No abstract

Electroless deposition process has been commonly used for the metallization of through holes of a printed circuit board (PCB). It is well-known that Sn/Pd colloid is widely used as the catalyst for catalyzing Cu electroless deposition onto the sidewalls of the through holes, composted of glass fiber and epoxy resin. However, Pd is very expensive. In addition, if IC substrate is fabricated using a semi-additive process (SAP), the residual Pd catalysts on the resin surface of the IC substrate, which has catalyzed a layer of copper electroless film, have to be removed from the resin surface after copper electroplating to prevent short between two copper lines. The SAP cost is accordingly high. To overcome these issues, copper nanoparticles (CuNPs) were synthesized using a heterogeneous reaction rather than a homogeneous reaction. Polyimide (PI) film was employed as a template to store copper ions. The CuNPs were gradually formed after the Cu2+-doped PI was transferred into a solution, containing a reducing agent and a ligand. The CuNPs were characterized by UV-Vis, FE-SEM, HR-TEM, ESCA, FT-IR, Zeta potential and XRD. Their mean particle size was smaller than 10 nm and varied from 4 to 6 nm and has a strong Cu (111) orientation. Therefore, they were able to perform a high catalytic activity for Cu electroless deposition.

High aspect ratio through holes are commonly designed in printed circuit boards (PCBs) with a high density interconnection. In order to achieve high copper throwing power of through holes by using plating, the copper plating bath has to be added with additives, like chloride ions, suppressors, brighteners and levelers. In this study, several brighteners were screened to understand their performance on copper throwing power of plated through holes (PTHs). The chloride ion concentration was fixed at 60 ppm and the suppressor, Polyethylene glycol, concentration was fixed at 200ppm.The brighteners were 3-mercapo-1-1-propanesulfonate（MPS）, 3-S-Thiuronium-propanesulfonate (UPS) and 3-(benzothiazolyl-2-mercapto)-proyl-sulfonate（ZPS）. Good throwing power of PTHs appeared at 1 ppm MPS, 20 ~ 100 ppm UPS or 20 ~ 60 ppm ZPS. Scanning Electron Microscope（SEM）shows that the electroplated morphology is related with the brightener concentration. Moreover, different brightener results in different plated copper orientation and stress in the copper deposit, which significantly influences the final thermal reliability of the PTHs. The results show that the PTHs with the highest copper TP, which were obtained at 1 ppm MPS, 10 ppm UPS, and 20 ppm ZPS, can pass the thermal shock for 5 cycles. Key words: Throwing power, Reliability, Brightener, Through hole. References: 1. W.-C. Tsai, C.-C. Wan, and Y.-Y. Wang, J. Electrochem. Soc., 150, C267 (2003). 2. C. Wang, J. Zhang, P. Yang, B. Zhang, and M. An, J. Electrochem. Soc., 160, D85 (2013).