8
\$\begingroup\$

I've heard about acid traps and that they are something from the days of yore. But what are they and how are they created?

\$\endgroup\$
  • \$\begingroup\$ I believe they refer to PCB technology: adding extra mask area in sharp bends to avoid overetching and so on - but I am not sure, hence why I typed this as commen, not actual answer \$\endgroup\$ – Joren Vaes Jun 27 at 8:21
10
\$\begingroup\$

This image contains an example of an acid trap:

Acid trap example.

Image borrowed from this earlier question.

That image also ended up on an AutoCad site as an example of an acid trap.

That yellow circled junction can cause etchant to collect. That will cause too much copper to be removed - this can cause an open circuit if it etches through the trace.

The trap is formed by the acute (less than 90 degree) angle.

As this other question and answers explain, this shouldn't be a problem with modern PCB fabricators.

If you make PCBs your self, then it can still be a problem. You can't use the techniques at home that the fabs use - it takes chemicals and equipment that you won't have at home. For "home made" PCBs, you'll want to avoid 90 degree and sharper angles so that you don't get acid traps.

Besides trapping etchant, I've also had sharp corners trap a bubble and block etchant. That caused a short circuit.


Since acid traps can also form on 90 degree angles, you will often see right angle bends that are made of two 45 degree bends.

That would look like this:

enter image description here

Image borrowed from this tutorial.

That tutorial mentions it in connection with the (supposed but unfounded) concern about the effect of 90 degree bends on RF signals but the image clearly shows how to route 90 degree bends as a pair of 45 degree bends.

\$\endgroup\$
  • \$\begingroup\$ Is the top left rectangular pad in the image another example of an acid trap? \$\endgroup\$ – Bergi Jun 27 at 21:46
  • 1
    \$\begingroup\$ @Bergi: The trace connected to U1 pin 1 would also be considered an acid trap. \$\endgroup\$ – JRE Jun 27 at 21:57
4
\$\begingroup\$

From https://community.cadence.com/

Acid Traps are generated when a gap from pad to trace is too small. They can be avoided by increasing the gap or moving the trace so that it exits the pad lower down the pad. Or you can use a gloss command to add a fillet from the pad exit.

and also from [https://www.edaboard.com/showthread.php?270524-PCB-Layout-diagonal-pad-exits-are-OK ]

It is not a technical fact, it's a descriptive term for structures that can't be exactly etched. Similar to same net copper features with too low clearance, they don't cause functional circuit problems, but affect the correct appearance of a board. They also make it difficult to control etching time by visual result.

Even if they are "the same net", they can cause problems (EMI problems, star grounding, 4-wire feel, etc.)

(collected from other forums)

\$\endgroup\$
  • \$\begingroup\$ One could argue that even if they are 'same net' they could cause problems (EMI issues, things like star grounding, 4-wire sense, etc) \$\endgroup\$ – Joren Vaes Jun 27 at 8:44
  • \$\begingroup\$ That's right, I edited my answer. \$\endgroup\$ – Joost Jun 27 at 8:55
  • 3
    \$\begingroup\$ You have directly copied part of your answer from another site, without quoting the copied material or citing the source. This is plagiarism and is not allowed on this site. Please read carefully the instructions at electronics.stackexchange.com/help/referencing \$\endgroup\$ – Elliot Alderson Jun 27 at 11:04
  • 1
    \$\begingroup\$ You're right,but I've put it under it comes from other forums. In the future I will add the sources if I copy something. Thanks for the link. \$\endgroup\$ – Joost Jun 27 at 11:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.