Effect of solder flux residue on the performance of silicone conformal coatings on printed circuit board assemblies

Abstract

Conformal coatings are applied on printed circuit board assemblies (PCBAs) in order to protect the assembly from environmental influence and silicone-based coating is commonly used. A systematic study on the performance of silicone conformal coating in connection with process-related contaminants is the focus of this paper. Different test substrates such as plain laminate surface and a test PCBA with and without conformal coatings were exposed to high humid environment at ambient temperature and the performance of the coating was evaluated using various parameters such as increase in leakage current across the components, morphology of the coating, and analysis of dendrite formation due to electrochemical migration under the coating. The morphology of the coating before and after exposure was investigated using scanning electron microscopy, and energy dispersive X-ray spectroscopy. Results show that the presence of flux residues greatly influences the electrical functionality of the components and also degrades the performance of the coatings compared to the clean substrates.     

Electrochemical migration of tin in electronics and microstructure of the dendrites

The macro-, micro-, and nano-scale morphology and structure of tin dendrites, formed by electrochemical migration on a surface mount ceramic chip resistor having electrodes consisting of tin with small amounts of Pb (∼2 wt.%) was investigated by scanning electron microscopy and transmission electron microscopy including Energy dispersive X-ray spectroscopy and electron diffraction. The tin dendrites were formed under 5 or 12 V potential bias in 10 ppm by weight NaCl electrolyte as a micro-droplet on the resistor during electrochemical migration experiments. The dendrites formed were found to have heterogeneous microstructure along the growth direction, which is attributed to unstable growth conditions inside the micro-volume of electrolyte. Selected area electron diffraction showed that the dendrites are metallic tin having sections of single crystal orientation and lead containing intermetallic particles embedded in the structure. At certain areas, the dendrite structure was found to be surrounded by an oxide crust, which is believed to be due to unstable growth conditions during the dendrite formation. The oxide layer was found to be of nanocrystalline structure, which is expected to be formed by the dehydration of the hydrated oxide originally formed in solution ex-situ in ambient air.     

Effects of electrolyte thickness,chloride ion concentration,and an external direct current electric field on corrosion behaviour of silver under a thin electrolyte layer

ABSTRACT

Effects of electrolyte thickness, chloride ion concentration, and an external direct current electric field (DCEF) on the corrosion behaviour of silver under a thin electrolyte layer (TEL) were investigated using electrochemical and surface techniques. The results indicate the corrosion rate of silver increases with the decrease of TEL thickness and the increase of chloride ion concentration. Moreover, an interesting conclusion was drawn that the corrosion rate of silver near the positive plate of DCEF first increases and then decreases with the increase of electric field intensity. In a DCEF, different polarisation behaviours of silver at different positions were attributed to the differences of the local corrosion environment.     

Electrochemical migration of tin in thin electrolyte layer containing chloride ions

The electrochemical migration of tin in thin electrolyte layers (TELs) containing chloride ions was investigated using in situ electrochemical and optical techniques, as well as ex situ characterization. The results show that tin dendrites co-exist with precipitates in both low and high chloride concentrations, however, in intermediate chloride concentration, no tin dendrites but only precipitates mainly consisting of stannic compounds were observed. The higher bias voltage is applied, the faster rate of ions migration is. Mechanisms have also been proposed to explain the electrochemical migration behaviors.     

Corrosion behaviour of TaN thin PVD films on steels

Abstract

The corrosion behaviour of tantalum nitride (TaN) thin films deposited on AISI 1018 carbon steel, AISI M2 tool steel and AISI 304 stainless steel has been investigated in an aqueous sodium chloride solution. The films were produced by means of magnetron sputtering plasma deposition. Morphological and microstructuctural characteristics of the films were studied by atomic force microscopy and X-ray diffraction analysis. Potentiodynamic polarisation, electrochemical impedance spectroscopy and cyclic voltammetry techniques were used for electrochemical characterisation. The TaN films displayed lower corrosion current densities than those of the substrates. A relaxation time in the impedance analysis was observed in the 1018/TaN system, two relaxation times were observed in the M2/TaN system, and a diffusion process was found in the 304/TaN system. This indicates that the substrates were not completely isolated from the solution and the defects in the films appear to be detrimental owing to electrolyte penetration through the defects which caused localised corrosion. The corrosion behaviour of the films depends mainly on the electrochemical properties of the substrate.     

