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Printed Circuit Board Plated Through Holes


Homebrew Through Hole Plating Station

Nice YouTUBE video showing a simple home based solution for plated through holes:

Markus Zingg says:

^You need a through plating station. I use one with five baths. It's actually not all that complicated, and does also not have to be expensive, but you must be prepared for initial costs of ~$1000 if you have nothing.
...
The advantage of this is that you get very consistent beautiful absolutely repeatable results. The disadvantage is that you have to buy the chemistry.
...
[ed: first you must drill the holes which are to be plated, this is before etching the artwork onto the board. Next, the chemical baths treat the drilled board.]
 
- The first bath micro etches the FR4 material (and of course also a little the copper but that's a side effect). The purpose of this is to assure that the palladium emulsion which is applied in the fourth bath (see below) can stick to the FR4 making it conductive to the degree needed for the effective plating process. You have to rinse the PCB with water after this step.
{ed: some people split this into sever baths: 5 min in Sodium Hydroxid to degrease the board, 30 secs in Ammonium Persulfate to micro etch, 5 min in 10% sulfuric acid solution then another 5 min in 35% sulfuric acid solution }

- The second bath is a so called pre-dip. It's a sodium chloride solution with additives and the same as the third bath, but there is no palladium emulsion in it. The purpose of this bath is to acclimate the PCB to the following bath and thereby avoiding contamination of the third, bath which is a bit critical and contains the most expensive chemistry (the palladium emulsion). There is therefore NO rinsing after this step.

- The third bath is the one making the holes conductive for the subsequent plating process. It's the same as above, but contains the palladium emulsion. The emulsion is somewhat expensive, but to give you an idea, half a litter costs ~80$, and said half litter will last you for the next ten years with a station of the size of the one I build, so costs are relative. You must rinse the PCB after this step.

- The fourth bath is hardening the palladium. This is having the effect that it sticks better to the FR4, but splitters of from the copper. Also rinse after this step.

{ed: some people will use another bath here with electroless copper plating solution to "prime the pump" for the main plating step which is next}

- The fifth bath is the effective copper plating. The attached picture^ ... shows the board in this bath. You apply ~2.5 amps per square decimeter of copper material to this bath and run the PCB in there according to the thickness of copper you want to end up with. After this the plating is done. What's left is to thoroughly rinse the PCB, and then of course continue with laminating dryfilm resist, expose and etch the outer layers (or only layers in case of a double sided board).
...
Regarding "keep the holes from being etched off ". That's the point why you have to use dryfilm resist. The resist "tents" the holes (this is called tenting) and thereby protects them from being etched away. That works very reliably. You just must take care to adjust the artwork precisely with the previously plated holes. Once you get the hang of working with dryfilm resist you realize how flexible this method is and you hardly will return to anything else.

[ed: Possible suppliers of the necessary chemicals:

Current Density

The amount of current passing though the Electrolyte is very critical. This is usually expressed in Current Density or Amps per Square Foot (ASF). This should be between 10ASF and 30ASF. To calculate the Current Density, use the formula:

 L*W*2 
-------  * 30 = Current Density 
  144 

This will be the current showing on the amp meter of your power supply when you are plating. The current can be set by varying the voltage between 4 to 6 volts. Do not allow the voltage to go beyond 6 volts. If you cannot get the current high enough at 6 volts, then just extend the plating time.

Example for a 12” x 12” board:

12 * 12 * 2 
-----------  * 30 = 60ASF 
    144 

In this case you would double the plating time. This is usually necessary for large boards as in the example.

If you cannot get the current below 30ASF at 4 volts, you will have to add some resistance in the circuit.

If the current is too high, the cathode will have a "burned" look at the sides. If the current is too low, the  surface will have a pitted look because of impurities in the solution.

Copper will plate at about 0.001" for every 36 minutes at 30ASF. This is thick enough for most PCB work.

Questions:

See also:


file: /Techref/pcb/plating/thruhole.htm, 6KB, , updated: 2013/12/2 17:56, local time: 2014/8/22 20:37,
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