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I am trying to do a home assembly of PCB but I have issues after reflow soldering. Especially I have bridges/shorts on the pins of QFN packages. I have assembled multiple boards other joins are okay but each time I have randomly bridged pins on QFN.

Probably the issue is the stencil printing. Can you give me ideas to improve my process?

I am using SN42BI58 paste, mixing it before apply, and using proper, suggested by manufacturer, temperature curve. So I think the soldering process is OK.

I am using a manual ZB3040H stencil printer. I have an 28x40cm big stencil and an unique 3D printed stencil frame.

I think the stencil is not to rigid/stiff and that produces more paste on board as should.

What can I do to improve my technology? How can I use 28x40cm sheet stencil with my ZB3040H frame based stencil printer?

Here are some pics:

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    \$\begingroup\$ Looking at the joins on R15..R13 and T2, I'd say there's too much paste being applied. \$\endgroup\$
    – Unimportant
    Commented Aug 5, 2021 at 7:03
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    \$\begingroup\$ The stencil jig looks very professional; I like this. \$\endgroup\$
    – mmmm
    Commented Aug 5, 2021 at 7:06
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    \$\begingroup\$ What is the thickness of your stencil ? You can reduce the amount of solder paste by reducing the thickness of your stencil. It may be that you chose the standard stencil thickness but you actually require a thinner one ... \$\endgroup\$
    – citizen
    Commented Aug 5, 2021 at 7:12
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    \$\begingroup\$ The thermal pad area should not have more than about 50% coverage (preferably dotted). More than that and there will be excessive solder used. See section 4.4 of this guide from TI. ti.com/lit/an/slua271b/slua271b.pdf \$\endgroup\$
    – Peter Smith
    Commented Aug 5, 2021 at 7:46
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    \$\begingroup\$ You may need additional support for the stencil around the PCB, placed around the PCB and at the same level, to prevent the stencil from flexing away from the PCB when you apply pressure on the sides. You should also try not to squeegee the paste multiple times over the stencil for one PCB - don't be afraid to put more paste on the top of stencil to ensure that you have a nice 'bead' of paste rolling across the stencil, and make sure to apply just the right amount of pressure too. \$\endgroup\$
    – brhans
    Commented Aug 5, 2021 at 12:47

1 Answer 1

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Solution:

I borrowed a new stencil printer and improved the process and I got proper results. I didnt need to change the stencil. Thank you for the tips.

These things must be done in my case:

  1. Wipe/clean stencil bottom after each board
  2. Mix paste before use
  3. Inspect and clean the cutouts after each board
  4. Make support sideway of the board to avoid bending of the stencil
  5. Stencil must be strongly stretched
  6. Only one swipe is permitted

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