Electroplated zinc finishes have been widely used in the packaging of electronic products for many years as a result of their excellent corrosion resistance and relatively low cost. However, the spontaneous formation of whiskers on zinc electroplated components, which are capable of resulting in electrical shorting or other damaging effects, can be highly problematic for the reliability of long life electrical and electronic equipment. To date, most research has focused on tin whiskers and much less attention has been paid to zinc whisker research. A number of mechanisms to explain zinc whisker growth have been proposed, but none of them are widely accepted and some are in conflict with each other. This study has investigated the mechanism for whisker growth from three commercial zinc electroplated coatings on mild steel substrates. Firstly, whisker growth from an alkaline cyanide-free zinc electrodeposit was studied. A reduction in deposition current density (from 50 to 5 mA/cm2) and an increase in deposit thickness (from 2 to 15 μm) both contributed to reduced whisker growth. In terms of the mechanisms of whisker growth, it was observed that the presence of spherical raised surface features (nodules) with cavities beneath, promoted whisker growth by markedly shortening the incubation time from ~ 5 months to ~ 4 weeks. More importantly, the time dependent recrystallisation of the as-deposited columnar structure is closely associated with whisker growth from both nodules and planar regions of the deposit. The formation of Fe-Zn intermetallic compounds (IMCs) was not observed at either the Fe/Zn interface or within the electroplated coatings and does not appear to be associated with Zn whisker growth. Whisker growth from an acid chloride zinc electroplated coating and a Zn-Ni alloy electroplated coating were also investigated. These two coatings were immune to whisker growth after 18 months of storage at room temperature, regardless of deposition current density, deposit thickness and substrate surface modification. Finally, the effect of potential mitigation strategies on whisker growth was evaluated. Exposure to elevated temperatures (50 to 150°C) for 24 hours promoted whisker growth by reducing the incubation time for whiskers to first appear from less than 4 weeks to less than 10 days, whilst whisker growth was significantly retarded when samples were subjected to a short period of thermal treatment (50 to 150°C) for 0.5 hour. More importantly, for a short period of thermal treatment, whisker mitigation became increasingly effective as the treatment temperature was raised. In addition, the formation of a trivalent chromium passive coating on the alkaline cyanide-free zinc electrodeposits immediately after deposition was not an effective mitigation method to retard whisker growth.

Physical, chemical and corrosion properties of a hexavalent chromium conversion coating (CCC) and that of a commercial third generation trivalent chromium system; Tripass LT1500, on zinc electrodeposited steel has been studied. Moreover, the role of additives has been studied to elucidate film formation and corrosion resistance mechanisms. Micro-cracking and self-repair corrosion protection behaviour commonly associated with hexavalent CCCs has also been investigated. Scanning Electron Microscopy (SEM) studies showed that for both hexavalent and trivalent CCCs were in general, flat with a spherical-like structure and in the case of the former microcracked beyond 122 nm conversion coating thickness. In general, the micro-crack pattern observed e.g. a dense crack network, depended upon the underlying zinc substrate morphology. The study has also demonstrated the effect of SEM imaging and prior specimen preparation conditions on hexavalent CCC micro-cracking and blistering. X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES) and Infrared (IR) data has indicated that the hexavalent CCC film formation appears to be a electrochemical/sol-gel mechanism given the lack of zinc content at surface and subsurface regions within the conversion coating, presence of H2O and in particular the contribution of Cr(OH)3 as opposed to Cr2O3. An alternative film formation mechanism may exist for trivalent CCC given a higher proportion of zinc at surface and sub-surface regions, IR data analysis indicating that chromium is possibly deposited from a chromium (III) complex ion such as [CrC2O4(H2O)4] + , moreover as Cr(OH)3 and Cr2O3 compounds as indicated by XPS data analysis. The role of cobalt nitrate during film formation is unclear given that cobalt was not detected within the trivalent CCC from XPS and AES data. Electrochemical LPR measurements, polarisation curves and XPS data has shown in general, self-repair corrosion protection properties for hexavalent CCC to be lacking. Instead, it is proposed that the corrosion protection behaviour for hexavalent and trivalent CCC to be barrier. Polarisation curves and LPR data showed that the corrosion resistance performance for trivalent CCC was higher than hexavalent CCC, in general. LPR data showed that the omission of cobalt nitrate and increased addition of sodium molybdate content within the Tripass LT1500 treatment solution formulation was found to overall decrease corrosion resistance within the trivalent CCC. In addition, silica based topcoat and black trivalent CCCs was also investigated and characterised using AES, SEM and LPR. Zinc whiskers was also observed from zinc electrodeposits following exposure to thermal treatment (150°C for 1 h). Elemental analysis and grain pattern investigations failed to help determine the cause of zinc whisker initiation. Zinc whiskers was seen to protrude out of hexavalent and trivalent CCCs, with the latter requiring a longer thermal exposure time.

