You've got some software, and you're ready to start laying out your design. How will your design file translate to a real PCB? Layers.

Layers

Your design software lets you work in different layers.  Each layer represents a different part of the circuit board and the design software helps you figure out What goes on each layer.

Boards start with a substrate - this is the thick, meaty part of the circuit board.  Usually you want to use FR-4: it's a fiberglass / resin compound with a dull yellowish tint. The first layer on top of that is the trace layer.  This is the layer where you're going to layout your circuit.  If your design only has traces on one side of the board, it's called a 1-layer design.  If you have traces on both sides of the board, your design will have 'top' and bottom' trace layers, so it's a 2-layer design.  You can do more layers, like 4 or 6 layers - these boards are constructed by making a PCB sandwich.    A 1 layer board is the cheapest and 2 layers is a bit more expensive.  4+ layer boards jump in price - I don't suggest doing a 4+ layer board unless you really have to.

FR-4 With a trace layer

You can stop there.  If you order a board with just a trace layer, you'll get something that looks a lot like a half-board or full-board.  You can see the substrate (dull yellow) and the traces.  The only problem with this is that it's fairly easy to make solder bridges.  So a lot of designs add another layer - a soldermask.

The Soldermask (a.k.a., Solder Resist) is layer of polymer applied on top of the trace layer.  The purpose is to protect the traces on the circuit board and to keep solder from making bridges and shorts while you're soldering.  Your PCB design software will automatically figure out where the solder resist should go as you lay down parts, but most software applications will also let you remove more soldermask just by adding a shape to the solder mask layer.  When you order boards, you can also choose what color of soldermask to use.  Most folks use green, but pcb manufacturers also offer white, black, blue, red, and a few other colors for a small charge.

A board with traces and a soldermask is complete.  You can also add a silkscreen layer to either side of the board.  The silkscreen is the printed stuff on top of the soldermask and the purpose is to show where parts should go and to aid in assembly.  If you're going to add soldermask, I suggest you also add silkscreen.  In your software, most parts you lay down will have an associated pattern on the silkscreen, but you can also add shapes, images, and text to the silkscreen layer.  I usually add the website address to the silkscreen and the name of the project, but it's up to you.  I usually will add silkscreen to the top of the board, only.  If there's an image or text I want to add to the bottom, I'll usually remove parts of the bottom soldermask to make lines and text.  The text and lines appear as shiny silver on the board.

Export in DipTrace
(click to enlarge)

Files

When your design is done, you'll save it in the default file format for your software (.brd for Eagle, .dip for Diptrace), but the pcb manufacturer will ask for Gerbers.

Gerbers are the standard file format for the production of PCB's.  Each design actually consists of several files - one gerber file for each layer.  So a 2 layer design with soldermask on both sides and silkscreen on one side will consist of 6 files: top_copper.gbr, bottom_copper.gbr, top_mask.gbr, bottom_mask.gbr, top_silk.gbr, board_outline.gbr.  A note about file names and extensions; different software programs use different names and extensions.  You just want to make sure each file is clearly named so the manufacturer knows which layer is which.

A gerber file is pretty simple - it defines
(1) A library of simple shapes (donut, square, rectangle, etc)
(2) Where on the design these simple shapes appear.

That's it - You can see that the Gerber file doesn't identify what layer it is, that's why identifying the layers in the filename is necessary.  Be sure to generate RS-274-X gerbers when exporting gerbers from your software.

In addition to the gerbers, the manufacturer will also want a 'drill' file.  The drill file tells the drill machine where to put holes and what the size of each hole is.  In DipTrace, you just need to go to file -> export -> N/C Drill.  In the export screen, hit the 'auto' button to assign each hole to a tool number.  You'll take the gerbers and drill file, zip them into a zip archive and send them out to the manufacturer for quotes.

If you're using ExpressPCB, there are no Gerber files.  ExpressPCB lets you order your boards directly from the software program by selecting Layout -> Order boards via the internet.  Obviously, this is a much easier way to order boards, but the downside is that your expressPCB design files are only good at ExpressPCB - you can't send the files to another manufacturer to get a competitive quote.  Advanced Circuits' PCB Artist software also doesn't generate gerbers.  You can email ExpressPCB after you place an order for your design, and they can provide gerbers for a $60 charge.  Advanced Circuits will provide gerbers for free, after you place an order.

Stay tuned for Part IV:Ordering PCB's & What to Expect!

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