Step one: make the hole conductive
After drilling, the barrel is cleaned and desmeared, then a thin electroless copper layer is deposited chemically. Without this seed layer, electroplating cannot start inside the hole.
Step two: plate to thickness
Electrolytic copper builds the barrel and the surface. The target is a minimum wall thickness, not an average: the centre of a deep hole always plates more slowly than the surface, and high-aspect-ratio holes are the limiting case.
Where it goes wrong
- Insufficient desmear — resin smear blocks the seed layer and the barrel voids.
- Poor throwing power — the hole centre stays thin and fails thermal cycling.
- Heavy copper — thicker surface copper needs longer plating, which widens the distribution problem.
We plate up to 20 oz outer copper and 1–38 layers on the same line, and cross-section the lot to confirm barrel thickness against IPC-6012B.
