What is the black sticky residue after heating copper hypophosphite?

I am experimenting with double sided PCB (printed circuit board) via (connections from one side to the other) holes activation.


The activation is the process of making the walls of holes electrically conductive enough to then electro plate copper. Forming a copper tube with good mechanical and electrical properties.


One activation technique consists of coating the surfaces to activate with copper hypophosphite mixed with amonia water. Then when heating at 150°C, the solution burns and leaves behind a conductive surface.


I understand the reaction produces nano particules of metallic copper. Annoyingly, this also leaves a sticky residue. Which is relatively hard to clean. But successfully cleaning it also takes most of not all of the conductive layer away. And of course it's pretty much impossible to clean within small (less than 2mm) holes anyways. Within holes this residue can leave too rough of a surface, which electroplates badly.


I know there are other techniques for via holes activation. This one has the advantage of being pretty fast with minimal steps. Which is why I am exploring it. Dip in activator making sure it gets within the holes. Cook past 150°C until black. It's now conductive. Sometimes as low as a few ohms.


The activator solution was made with calcium hypophosphite, copper sulphate, amonium hydroxide, and water. There is a bit of excess calcium hypophosphite compared to the sulfate. I filtered the calcium precipitation out. And I diluted the solution to about 15% of amonia. Finally I added 1% of non ionic surfactant (triton X).


What is the sticky residue made of? What could I try to reduce the amount of residue?


PS: I am no chemist.




My leading suspect is decomposed TritonX. [TritonX](https://en.m.wikipedia.org/wiki/Triton_X-100) is an organic compound used as a surfactant, or synthetic "soap".


You might try to clean your substrate first with TritonX (only) solution as a *separate* step first, then rinse with distilled water.


Make your activator solution *without TritonX*, and apply it after cleaning.


After heating, there should be less undesirable residue.


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Three months later I hope you are still interested and testing. The black residue is one (if any) of the disadvantages of this method of activation. From the literature and internet info available, there is no clear states about the need (or not) to rinse/blow/dry this residue to a minimum.


I also never trusted about the use of soap (no matter how pure), because is for sure that there are some extra ingredients added. I always believed that the use of soap is only as a surfactant and to ease the surface tension of the liquid activator, so I will be using ethoxylated nonyl phenol instead of soap (warning: like you, I am not chemist, so DO NOT trust what I am saying).


Your industrial 30% ammonia is enough and very good for this application. You do not need reactive lab grade purity for this process. TritonX seems to me very overloaded for this, so I prefer a purest surfactant like ethoxylated nonyl phenol (hope to test it in a few days or weeks).


Another not very clarified step is the oven process. When I was into PCB shop manufacturing floor, all the oven processes were carried through batch racks, so it is uncommon in this process to enter the PCBs into the oven with the activator still wet (a no-no).


You stated that you post-diluted the ammonia to 15% ¿why? ¿it was because of irritating odor? ¿another reason? Let me ask you some questions so maybe we could find some causes... (also please, clarify, at which stage, if any, did you post-diluted ammonia)


1. Of course, at first, when adding ammonia water to the solution you must be at outdoors and with some respirator mask if possible, because of the pungent odor of ammonia.
2. Then, after the complete activator solution is ready to use, and when you were hole-activating a PCB, how strong was the ammonia odor? This is a good indicator if the solution was prepared correctly. Did you was indoors? Did you make the activation process without using a respirator mask? It was odor tolerable? Did you perceive ammonia odor at this stage? If so, how much, none, low, medium, high odor, not-tolerable?
3. At the oven process, again, how pungent was the ammonia odor, if any? Did you had the oven indoors, so you can perceive the heating process odors? Take in consideration that a very precise monitoring of the oven temperature is needed since you do not want to overheat the ammonia and enter some decomposition (for health issues and for the activation process itself).


A convection oven is mandatory so you can place your PCBs vertically (NOT horizontally), and again, this is influenced if some drying is needed (or not) into the activated boards (more testing needed at this stage to observe results).


Regards.


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