Thanks to everyone for all the good advice!
I'd hoped to get my board design submitted last week, but I think I need to sit on it and think for a while longer first.
I've got other obligations this week and may not get to work on it for a few days now.


Melanie,
Using more vias instead of more layers sounds great! But I just couldn't get there.... I ended up with too many traces on the backside and carved up my ground plane and had problems with islanding and I *still* couldn't quite get it all to fit.
With 4 layers I can run traces on both outer layers with power and ground internal and it gets LOTS easier for this poor old boy...
I'm sure the problem is all about my lack of skill. I haven't done all that much board design and I've usually got plenty real estate to work with.


Ioannis,
Yes, I think I need to shuffle a few pins around. But some are pretty much fixed by function (many of the sensors need an ADC pin), and some pins are fixed by... habit (I use the same pins for the LCD on several projects so it's easier to remember what's what.)
Part of the problem is that the "core" of the circuit was stolen from a previous project, so the layout on that chunk might not be ideal for this project.

Anyway, I'm going to toss all the parts down on the board again and start over and see if I can cram it onto two layers.


Robert,
Thanks for the link to the PCB Design Tutorial. That was a worthy read and may provide some useful tips for me. Much appreciated!
Yeah, I know I can ditch the programming connector. I eliminated it on the 2 layer design (still couldn't make it all fit), but there was enough space after going to 4-layer that I snuck it back in again. If nothing else it brings those PIC pins out where I can use them If I want.


Ron,

The power dissipation on the voltage regulator and transistors should be fine. The regulator has already proven itself to not need any more heat sinking (in another project) and the power dissipation on the FET's is low, and the duty cycle is also low. Should be fine.
I did add an extra set of through holes for those parts so I can use T0-220 parts on a heatsink if I ever need to.

Yes indeed, got a contrast pot for the LCD. It's digital (U4) so the user can have some control over it without needing to access the board with a screwdriver.
Got a single big ugly mounting hole right above Q3.

You use BGA parts? Wow. Way too scary for me still.


Anyway, I appreciate all of the good advice you folks have given,and later this week I'm going to try and do a better job on layout and see if it'll all cram onto 2 layers.
You folks have inspired (or shamed) me into trying harder.


steve