I was amazed lately to find out about solder paste – so we can assemble our PCB prototypes at home relatively easily.
The process seems easy, but now I started to suspect I am going to have a problem with small 0402 and 0603 parts. I also have the main part which is QFN-48 (no pins, only pads underneath).
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To make the stencil with such small sizes that you can't even print seems like a real problem – e.g for 0603 is less then 1 mm so most printers will not do it, even laser.
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If I do these parts by hand, can I make a stencil for the QFN-48 pins (600×600)? or is it too small for that? 12 pins per 6 mm is one pin for 0.5mm!
How do you do these things? Can I just splash all pins with the paste without stencil fixture? Assembly for 2-5 parts can cost more than $350, and some parts such as QFN can't be assembled by hand.
Best Answer
(disclaimer: this is not a "how to", more like a "how I ended up doing it".)
The stencil
Where I live, the cost of what they call a "proto" stencil (no frame, limited thickness options, etc) would be very close (about 60~70%) to the cost of a "real" stencil (framed, tensioned, coated, etc).
So I went with a "real" stencil, 23x23in. I figured that having it tensioned would help improve pasting quality.
The setup
The first panel I tried was taped directly onto a stone counter, and the stencil frame put on top of it with weights (~10kg) so that it wouldn't move during stenciling.
I found out that the pasting quality can be much better if the stencil "sheet" recedes into the frame when you press the stencil onto the PCBs, so I started using 2 used panels underneath the panel being stenciled to improve surface contact: YT video. I didn't record this intending it to be a how-to so not much is visible.
Pasting
Due to the size of my PCBs and panel I was able to use a standard credit card sized plastic tag instead of a proper squeegee.
IMO there are 2 main steps:
1 - Loading the stencil
2 - Removing the excess
Loading is when you pass with very light pressure with the "squeegee" at an angle, making sure all pads are loaded with solder paste. When removing excess you should use a more vertical angle with your squegee and apply enough pressure so that all excess paste is removed.
Then I removed the weights and pulled the frame as vertically as possible so that I wouldn't smear the paste: YT video.
I can post more info on placing the components and reflowing the PCBs later if someone wants it.