An Investigation into the Effect of a Post-electroplating Electrochemical Oxidation Treatment on Tin Whisker Formation

Abstract

Since the 'cracked oxide theory' was proposed by Tu in 1994 [1], there has only been a limited number of studies that have sought to investigate the effect of the Sn oxide on whisker growth. The current study has used electrochemical oxidation to produce oxide films, which has enabled the effect of the surface oxide thickness on whisker growth to be established. The effect of oxide thickness on whisker growth has been investigated for tin electrodeposits on both Cu and brass substrates. The influence of applied oxidation potential on the thickness of the Sn oxide film has been investigated using x-ray photoelectron spectroscopy (XPS) for potassium bicarbonate-carbonate and borate buffer electrolyte solutions. Whisker growth from electrochemically oxidised Sn-Cu deposits on Cu and Sn deposits on brass has been investigated and compared with samples left to develop a native air-formed oxide. XPS studies show that the thickness of the electrochemically formed Sn oxide film is dependent on the applied oxidation potential and the total charge passed.Subsequent whisker growth studies demonstrate that electrochemically oxidised Sn-Cu deposits on Cu and Sn deposits on brass are significantly less susceptible to whisker growth than those having a native oxide film. For Sn deposits on brass, the electrochemically formed Sn oxide greatly reduces Zn oxide formation at the surface of the tin deposit, which results in whisker mitigation. For Sn-Cu deposits on Cu, the reduction in whisker growth must simply derive from the increased thickness of the Sn oxide, i.e. the Sn oxide film has an important role in stemming the development of whiskers.

Tin whisker mitigation by means of a post-electroplating electrochemical oxidation treatment

“…It is evident from figure [2] that the native air-formed oxide is very thin compared with oxide films that are formed electrochemically. Results show that the thickest electrochemical oxide film was produced at a potential of 1.2 V vs. SCE, which is consistent with previous trials 17 . The Sn oxide and Sn metal contributions as a function of electrochemical oxidation potential are summarised in table [1], which clearly indicates that the thickness of the Sn oxide increases as the oxidation potential is increased.…”

“…The slight reduction in whisker growth at the higher oxidation potentials can be directly related to the relative intensities of the Sn3d5 oxide and Sn3d5 metals peaks shown in previous work 17 which indicated that oxidation at potentials greater than 1.2 V vs. SCE resulted in an increase in oxide thickness.…”

“…Oxidation peak I (-0.83 V vs. SCE) was not studied because it had previously been shown to produce an oxide film comparable to a native oxide 17 . The electrochemical oxidation potentials -0.4 V and -0.5 V vs. SCE were chosen as they occurred upon the broad trailing slope of the second oxidation peak which would suggest large amounts of activity within this region and it was suggested by Drogowska et al 19 that this broad slope corresponds to a dehydration reaction that results in the formation of SnO 2 .…”

“…The electrochemical oxidation potential of 0 V vs. SCE was chosen as it occurred on the part of the voltammogram that had the lowest anodic current, which was suggested by Drogowska et al 19 to be the region of passivity. The electrochemical oxidation potential of 1.2 V vs. SCE was chosen as it was previously shown to produce a thick oxide film 17 .…”

“…in a previous study 17 the authors presented some initial results, which indicated that whisker growth could be reduced by the application of an electrochemical oxide film. The current paper builds upon this initial work to more fully investigate the influence of applied potential and charge passed on the oxide film developed and its effect on whisker growth.…”