Manipulating Crystallographic Texture of Sn Coatings by Optimization of Electrodeposition Process Conditions to Suppress Growth of Whiskers

Abstract

The effects of two major electrodeposition process conditions, electrolyte bath temperature and current density, on the microstructure and crystallographic texture of pure tin coatings on brass and, ultimately, on the extent of whisker formation have been examined. The grain size of the deposited coatings increased with increasing electrolyte bath temperature and current density, which significantly affected the dominant texture: (211) or (420) was the dominant texture at low current densities whereas, depending on deposition temperature, (200) or (220) became the dominant texture at high current densities. After deposition, coatings were subjected to different environmental conditions, for example isothermal aging (room temperature, 50°C, or 150°C) for up to 90 days and thermal cycling between À25°C and 85°C for 100 cycles, and whisker growth was studied. The Sn coatings with low Miller index planes, for example (200) and (220), and with moderate aging temperature were more prone to whiskering than coating with high Miller index planes, for example (420), and high aging temperature. A processing route involving the optimum combination of current density and deposition temperature is proposed for suppressing whisker growth.

Alkoxylated β-Naphthol as an Additive for Tin Plating from Chloride and Methane Sulfonic Acid Electrolytes

“…Such random-textured Sn films are obtained when the deposition is performed at relatively low current density resulting in a slower film growth rate. Applying high current density favors the deposition of Sn coatings with different texture, commonly along low Miller-indexed planes such as (200) or (220) planes [33]. Such coatings show weak or no diffraction peaks from higher Miller indexes planes, as also seen in previous studies on Sn deposited from chloride based [11,34] and MSA based electrolytes [28].…”

“…From the analysis of the XRD spectra, all the Sn films deposited from both electrolytes can be considered as randomly-textured. The X-ray spectra of randomly-textured Sn films are commonly characterized by several minor peaks of high Miller-indexed planes [33], as in the case of the XRD spectra shown in Figure 11a,b. Such random-textured Sn films are obtained when the deposition is performed at relatively low current density resulting in a slower film growth rate.…”

“…Such random-textured Sn films are obtained when the deposition is performed at relatively low current density resulting in a slower film growth rate. Applying high current density favors the deposition of Sn coatings with different texture, commonly along low Miller-indexed planes such as (200) or (220) planes [33]. Such coatings…”

Critical Evaluation of Relative Importance of Stress and Stress Gradient in Whisker Growth in Sn Coatings

“…ZnO forms due to the fast diffusion of Zn atoms from the brass substrate along the grain boundaries of Sn and the subsequent reaction with SnO2. The fast diffusion of Zn was confirmed using electron probe microanalysis (EPMA) in one of our previous works [43] and also in a few other studies [44,45], where considerable concentration of Zn was noticed near the free surface of the Sn coating. The molar volume of ZnO is 14.5 cm 3 /mol [42], which is considerably smaller than the molar volume of SnO2, which is 21.7 cm 3 /mol [41].…”

“…It should be noted that the growth rate of IMC slows down with time, t (as hIMC ~ t n , where n < 1, e.g., ½, etc.). This reflects also in the slowing down of the growth rate of whiskers in Sn coatings with time [43]. This balance between stress generation due to IMC and stress relaxation due to whisker growth seems to be manifested as a steady state compressive stress in Sn after prolonged aging.…”