EEs: PCB Layout for Newbies

[jmcoreymv]

I'm doing my first board layout and basically I'm having a lot of trouble getting all the traces routed in two-layers. There's no way I can afford to have a 4-layer board fabricated so I was wondering if anyone has useful tips for routing. I know its a pretty general question but heres some information about the board:

5x6 in
1 112-pin LQFP IC
1 28-pin TSSOP IC
2 14-pin SOIC
1 8-pin SSOP
1 8-pin SOIC

And many other SMT parts/connectors including 0603's, 0805's, 1206's and others. I have to keep all my trace widths and spacings to at least 8 mils so that limits the number of lines I can run underneath the ICs and doing the via fanout just takes up so much space.

Any advice is appreciated.

 

[Oscar1613]

how many copies do you need? just 1?

 

[jmcoreymv]

Just 1. If I can do it in 2 layers its only $33. If it takes 4 layers, then its like $500.

 

[JohnCU]

i'm a first year senior in EE and have no clue what you're talking about.

well kind of, but we haven't really done much on PCB design...

 

[jmcoreymv]

Here is a component placement. I also have a picture of one I finished routing, but it was using 4 layers and then I found out how expensive it was. The parts are spaced far apart because I had them close together before and could barely route any traces.

 

[Oscar1613]

Originally posted by: jmcoreymv
Just 1. If I can do it in 2 layers its only $33. If it takes 4 layers, then its like $500.

so i take it you're going thru 4pcb for the student special... they have another special that gives you a $500 credit to your account to use on your 1st two orders if you havent seen that already

 

[Oscar1613]

make sure the components that are connected together are near each other

oh and all those SMT devices are NOT gonna be fun to put on

 

[jmcoreymv]

Originally posted by: Oscar1613

Originally posted by: jmcoreymv
Just 1. If I can do it in 2 layers its only $33. If it takes 4 layers, then its like $500.

so i take it you're going thru 4pcb for the student special... they have another special that gives you a $500 credit to your account to use on your 1st two orders if you havent seen that already

I saw that, but had already registered. Also the board is going to be somewhat general purpose and the design may be reused in the future so I need to get it onto two layers in case someone else wants to use it to make another prototype.

 

[jmcoreymv]

Originally posted by: Oscar1613
make sure the components that are connected together are near each other

oh and all those SMT devices are NOT gonna be fun to put on

The smallest pitch components are 25 mil and I know they will be a PITA. However, I have done 20 mil by hand before so I'm not too worried.

 

[jmcoreymv]

Another question. Let's say I have a thru-hole component with a pin diameter of 1 mm. How big should the drill diameter be and how much do I have to account for the space taken when the hole is plated?

 

[OrganizedChaos]

<--not an ee so probably a stupid suggestion.

split the project into two boards and connect with ribbonwire

 

[jmcoreymv]

Originally posted by: OrganizedChaos
<--not an ee so probably a stupid suggestion.

split the project into two boards and connect with ribbonwire

I could split them into separate circuits on the same board and connect with jumper wires. I may end up having to do that, but it could get really messy and be a PITA.

 

[kirkaptain]

What PCB layout software are you using?

 

[NutBucket]

Originally posted by: jmcoreymv
Another question. Let's say I have a thru-hole component with a pin diameter of 1 mm. How big should the drill diameter be and how much do I have to account for the space taken when the hole is plated?

The board house should provide specs for how thick the plating is on through holes. 1mm pins? What are you mounting, power transistors?

At any rate, when we send out board for through-hole parts, say 1/4 watt resistors or 74LS dip parts, we usually specify 35mil (0.035 inch) holes and it works out fine. Incidentally that's the same size we use when we make boards in house.

Routing....well, seeing as that looks like a digital board to me you don't have to take extreme care in routing. Most important tip I can think of is don't leave isolated islands of copper between traces. Tie ground areas together with plenty of vias.

 

[jmcoreymv]

Originally posted by: kirkaptain
What PCB layout software are you using?

OrCAD Layout

 

[jmcoreymv]

If I have Pad Exit Location violations (for SMT parts), is that a big deal if they are not high-speed lines? For some of the SMT parts, I need to connect the trace to the side of the pad and not axially.

 

[NutBucket]

How fast is this circuit?

 

[jmcoreymv]

The fastest thing on the circuit is the 16 MHz crystal which connects to the microcontroller. Everything else is a few MHZ or less.

 

[NutBucket]

Bah. You could wire wrap the damn thing without any problems.

 

[WhoBeDaPlaya]

What's the speed involved? RF, u-wave, audio, etc?
That'll determine how ghetto you can be

 

[WhoBeDaPlaya]

Oops, nevermind
Forget about niceties like mitering (if you're short on time). At that speed, you can seriously consider using wires as another "interconnect layer" and impedance matching won't be critical.
The only other thing that comes to mind right now is to remember to put sufficiently large pads for SMTs and to not inadvertantly flip the layout of a chip (ie. you have to mount it upside down). You'd be surprised how many seniors / grad students make this fsck-up.

 

[NutBucket]

Originally posted by: WhoBeDaPlaya
Oops, nevermind
Forget about niceties like mitering (if you're short on time). At that speed, you can seriously consider using wires as another "interconnect layer" and impedance matching won't be critical.
The only other thing that comes to mind right now is to remember to put sufficiently large pads for SMTs and to not inadvertantly flip the layout of a chip (ie. you have to mount it upside down). You'd be surprised how many seniors / grad students make this fsck-up.

Students? We do it on occasion and we're "professionals"

 

[So]

<bump> for ya. I recently (last week) did my first PCB design. Same 2 layer $33 deal

Mine was a *LOT* simpler than yours though.

 

[jmcoreymv]

Completed Layout

I was able to do all the routing in two layers. It's pretty much done except I'm waiting on the footprint for the component at the top right.

 

[So]

Originally posted by: jmcoreymv
Completed Layout

I was able to do all the routing in two layers. It's pretty much done except I'm waiting on the footprint for the component at the top right.

Which is a terminal for off board connections?