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:
- Clean board and stencil with IPA, let both dry
- Align stencil and pull flat (Kapton stencil has curvature, I'm guessing they cut it from a roll)
- 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.
- Press plastic card at ~45deg angle, scraping the excess paste off the stencil with 1 swipe
- 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?