碱性介质中吐温20对锌电极电化学行为的影响

解决锌电极的枝晶问题对延长电池使用寿命、资源综合利用和减少固体废物有积极作用.利用循环伏安法以及动电位扫描法考察了在1.0mol/dm3 KOH溶液中,TWEEN(吐温)20作为电解液添加剂时锌电极的电化学行为,结果表明:适宜浓度的吐温20能显著影响锌电极的电化学行为.吐温20对锌电极具有一定的缓蚀作用,其缓蚀作用主要是由于覆盖效应,吐温20在锌电极表面的吸附行为符合Freundlich等温式.适宜浓度的吐温20对锌电极的阴极过程有显著的抑制作用,可以有效减缓锌电极的枝晶形成和变形.     

The effect of additives on the corrosion of tin in alkaline solution

In order to study the effect of potassium chloride, sodium nitrite and sodium chloride on the corrosion of tin, in solutions of sodium hydroxide, the method of galvanic couples has been used. Cathodic and anodic potentials and the limiting current densities of the tin-platinum couple have been measured in order to determine the corrosion rate and type of control exerted by additives. The results indicate that the nature and concentration of additives exert an influence on the dissolution process of tin. Weight loss experiments have been also performed; data obtained are compared with electrochemical results. The significance of such additives on the corrosion of tin is discussed.     

Effect of direct current electric field on atmospheric corrosion behavior of copper under thin electrolyte layer

A thin layer electrochemical cell was successfully developed to study the atmospheric corrosion behavior of copper film in printed circuit board (PCB-Cu) under thin electrolyte layer (TEL) and direct current electric field (DCEF) by electrochemical impedance and electrochemical noise analysis. The electrochemical measurements and SEM morphologies after corrosion test indicate that DCEF decreases the corrosion of PCB-Cu under TEL. The corrosion rate and probability of pitting corrosion of PCB-Cu under DCEF decrease due to the electric migration of aggressive Cl⁻ ion out of working electrode surface.     

Electrolytic Codeposition of Nickel and Phosphorus from Methanesulfonate Electrolyte

The characteristics of Ni–P alloy electrodeposition from a methanesulfonate electrolyte have been investigated. It has been found that the phosphorus content of the alloy increases with increasing the concentration of sodium hypophosphite in the electrolyte and reducing the electrodeposition current density. A mechanism of codeposition of nickel and phosphorus has been suggested. It is shown that phosphorus was formed by electrochemical reduction of hypophosphite anions and their disproportionation at a catalytically active surface of a nickel cathode. It has been shown quantitatively that the most likely path for the formation of phosphorus through the electrochemical mechanism is the direct electrochemical reduction of a hypophosphite anion to atomic phosphorus. The rate of phosphorous formation from hypophosphite anions is dependent on the concentration of hydrogen ions in the near-electrode layer. Therefore, the phosphorus content of the coatings obtained from the methanesulfonate electrolyte is slightly decreased as compared with that from the sulfate electrolyte which exhibits higher buffering properties. It has been revealed that codeposition of nickel and phosphorus reduces the kinetic difficulties of electrochemical reduction of the nickel ions. This might be due to an increased near-electrode concentration of nickel hydroxyl complexes discharging at the cathode, which is the result of an increased near-electrode pH caused by the reactions involving hypophosphite anions and hydrogen ions.     

席夫碱基季铵盐型双子表面活性剂对碱性锌电极电化学性能的影响

目的选择席夫碱基季铵盐型双子表面活性剂作为电解液添加剂,改善碱性锌电极的电化学性能,提高其耐蚀性能。方法通过电化学分析法,如失重法、塔菲尔极化曲线法、交流阻抗法等分析研究三种席夫碱基季铵盐型双子表面活性剂(D1、D2、D3)对锌电极电化学性能的影响,利用X射线光电子能谱(XPS)和扫描电子显微镜(SEM)研究在6 mol/L KOH电解液(饱和ZnO)浸泡48 h后,锌片表面的成分和形貌。结果室温下,缓蚀率随席夫碱基季铵盐型表面活性剂浓度升高而增加,当浓度进一步增大,缓蚀率变化不大。三种席夫碱基表面活性剂中,D3缓蚀能力最强,缓蚀率最高达95.67%,抑制腐蚀的效果顺序为:D3D2D1,属于抑制阳极型缓蚀剂。结论 D1、D2、D3作为碱性锌电极的电解液添加剂,可以有效减缓锌电极的腐蚀、变形、钝化及枝晶形成的能力,改善了碱性锌电极的电化学性能,D1、D2、D3适合作为碱性锌电池的缓蚀添加剂。     

