Any tips for 4-layer boards?

[Byte_Butcher]

Well... In the "old days" I used to build projects with through-hole parts on "vectorboard" (or perfboard, or punchboard, or whatever other names it goes by).
That worked for me for prototyping projects for about 30 years.
And if I made a mistake I could unsolder something and "move it over there" and re-solder it...

Then one day I woke up to find that a lot of the "cool" new parts that I'd like to use are only available in surface mount packages. Sigh.

So I finally set myself up with enough equipment to do SMD's and I started laying out (and paying for ) boards for most of my projects. But small runs of boards are EXPENSIVE, so I've gotten pretty good at maximizing my board usage to get the most parts on the least amount of real estate. I've done quite a few pretty dense 2-layer boards with good success. (Don't ask about the RTC that got laid out "upside down" on one board... I felt really stupid with a magnifying glass and tweezers bending the leads on the IC's to point upwards so I could mount it upside down on the board. )

Anyway, now I've got a new project that needs to fit in a very tight space.

I've got 6 IC's including 16F887, an RTC and xtal, EEPROM, voltage regulator, digital pot, and IR detector. (mostly TQFN, TSSOP, and DPAK packages)
There's also 5 MOSFETS, 4 in DPAK and 1 in SOT223.
And there's a few diodes, a crystal, a 470uF electrolytic cap (big!), and a metric ****load of resistors and caps, mostly in 0603 packages.

All together there's 59 parts that live ON the board, plus connectors to connect a 2x16 LCD, several external sensing devices, and power and programming connectors.

The problem? It's all gotta fit on less than *5*square inches of real estate. I've got about 2" x 2.5" of space to work with.

I've banged my head against it for days now trying to make it fit on a 2 layer board, but I just can't quite get there. I'm using the smallest packages available (or that I can work with) but it just ain't gonna go on 2 layers in the space I've got to work with.
If I go to a 4 layer board it should work . It's tight... but it'll fit.

SO... Are there any special tips I should know about for laying out 4 layer boards? I've done plenty of 2 layer, and in theory 4 layer should be easier... But 4 layer is so much more expensive and I don't want to do something obviously stupid and wreck it.
I've got power and ground on the inner layers and all my signal stuff on the outers. Everything *looks* like it's OK, but I think I've got the fear in me because it's my first time. :

Any words of wisdom before I jump into the world of 4-layer and blow $120 for 6 boards that are only 5 square inches each?



steve

[mackrackit]

I do not have any good suggestions about a four layer, but if you have the height why not two two layer boards stacked?

[Pic_User]

Hi guys,

Sounds like you guys have come up with some good ideas for squeezing the most components into the least real-estate. What might work would be insulated jumpers in place of the added layers. Circuit board designers have a prejudice against jumpers, but I have used them to avoid additional layers. Just force the layout with two layers then rearrange for the least and shortest “air-wires”. The jumpers would replace the air-wires.

We want to see pictures of your projects! Past and future.

-Adam-

[ScaleRobotics]

When I need to pack the most parts per inch, I start populating the bottom side with a few of the parts I can't fit on the top. I never had to go to 4 layer.

[Ioannis]

Triple check. What else can I suggest?

What is your design program? In Eagle there are some tools that help to find shorts or unconnected parts.

Use these tools to make it easier for checking.

Photos would be great for this!

Good luck and keep us posted.

Ioannis

[Byte_Butcher]

Thanks Guys for all the good suggestions!

If this thing works well I hope to make a commercial product out of it, so ease of assembly and a minimum amount of "external" wiring that needs to be hand soldered is important.
I want 6 boards to build prototypes, and if all goes well I'll order 100 more for my second round. So it's not just a "one of" project. and It needs to be easy to assemble and reasonably "proffesional".

In the long run, I think that Ioannis had the most practical suggestion for this board.... Triple Check!! (I knew that anyway)

I don't think that a pair of stacked 2 layer boards saves any money on board costs (4-layer costs less than twice as much as 2 layer), but probably does add quite a bit to assembly time.

I already tried putting some parts on the backside of the board, but that didn't seem to help. The back is already dense with traces and parts on the back just eats up space where traces need to go and nothing is really gained. Besides, I dislike populating both sides on SMD boards because it's a bitch to keep parts from falling off the bottom when the board is in the oven and I don't relish the idea of soldering one side by hand. I know there's special glues available for that, but that takes more time so I'd rather not populate both sides if I can avoid it.

I'm not a big fan of jumpers and "air wires". I DO use them sometimes, but again, it means more hand soldering and possibility of problems. And I'm afraid I'd need many of them for this board. Besides, jumpers require through holes, which kills space on BOTH sides of a board.

I think I'm doomed to go with 4-layer boards on this one. It's OK. I need to learn to become proficient at 4-layer anyway since parts keep getting smaller and smaller, and 4-layer does have some advantages with noise immunity.

I use DIPTrace for my schematic layout, and if I want quite a few boards I use DIPTrace for the board layout also. However, if I just need a few small boards at the lowest possible cost, then I order them from ExpressPCB, which means I'm stuck using their irritating board layout software. (Love their board quality and prices, hate the layout software)

DIPTrace does have some good tools for verifying the integrity of networks and the like. And it IS possible (with a little work) to import the netlist from DIPTrace to ExpressPCB, so I'm pretty sure my board layout matches the schematic OK.

I tried Eagle for a few boards, but I just didn't like it much. It seemed difficult and non-intuitive to learn and I finally gave it up. DIPTrace is more compatible with my brain I think.

I worked on the board a bunch last night and I think I'm about done. I need to double check the footprints of a few items one more time, and also change the pad sizes where the LCD connects, but I think it's about ready to submit... I hope.

Here's a pic of the top copper layer and silkscreen. Keep in mind that the entire board is only 2.5" (63.5mm) tall.





Some of the patterns look a little "funny" because they are laid out to accept more than one package style. For example, Q's 2-5 can accept MOSFET's in either DPAK, or TO-220 (stood up)

Thanks for all the suggestions. I'm gonna go with the "triple check" method and hope for the best!


steve