Category: PCB

There are several kinds of etchants; I am using Ammonium persulfate solution from MG chemicals. It comes as loose powder or crystals. The content releases gasses over time, so you should punch a small hole on the lid of its plastic bottle. These are dangerous and corrosive chemicals, so use a lot of common sense when working with them. There are several variables in the process that you can try to control for the most efficient etching. The goal is to have the shortest etching time and minimal use of etchant chemicals. There is a sweet point in that compromise and you can reach it by experimentation.

This step is optional, and many people skip it. Copper will oxidize over time, and this simple step will protect it. I’ve found several methods to be reasonable but I always use the liquid tin to start.

The best bits to drill holes in your FR-4 board are tungsten carbide bits. They are, however, more expensive. You can also use HSS (high-speed steel) bits, but they will quickly dull. To drill larger holes start with a small bit and then progressively go larger.

Have a good working light shining at the board. This is crucial to get the holes well centered, especially for smaller bits.

It is time to solder through-hole and surface-mounted components.

I use Chip Quick solder paste for SMT components. When not in use, keep it in the freezer to extend its shelf life (it should not dry up). Chip Quick has a very low melting point and it’s perfect to use for SMT since you can apply tiny drops of it through a syringe it comes with. Apply small drops to pads and then position SMT components on them using tweezers.

Even tinned, the PCB traces will still corrode. Soldered joints will also oxidize since the solder is exposed to air.

After soldering, the board could be further protected by applying a coat of clear lacquer. The lacquer dries fast and can be soldered through afterward if needed.

I use this lacquer for final board protection:
http://www.testors.com/products/137179

This particular board did not need an enclosure. Instead, I mounted it on raisers. Holes for raisers were already drilled.

If you do have an enclosure, you may want to try to fit it: drill the holes and/or cut openings using a tool like a nibbler.

This is general guidance for the initial testing.

Thoroughly examine the board for physical defects: are all traces complete or there are breaks?
Are all pins soldered and is the quality of the solder joints acceptable? The soldering point may be missing (forgot to solder it), may have a bridge or a short, or it may be a poor joint (cold point). Just redo it.
Is any component missing? (Something blunt but may happen!)
You may need a magnifying glass or a loupe with a strong light.

Check again the orientation of components: LED, diodes, connectors, chips – where is pin 1 located. Check the orientation of tantalum and electrolytic caps. On SMT components, this can sometimes be confusing.

The quality of the tools that we use to write software for projects like this is very important when selecting which MCU we may want to use. Atmel has a very robust IDE based on Microsoft’s Visual Studio (which is arguably the best development environment out there). Coupled with a JTAG interface, writing and debugging firmware with the Atmel toolset becomes a pleasure.

Their ASF (“Atmel Software Framework”) is a good collection of ready-to-use projects and components. Investing time to learn that environment is the best way to become very efficient when using it.

There are a few things that I did not get to try this time around:

• Adding a bitmap (for example, a logo) to the board’s layout. I believe you create it as a custom part and use it from your library? Or you somehow download an image.
• Similarly, it would be useful to make bitmaps for parts like diodes to help orient them as a guide when soldering.
• How do you remove Eagle layers like Cream, Glue, etc. which are populated? There are also many other tricks to learn with Eagle.

If a project is large enough, one could add a board ID that is readable by the software. That could be done by GPIO pins connected to, or “fused”, to certain voltage levels creating a short ID. This may be overkill for simple boards, but still, it is a known industry practice.

I really wanted to find out exactly how much etchant would I need for a given board. That would be at least a theoretical minimum to etch all exposed copper, so after some calculation, I made this table. In practice, you would probably want to double that amount to get it to etch faster, but that’s a good starting value to measure.