[time-nuts] pick and place problems/design

Bob Camp kb8tq at n1k.org
Mon Jun 27 19:58:58 EDT 2016


Hi

This whole thing *is* a multi solution problem. There is no one right answer for everybody and everything. It
is refreshing to get past the whole “the world ends when we run out of DIP packages” hysteria that often grips
us when talking about *building* a Time Nut gizmo. A *lot* of the things that we would all like to play with start off
with “something” and then tie it to an FPGA and/or an MCU. Both of those parts are rapidly becoming a fine pitch
SMD sort of thing for newly designed parts. Moore’s law still makes sure that the new part is the cheap one to 
use.

To try to wrap this up:

1) You can do manual placement with or without a stencil. The stencil process does not add a lot of cost. Results
are quite acceptable *if* you have the patience and eyesight to do the job.

2) There are some really interesting short run outfits popping up that may change the economics of short run boards.
It remains to be seen if they are viable in the long run. It only makes sense if they can make enough money doing it to 
stay in business. 

3) Various forms of pick and place gear are getting into the range of “same price as XXXX” that you already have on your
bench. Right now, they are a hassle to use. Will they make sense … who knows. 

Getting anything working is always a process of multiple trials. Nobody ever gets it right the first time. Anything that takes
the “cycle time” from months and years to weeks and months is a good thing. We have spent a lot of time talking about GPSDO’s, 
super duper counters, ADEV boxes, and phase noise test sets. I believe I can count the number of projects actually moving forward 
on one hand. I hope the increased awareness of how many ways there are to “build stuff” will significantly increase the number
of projects we all can share in.

Bob 

> On Jun 27, 2016, at 12:19 AM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 
> 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.
> 
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