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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

John U
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    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. – tobalt Mar 23 '22 at 10:49
  • 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? – Rokta Mar 23 '22 at 11:13
  • @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. – John U Mar 23 '22 at 11:20
  • @Rokta the white bits in the corners are silkscreen. – John U Mar 23 '22 at 11:20
  • In my experience, the application *looks* bad, but it will likely reflow just fine. Agreed, looks too liquidous. Perhaps agitate the syringe and dispense more and see if it thickens up (liquids settled to the tip?) – rdtsc Mar 23 '22 at 11:59
  • Well, the flux shouldn't melt the silk screen...? – Lundin Mar 23 '22 at 12:19
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    @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. – John U Mar 23 '22 at 12:46
  • @rdtsc are you saying that plonking a chip down on this and running it through the oven would actually be likely to flow onto the pads & work OK? – John U Mar 23 '22 at 12:47
  • Probably, yes. But I wouldn't run production boards that way. The solder will tend to wick into the pads really well, but some balls of it can remain under the IC. It might bridge pins directly, or increase the likelihood of "[conductive flux](https://electronics.stackexchange.com/questions/159492/chipquick-smd291-no-clean-paste-flux-conductive)" phenomena. – rdtsc Mar 23 '22 at 14:00

1 Answers1

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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.

Sim Son
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  • 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. – John U Mar 23 '22 at 14:16