6
\$\begingroup\$

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.

Dendrites-2

Dendrites-1

\$\endgroup\$
2
\$\begingroup\$

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

\$\endgroup\$
  • 2
    \$\begingroup\$ Well, that "flux" in your picture is actually never a good idea for electronic soldering. It's quite a strong beast for plumbing, gutters and other "heavy metal"; the label says: "iron, steel, copper, brass and tinned sheet metal" And it says: "Remove any remaining flux [after soldering]!" \$\endgroup\$ – zebonaut Oct 15 '15 at 11:13
  • \$\begingroup\$ +1 at @zebonaut's comment. It also recommends safety glasses and gloves and extractor hood. S26 means very toxic when in contact to skin or inhaled. This is really good stuff to make iron or heavily oxidated stuff solderable, but residues of the zinc-chloride may corrode tiny structures on a PCB. \$\endgroup\$ – sweber Oct 15 '15 at 12:07
  • \$\begingroup\$ absolutely, lesson learned. In fact, datasheet is somehow missing those guidelines which are on the bottle (I don't read German). Nor it seems to have characteristics on conductivity etc. \$\endgroup\$ – FlegmatoidZoid Oct 15 '15 at 20:11
  • \$\begingroup\$ Thanks for taking the time to post your findings. Often enough here someone will post a fairly mysterious problem, gets a bunch of suggestions, but we never hear back on what the problem actually was... Interesting enough this cause of dendrite growth (acid medium left on the board) didn't get much coverage in the sources I've looked at. They all emphasized humidity. But higher currents/voltages were definitely known to be a factor. \$\endgroup\$ – Fizz Oct 28 '15 at 11:34
1
\$\begingroup\$

Maybe this link might help

http://www.edifgroup.com/case-studies/contamination-and-moisture-effects-on-printed-circuit-board-reliability

Certainly looks like some form of contamination though

\$\endgroup\$
  • \$\begingroup\$ When this link dies, your answer won't have any information in it. Can you add the important details from this link to your post? \$\endgroup\$ – Greg d'Eon Oct 15 '15 at 12:24
  • \$\begingroup\$ Among the causes listed in that paper are "Ionic contamination [that] can arise from fluxes used during soldering processes". Which was actually the main cause here. The paper also mentions the testing standards in this respect that are performed at factory for boards: "The development of standards of cleanliness and standard test methods date back to the early 1970’s in the US, culminating in the publication of the original American MIL standards. It was from this work that the original pass/fail criteria of 10µg NaCl equivalence per square inch or 1.56µg per cm2 were first proposed." \$\endgroup\$ – Fizz Oct 28 '15 at 11:40
0
\$\begingroup\$

This comes about due to the reduction of lead in solder and tin alloys. NASA has done research into this and one solution is to coat the board with a 'conformal' coating: " NASA GSFC experiments have shown that use of Arathane 5750 (formerly Uralane 5750) conformal when applied uniformly to a nominal 2 to 3 mils thickness can provide significant benefit by containing whisker growth outward through the coating. "

More can be found on NASA's page about Tin Whiskers (dendrites) here: http://nepp.nasa.gov/Whisker/background/index.htm

\$\endgroup\$
  • \$\begingroup\$ The article you mention is about whiskers, not dendrites... And as NASA says in its own article, they are not the same. The OP looks confident his problem is not about whiskers but dendrites, so your answer might be out topic. \$\endgroup\$ – Macmade Oct 9 '15 at 0:10
  • \$\begingroup\$ ... Or maybe not, but then I think you should really explain why you think the issue is actually whiskers... : ) \$\endgroup\$ – Macmade Oct 9 '15 at 0:12

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.