Electronic – Dendrites (ionic contamination) on home-brew PCBs


I've been making PCBs myself for 5 years already.

  • I use UV curing
  • develop in NaON (SodiumHydroxide)
  • etch in Na2S2O8 (Sodium Persulfate)
  • clean finished PCB with acetone

Recently, I've noticed variety of weird glitches with newly made PCBs – hundred kilo-ohm resistance across separate tracks, power leaks, self-resetting MCUs and self-flashing LEDs or self-turning FETs.

When inspecting PCBs under volates (up to 12V) i've noticed weird spots growing in front of my eyes. Absolutely clean PCB gets dirty in approx. 60 seconds.

Further inspection led to a single conclusion – DENDRITES (or ionic contamination).

I've thrown away all my chemicals and ordered a new set of blank PCB material.
I've event replaced standard water with the distilled-one as well as bought new welding Flux (Weller).

I do still get the same effect.

What else should I try?

ps: PCB in the photos below was crystal clean 60 seconds before powering it up with a 12V rail.



Best Answer

I am answering my own question as I was able to identify the root cause.

It is due to Flux. I am using Weller F-SW12 (datasheet) I haven't experienced problems in the past because my PCBs were mostly for 3.3V/miliAmpere range (however, quartz filtering capacitors were often going crazy, now I know why). The dendrites have appeared due to multiple factors:

  • High Voltages (relatively) - up to 16V
  • High Currents (relatively) - 30+ Amperes
  • Non-perfect Acetone bathing (for excess Flux cleaning)
  • Not washing final PCBs with distilled water

Normally, after acetone bathing I was drying PCBs. That process was leaving a thin layer of highly diluted Flux, thus spreading that Flux across the entire board.

I fact, I was able to recover "contaminated" PCBs by re-washing those with acetone and then intensively washing it all under the running water, finally rinsing everything in the distilled water.

To prevent future PCBs from dendrite formation I have replaced the Weller flux with simple rosin/alcohol fluid and have not observed dendrite growth ever since. Furthermore, this mixture doesn't require to be carefully treated/washed as is not conductive to electricity (It seems).

Weller F-SW12