Study on Non-cyanide Co-electrodepositing Mechanism and Properties of Au-Sn Eutectic Alloy
Author:Huang Zuo Zuo
Supervisor:huang ming liang
Flux-less solderable Au80-Sn20(wt.%)eutectic alloy with the performance of high strength,good thennal conductivity,superior fatigue and creep resistance is considered as one of the most prospective lead-free solders for optoelectronic assembly.Nowadays,flip chip has become to the mainstream of LED chip bonding technology due to the better heat dissipation and luminous rate,in which solder bumps play a key role in heat conduction,electrical conduction and mechanical support.With the continuous miniaturization of electronic devices,solder bumps have been downsized to the microscale.Therefore,the development of a convenient and efficient bumping method is crucial for high-power LED packaging.Compared with evaporation and sputtering deposition methods,electrodeposition with the advantages of simple operation,low cost,high plating speed,controllable film thickness and micro-pattern formation capacity,has become a reliable preparation method for Au80-Sn20 eutectic alloy.At present,the electroplating bath of Au-Sn is still cyanide(CN-)containing,while the high toxicity and the corrosion of CN-to photoresist limit its application.Meanwhile,there is a concern regarding environmental issue and production safety considerations in the earlier plating method based on cyanide included solution.Therefore,it is necessary to develop a highly stable non-cyanide electroplating bath for the preparation of Au80-Sn20 eutectic solder bumps.In the present work,a highly stable non-cyanide electroplating bath of Au-Sn alloy is developed.The influences of the bath constitute variations and processing parameters on the composition and morphology of the electrodeposits were systematically studied.Au-Sn eutectic with mirror-like appearance was obtained through robust experimental design.Combined density functional theory simulation with electrochemical measurements,the coordination behavior and the existed forms of Au and Sn ions in the non-cyanide plating solution were studied.The mechanism of high stability of the electroplating solution and the co-deposition mechanism of Au-Sn alloys were clarified.The co-deposition kinetics and nucleation mechanism of Au-Sn alloys were studied.The wettability and soldering properties of Au-Sn eutectic electrodeposits were evaluated.Moreover,the interconnection reliability of the electrodeposited films with Cu and Ni substrates was investigated.The main conclusions are drawn as follows:1.A green,non-cyanide and high stable Au-Sn electroplating bath was developed which remained clear for three months without any precipitates.Gold ions and stannous ions were used as main metal salts.5,5-dimethylhydantoin(DMH),pyrophosphate ions(P2O74-),sulfite(SO32-)and ethylenediamine tetraacetic acid(EDTA)were used as the complexing agents,and catechol was used as antioxidant in the bath.The morphology and composition of the Au-Sn electrodeposits were optimized by altering the plating bath constitutes and processing parameters.Finally,pH,concentration of EDTA,concentration of catechol and pulse peak current density were considered as the key factors.As a result,compact,smooth and mirror-like Au-Sn eutectic alloy was obtained through the robust design of experiment.The optimized parameters were obtained:Au-DMH was 0.01 mol/L,stannous ions was 0.03 mol/L,pyrophosphate ions was 0.18 mol/L,sulfite was 0.12 mol/L,EDTA was 0.012 mol/L,catechol was 0.03 mol/L,the pH value of the bath was 7.50.Pulse electroplating mode,ton:toff=2 ms:8 ms with a peak current density of 20 mA/cm2.2.Combined the density functional theory(DFT)simulation with the electrochemical measurements,the mechanism on the bath stability was revealed.The multiple interaction of metal ions with various alternative complexing agents effectively inhibited the decomposition of monovalence gold complex and prevented the spontaneous oxidation and reduction reaction between gold and tin ions.It is determined that gold ions existed in two valence i.e.,Au3+ and Au+ with the complex forms of[Au(DMH)4]-and[Au(SO3)2]3-.[Au(DMH)4]-could inhibit the spontaneous decomposition of[Au(SO3)2]3-.Moreover,EDTA could coordinate with[Au(SO3)2]3-in some complex form,and further stabilize[Au(SO3)2]3-.Sn2+ coordinated with P2O74-and EDTA,and existed in three complexing forms i.e.,[Sn(P2O7)]2-,[Sn(P2O7)2]6-and Sn-EDTA in the bath;formation of metal complexes changed the reduction potential of metal ions,which was far from their standard electrode potentials,and thus gold ion could not be spontaneously reduced by stannous ion to generate metallic gold in thermodynamics.The proposed stability mechanism provides an insight into designing stable cyanide free Au-Sn plating bath.3.The cathodic polarization revealed the mechanism of Au-Sn co-electrodeposition,for which the deposition potential of both Au and Sn ions changed towards to the negative one and was close to each other due to the complexation of DMH and Na2SO3 with Au ion and that of K4P2O7 and EDTA with Sn ion.The onset potentials of Au and Sn ions deposition were-0.76 V and-0.98 V,respectively.The deposition potential gap of Au and Sn diminished from 1.636 V(standard electrode potential)to 0.220 V and thus realized co-deposition of Au and Sn.The dynamics and nucleation of Au-Sn co-deposition were studied by cyclic voltammetry,impedance and spectrum chronoamperometry.The Au-Sn co-deposition was a diffusion-controlled process,and the activation energy of reaction was 10～35 kJ/mol at the studied deposition potentials.The model of Au-Sn nucleation was a three-dimensional progressive nucleation with diffusion-controlled growth on both glassy carbon electrode and gold film electrode.The co-deposition type of Au-Sn alloy was a regular co-deposition.4.The properties of Au-Sn eutectic electrodeposits were studied.The surface of the coating with mirror-like and nanocrystalline electrodeposit was obtained with the grain size of 10 to 20 nm.The DSC measurement showed that melting point of the Au-Sn electrodeposit was 278 ℃,corresponding to the eutectic composition,and the wettability improved with the decreasing surface roughness.The corrosion resistance of the electrodeposited Au-Sn eutectic is better than that of Sn-3.0Ag-0.5Cu alloy.The deposited Au80-Sn20 electrodeposit consisted of δ(AuSn)and ζ(AusSn)phases,and the Au-Sn coating phase diagram agreed with the Au-Sn thermodynamic equilibrium phase diagram basically according to the XRD measurements of different alloy compositions.The Au-Sn eutectic electrodeposits formed metallurgy bonding with Cu and Ni substrates,indicating that the electrodeposited Au-Sn eutectic alloy had superior solderability.