Les whiskers, filaments métalliques qui poussent sur des surfaces métalliques, sont unproblème très important pour la fiabilité des composants électroniques. Depuis ces dernièresannées, il y a eu un regain d’intérêts industriels dans le domaine de la croissance des whiskers,principalement en raison de la miniaturisation des dispositifs électroniques et des réglementationsenvironnementales interdisant l'utilisation du plomb.Alors que la plupart des recherches concernent les whiskers d'étain, il y a encore peu detravaux sur les whiskers de zinc. Les revêtements d’électrodéposés de zinc sont utilisés commeprotection anticorrosion pour les aciers faiblement alliés dans diverses industries, commel'automobile, l'aéronautique ou l'énergie, ainsi que dans les structures de soutien ou les planchersfaux plafonds dans les centres de données informatiques. Afin d'atténuer, de prévenir et deprédire les défaillances causées par les whiskers de zinc, les mécanismes de sa croissance doiventêtre compris.Grâce à des tests de stockage accéléré et à des observations par microscopie électronique àbalayage (MEB), la cinétique de croissance des whiskers de zinc a été étudiée sur des tôles d'acierau carbone faiblement allié, galvanisé et chromé. Afin de comprendre les mécanismes de lacroissance des whiskers de zinc, la caractérisation quantitative ainsi que les excroissances (densité,volume et vitesse de croissance) ont été reliées aux paramètres suivants: la température, le bainpour l’électrodéposition du zinc, la chromatation, l’épaisseur du substrat d’acier, l’épaisseur durevêtement de zinc ainsi que la contrainte résiduelle.En outre, la microstructure et la cristallographie du revêtement de zinc, des racines deswhiskers ainsi que des whiskers elles-mêmes ont été étudiées par diffraction des électronsrétrodiffusés (EBSD), microscopie électronique à transmission (MET), microanalyse par rayon X(EDX) et le dispositif ASTAR pour l'orientation locale des grains; la préparation des échantillonsa été réalisée à l’aide d’un faisceau d'ions focalisés (FIB). La recristallisation ainsi que lesdislocations dans les whiskers et les excroissances ont été observés; aucun composéintermétallique n’a été observé que ce soit dans les échantillons issus de différents bainsélectrolytes ou encore dans les films / whiskers.Il a été montré que la relaxation de contrainte de compression résiduelle et la croissance deswhiskers sont deux phénomènes différents mais fortement reliés et thermiquement activés.Chacun d'entre eux suit un mécanisme différent; les énergies d'activation apparentes des deuxphénomènes ont été établies, et la diffusion aux joints de grains est proposée comme le principalmécanisme de diffusion pour la croissance des whiskers.Des cinétiques de la croissance des whiskers, à la fois analytique et phénoménologique sontproposées. Une bonne estimation de la croissance des whiskers et de leur vitesse de croissance àdes températures proches des conditions de fonctionnement est obtenue par comparaison avecles données expérimentales. / Whiskers, conductive metallic filaments that grow from metallic surfaces, are a very importantissue for reliability of electronic components. Through recent years, there has been a renewedindustrial interest on whisker growth, mainly due to the miniaturization of electronic devices andthe environmental regulations forbidding the use of lead.While most of the research has been focused on tin whiskers, there is still little reference tozinc whiskers. Electroplated zinc coatings are actually used as anticorrosive protection for lowalloy steels in diverse industries such as automotive, aerospace or energy, as well as for supportstructures or raised-floor tiles in computer data centers. In order to mitigate, prevent and predictthe failures caused by the zinc whiskers, the mechanisms of growth must be understood.By accelerated storage tests and Scanning Electron Microscopy (SEM) observation, kinetics ofgrowth of zinc whiskers was studied on low alloy chromed electroplated carbon steel.Quantitative characterization of both whisker and hillocks (density, volume and growth rate) wasrelated with the parameters