Board is usually made from dielectric materials such as epoxy, paper etc. which is not capable to pass the electric current. The electronic parts which have been installed on the two layers and multi layers boards will be connected via pads and cracks while the wall of the holes is dielectric so cannot be electroplating directly.
They must be electric at first. For this reason there is essential to deposit a thin metallic layer on the substrate. Since deposition a metallic layer by electroplating is not possible so electroless solutions are used. The surface must be act for suitable reaction with metallic ions.
Catalyst solution is used for activation the surface. The Palladium – Tin catalyst system is common in industrial. In this system, tin ions are deposited on the wall of the holes then palladium ions oxide two Valente tin ions and will be reduced on the surface. In fact the inner part of particle is full of palladium and the outer layer is formed by hydrated SnCl4. The charge of the whole of particle is negative. In electroless (nickel or copper), positive ions absorbed to the negative particles and in this way a thin metallic layer will be deposited on the wall of the hole.
Activation solution is expensive also sensitive so the pre-activation solution is used before activation step. This step prevents drog-in contamination particles into the activation solution.The pre-activation is directly prior to the activation step and the parts go to the activation from this step without rinsing. Pre-activation has the same components of activation solution except catalyst.