Electronic – pitch limit to applying solder paste through a PCB stencil by hand

pcbpcb-assemblysolder-pastestencilsurface-mount

I've ordered two stencils from two different companies, two different materials. Both times I've been completely unable to effectively apply solder paste to the pads of a QFP-64. I end up with a mess of solder all along one side, instead of a discrete amount of paste on each pad. Applying more pressure to the stencil or to the squeegee does not seem to help. I suspect I'm simply doing something wrong, perhaps missing some vital piece of equipment everyone assumes I have for stencil work, or I'm pushing stencils past their reasonable limits.

Can someone explain to me how I can, by hand, repeatedly and consistently, apply solder paste to a QFP-64 footprint through a stencil, without bridges? Or if that's not possible, what's the next best thing?

Best Answer

QFP-64 shouldn't be too much of a problem. I'm doing 0.5mm pitch 100-pin packages by hand, though I do need to touch them up every now and again.

  1. Use stainless steel stencils, and ignore all the Kapton or other plastic sheet materials***
  2. Use smaller stencil apertures than the default size for fine pitch components. I find in eagle I have to use NOCREAM on the SMD pads in my library, and then manually add polygons for the cream layer. I use a macro in Octave to build an Eagle routine to do this
  3. Let your paste come to room temp. I leave a blob out on a stencil and put my big stash back in the fridge. It's faster that way.
  4. tape the stencil down really well, and practice lifting it without smearing before spreading paste
  5. Try to use stencils with side ridges to stiffen them and prevent bowing in the middle.
  6. Get good with removing extra solder with braid
  7. Be sure to leave enough space between pads on your boards to have some solder mask between them.
  8. Pick packages with the biggest pitch you can. The dspics I'm using come in 12mm and 14mm sizes, and I go for the 14. By the same logic, use packages with legs you can see and get at.

***pcb-pool.com provides steel stencils free with your board order.

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