Electroless Ni/Au plating as a solderable finish on top of sequential buildup layers: overplating reliability considerations for the solder mask

The plating of electroless Ni/Au as a solder for sequential buildup layers is investigated. Advanced printed circuit boards (PCB's) using sequential buildup layers with microvias require alternating dielectric and copper layers. This can be achieved by laminating of RCC, dip coating of dielectric and deposition of copper or lamination of a polymer layer and deposition of copper. On top of these buildup layers a solder mask polymer has to be applied in order to separate solder pads. Parts of the underlying buildup layer that are exposed to the electroless Ni plating solution, even though they are etched, can grow electroless Ni on top of them, even though they are not activated by the catalyst. Experiments showed that in the case of RCC this phenomena does not occur, and does occur for the processes that use electroless copper deposition during plating of the buildup layer under the solder mask. Investigation showed that colloidal Pd catalyst remaining from a preceding electroless copper deposition catalyzes the electroless Ni deposition, which uses metallic Pd catalyst. These overplating phenomena could be traced back to the design of the solder mask. All parts of the buildup layer under the solder mask that require no electroless Ni plating have to be completely covered by solder mask polymer to prevent overplating if the copper layer from the buildup layer was plated. This puts more constraints on the fine pitch capability of the solder mask. This is important for the design of the solder mask, because electroless Ni/Au plating grows in popularity because of its environmental friendliness.