Eric Sorensen

Step Two: Design & Preparation

A complete tutorial about designing circuit boards is out of the scope of this guide- but I can give you specific advice about designing a board for home etching. Cadsoft Eagle is the most widely used software used to design circuit boards at home and professionally. I recommend using this software and my advice is tailored specifically to it.

One of the limitations in etching at home is the resolution you can achieve. Designing boards with wider traces and higher clearances will prevent shorts or breaks. This can be done manually using the “changth width” command in Eagle. Fortunately,  there is an easier way for those of us that use the auto router. By tweaking Eagle’s design rules (DRC), its possible to have Eagle automatically output an at-home friendly circuit board.

1. Placing the parts and auto-routing.

First place the components where you’d like them on the board. How to do so is beyond the scope of this guide. After you have done so, type “auto;” to auto-route the board. The image abouve is a typical output of Eagle auto-routing. The red wires make up the top layer and the blue make up the bottom. This by itself would work well for a commercially produced unit, but since this board is intended for home etching we can tweak it for better results.

2. Disable the top layer.

The first step is to disable the top layer. In order to do so, click on the “Auto” button or type “auto” and hit enter. On the left side of the window are the options for preferred direction. Using the drop down menu set the preferred direction for the top layer to “n/a”. This completely disables the top layer so that next time you route the signals they’ll all be on the bottom. Try this by typing “ripup;” and then “auto;”.

3. Update the DRC and manually edit

Type “drc” to bring up the design rules. This is where we can tweak the rules Eagle uses when automatically routing a board. There are 10 tabs in this dialog. Four of them are useful for an at-home etcher. Please note: The values I propose here are a starting point. Please tweak them as you see fit. For instance, simpler boards can often handle wider traces.

Clearance:

Wire x Wire: 25mil

Wire x Pad: 25mil

Pad x Pad: 25mil

Distance:

Copper/Dimension: 40mil

Sizes:

Minimum width: 16mil

Restring:

Min (Top and Bottom): 16mil

Next, ripup and auto again. You should see the routes are much larger now. Now is your chance to manually tweak any of the route widths using the “change width” command. Particular attention should be paid to any traces involved in power transmission because  these often need wider traces to handle higher currents.

4. Fill the empty space

The  last step in preparing a design for etching is using the “polygon” command to fill in as much space as possible. During the etching process anything you see as black will be removed. By filling all of the black space in you will limit how long etching takes and get the most out of your etching solution. Also, since eagle allows you to connect the filled in layer to a specific signal you can create a ground plane for your circuit.

In order to use the polygon command, type “poly” and hit enter. If you want to tie the polygon to a particular signal you should type the signal name afterward. For instance, “poly gnd” will create a ground plane. The poly command will allow you create any kind of polygon, but a rectangle around the entire board is usually sufficient. After you have mapped out your polygon, type “ratsnest” to fill it in.

5. Print

At this point, you are ready to print and copy the design.

If you haven’t already, type “disp” and disable all the layers not actively involved in etching. The only layers you should have visible are “Pads”, “Vias” and either “Top” or “Bottom” depending on which side you are printing. Select file->print and select black and solid from the dialogue that appears. Mirror the document only if you are printing the top layer. Print a copy of the document at real-size.

6. Copy

The last step of the process is to copy the document to some paper with a laser copyer. Everyone has their own opinions about this step- here is what has repeatedly worked for me.

Go to Kinko’s and ask to use some “Glossy Cardstock” in the black and white machines. Warning: they might give you a funny look. Tell them you know what you are doing and head over the the machines. Using a black and white copyer, copy your document to the cardstock. Make multiple copies because they are cheap and it’ll save you a return trip if the transfer doesn’t work well.

At checkout, the Kinkos employee should charge you for the cardstock and for a black and white copy. They should NOT charge you for a color copy. Plan on spending no more than $1.50 to get your copies.

Head home. At this point, you are ready to print and copy the design.

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Tags: howto, howto etch circuit guide