Next we electroplate the boards with copper. The operator loads the panels onto the flight bars. He checks all the clamps to ensure a good electrical connection. The panels themselves act as cathodes for electroplating and we can plate the hole walls thanks to the conductive carbon layer already deposited there. The operator starts the automated plating line. The copper surface of the panels is cleaned and activated in a number of baths and then electroplated. The whole process is computer controlled to ensure that each set or flight of panels stays in each bath exactly the right amount of time. You can see the copper anodes in their bags.
To ensure good conductivity through the holes we need to plate an average of 20 microns of copper on the hole walls (18 µm copper is the defined minimum in the IPC 600J-Class 2 standard). This means that we also plate +/- 20 microns on the surface tracks. For detailed info see Tolerances on Copper Thickness.
The baths are designed to produce an even copper thickness across the panel. Modern chemical solutions also have good “throwing power” to produce an even thickness of copper right through the hole.
Once we have plated the copper onto the board we then plate a thin layer of tin. This we will use in the next step of the process when we etch off the unwanted copper foil.
When plating is completed the flight of panels is returned to the operator and he unloads and stacks the plated panels. He then uses non-destructive testing to check a sample of each flight to ensure that the copper and tin plating is the correct thickness.