Solder paste stencil application

Question

I am trying to do reflow soldering with a stencil for the first time. I have done some reflow soldering before, but every time I manually applied the paste using a small syringe + needle. These attempts went well and I was able to solder some QFN and 0603 components. In my newest PCB I am using 0402 passives, a 0.5mm LQFP64 IC, an LGA IC, a BGA IC, and overall significantly more components. As a result, I'm trying out using a stencil for the first time.

I had double-sided, 4-layer boards made by OSH Park and 5 mil Kapton stencils made by OSH Stencils. I am using Chip Quik lead-free solder paste , which is the same as I used in my previous run, but a fresh batch . I am using the lead-free version because the BGA is lead-free and am trying to reflow it at the same time as the other components.

The problem I have run into is that during solder paste application, the solder paste doesn't seem to want to stick to the pads. I am doing the following steps:

1. Clean board and stencil with IPA, let both dry
2. Align stencil and pull flat (Kapton stencil has curvature, I'm guessing they cut it from a roll)
3. Apply solder paste (small amount was dispensed cold and left for ~3hrs to warm up) to stencil using a needle, making sure to cover each aperture.
4. Press plastic card at ~45deg angle, scraping the excess paste off the stencil with 1 swipe
5. Gently lift stencil starting from one side, trying to keep it from moving in areas that haven't been lifted.

The problem becomes apparent at step #5. When I lift up the stencil, some of the paste is applied (mostly on the larger pads), but a significant amount of the paste is kept in the stencil apertures, most notably on the fine pitch components. Right after scraping off the solder paste, I have checked to make sure that all the apertures are filled with gray paste. The paste is definitely inside the apertures before I lift up the stencil (before starting step #5) and it stays aligned on the pads. I designed the paste layer to be ~80% of the area of the pad with a rectangular aperture shape.

Does anyone notice any glaring errors with my procedure or have any hints on what I can do to make the paste stick to the pads better during stencil removal?

Answer 1

First, for these small apertures, a 3-mil stencil thickness would work better than the 5-mil. OSH Stencils offers both. The extra thickness adheres more to the solder, lifting it from the PCB. Also, even when it works, the thicker stencil can leave too much paste on the pads.

Other than that, I recommend a few things:

1. Surround your pcb with other material of the same height. Other blank PCBs are good for this. This keeps the flexible stencil from bending down around the edges, lifting away from the surface of the board.

2. Tape these supports to the table with masking tape or Kapton. Tape the PCB to the supports. Tape the stencil to the supports. Tape, tape, tape!

3. Make a thick line of solder along the left side of the board. This assumes you are right-handed and will drag the squeegee from left to right. Don't bother putting paste down near each hole.

4. Drag the paste across the board in one motion. As you go, make the angle of the squeegee more and more acute. Start around 45 degrees and end up at 25-30 degrees. This forces more solder towards the holes.

5. Peel off the stencil.

If you end up with a few holes unfilled, I would remove the stencil anyway and apply paste manually with the syringe. Swiping over the stencil again often messes up the previously-good solder deposits.

Also, if your board is small, you might find that a utility knife blade makes a better squeegee than the supplied plastic card.