Electronics > Manufacturing & Assembly

Stencil performance

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davegravy:
My first time trying to use solder paste and a stencil.

Attached are a couple microscope pictures of how I did. I feel like it deposited well in some areas but not so well on some fine pitch (especially the 8-pin DFN). It's like it seeped under the stencil or something.

The stencil is stainless 4mil from OSH Stencil, and the solder paste is: http://www.chipquik.com/datasheets/SMD291AX.pdf

With the stencil securely taped in place I applied firm downward pressure and only made 2-3 passes with the applicator. Does it seem like there's too much applied? Is my paste maybe not sufficiently viscous?

48X24X48X:
Paste not mixed well, should limit to 1 pass, and squeegee angle should be around 60 degree. It also looks like your stencil are not full flat. Is this a frameless stencil?

davegravy:

--- Quote from: 48X24X48X on January 15, 2022, 05:29:29 am ---Paste not mixed well, should limit to 1 pass, and squeegee angle should be around 60 degree. It also looks like your stencil are not full flat. Is this a frameless stencil?

--- End quote ---

It's a frameless stencil, yes.

Not mixed well? Hmm, it came in a syringe, I don't supposed I'm meant to take it out and stir it, am I?

Thanks for the other tips.

mairo:
Yes, paste is too much. Still you can try reflow it and fix any issues post reflow, or clean the board from the paste and do it again. Try aiming for just a single pass. Paste and room temperatures is also important - if it is too warm the paste will be more runny.

48X24X48X:

--- Quote from: davegravy on January 15, 2022, 05:35:03 am ---
--- Quote from: 48X24X48X on January 15, 2022, 05:29:29 am ---Paste not mixed well, should limit to 1 pass, and squeegee angle should be around 60 degree. It also looks like your stencil are not full flat. Is this a frameless stencil?

--- End quote ---

It's a frameless stencil, yes.

Not mixed well? Hmm, it came in a syringe, I don't supposed I'm meant to take it out and stir it, am I?

Thanks for the other tips.

--- End quote ---
If it is a frameless stencil, you had to ensure it's fully pressed onto your PCB during the stencilling process. This is one of the reason there are stencil printer that holds these frameless stencil in place, to give a proper tension across the entire sheet. I mentioned it's not mixed well because the paste looks too runny. Whether it's a paste in a jar or syringe, the liquid portion will always follow the gravity. Therefore it has to be mixed properly to get the correct consistency. The stencil thickness is okay for the IC pitch you are using.

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