Etching In House

How to use a very expensive etching setup to make nice PCBs
Intro

For a certain number of projects, including first-prototype, surface mount 'breadboarding', layout experimentation, rapid multiple-revisions, and 1-hour deadlines doing your own PCB etching can be quick, clean, and very very inexpensive. The method set up in the Media Lab basement is somewhat-similar to that of large pcb manufacture shops, except in scale and automation. There is no system for through-hole plating, automated drilling/routing or multilayer design. However, you can make precisely aligned doublesided boards with simple registration techniques.

Added 2/7/04: The etching setup is now in the Holography darkroom, in the LL across from the shops. To get access/training please email or visit me (ladyada@ml in office 020D)

Added 2/7/05: Ha! I graduated. Bug someone else in the Computing Culture group...

When To Use

A tool is only good towards the application it was designed for. Using the etcher for something other than what its good for will frustrate you and waste time.

When using the spray etcher is a good idea:

• You need the boards quickly
• The substrate is 1/16" with 1oz or 2oz copper either FR4 or paper phenolic (preferred)
• The design is single sided
• There are many surface mount parts
• The design is double sided, with no hidden vias (vias underneath surfacemount parts) or through-hole part vias (vias that also serve as through-hole component holes) that you cannot solder on both sides
• You can visually scan for shorts
• The design follows a 12/12 mil rule (this is probably not true, I'm sure 10/10 or even 8/8 is fine, but yield will drop) That is, 12mil minimum trace width, and 12mil minimum trace distance.
• You only need a few boards, or are willing to live with a yield as low as 50%
• Drill hole locations can be imprecise (up to 10 mils off!)

Conversely, when using the spray etcher is a bad idea:

• You dislike wet chemistry/cleaning
• Are using exotic substrates
• There are many vias, or throughhole parts, and you dont want to drill holes/solder vias.
• You want the boards to be perfect without checking for shorts or opens
• You need printed overlays, multiple layers, through hole plating or solder masks.
• You want many parts. Remember that for $200 you can just about get as many boards as your heart desires, and from a reputable PCB manufacturer.

Getting the Layout Ready

In this step you will prepare your layout for etching. For this step you will need: a quality laser printer, a sheet of transluscent paper.

The photoresist method we use in PCB etching is a positive process, which means that when UV light hits the resist, it softens, and then is washed away. What remains is a positive of the PCB design. What this boils down to is that you will want a positive printout of your PCB design (black where there will be copper). Preferably mirrored. Most all PCB design tools let you print out your layout mirrored. Or you can mirror your entire design in the software. Whichever. Also, if possible, have it print white holes where you will drill, these will be your drill guides. Because the drill holes will not be precise, make your annular rings (the copper around a drill hole) larger than normal. Since you will lose as many as half of the design to exposure/development/etching flaws, tile 2-3 times layouts as many as you want.

In Eagle: After your design is ready, go to the CAM Processor, and open the "layout2.cam" job. As output select PS. Change the extention to ".ps" as well. Be sure that Mirror is selected but that Fill Pads is not. Then open the ps with any free postscript viewer and print it. (Also you could print it to "Gerber274x" and use a gerber viewer, or any other format you can print.)

After you have verified the above, print your design to a high quality (600dpi at least) laser printer, in monochrome mode, onto a white piece of paper. Double check that it is as you want it, in the correct orientation, enough tiling, mirrored, dark ink, slightly smaller than the PCB you have, etc. Now print it onto translucent paper, there is a box of it in the cabinet underneath the etcher, in a thin cardboard sleeve.

Left, plain paper test. Right, translucent paper. Both are mirrored.

In this picture, I have two layouts I want to etch. I tiled one three times and one twice. The one I tiled three times has a very fine pitch IC (TSSOP-16) so it is less likely to come out. On the left is opaque laser print. On the right, translucent paper. Note that the design (noticably the text) is mirrored.

Exposing the Board

This step transfers your layout design to a positive-resist PCB by exposing UV light to the sensitized PCB with the printout as a mask. For this step you will need: a presensitized positive photoresist copper clad board, scissors, tape, a UV bulb and thin plate glass or exposure unit. This step takes 5-10 minutes.

First cut out the layout leaving a few millimeters of space on the edges. If you are going to perform the exposing and etching in the same session, go ahead and turn the etching machine on now, since it takes 10 minutes to warm up.

Cut the translucent paper, leaving at least a few mm boarder

Next, locate an unused PCB. There is a bag of them in the cabinet underneath the etcher. You may need to cut a larger board down using a metal shear. Using a larger PCB than necessary is wasteful, and exhausts the chemicals faster than a board that is well-fit. You can also purchase your own stash of PCB material in many different substrates, thicknesses, etc. and, of course, that is encouraged. (See "Where to Purchase Supplies, at bottom")

The best PCBs are those that have a plastic sticky sheet protecting them (some are just sold in a plastic bag) so that they can be handled, drilled and sheared without excess UV exposure. The plastic is easy to peel off:

The underlying substrate should be a greenish colour

Place your design on top of the exposed resist, ink-side down, so that it appears not-mirrored when you look through the paper at the board. Placing the ink closest to the resist means less light can leak around and cut into thin traces. Next, tape down the design on two edges, so that the tape does not overlap any of the layout, but holds the paper flat...

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