[time-nuts] PCB design questions
brooke at pacific.net
Mon Jun 2 17:15:50 EDT 2008
1) There are many benefits of using Surface Mount Devices (SMD). In addition
to the reduction in board area just because the part is smaller there's also a
major reduction in board area because of the lack of through holes, i.e. you
get to use both sides of the board (doubling the area). Classic through hole
parts are on a 0.1" pitch and most surface mount parts are on some sub multiple
of that such as 0.05", 0.025", etc. and either true inches or metric
equivalent or rounded metric equivalent pitch. With a fine pointed soldering
iron I (old guy) can solder 0.05" pitch parts.
2) If you use a low cost commercial service like ExpressPCB then you have your
choice of double sided or four layer. The four layer has ground and Vcc on the
I like their service which now includes schematic and layout applications.
Before when they had just the layout it was purely a mechanical process and
easy to make mistakes (shorted or open traces). Now, when the schematic is
linked to the layout you can check each node's connections and mistakes are
much less likely. Both packages are free and in my opinion much easier to
learn than the generic PCB design software packages. I choose ease of use
rather than to have generic board and drill files. The latter would make sense
if you were going to make thousands of boards. I at most make hundreds of boards.
2b) If working with a double sided board I think you'll find that many ICs
have a recommended layout that requires ground on the top surface. For example
op amp inputs may need a guard ring around the inputs or a switching circuit
may need ground immediately adjacent to caps.
It's not uncommon to have a board with an analog section and a digital section
and the layout is different in the two sections. If you look in the archives
there was a comment about how grounds are handled in one of the small boards
that John Ackerman makes I think relating to using a loop for ground or a tree
structure where there are no loops.
I haven't been doing PCBs recently but think the eight inch shear, which has
not worked too well so far, might work for cutting PCBs if the blade is aligned
properly. http://www.prc68.com/I/8MSB.shtml With a good way to cut boards you
can put multiple designs on a panel then you cut them apart. A money saver for
http://www.prc68.com/P/Prod.html Products I make and sell
http://www.prc68.com/Alpha.shtml All my web pages listed based on html name
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
David C. Partridge wrote:
> I've been working on the design for a frequency divider to complement the
> Thunderbolt I recently bought from TVB (thank you Tom, it's working very
> well as far as I can tell, though of course I've no other standard to
> compare against).
> Thanks to lots of advice and guidance from Bruce Griffiths (many thanks
> again Bruce), I've got the design near completion.
> I'm not aiming for NIST or equivalent perfection in terms on minimising
> jitter and other noise, but would like to at least make a at least a
> half-way decent job of this.
> I'm now thinking ahead to the PCB requirements,with the caveat that I've
> only ever designed one PCB before and that was a single layer board done
> using double sized mylar and sticky black tape (Yes, it was a good many
> years ago).
> Now to questions:
> 1. Surface mount or through hole? I don't have a re-flow oven (or even a
> hot air soldering system), so my inclination is to use through hole CMOS
> (74HC163s with 74AC glue logic and flip-flops), with the surface mount
> restricted to the clock shaper using a BAV99 and either an ADCMP600 or
> MAX999 and surrounding components. Will using through hole cause me grief?
> 2. How many layers? In an ideal world with money no object, if I
> understand the current art correctly, I think I'd probably aim for a five
> layer board with Vcc, Digital Ground and Power Ground being separate
> internal planes, and trace routing on the top and bottom of the board with
> as few vias between top and bottom as possible. Does that sound right?
> Do you think I can safely restrict myself to two layers, and if so does it
> make most sense to make one side of the board digital ground, and route
> everything else (Vcc, Power/Analogue Ground, and signals) on the other side.
> Or is there a better approach (always assuming that a two layer board is a
> viable option).
> Dave Partridge
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