Study On Molecular Simulation And Mechanism Of The Additives Used For Through-hole Copper Electroplating On PCB

Printed circuit board （PCB） is the necessary component of electronic products,and the emergence of multi-layer PCB helps to develope small, convenient,intelligent electronic products. Through-hole （TH） copper electroplating is one ofthe main methodes to realize the conduction among PCB layers, which is also veryimportant technology of PCB manufacturing process. It is very difficult to obtaincoatings with uniform thickness using traditional plating bacause of the unevendistribution of current density in the TH during DC plating process. For obtaining anuniform plating in the TH, using organic additives is an effective and economicalway. However, choosing and exploration of additives by experiments are verytime-consuming and laborious. In this paper,4levelers and7additive systems werepredicted using molecular dynamics （MD） simulation, and the TH wereelectroplated using these additives. The uniform power （UP） of additives wereexamined, and the actions of additives on the surfaces of cathodes and mechanismswere explored by electrochemical test, atomic force microscopy （AFM）, X-rayphotoelectron spectroscopy （XPS） and quantum chemical calculations.The accelerator thiazoline dithio-propane sulfonate poly （ethylene glycol）（SH110） which were considered as a combination of accelerator and inhibitor orleveler agent and inhibitor PEG, MW=10000were selected by electrochemicalcomparative experiments. Two new levelers of N-butyl-N-bromide in piperidine(PP₁₄Br) and nitro blue tetrazolium （NTBC） were predicted using MD, andSafranine T （ST） with the structure similar to Janus Green B （JGB） was predicted asan uneffective leveler. The results of MD simulations and electrochemical testsabout SH110indicated that SH110could be used as a single TH plating sealingadditive. Accelerator-inhibitor-leveler and accelerator-inhibitor-walk agent systemswere designed by using SH110and PEG as accelerators and inhibitors, respectively.THs on PCB with different aspect ratio and hole diametres were electroplatedby DC using predicted and designed additive systems. The previous analysis and theproperties of of additive systems were verified and the preferred concentration scopeof additives were explored. The uniformity of the plating in TH was examined fromthe view of cross sections of THs. SEM results indicated that butterfly technology（BF） emerged after short-time electroplating and superfilling perfect sealinggenerated after18h electroplating when the concentration of SH110was10mg/L.Four excellent additive systems with UP more than90%were obtained for THelectroplating with the aspect ratio of10by adjusting the concentration of additives. These4kinds of additive systems were SH110（1mg/L）-PEG （100mg/L）-PP₁₄Br（20mg/L）, SH110（1mg/L）-PEG （200mg/L）-NTBC （3mg/L）, SH110（1mg/L）-PEG（200mg/L）-JGB （1mg/L） SH110（1mg/L）-PEG （100mg/L）-fatty amines, andpolyoxyethylene ether （AEO,10mg/L）, respectively. The prediction of ST whichwas not an effective leveler was also verified. The UP was76.1%in TH with aspectratio of20using SH110（50mg/L）-PEG （200mg/L）-PP₁₄Br （40mg/L）, which metwith the requirement of the TH with a aspect ratio of20in TH electroplating.The electrochemical behavior of the additives were tested by rotating diskelectrode combination of dynamic polarization curves and constant current E^tcurve, and adsorption behaviors of the additives were characterized using AFM andXPS. Electronic structure informations about some additives were obtained andcompared by quantum chemical calculations and the mechanisms of actions of theadditives in the Th electroplating process were studied. The results showed thatSH110was a combination of an accelerator and inhibitor or a leveler, which may ledto the formation of adsorbed film on the copper surface and reflected theperformance of SH110as an inhibitor. The inhibition of SH110could take part instrong convection zone of the cathode more easily, and the acceleration mainlyappeared in weak convection zone of the cathode, which accounted for BF andsuperfilling during the TH eclectroplating. The addition of levelers of PP₁₄Br andNTBC not only increased the cathodic polarization of the bath, but also increasedcathode potential differences at different convection strength, which inhibitedcopper ions in the area of convection at the mouth of TH more strongly and helpedto inhance the uniformity of plating inside the holes. NTBC could lead to theformation of an absorption layer on the copper cathode surface, which caused stronginhibition of copper ion deposition. JGB had an impact on the cathode potential andcathode polarization, and ST had little effect on the cathode potential and cathodepolarization. The highest occupied molecular orbital （HOMO） value of JGBmolecular was larger than that of ST, which demonstrated that JGB having astronger electron-donating ability could be adsorped by contributing electrons toempty d orbital of copper atom and bonding. The adsorption made JGB as a levelereventually but ST did not have the adsorption. The actions of aminoazobenzene andN=N were much stronger than that of quaternized N atom during the chemicalsorption of JSB on copper surface, and ST did not have such a structure, whichexplained that ST was not effect leveler.