Study On The Technology Of Simultaneous Through-hole Metallization And Blind Hole Filled Copper

High density interconnection （HDI） board requires buried via, blind or throughhole to carry out interlayer connections. With the increasing demand of HDI board, thetechnology of through-hole and blind-hole metallization should be improved to meet thereliability of products. Generally, the process of metallization includes two steps. Microblind hole should be filled with plating copper firstly then through-hole was plated. Themetallization requires two plating processes, resulting in a waste of energy and time. Tosolve the problem, the simultaneous metallization technology of through hole and blindhole was studied in order to improve production efficiency of HDI board.The effect of plating solution to the plating quality was studied by CyclicVoltammetry Stripping （CVS）. The effect of Fe³⁺, Cu²⁺and Cl-with differentconcentration to the Cu electro-deposition process on the platinum electrode and theassay value of brightener have also been studied by CVS method in the medium ofCuSO4-H2SO4. Results revealed that the concentration of brightener changed along withthe concentration of Fe³⁺, Fe²⁺, Cu²⁺and Cl-, and Fe³⁺has the greatest influence. Theextreme concentration value changes were shown RFe³⁺=10.33ml/l, RFe²⁺=3.4ml/l,RCl-=1.22ml/l, RCu²⁺=3.1ml/l, respectively. With the concentration of Cu²⁺increasedthe measured concentration of brightener would be low. Fe²⁺concentration changeparabolic trend and at6g/L, the brightener’s concentration was higher. Pump frequencyequivalent increase in the electroplating process and the exchange rate of the syrup.Then, the brightener needed to accelerate the copper plating speed. So the brightener’sconcentration would increase with the increasing concentration of Fe³⁺. In a result, theconcentration of the inorganic components had a great influence to the brightener,which ulteriorly affected the production of PCB.The copper filling in blind hole was researched by horizontal pulse platingexperiment. The experimental optimization combined with Minitab was mainly used tooptimize the factors that influence horizontal plating quality. The optimal pulse platingparameters were gained after analysis as average current density5.6/28.6（Forward/Reverse, A/dm²）, pump frequency36/29/40（I/O/AFD, Hz）, line speed0.3m/min, and pulse time80/4（Forward/Reverse, ms）. Thermal stress test for pulse plating sampleswere done for three times at288℃. SEM of samples indicated that the deposited surfacewas compact and uniform.HDI PCB through-hole plating needed high acid and low copper plating solution,while high copper and low acid plating solution was required for blind-hole platingprocess. However, the flow speed of the plating solution near the bland-hole would beincreased during the simultaneous metallization due to the existence of through hole.Therefore, a special technology and plating solution were adopted to avoid thedisadvantage. In this study, the horizontal pulse plating line and vertical continuousplating line combined with the experimental optimization design were used. Theproportion of the organic additives is fixed at1:14:50（brightener: leveling agents:inhibitor）. Besides organic additives, the main influencing factors are current density,pump frequency, line speed and plating time. Optimized experimental analysis was usedto explore the best combination of parameters for filling copper in blind hole. Bestparameters of filling copper in blind hole were shown as current density1.5ASD, pumpfrequency30Hz, line speed0.37m/min and plating time65min. Current density andpump frequency were main influencing factors to filling copper in blind hole, the size ofimpact is98.6%and98%, respectively. The optimal parameters for through-hole copperthickness were shown as current density1.5ASD, line speed0.37m/min, pumpfrequency25Hz and plating time65min. Current density as the main influencing factorto through-hole copper thickness, the size of influence is99.3%. Based on the aboveanalysis we could conclude that the current density was the main effecting factor bothfor through-hole and blind hole. Minitab software and comprehensive balance methodwere used to find out the optimal parameters of simultaneous plating as the currentdensity1.5ASD, line speed0.37m/min, pump frequency30Hz and plating time65min.Results showed that the Dimple value of blind hole was≤15μm, through-hole copperthickness was about25μm and copper thickness of the board’s surface was29.3μm.Plating results have a good agreement in quality requirements of through hole and blindhole. The thermal stress and the reflow performance test were taken to the sampleboards. Results revealed that there were no explosion board, blistering and pointdefects.