Qualitative electroless Ni/Au plating considerations for the solder mask on top of sequential build-up layers

Advanced printed circuit boards (PCBs) with sequential build-up (SBU) layers require alternating dielectric and copper layers on top of a core substrate. This can be achieved by lamination of resin coated copper (RCC) or by coating of dielectric polymers followed by copper deposition. The plating of electroless Ni/Au used as a solderability preservative on top of sequential build-up layers is investigated. For this application a solder mask polymer has to be applied in order to separate solder pads. Experiments showed that on parts of the underlying build-up layer exposed to the electroless Ni plating solution electroless Ni can grow. This overplating is caused by the remains of colloidal Pd/Sn catalyst on top of the build-up layer from preceding electroless Cu deposition. At very small features skipping of the plating can also take place. The overplating and skipping phenomena are influenced by a number of parameters, such as the temperature, the concentration of the stabilizer and pH. The dimensions of features on the board and the thickness of the solder-mask polymer also influence skipping. Based on qualitative analyses of the skipping and overplating phenomena rules of thumb for the solder mask design based on the plating conditions are proposed.