Crazy Idea of the Day: Solder fingerprinting dust

Equipment
1

IMHO, the hardest part of doing SMT/SMD soldering at home is to get the solder into all the right places. I’ve never quite gotten the hang of stencils, and while I don’t mind depositing solder manually, it’s always a smeary mess, and I haven’t found any tools that can make it better. (Not sure about digital glue dispensers, since they seem to be fairly expensive.)

Anyways, my idea is this: Use a super-fine syringe to to deposit flux instead of solder paste. Flux tends to be sticky, but can be bought in any viscosity you like, from water-like to paste. I’m thinking that something that flows relatively easily should be much easier to deposit with a syringe, and in far smaller droplets than solder paste. (because I can use finer needle) Also, it should be possible to “paint” a surface with the liquid I think. Maybe something like a super-tiny brush could also work.

Assuming step #1 above works well, step #2 would be to take some finely ground solder power and sprinkle it over the board. The power should stick where the flux is, but not so much to the dry parts, so turn the board over to get rid of the extra powder, then place components and bake…

I haven’t found solder powder for sale, so I I would probably have to grind some up if I want to test this. Not sure what to use since I don’t want to sacrifice the mortar and pestle from the kitchen. Any ideas?

PS: If anybody else decides to try this; Do NOT grind up leaded solder, the dust would be extremely hazardous. Non-leaded solder might also be bad for you, but lead is the worst.

2

I get the idea. It’s like making glitter art with white glue.
My initial thought is all the loose dust would maybe be pretty hard to be sure is all gone by dumping it over, even blowing. Risky, and more messy than the original plan?

I find a sewing needle tip works well, extremely small dip into the paste tube end, and less is more. can always add more.
It’s certainly time consuming though for many components.

Stencils are great in my experience. A metal one, with serious masking tape securing it as a hinge on one side works all the time. Just need to have it at the same height as the target PCB, so 1.5mm surrounding riser pieces, also taped in place. I make sure to clean it of course, including between immediate uses.

3

It doesn’t matter if there is a little bit of solder dust left. It will either go where it needs to go when the board is heated up, or it won’t. The only place I would be concerned would be in places where I couldn’t see, like under an LGA or BGA component. In those cases I would check with the microscope for stray dust while I’m placing the components.

Sewing needle is an interesting idea. I have not tried that.

I would certainly use stencils every time if I was making multiple boards at a time. As it is, I feel that neither stencils, nor my existing tools for depositing solder paste are satisfactory. Maybe I just need better paste…

If I had a tool that could extrude a 0.1mm solder paste bead, making prototypes would be much easier.