiconi
author_rank: Frequent Contributor
timestamp: July 22, 2020, 02:49:01 pm
content: Reflowed another board tonight. I got four bridges on the QFN chips, but easily fixable with a soldering iron, and it was already much more successfully than previous attempts (which all ended up with massive short circuits). It's only a matter of practice to improve from here.I switched from Type 3 solder paste to Type 4. And I also found the key to successful solder paste printing is applying it at a 45-degree knife angle, andin a single pass. Never attempt to apply the paste repeatedly,I think it was largely responsible for my previous failures. With these two improvements, the solder paste can be printed in a clean and consistent manner.And a finally note: the "junk PCB coplanar template" actually made the warping problem worse for me, it shifted the warping position towards to outer edge of the stencil and created a bigger gap at the center where the board lies. Nevertheless, I think it's still a workable method, but if and only if the junk PCB template is as large as the frame of the stencil. But it was simply inconvenient for me, so I decided against it and didn't experiment it further. Ultimately, I found the warping of the stencil on the original PCB is, in fact, not a big deal. Due to the small size of the board, it will bend the stencil just enough to close the gaps if you press both sides of the stencil down.In conclusion, case closed and problem solved. Thanks to everyone's suggestions.

author: jmelson
author_rank: Super Contributor
timestamp: July 23, 2020, 06:26:34 pm
content: Quote from: niconiconi on July 22, 2020, 02:49:01 pmI use a thin (about 0.07mm) stencil, and reduce the apertures to lees than the pad size for fine pitch parts. You need just enough solder to flow between lead and pad, but no more.Jon