temperature, electroplating electrolyte, presence of chrome, steelsubstrate thickness, zinc coating thickness and residual stress, in order to understand themechanisms of growth.Additionally, both microstructure and crystallography of zinc coating, whisker roots and actualwhiskers were studied by Electron Backscatter Diffraction (EBSD), Transmission ElectronMicroscopy (TEM), Energy-dispersive X-ray spectroscopy (EDX) and local grain orientationwith ASTAR setup, using Focused Ion Beam (FIB) for samples preparation. Recrystallization aswell as dislocations were observed in both whiskers and hillocks; no intermetallic compoundswere seen in neither electroplated nor whiskers.It is found that compressive residual stress relaxation and whiskers growth are two differentbut strongly interconnected phenomena both thermally activated, an each of them follows adifferent mechanism; apparent activation energies of the two phenomena are calculated, andgrain boundary diffusion is established as the main diffusion mechanism for whiskers growth.Whiskers growth kinetics, both analytical and phenomenological is proposed. Goodestimation of whiskers growth and whiskers growth rate at temperatures close to operationconditions is obtained when compared with experimental data. / Whiskers, filamentos metálicos que crecen en superficies metálicas, son un problema muyimportante para la fiabilidad de componentes electrónicos. Durante los últimos años, ha habidoun renovado interés industrial en el crecimiento de whiskers, debido principalmente a laminiaturización de dispositivos electrónicos y a las regulaciones ambientales que prohíben lautilización de plomo.La mayoría de las investigaciones se concentran en los whiskers de estaño y hay todavía pocostrabajos sobre los whiskers de zinc. Los recubrimientos de zinc electrodepositado son utilizadoscomo protección anticorrosión para los aceros de baja aleación en diversas industrias, comoautomotriz, aeronáutica o energética, así como en la estructuras de soporte o tejas de techosfalsos en los centros de datos informáticos. Para atenuar, prevenir y predecir las fallas causadaspor los whiskers de zinc, los mecanismos de crecimiento deben ser comprendidos.Gracias a experimentos de almacenamiento de muestras y a observaciones por microscopíaelectrónica de barrido (SEM), la cinética de crecimiento de whiskers de zinc ha sido estudiada enaceros de baja aleación recubiertos de zinc y cromados. Para comprender los mecanismos decrecimiento de whiskers de zinc, la caracterización cuantitativa de whiskers y de protuberancias(densidad, volumen y velocidad de crecimiento) fue relacionada con los parámetros siguientes:temperatura, electrolito usado en la electrodeposición de zinc, cromado, espesor del substrato deacero, espesor del recubrimiento de zinc al igual que el estrés residual.Adicionalmente, microestructura y cristalografía del recubrimiento de zinc, de raíces dewhiskers así como de los propios whiskers fueron estudiadas por medio de la difracción deelectrones por retrodispersión (EBSD), microscopía electrónica de transmisión (TEM),microanálisis por rayos X (EDX) y el dispositivo ASTAR para la orientación local de granos; lapreparación de muestras fue realizada con la ayuda de un haz de iones localizados (FIB). Larecristalización así como las dislocaciones en whiskers y protuberancias fueron observadas;ningún compuesto intermetálico ha sido observado en los recubrimientos ni en los whiskers.Se determinó que la relajación del estrés residual de compresión y el crecimiento de whiskersson dos fenómenos diferentes pero fuertemente interconectados y térmicamente activados. Cadauno de ellos sigue un mecanismo diferente; las energías de activación aparentes de los dosfenómenos han sido establecidas, y la difusión por bordes de grano es propuesta como elprincipal mecanismo de difusión para el crecimiento de whiskers.Cinéticas de crecimiento de whiskers, a la vez analíticas y fenomenológicas son propuestas.Una buena estimación del crecimiento de whiskers y su velocidad de crecimiento a temperaturascercanas a las condiciones de operación es obtenida por comparación con los datosexperimentales.