Electronics > Beginners

Construction techniques for prototypes / one-offs

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baljemmett:
Hi all,

Hoping I can canvas some opinions on methods of building one-off or prototype designs; I'm trying to decide how to put together my next project, and I get the feeling there are better ways to do it than sticking to stripboard (as I have with all my other contraptions).

The project is a homebrew CPU built from 74-series chips -- think Magic 1 (and plenty of other examples) but smaller and simpler! -- so it's going to consist of a fair number of logic chips running at maybe a couple of megahertz (if I'm lucky).  The various ways to build things that I've seen around are:


* Stripboard -- I think this might prove to be more hinderance than help, since most interconnections are unlikely to line up conveniently so there's a lot of track cutting going on (unless using the more expensive DIP or tripad varieties) -- and of course the issues Dave's discussed in the past, with maintaining signal quality etc.
* Matrixboard -- using jumper wires for nearly all interconnections -- strikes me as a lot more flexible and probably not much more fiddly than stripboard overall?
* Home-made PCBs -- I've never etched and drilled my own before, but I suspect it'd probably work out about the same as strip/matrix board in terms of effort (drilling vs. making jumper wires?)
* Professionally-made PCBs -- I think this will be prohibitively expensive in terms of setup costs, since there are likely to be several reasonably-large boards and only one small one will be reused in any quantity -- but it'd look good!
* Wirewrap -- this sounds expensive, fiddly and frustrating, but very flexible
* Wiring pen(cil)s -- I'd forgotten these until recently, but the technique looks like a cross between wirewrapping and matrixboard construction
So it looks to me like there's going to be some tooling cost whichever route I go, apart from simple strip/matrix board techniques; PCBs would probably look the best, but also require a lot of confidence in the design before committing anything to copper or it'll get expensive to respin them.  Surface-mount PCBs would significantly reduce the amount of drilling I'd need to do, but I've not had much practice at SMT soldering (could always construct some other bits and pieces this way first to get my hand in, of course!)  I'm currently leaning towards the wiring pen, since it seems to offer plenty of flexibility, and possibly some PCBs for one particular board that'll be useful in quantity.

Does anybody have any thoughts or experiences that might be helpful?  Perhaps I should just try a few things out and see which I'm most comfortable with, but since I've little experience of this sort of thing I'm hoping those of you with more might know if any of them are always going to turn out badly or suchlike.

Thanks for any words of wisdom!

Bored@Work:
I know, I should not answer this, because this is one of those questions where a second after I answer this new requirements are presented with ifs and buts, but anyhow, in decreasing order of preference:

1. Professional made PCB

2. Professional made PCB

3. Professional made PCB

then

4. Pre-made, off-the-shelf prototype board, Eurocard format, pattern depending on chosen IC package format. E.g. for THT with DIP patterns. And what you call wiring pencil, but without any of those far too expensive and relatively useless plastic combs.

5. Perfboard and wiring pencil (see above).

6. One of the Manhattan construction techniques with dead bugs.

Psi:
It really depends how much free time you have and how quickly you want a working prototype.

It's true that its quite hard work to etch your own pcbs, however once you've done it a few times you get fast enough to go from a pcb design to an assembled board in a day.  Now true, you could just sit down at your computer for 15min and email off the gerber files to get the board made professionally and you get all the benifits of that. A much better quality board and no problems doing 2-layers but you wont have the board at the end of the day or even the next day you have to wait anywhere from 3 days to 2 weeks for them to make and post it to you.

When i have a circuit i want to build/test I start using throughhole components whereever possible and just solder their legs together in a grid pattern to test out various individual small sections of the circuit. I find this a really quick way to get a small circuit built, changing values is fast too.
Once i've tested all the sections and i'm happen with the way they are working i will either...

A) Built the whole circuit out on breadboard (can bit tricky if you want/need to use smt components or ICs)
or
B) Create an inital pcb design and print it out on press-n-peel-blue pcb paper, iron it on a blank pcb and etch. Then drill holes and solder the components on.

After i've tested the board and probably changed or added a few components and/or added wires for extra tracks i will fix up the pcb design with these changes and either make a new final board using press-n-peel blue (if its just a one off hobby project and i have time) or send the pcb layout to ourpcb.com and get some professional boards made up.


NOTE: When making your own pcbs drilling the holes for throughhole components is the most annoying part. It takes time and effort to dent all the pads so the drill will be centered and even then the hole will often go off-centre anyway. So it's good to use surface mount as much as you can to reduce the number of holes you need to drill.

baljemmett:

--- Quote from: BoredAtWork on January 26, 2011, 09:15:00 pm ---I know, I should not answer this, because this is one of those questions where a second after I answer this new requirements are presented with ifs and buts
--- End quote ---
Don't worry, if I was confident enough in my requirements to state any more I'd have included them :)  Or maybe they'd have answered my own question -- at the moment I'm too inexperienced in the practical side to have nailed anything down firmly yet.


--- Quote ---but anyhow, in decreasing order of preference:

1,2,3. Professional made PCB
--- End quote ---
Yes, I suspected that would be the right/ideal answer; what worries me is cocking something up and having to go through hideously-expensive numbers of iterations.  I might play with some smaller parts of the design and see if I can get the hang of it, and maybe prototype the system using another method and transfer it to PCB for a 'finished product' if it goes well!


--- Quote ---4. Pre-made, off-the-shelf prototype board, Eurocard format, pattern depending on chosen IC package format. E.g. for THT with DIP patterns. And what you call wiring pencil, but without any of those far too expensive and relatively useless plastic combs.

5. Perfboard and wiring pencil (see above).
--- End quote ---
Ah, now that's interesting; I had been eyeing up Eurocard format boards, especially since there are plenty of varieties with spaces for DIN41612 backplane connectors.  Seemed like a sensible way to spread a system across boards in a nice orderly fashion, as opposed to using ad-hoc headers and cables.  Two I had been looking at were the Roth RE-315 and RE-060; I think these are the sort of thing you mean for 4 (with DIP packages) and 5, respectively?

Thanks for the advice; I think I'll order a wiring pencil (the combs do indeed look rather expensive so I'll skip them if they're of questionable utility!) and some boards over the weekend and see how I get on.  If nothing else, it'll give me an order reference with which to enter Farnell UK's current prize draw ;)

RayJones:
LOL, I was thinking of starting a post just yesterday bemoaning about the difficulty of using veroboard (stripboard)

I just lashed up a real simple one off job yesterday using this devil spawn, and spent about 1/2 hour debugging all the fine solder bridges I left between tracks.
Should have worn the reading glasses methinks ::)

Being a simple design, the fixed nature of the board was not a great hindrance, but yeah it pays to be careful how you run the tracks, and making sure you do cut where required.

In the end I should have used the simple matrix board without copper that I have heaps of and would have done a better, neater, and probably more reliable job - the veroboard tracks are easy to unintentionally break too.

My conclusion is veroboard is most definitely devil spawn. It seems tantalising at first glance but no end of pain.

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