[time-nuts] pick and place problems/design

[Nick Sayer]

I read through the thread so far and thought I’d throw out there some of how I do it. I’ve done a lot of manual pick-n-place in a very short career. :)

I used to always use stencils for paste, but now most of the time I use a dispenser and place the paste by hand. The exceptions are some of my smaller boards with QFN packages. Those get pasted with stencils always, but there are only a few designs that do that, and all but a couple I get made in batches by SBA.

I place most things with tweezers, and I’m pretty good at it by this point. I have a vacuum tool that I use for QFN and for SSOPs that are too large to fit entirely in the tweezers. I grab SOICs by a couple pins, but SSOPs are too fine for that technique.

I tend to use too much paste rather than not enough, which sometimes means SSOP devices need to be reworked with braid to remove bridges. Just part of the process for me.

My passives are 0805 (when I have a choice). I probably could do 0603, but it turns out that the footprints aren’t terribly different between those two sizes. I don’t design smaller than that because I simply have no need. I do surface mount now because it’s *faster* for me than through-hole, and because you have a far wider variety of devices available than limiting yourself to through-hole devices exclusively. The fact that it’s smaller is kind of a nice side benefit.

My rule now is that I don’t buy “ordinary” resistors now other than as a full reel, 1/8W 1% 0805. Specialized parts, like current sense or high power resistors don’t count. I have reels of MMBT3904/6 and MMBT4401/3, as well as 1N4148 SOT323, and BAT54. For caps, I keep 22µF, 10µf, 2.2µF, 1µF, 0.1µF, 0.01µF, 0.001µF and a few selected pF values on reels. I’ve also got a lot of other parts on cut tape. When I need parts for SBA that are too expensive to stock reels, I buy DigiReels or MouseReels. It’s cheaper to get them to treat cut tape thusly than to pay SBA the premium for them to deal with cut tape.

My workbench is a *mess*. Tidying up is just never a priority somehow. My current inventory management system is best described as restrained chaos. I have “project bags” where I store non-common cut-tape parts for each particular product. I have a laundry tub where most of my reels are held, and a “stack” of commonly used values on the bench. I have 5 other bags of cut-tape parts that are categorized as integrated circuits; discrete semiconductors and LDOs; caps, inductors and crystals; miscellaneous parts (like trimmers, battery clips, tactile switches, etc); and through-hole parts - mostly things like screw terminals.