Table of Contents
Introduction
If you want to do small scale prototype SMT manufacture, it may be faster to just deposit paste by hand onto the PCB instead of getting a stencil made and shipped. We use this for our prototypes and its very fast once you get the hang of it. Instead of stenciling paste on, a small syringe of paste is squeezed onto each pad, sort of like a cake decorator (but with toxic metal).
After the paste is deposited on the pads, each component is handplaced with tweezers. Then the entire board is reflowed. you can use hot air but a plate or oven works best (we'll have a tutorial about that some day)
This technique works for both lead and lead-free PCB/parts/paste but of course leaded solder is easier to work with.
Tips and Plunger
We use LF-4300 from Amtech for our in-house lead free paste. Its not necessarily the best, just what we use so if you have a favorite brand go with that! We get a syringe of 50g no-clean paste from solderandmore.com altho you can get paste syringe from anywhere.
You'll also need a plunger and tips. You can get a huge assortment of different tips from McMaster. For every day use we like using the 20ga pink plastic tip from the shop above - best for larger parts like SOIC, passives and some larger pitch TQFP.
For finer pitch parts, and leadless parts, we go with a luer-lock 24 gauge. Its a bit tough to push on but gives a nice fine bead.
For the plunger, we suggest shelling out for the nice plunger we have shown here, its $25 but makes hand-dispensing trivial and you can just reuse it when the syringe is empty
0.4mm pitch TDFN
When hand assembling fine pitch or leadless parts, the syringe dispener makes an impossible job easy!
For this board we'll be placing a 0.4mm TDFN leadless part. Virtually impossible to do by hand! Using the fine needle, place a blog on each pad and lay a small bead on the TDFN pads. You can also place a small dot in the center of the TDFN pad - sometimes this helps with placement but its a bit of a toss up so try both ways and see which you prefer
Place the large parts first (so you dont bump the tiny parts as you place them)
Carefully place the fine pitch parts in the correct orientation. Be careful about placement so that the pads line up, a difficult task as you get to smaller pitches
After you reflow, there will be tiny bridges, you can fix those with a very fine tip soldering iron and some wick
