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SMD at home
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wraper:

--- Quote from: mayor on April 15, 2019, 08:27:21 am ---@wraper: Stainless steel.

--- End quote ---
Then either your stencil is too thick, has too large hole aperture, was misaligned or your paste application method suck so it gets smeared. It would be best if you make a photo of PCB with solder paste applied.
mayor:

--- Quote from: wraper on April 15, 2019, 11:13:56 am ---
--- Quote from: mayor on April 15, 2019, 08:27:21 am ---@wraper: Stainless steel.

--- End quote ---
Then either your stencil is too thick, has too large hole aperture, was misaligned or your paste application method suck so it gets smeared. It would be best if you make a photo of PCB with solder paste applied.

--- End quote ---

Quite possible my method sucks! Basically:

0. Put a few boards down to keep stencil same height as target board;
1. Tape boards together so they form a "solid" board;
2. Align stencil
3. Tape edge of stencil to one of the boards
4. Use a hotel card to smear some paste
5. "Rotate" stencil out of the way

I admit, it doesn't produce the cleanest application. But I can't say for sure if it's paste, technique, stencil characteristics...
Kjelt:
Aaaaaaah QFN packages  :scared:  ,I regognize the horror of qfn reflowing  :)
I solved my issues with these packages, by changing the default stencil layout, cut the openings down to max 60% of the pads and a bit smaller than the pads.
You can also make the pads a bit longer, whatever suits you but I would not recommend putting paste as broad as the pad. Putting on the ic already is enough to cause bridges.
Then I also added soldermasking between the pads, not all cheap boardhouses can or will do this but to prevent bridges it is kind of a must with small pitch.
Last but not least , don't use much pressure when putting down the ic, let it barely touch the paste and then release the vacuum of your vacuum tool.
If you have a P&P machine you do not have that option.

Psi:
yep, few tips from my experience doing paste and reflow at home

- Use the thinnest paste stencil you can get
- Make sure the stencil is flat on the PCB and stays flat, push down relativity hard with the squeegees.
- The paste image should be clean with sharp lines. If it's messy then either your stencil moved around during pasting or your paste is bad
- Don't use the cheap solder paste, get something good from a good brand and from a good supplier to you know its legit
- If you get any little explosions and parts moving or 'popcorn' during reflow then your paste has got water in it. The water is boiling and causing your problem. Get some better quality paste
- Ideally use a toaster oven for reflow. If you have to use hot air use a big nozzle and go much much slower than you would for smt rework. Your goal is to slowly bring up the entire pcb to solder temp.
- When you place the parts on on the pasted PCB push them down a little. You dont want them sitting on the paste you want them pushed into it.
mayor:
Excellent advice, thanks. I will see what can be done about the stencil.

I did wonder about how much pressure to apply when putting the parts on. So it sounds like just a bit of pressure, then. Maybe I've been pushing them down too hard.
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