A Study on the Effect of Pulse Electrodeposition Parameters on the Morphology of Pure Tin Coatings
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MICROELECTRONIC components made of Cu and Cu alloys are electroplated with Sn in order to maintain their solderability. However, pure Sn platings are prone to the formation of whiskers which are approximately 1 lm in thickness, and up to several millimeters in length. Their spontaneous formation on microelectronic components can cause short-circuiting, and thus studies based on their mitigation have been conducted since 1950s.[1,2] In the past, lead has been used as an alloying element for tin plating for controlling whisker growth. However, recent environmental regulations, e.g., the European Union’s Restriction of Hazardous Substances (RoHS) act have called for the restriction of hazardous substances from electrical and electronic components. RoHS directive is adopted by European Union in February 2003 which restricts the use of six substances, (1) Lead (Pb), (2) Mercury (Hg), (3) Cadmium (Cd), (4) Hexavalent Chromium (Cr+6), (5) Polybrominated Biphenyls (PBB), and (6) Polybrominated Diphenyl Ethers (PBDE). RoHS act not only contributes to the protection of human health, but also deals with the environmentally sound recovery and disposal of waste electrical and electronic equipment. These concerns create interest in other methods of whisker growth mitigation.[3] Various factors that affect whisker growth are surface morphology and microstructure described by plating thickness, grain shape, size and orientation, addition of alloying elements in the Sn plating, external applied stress, and lastly, composition of the ASHUTOSH SHARMA, Research Scholar, SUMIT BHATTACHARYA, Graduate Student, SIDDHARTHA DAS and KARABI DAS, Professors, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India. Contact e-mails: [email protected]. ernet.in; [email protected] Manuscript submitted December 6, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
substrate.[1,3–8] The surface morphology of electrodeposited metals and alloys is governed by the various electrochemical processes, such as, diffusion and transport of ionic species from the electrolyte toward the electrode, formation of nuclei and growth of the surface.[9,10] The correlation between these phenomena and the evolution of morphology is therefore important in order to manipulate the film microstructure by varying the pulse electrodeposition parameters. For example, in typical applications, dendritic growth is avoided to obtain dense and compact films[11]; however, it is needed where formation of materials with high surface areas is desired.[12] There are several reports on the effects of pulse parameters such as current density, additives, duty cycle, frequency, and stirring rate of bath,[13–16] however, very limited work has been carried out to study the effect of temperature as well as pH on Sn electroplating from acidic baths. This article documents the effect of plating parameters, i.e., current density, additive concentration, duty cycle, frequency, pH, temperature, and stirring rate on
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