Through Hole Plating .. actual procedure ?

[donkey41]

hello there,

can anyone please explain me, what is the actual procedure of THP in a simple double sided pcb ?

according to my understanding, THP needs firstly an electroless hole plating and then electrolytic plating .. correct ?

if the above point is correct then there could be 2 options for THP

1 . first drill the pcb, and do THP, then make traces of the actual circuit and etch the rest of the clad-board.

2. first make traces and etch the board, and then do THP.


in the first option, i have the advantage that every hole is reachable for electroplating process because the board is not etched yet - but if i etch after plating holes, this will etch holes as well ! .. am i right ?

exactly the same, if i make traces and etch the board first, as above in second option, then not every hole is reachable for later electroplating !!??

i may have absolutely missed something here and my understanding could be completely incorrect.
please help me understanding this and solve this "egg-chicken" like problem

i want to make double sided THP'ed pcb at home, that is why i am asking this.

 

[FvM]

Your consideration is basically correct, galvanic plating can only occur at areas connected to the cathode. Thus the usual
THP process is performed on the copper laminated board.

There is however a principle option to make the THP with chemical copper only. It's not used in industrial production,
because it consumes too much chemicals and is rather slow, but have tried it a longer time ago. I used a chemical copper
process starting with a standard silver "seeding" and a special higher concentrated disposable reductive copper bath.
The special trick was to put a thin coating on the etched board before drilling, that was washed off after the seeding
process to avoid plating of the copper-free board areas.

Although I could prove the basic feasibility, I found the process rather awkward. The reductive copper bath is also dealing
with unhealthy chemicals, e.g. formaldehyde in my version. I had the opportunity to use a laboratory fume hood, but I wouldn't
suggest it for processing at home.

 

[donkey41]

Thanks FvM for your reply.

As you've written, you have tried non-electrolytic process can also be applied but you've also mentioned the possible hazard of unhealthy chemicals.

There are some points which I could not get :

a. what is exactly _standard silver "seeding"_ ?
b.

The special trick was to put a thin coating on the etched board before drilling, that was washed off after the seeding process to avoid plating of the copper-free board areas.

assuming you put that thin coating AFTER etching the pcb, you drilled the pcb and "silver seeded" holes and then WASHED that thin coating .. am I right ?
If I understood correct, why did u need a thin coating and how did you plate holes ? I would be very thankful if you could explain it

I have also read about similar process. I don't want to use those chemicals.
Actually, I've come across an idea, rather exciting, to make a thin layer of conducting material in hols without unhealthy chemicals, but it (probably) will only make a very thin conductive layer. It means that I have to use electroplating later for having sufficient copper (or conducting material) in hols.

As far as i can remember, the process I read on internet, THP conducted in multiple steps.

1. chemical (very thin conducting layer) plating in holes.
2. electroplating the copper in holes.
3. electroplating holes with some copper-etch resisting material.
4. etching the copper for traces.
5. etching the "some other copper-etch resisting" material from holes.

unfortunately I can not remember where exactly I read this. If it was not exactly the same then at least it was similar to the above mentioned process.

can this be correct ?

have you any other suggestion for covering plated holes for copper etch process ?

 

[FvM]

I was under the assumption, that have read some basic literature about electroplating before. As for any industrial technique, there are text books with formulations and descriptions of processes.

In a short, metal seeding (sometimes also called activation) is necessary to restrict the chemical deposition to the product area

Generally, chemical copper plating always involves multiple process steps. This is also the case with standard industrial PCB processes. As you mentioned before, electroplating can be only performed before etching the traces, this also implies drilling before etching and lamination of a photo resist.