Stencil printing small number of 7x5 cm boards without stencil holder

[Axk]

Originally posted in Manufacturing & Assembly but I figured it's a hobby question and not proper automated Manufacturing & Assembly

Hi everyone,

I need to assemble 2-10 small PCBs with a big number (~150) of small 0402 components and a few 0.5mm pitch QFNs and one USB-C connector (the smallest pitch of them all).

I have a hot plate that I can reflow these on, which appears to work fine.
Now I need to apply solder paste to all the 0402s and I'm looking for a way to easily do this. I do have an HDMI microscope.
Have tried to dispense solder paste with a 23G needle cut to short length but it's difficult and it clogs all the time (ordered some 22 and 21G needles to try).

Now also thinking about ordering a frameless stencil and trying with the stencil.

Considering the small board size, will it be very difficult to properly print with a small frameless stencil (stainless?) without a special jig just using other PCBs as spacers and tape (like people do) with this size of components?

[Pinkus]

Forget the manual solder placement with the needle - that will never work well with such a fine pitch.
A stencil is a must. If you order one, I highly recommend nano coating to the stencil.
I would propose a framed stencil and a stable stencil printer.  A good performance for a (relatively) low price (300€) is the '3040 stencil printer' (just google).
If you are on a budget and just need this one PCB, you may try a non-framed stencil and paste on a table, though if you need to paste PCBs more often in the future, get the 3040 stencil printer now. It is worth the money.

[Axk]

It's just a one off thing, I won't be doing more in the near future so trying to keep equipment to a minimum.
Just 10 PCBs max depending on if it the design will work.

I figure it shouldn't be too difficult to print 0402s? Even if I cannot print QFNs it's not a big deal, I can just drag apply the solder with an iron add flux and reflow the second time.

[Psi]

0402 is doable but hard without something to keep the alignment perfect.
The smallest i've done with a DIY method holding my stencil is 0603

The way I do it is to take some blank PCBs (minimum 4. ideally 6-8) and double sided tape them to some rigid cardboard to create a box around where the PCB to be paste'ed can 'drop' in.  Then I tape the stencil to the top of the thing so i can lift it up and it will drop down in the same location.  You do have to keep an eye on it, even doing it this way its location drifts a bit and this will be worse for 0402.
I usually use the microscope to check it between PCBs and nudge as needed and you will definitely need to keep checking it for 0402.

The key is, when you're doing the actual pasting, you need to stop the stencil sliding as you move the paste squeegee across the pcb.
One option is just pressing down hard so friction locks it and another is to put a few tiny dots of double sided tape between the stencil and some of the blank PCBs. You have to overpower them to lift up the stencil but if they are small that is usually ok. But you have to keep them well away from the main PCB area because they are adding a gap between stencil and PCB.

Also you need to be using the right size solder paste balls so you can paste through tiny apatures.  T4 or T5 for example. I think my stuff is T4.

Also the temp of the solder paste will affect your results abit, i find it works better when the paste is colder than room temp but YMMV.
When it gets hot/runny it tends to produce more smudged transfers. That isn't always a problem for 2 pad devices as they tend to fix themselves with a little paste alignment error, but it can cause bridges for ICs.

Oh, and if you get it to work (and produce good results) but then for some reason it stops working properly, the cause is usually that the under side of the stencil needs to be fully cleaned with IPA and dried. Solder has come through to the other side of the stencil and is now preventing it from being totally flat against the PCB.

There is some art/skill to it, if you've never done it before then doing 0402 and getting good results will be challenging.
If you don't mind lots of rejects, and having to clean them and try again, then give it a try.
Just be aware that solder paste makes a mess and it gets everywhere, including under your fingernails and it contains lead.
So you want to control the area and make sure cleanup is easy. I would highly recommend always wearing gloves to avoid having to clean paste out from under your fingernails.    The pasting itself isn't that messy but the cleanup is for any PCBs you ruined and need to clean and try again.

The squeegee doesnt need to be super special, a debit/credit/library/airpoints card works ok as long as it's new and has a totally smooth edge.