NaOH Electrolytic Problems: Where did my gold go??

[Bertho]

A strange problem occurred while I was trying to remove solder masks. I did a lot of reading and watched LazerSteve's video using the hot NaOH. I saw several other comments that said it was safer to use room temperature NaOH and wait a couple of days and I chose the safe way. I used the same ratio NaOH to H2O as LazerSteve.
A day later the solder mask was untouched (as expected after only 24 hours) but I noticed a big problem: Most of my gold plating was "gone"! More about that below.

I heated up the NaOH to 80C and after 20 minutes I pulled the boards. The LazerSteve video showed that less than 10 minutes was needed so soaking for 24 hours in hot Florida weather and then gradually warming up to 80C and sitting at that temperature only removed the solder mask on two out of about 10 boards. On a couple of other boards I can partially scrape off the solder mask using a finger nail. These boards are ancient and the solder mask material unknown.

There has been many comments that NaOH will not attack gold and I believe that to be true.

I am pretty sure an electrolytic process was involved. Either the gold was "unplated" and plated somewhere else or some other metal was plated on top of the gold. All gold plating is untouched and beautiful on pads that are not electrically attached to other circuits. Look at the picture. For example, there are four fingers that are perfect. They are unconnected on both sides. Similarly the top and bottom stripes are unconnected. All the traces and pads are still in place, they turned bluish/dark. That is also true for the white looking tracks, just a light reflection in the photo.
The solution PH is 11 as expected. I did a Stannous Chloride test and there is and it showed no gold in the liquid.
Also note that the container was stainless steel so it was conductive.

Questions:
What happened?
How shall I proceed?
How to prevent it in the future?
How to remove old solder mask?

My original plan was NaOH solder mask removal.
Follow with hot HCl to remove solder and then shake off the components.
Follow with AP ..........

There is a very significant “detail” that I have not mentioned: I ran out of lab-grade NaOH and I used a can of Drano "Pro strength crystals". The ingredient listed on the can is Sodium Hydroxide and nothing else. I was surprised when I poured the crystals that they were greenish so apparently they have added some coloring agent. I have already ordered more pure NaOH for the future use. After looking up the MSDS I got a big surprise. The MSDS lists the ingredients as:
Sodium Hydroxide 30-60%
Sodium Nitrate 15-40%
Sodium Chloride 7-13%
Aluminum 1- 5%

I am doing this as a hobby with good results on clean items like fingers or CPUs (Clean stuff is always used for demos). Unfortunately, I ran into problems with the old through-hole PCBs.

I am keeping my fingers & toes crossed that it will not be too difficult to recover and proceed.
Bertho

 

[Lino1406]

I assume acid rinse will expose the gold. Tin solder will be removed with hot and prolonged NaOH treatment, besides other possibilities

 

[g_axelsson]

The gold has been plated with tin. Different metals creates different potentials and some tin dissolved as stannate is plated back onto the exposed gold. When there is no other metal than gold exposed there is no potential difference to plate the tin.

There are a couple of threads about gold in lye and that it seems to disappear. It's a topic that pops up every couple of years.

Göran

 

[Bertho]

Thanks Lino and Göran!

For the solder stripping I see many suggestions for hot HCl. What temperature and concentration should I use?

Göran: I am from Sweden too but live in Florida now.
Bertho

 

[Lino1406]

If the solder is lead less, higher temperature/concentration only shortens time

 

[Bertho]

The PCBs are very old through-hole type so the solder is tin-lead.
For a very little extra safety I I am diluting the HCl with equal amount of water so Is should have 16% HCl.
Now I am waiting for it to warm up to 80C so I can do some testing.
Bertho

 

[Findm-Keepm]

It's simple - HP used Parylene conformal coating on those boards. NaOH is not the removal medium - you need to use either Tetrahydrofuran or abrasion.

And yes, for about 80-90% of the boards they made, there is a gold plating under the Parylene - but it's often uneconomical in getting to it.

 

[goldsilverpro]

Concentrated H2SO4, overnight at room temp., might work. I played with it several times with success, but I have no idea what mask was on the boards I used it on. It's messy. Wear a full face shield if you try it. If I remember right, the H2SO4 could be reused.

 

[Bertho]

Thanks guys!
Parylene is tough stuff. I will try the H2SO4 to see what happens. It is not economical since I am doing this a as hobby and the amount of time spent can never be economically justified.

An OT question: I have been searching for an option to get notified by email when there is a response to my questions but have not been able to find that option. Where is it?
Bertho

 

[FrugalRefiner]

An OT question: I have been searching for an option to get notified by email when there is a response to my questions but have not been able to find that option. Where is it?
Bertho

Unfortunately, it's not available here.

Dave