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A short question this - I've got a board with a QFN-32 0.4mm pitch footprint, I ordered this stencil, and a tube of this solder paste, and the result is in the picture below.

Now, I'm new to this, but I've done some homework about process and indeed followed the instructions that came with the stencil closely - line up under a microscope, tape the stencil down, squeegee the paste on in one smooth wipe and lift the stencil cleanly off.

However, the paste then just all runs together, and under the microscope it looks to me like the paste is almost too coarse for the size of footprint.

Is this just a very runny blob of paste from the first squirt out of a fresh syringe, have I got the wrong paste or wrong stencil or is there something else I'm missing here?

Solder paste misbehaving

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    \$\begingroup\$ looks like the paste flux is already melted. Did you heat the board? Anyway i guess it would still reflow fine because the cover is on average thin and even. \$\endgroup\$
    – tobalt
    Commented Mar 23, 2022 at 10:49
  • \$\begingroup\$ I got similar results (but never that bad) in 2 cases. either solder paste had too much flux or stencil flexed and allowed me to put 2x the normal amount of solder paste on PCB. What is that white stuff mostly visible on all 4 corners? \$\endgroup\$
    – Rokta
    Commented Mar 23, 2022 at 11:13
  • \$\begingroup\$ @tobalt it's not melted, although it wasn't refrigerated before application if that matters much? It's not exactly hot in our factory, maybe 20 degrees C. \$\endgroup\$
    – John U
    Commented Mar 23, 2022 at 11:20
  • \$\begingroup\$ @Rokta the white bits in the corners are silkscreen. \$\endgroup\$
    – John U
    Commented Mar 23, 2022 at 11:20
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    \$\begingroup\$ @Lundin the silk screen is fine, it's just a reflection of the light off the flux I think. It's definitely not melted, I can say that for sure. \$\endgroup\$
    – John U
    Commented Mar 23, 2022 at 12:46

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It looks like the ratio flux/alloy is much too high, but I don't think it's a problem with the paste itself. Flux and alloy balls tend to separate from each other due to their different density. Unmixing is even stronger when the paste is stored at room temperature because of the flux having a lower viscosity.

Most manufacturers recommend to keep their paste in a fridge and to not store syringes in an upright position.

In your case, the solder paste is likely still fine. Just squeeze out a little bit more than you actually need and mix it again. As it is on thr picture, the paste might still reflow just fine as long as the flux is still ok.

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  • \$\begingroup\$ Thanks, I'll give it another go, the flux was cheap enough that I can squirt a bit of it away to see if it comes out in a better texture. \$\endgroup\$
    – John U
    Commented Mar 23, 2022 at 14:16

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