Design Optimization And Function Of Additives Using In Methanesulfonic Acid Electrolyte On Tin Electrodeposition

Methanesulfonic acid（MSA） has gradually become the popoular electrolyte for tin plating process due to its non-toxic and biodegradable characteristics. Up to now, the tinplating additives for MSA in the domestic market is dominated by the foreign brands, which goes against the development of tinplating industry of our country. So, it is imperative to develop the tin additives with our own intellectual property and high efficiency for tinplating. In this paper, the stability of MSA bath, additives selecting, bath performance optimization, initial nuclear process and its function were explored by quantum chemical calculations and molecular dynamics simulation（MD）, and combined with electrochemical measurements, plating performance test s and scanning electron microscopy（SEM）, etc.The electronic properties and orbital information of MSA indicates that the sulfonic group（-SO₃） is the active site for electrophilic and nucleophilic part. In terms of EHOMO（-1.53 e V） and ΔEHOMO- LUMO（6.837 e V） of MSA-, it have stronger adsorption ability than sulfuric acid（SA） based on the frontier orbital theory. Electrostatic potential（ESP） distribution shows that MSA- tends to perpendicularly adsorb on metal surface. The structure optimization confirmed that three complexing structures of Sn[MSA]+, Sn[MSA]3- and Sn[MSA]2²⁺ could exist in the MSA bath. Cyclic voltammetry（CV） and Sn²⁺ degradation showed that the cathodic polarization of MSA bath has been greater than that of sulfuric acid bath, but it can fail to surpress the growth of tin dendrite. Hydroquinone as the selected stabilizer can be oxided by oxygen firstly to protect Sn²⁺. The structure of phenolic compounds can aid the reversible electrochemical reaction, which help maintain stability of MSA bath for a long time. The influence of iron ion on oxidation-hydrolysis of Sn²⁺ was studied in methanesulfonic acid solution. It has been showed that Sn²⁺ was oxidized to Sn⁴⁺ firstly and then hydrolyzed to form Sn O2 precipitation. Fe³⁺ can be directly oxidized Sn²⁺ to Sn⁴⁺, however, Fe²⁺ within 5¹5 g/L have function of slowing the rate of Sn²⁺ degradation, and 10 g/L Fe²⁺show the strongest effect.In order to obtain a applicable coating from MSA bath, the Sn²⁺ cathodic process must be suppressed. Different additives were employed to improve the polarization of MSA bath, and achieve tin coating with matt and bright appearance. The design optimaztion has been carried out firstly in that a block copolymer of EPE4600 was selected from polyethers on account of molecular characteristics and frontier orbital theory. EPE4600 is a block copolymer made of 40% ethylene oxides（EO） and molecular weight is 4600. In static condition, Hull cell test showed that 0.8¹.6 g/L EPE4600 as single suppressor in MSA bath containing 15 g/L [Sn²⁺], has allowed a maximal current density of 8.0 A/dm2（commercially available high speed tin additives TPG7 is 3.0 A/dm2）. In addition, cloud point（79 ℃） of EPE4600 indiacted the bath can be worked in wide temperature range which comply the requirements of high speed electroplating. The voltammetry analysis on the rotating disk electrode（Pt-RDE） show that EPE4600 can form an adsorpting barrier film to improve polarization and suppress tin dendrite growth under different hydrodynam ic modulations. Finally, barrier mechanism model were established. DFT calculation and MD showed the PO part of EPE4600 has a strong adsorption activity and a synergistic adsorption with MSA-.The current transient curves were employed to evaluate the initial tin nucleation behavior of MSA-SA bath containing EPE4600. In MSA bath, it is found that tin nucleation approximately follow the 3D progressive nucleation model under diffusion control. But with SA bath mixing with MSA bath, the nucleation model transform into the instantaneous nucleation model. According to the model, the kinetic nucleation parameters involving nuclear active site（N0）, growth rate（A）, critical Gibbs nuclear free energy（ΔGcrit） and critical nucleus size（Ncrit） were calculated. And the results showed that MSA bath had 5 10 times of surface active sites more than SA and MSA-SA bath. The number of atoms forming the critical nucleus（Ncrit） consist of 0¹.5 atoms, suggesting that the substrates have similar activities, and the small size of critical nuclei suggests strong bond energy between tin atoms and the substrates. The morphology of initial tin deposition followed the nucleation characteristics. A thin coating with a loading of 0.7 g/m2 tin was fabricated with excellent uniformity and coverage on wet temper rolling steel. EPE4600 can then be used for ultra-low tin coating in tinplate steel to save tin.In order to further improve the cathodic polarization and obtain a bright tin coating appearance, brightener combination were selected based on the molecular characteristics and frontier orbital theory. The selected brighter contain furfurylidene acetone, benzylidene acetone and glutaraldehyde as inhibitor, NP-10 and NS-665 as mixing carrier. An formaldehyde free bright tin plating additive, HIT-2, were developed, with which MSA bath allowed the maximum current density of 5.1 A/dm2 and bright tin coating with a single stronge indice of（112） crystal face was realized. HIT-2 additive can be applied in continuous plating process of reel to reel.In terms of the electrochemical behaviors of HIT-2 additive, a co-adsorption between benzalacetone and furfuralacetone have been discovered, which co-inhibit the discharge of Sn²⁺ in MSA bath. However, glutaraldehyde reduce the adsoption of benzylidene acetone and furfurylidene acetone, and its counter action contributes to achieve the electrodeposition of bright tin coating. MD reveal that the benzalacetone and furfurylidene acetone molecules tended to parallelly adsorb on metal surface and an interface adsorption model of additive molecules was proposed. The matt and bright additives were used in the through-hole plating reseparately. A selective tin electrodeposition of inner and outer were observed. And this process could be applied in bumps electrodeposition for high integrated chip interconnection package.