There’s an old rule of thumb circling around the PCB designer communities — avoid acute angles (<90°) in PCB design, lest you incur the wrath of the acid trap. While that may have made sense over ten years ago when thermally activated etchants were widely used in PCB fabrication, today’s manufacturing methods have greatly improved.
So what exactly are acid traps and are they still a problem in 2019? In this post we’ll get to the root of this old engineering heuristic.
Before we can explain what an acid trap is, it helps if you’re familiar with a critical step in the PCB manufacturing process: etching.
Ever wonder how the traces in your design are placed on a physical PCB? While it might look like those copper traces are drawn on an empty slab of fiberglass, in reality they are etched out of a uniform copper surface using chemical solvents.
A typical DIY etching process might look something like this:
Start with a clean board of fiberglass (e.g. FR4) plated on one or both sides with copper.
Transfer the PCB trace layout to the copper surface with an etch resist or toner.
Dip the board into a bath of etching solution to remove the excess copper not protected by the toner.
Clean the board of excess etching solution and dry it.
Remove the toner to reveal clean copper traces on fiberglass board.
The key factor in these wet etching processes is the time it takes to etch a board. Reaction times are well tabulated, allowing you to control the amount of copper removed by keeping track of the exposure time. Soak too long and you run the risk of etching solution eating the copper substrate under the masking material (etch resist/toner).
A number of etching solutions have been used over the years, but these are the most common you’ll encounter today:
Ferric Chloride (FeCl3) is cheap and aggressive. The solution is dark brown making it difficult to see the product being etched. It also reacts with tin, a popular etch resist.
Ammonium Persulfate (NH4)2S2O8 is a strong oxidizer. It’s clear and plays well with tin based etch resists. It is generally considered faster than FeCl3.
Sodium Persulfate Na2S2O8 is a strong oxidizer. It’s clear and plays well with etch resist pens and toners. It is generally considered faster than FeCl3.
Cupric Chloride (CuCl2) may only have half the etch rate of FeCl3, but it has excellent regenerative properties on the production line.
An acid trap is simply any sharp corner in your trace pattern that could trap the harsh chemical etchants used to strip excess copper from a board during manufacture. When etching solution pools in a corner, there’s a risk of it tunneling through the etchant resist and corroding your traces and creating a faulty connection or open circuit.
As you may have noticed, examples of right or acute angles in modern PCBs are not too hard to find. In fact some designers take pride in creating unusual designs and shapes. What gives?
The reality is that a majority of manufacturers have greatly improved the traditional etching process in a number of ways:
Photoresist: Manufacturers can uniformly coat a copper surface in photoresist, apply a dry film with a negative print of the trace layout, and use UV light to harden the resist on the exposed traces. The board is developed (wash away the excess photoresist) before being subjected to the etching bath. The photoresist protects the traces from etching and can be removed after the board has been cleaned of etching solution.
Dry Etching: Some state-of-the-art fabrication houses use the plasma of fluorocarbons, oxygen, chlorine and other gases to remove the excess metal. This method is precise and cleaner than conventional
Precision: Today’s fabrication houses have tighter control and better equipment. Even in situations where photoresist isn’t used, manufacturers are still capable of cleaning away any excess etching solution from your board.
Photoresist has been around for a long time, but its cost has come down considerably in the last couple of decades. As a result, it’s not too hard to find affordable fabrication houses that use photo etching in their production processes. So the next time you hear someone warning you about acid traps, you’ll know what to say.