Any tips for 4-layer boards?

[rsocor01]

Hi,

I have done many 2-layer SMD designs, but I have never done a 4-layer design. I agree with Melanie that the preferred way to go is to use many vias with 2-layer boards. There are many useful design tips in the next PDF document.

http://www.alternatezone.com/electro...torialRevA.pdf

I use pad2pad instead of expressPCB. Their software is very user friendly and it has features to check your design for flaws.

Also, there are ways for programing your chip on board without having to solder a connector for programming purposes. Well, keep us posted about your project.

Robert

[ronsimpson]

Steve,

Will U5 get hot? I usually use a larger pad as a heat sink.
Is there much power lost in Q2,3,4,5? I usually have a large collector pad on all layers. This works as a heat sink and carries the heat to the other side of the board.

U3,C12 spacing?

Often a LCD display needs a “pot” to set contrast.

Mounting holes?

When I do 4 layer boards I usually use one of the inter layers as ground.
The other inter layer may have large areas of +5, +3.3 and a hand full of hard to rout traces. I usually have signal traces on the outside.

If there are noise issues, I may have the outside layers as ground and power planes. This sandwiches the signal traces between planes of copper. (FCC issues)

Using ball-grid arrays I need two signal layers, power, ground, and two more signal layers.

ron

[Byte_Butcher]

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

[Ioannis]

Can you get rid off of the U4 and replace the function with some PWM from the PIC itself?

Ioannis

[Byte_Butcher]

Can you get rid off of the U4 and replace the function with some PWM from the PIC itself?

Ioannis

Mmmmm.... I don't know. The contrast pin for the LCD wants to be on the "wiper" of a 10K pot with the other legs connected to Vdd and Vss.

I'm not sure what would happen if I fed it from a filtered PWM output... ?

U4 actually eats up about the same amount of real estate as the 10K pot that it replaced on my original design, but it adds easy user adjustment.

Actually, I've been working on it some today and I'm starting to think I just might get the whole thing on 2 layer now that I've got a bit better layout.

We'll see....



steve

[Melanie]

http://www.picbasic.co.uk/forum/show...=Easy+Contrast

Actually, six years on from that post I standardised with 1K5 for R1 and 470R for R2... works well...

[Byte_Butcher]

http://www.picbasic.co.uk/forum/show...=Easy+Contrast

Actually, six years on from that post I standardised with 1K5 for R1 and 470R for R2... works well...

Errrr.... seriously? It's that easy?

Melanie, you never fail to amaze me.

No need for a filter cap even? Any benefit to using one?
Any restrictions on the max/min PWM frequency?

Thanks VERY much for that tidbit. That's excellent.

Ahhh rats... now what am I going to do with all those digital pots I've got in stock?


steve