射流电沉积镍中晶体形态的可控生长

利用分形理论编程模拟了射流电沉积中沉积几率较小时粒子簇的生长形貌。基于模拟的原理,利用摆动射流电沉积改变了枝晶的树枝状分形生长特性,制备不同电流密度、摆动速度、NiSO4浓度和电解液温度时的二维多孔交织的金属镍枝晶簇。结果表明:随着电流密度的增大,枝晶簇开始由分形生长形态向多孔交织形态转变,分形维数也随之增大。随着摆动速度的减小,枝晶簇向致密、均匀的多孔交织形态转变明显,分形维数逐渐增大。NiSO4浓度较小时,枝晶簇的分枝较多,形貌较为致密;NiSO4浓度最大时,气泡的析出量大大减少,枝晶簇的分枝显著减少,难以形成多孔交织的组织;分形维数随NiSO4浓度的变化先增大后减小;电解液温度的升高使枝晶簇的形貌向致密型转变,分形维数逐渐增大。     

Effect of polystyrenesulphonate and electrolyte concentration for electrical properties of polypyrrole film on aluminium alloy using conductive AFM

The electrochemical synthesis of polystyrenesulphonate (PSS) doped polypyrrole (PPy) film onto aluminium alloy (AA 2024-T3) under galvanostatic conditions at current densities of 1 mA cm^?2 was studied. In this study, conductive atomic force microscopy (C-AFM) was performed to investigate the electrical properties of PPy film on AA 2024-T3 depending on the concentration of PSS as dopant and nitric acid as electrolyte during electrochemical synthesis. The addition of HNO₃ to aqueous electrolyte solution is found to allow the electrochemical synthesis of well adhering homogeneous PPy film in the presence of PSS on AA 2023-T3. The PPy film in the presence of nitric acid alone can be synthesised, although this film showed the poor quality of the electrical properties of PPy film. According to C-AFM, the study confirmed that the conductivity of PPy film is significantly increased with increasing the PSS, nitric acid concentration and electrochemical deposition time.     

A study on zinc electrodeposition onto zinc cathode

Abstract

The electrodeposition of zinc onto a zinc cathode was studied through the comparison between theoretical and experimental polarisation curves. The former was constructed using the Butler–Volmer equation with electrochemical parameters, such as Tafel slope values, exchange current densities, equilibrium potential values, etc. The latter was obtained from data derived in a chloride based electrolyte of 0·6 mol dm^?3 Zn²⁺ ions with boric acid (25 g dm^?3), pH = 5. The electrochemical parameters such as exchange current density, limiting current density, equilibrium potential, Tafel slopes, can be obtained with good agreement with the values from the literature.     

脉冲漏磁检测中的涡流效应

为了解脉冲漏磁检测中涡流效应的特点,奠定进一步分析脉冲漏磁检测信号的基础,建立了脉冲漏磁检测的有限元仿真模型,观察了检测中瞬态磁场和感生涡流的分布,分析了感生涡流特征量的特点及影响因素。结果表明,脉冲漏磁检测中,瞬态磁场和感生涡流总体上符合集肤效应并相互影响,其中感生涡流具有渗透深度浅、感应强度大的特点,涡流密度峰值时间在深度方向上有较强的分辨率。电导率和磁导率影响感生涡流的渗透深度和密度峰值时间在深度方向上的分辨率;脉冲激励上升时间常数只影响感生涡流的渗透深度,而和密度峰值时间在深度方向上的分辨率无关。     

Effect of chloride concentration on passive film properties on copper

The semiconducting properties of passive films formed on copper, in anaerobic alkaline sodium chloride solution, were studied using Mott–Schottky analysis and electrochemical impedance spectroscopy, based on the point defect model. Results showed that the corrosion resistance increased with increasing potential, which was attributed to a well crystallised, refined grain structure, and a thicker passive film at higher potential. P-type semiconducting characteristics were obtained with or without chloride. The density of copper vacancies was approximately 10²⁰?cm^?3, and increased with the increasing chloride concentration, which was attributed to faster film-formation in a higher chloride environment. The diffusion coefficient of the defects, a key dynamic parameter for passive film breakdown, was in the range of 10^?16–10^?15?cm²?s^?1, and increased with increasing chloride concentration, thus leading to a greater probability of pitting.     

