Investigation Of Copper Electrodeposition For Printed-circuit Interconnection Based On Multiphysics Coupling Method

The functionalization, integration, miniaturation and high reliability of electronic information products drive printed circuit board(PCB) which achieves electrical interconnection of component, functional module and chip to develop in the direction of high density and high reliability. The hole metallization has become a key to the high reliability of high density interconnect(HDI) PCB due to finer lines and smaller holes of interlayer interconnection. Hole metallization of high density interconnection PCB is achieved by copper electrodeposit with high density and high performance. Uniformity of electrodeposit layer and the flatness of filling are demanded to achieve good electrical interconnection and the reliability for PCB products. Copper electrodeposition acts as a crucial step for the manufacture of HDI board. The stability of plating solution and the uniformity of copper electrodeposit are the hotspot and nodus for the research of electrodeposition. However, mechanism of copper electrodeposition is complex. Because a large number of factors such as geometry of plating bath, convection, properties of additives, electric field distribution and temperature are included in electrodeposition, experimentally determining all parameters and electrodeposition conditions become unmanageable. Therefore, it is very necessary to study the electrodeposited process with the aid of multiphysics coupling technology. The article focuses on copper electrodeposited interconnect of PCB to investigate microvia filling, copper electrodeposition of through hole and pattern plating. These researches have important scientific significance and practical value for copper electrodeposited process and simulation of electrochemical system.Electrochemical behaviors of electrodeposition containing bis-(sodium sulfoproply)-disulfide(SPS)、ethylene oxide and propylene oxide block copolymers(EO/PO) and polyethylene oxide-polyimide(PEOPI) were investigated by cyclic voltammetry(CV), galvanostatic potential transient measurement(GM) and dynamic potential polarization technology. GM showed that EO/PO significantly inhibited the copper electrodeposition owing to adsorption of EO/PO-Cu+-Cl-. During copper electrodeposition, SPS and EO/PO showed different adsorption constants on the electrode surface. SPS could adsorb on the active site of the electrode, as well as replace EO/PO adsorption site on the electrode to accelerate copper electrodeposition. CV curve showed that the critical concentration of EO/PO on the electrode was less than 5 mg/L. When 200 mg/L EO/PO was injected to the plating solution system, EO/PO was in a state of over saturation and its consumption was negligible in the process of electrodeposition. CV test and dynamic potential polarization test were accomplished to various electroplating system. The results showed that the concentration of PEOPI and rotating speed of rotating disk electrode(RDE) had great influence on the mechanism of copper electrodeposition. The relationship of variables was obtained by curve fitting of electrochemical data. These variables included the coverage of additive, concentration of additive, rotating speed of RDE, exchange current density and slope. The mathematical model was constructed for in-depth study for copper electrodeposition.Based on multiphysics coupling of copper electrodeposition, the mathematical model of microvia filling was built to simulate the filling process by a finite element method, and a systematic analysis of the whole filling process was carried out. The results showed that the pinch-off effect occurred when microvia was filled in the base plating solution with CuSO4?5H2O, H2SO4 and Cl-. The high current density region was formed around the orifice due to aggregation of electric field lines. When microvia filling was achieved in the electroplating system containing 1 mg/L SPS, 200 mg/L EO/PO and 6 mg/L PEOPI, copper electrodeposition was accelerated at the bottom of microvia. Copper deposited reaction resulted in the decrease of effective deposited area in the microvia. So, accumulation of SPS led to a fast copper electrodeposition in the microvia. And SPS became invalid due to adsorption of PEOPI in the later stages of microvia filling. This avoided the overfilling of microvia. Finally,bottom-up filling of microvia was achieved by the synergy of SPS, EO/PO and PEOPI. Numerical simulation showed that the process of microvia filling included initial growth period, the bottom-up period and the stable growth period. The performance of microvia filling reached to 95%. Subsequently, the experiment of microvia filling was achieved and the results were in agreement with the results of numerical simulation.The mathematical model of copper electrodeposition for high aspect ratio through hole(TH) was constructed. The characteristics of copper electroplating in Haring cell and novel plating bath were described by multiphysics coupling technology and experimental method. The results showed that electrolyte flow played an increasingly important role in the uniformity of current density distribution and electrodeposited thickness. The flow velocity inside the TH was improved in the novel plating bath configuration. It was beneficial to uniformity of the diffusive boundary layer and copper electrodeposit inside the TH. At the same time, numerical simulation result showed the metallization of TH with aspect ratio(AR) of 12.8 was achieved by a revised plating bath. The uniformity inside the TH was obviously improved with the increase of liquid level difference between inner bath and outer bath for the optimized plating bath. Compared with Haring cell, the value of uniformity reached to 83% when the liquid level difference was 10 cm. Further, experiments of TH electrodeposition were achieved, which was in good agreement with the results of simulation.The mathematical model of TH copper electrodeposition for flexible circuit board was constructed and the electrodeposited process was simulated. During copper electrodeposition, SPS and EO/PO had a different diffusion coefficient in the electrolyte and the difference of adsorption time on the electrode. Therefore, coverage of SPS was high in the middle of TH, which accelerated the deposition rate of the center of TH. Conformal deposition mode of TH filling was observed because the adsorption of additives formed a specific concentration gradient. And throwing power was more than 100% for TH electrodeposition of flexible circuit board.Taking the pattern plating process of PCB manufacturing as the research object, the uniformity of electrodeposit layer in the pattern plating process was discussed by multiphysics coupling technology based on the finite element method. Finally, a uniform copper layer was achieved in pattern plating. The results of simulation showed that this was advantageous to improvement in uniformity of pattern plating by increasing the distance between cathode and anode. The suitable distance between cathode and anode was 20~40 cm. And the uniformity of electrodeposit layer was better when anode and cathode were just in the plating solution. When the distance between anode and cathode was fixed and area ratio was larger than 2:1, uniformity of electrodeposition was improved obviously by using an insulating shield and an auxiliary cathode owing to changing spatial distribution of electric fields in plating bath. Furthermore, graphic design of plating pattern also affected the uniformity of electrodeposition. Uniformity was excellent and the roughness of plating thickness was 0.02 μm when the graph was evenly distributed in the PCB.