Corrosion behaviour of electrode materials in chloride migration test

Abstract

Graphite, stainless steel and brass were selected as electrode materials in the chloride migration test. The diffusion coefficients were calculated from the steady state of chloride flux by the Nernst–Planck equation. X-ray diffraction was used to characterise the corrosion products in the chloride migration tests. The results show that the types of electrodes have significant effects on the diffusion coefficient calculated from the migration test. The diffusion coefficient may trend closely to the real value when graphite electrodes are used. Graphite is a suitable electrode material used in the chloride migration test.     

Electrochemical behavior of copper-nickel alloys in acidic chloride solutions

The electrochemical behavior of Cu-Ni alloys in acidic chloride medium was investigated. Commercial Cu-Ni alloys were investigated using potentiodynamic techniques, complemented by electrochemical impedance spectroscopy. The influence of alloy composition, chloride ion concentration and immersion time on the electrochemical response of the alloys was analyzed. Results of present investigations with pure metals (Cu and Ni) are also considered in this paper for the sake of comparison. Potentiodynamic measurements reveal that the increase in nickel content decreases the corrosion rate of the alloy and when the nickel content exceeds 30%, an increase in the corrosion rate was recorded. Also, the corrosion current density increases with increasing the concentration of chloride ions up to 0.6 M.The experimental impedance data were fitted to an equivalent circuit model representing the electrode/electrolyte interface. The relevance of the proposed model to the corrosion/passivation phenomena occurring at the electrode/solution interface was discussed.     

Use of Electrochemical Noise (EN) Technique to Study the Effect of sulfate and Chloride Ions on Passivation and Pitting Corrosion Behavior of 316 Stainless Steel

In the present paper, studies were conducted on AISI Type 316 stainless steel (SS) in deaerated solutions of sodium sulfate as well as sodium chloride to establish the effect of sulfate and chloride ions on the electrochemical corrosion behavior of the material. The experiments were conducted in deaerated solutions of 0.5 M sodium sulfate as well as 0.5 M sodium chloride using electrochemical noise (EN) technique at open circuit potential (OCP) to collect the correlated current and potential signals. Visual records of the current and potential, analysis of data to arrive at the statistical parameters, spectral density estimation using the maximum entropy method (MEM) showed that sulfate ions were incorporated in the passive film to strengthen the same. However, the adsorption of chloride ions resulted in pitting corrosion thereby adversely affecting noise resistance (R _N). Distinct current and potential signals were observed for metastable pitting, stable pitting and passive film build-up. Distinct changes in the values of the statistical parameters like R _N and the spectral noise resistance at zero frequency (R°_SN) revealed adsorption and incorporation of sulfate and chloride ions on the passive film/solution interface.     

Influence of ultraviolet light irradiation on the corrosion behavior of carbon steel AISI 1015

Corrosion of carbon steel in sodium chloride solution was studied under ultraviolet illumination using weight loss, polarization, electrochemical impedance spectroscopy and current transient tests. The polarization test revealed an increase in the corrosion current density observed under UV illumination. The impedance spectroscopy indicated that the charge transfer resistance of the system was decreased by irradiation of UV light on a carbon steel electrode. The weight loss of carbon steel in solution increased under UV light, which confirms the results obtained from electrochemical measurements. We propose that the main effect of UV irradiation is on the oxide film, which forms on the surface. Thus, in presence of UV, the conductivity of oxide film might increase and lead to higher metal dissolution and corrosion rate.     

Magnetic field effects on anodic polarisation behaviour of iron in neutral aqueous solutions

Abstract

The presence of a magnetic induction field with a flux density of 0·4 T caused the anodic active-passive transition potential of iron in neutral aqueous solutions of sodium sulphate to move in the noble direction and resulted in a reduction of the passive potential range. Anodic current densities at the active-passive transition potential and in the passive potential range increased in the presence of the magnetic field. There was a synergistic effect between the magnetic field and the presence of chloride in solution on the breakdown of iron passivity.