An Investigation into Zinc Diffusion and Tin Whisker Growth for Electroplated Tin Deposits on Brass

Abstract

It is widely documented that whisker growth is more rapid for tin deposits on brass compared with deposits produced on other substrate materials, such as copper. As a result, studies investigating the effect of process variables on tin whisker formation are often conducted on brass substrates to take advantage of the increased whisker growth rates. Although it has been understood since the 1960's that the increased whisker growth results from zinc diffusion, to date there has not been any detailed analysis of the zinc/zinc oxide distribution at the surface of the tin deposit. Using a commercial bright tin electroplating bath, the formation of zinc oxide at the surface of tin deposits on brass has been investigated. The distribution of the zinc oxide and its development as a function of storage time has been studied. The effect of an electrochemical oxidation treatment, immediately after tin deposition, on zinc oxide formation and subsequent whisker growth has also been investigated. Analyses show that zinc oxide is present on the surface of the deposit within one day of electroplating. During storage at room temperature a network of zinc oxide is formed at the surface grain boundaries, the extent of which increases with time. The critical role that zinc diffusion plays in whisker growth for tin deposits on brass has been demonstrated by electrochemical oxidation of the tin shortly after electroplating. This develops a tin oxide film that is thicker than the native air formed oxide and subsequently serves as a diffusion barrier to zinc surface diffusion, thereby mitigating whisker growth.

The effect of electroplating parameters and substrate material on tin whisker formation

“…12 shows the cause of the increased whisker growth from the tin deposits on brass. The light contrast phase at both the deposit surface and along the surface of the short whiskers corresponds to zinc oxide, which forms as a result of zinc diffusion from the brass substrate into the tin deposit [33]. Considerable intermetallic formation has also occurred, although compared with equivalent tin deposits on copper the distribution is less uniform and the coverage along the Sn-Cu interface is less complete.…”

“…Whisker growth has been evaluated for both tin deposits on copper and tin deposits on brass, the latter used to promote accelerated whisker growth. An increased whisker growth rate from tin deposits on brass occurs as a result of zinc diffusion through the tin and the subsequent formation of zinc oxide both on the surface of the deposit and at tin grain boundaries [30][31][32][33][34]. The effect of storage at elevated temperature and humidity on whisker growth has also been investigated for both tin deposits on copper and tin deposits on brass.…”

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

“…Figure [10a] clearly shows both zinc oxide, at the deposit surface, and Cu 6 Sn 5 intermetallic formation at the Sn-brass interface, both of which are known to cause whisker growth 8,17,[23][24][25] , by generating internal stresses within the electrodeposited coating. In comparison figure [10b] shows that no zinc oxide formation has occurred at the surface of the electrochemically oxidised electrodeposit and only limited intermetallic formation has occurred at the Sn-Cu interface after ~30 months of storage.…”

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

“…48 Formation of ZnO at Sn grain boundaries has recently been confirmed in an x-ray photoelectron spectroscopy (XPS)-based study. 49 Interestingly, Sn atoms released by this reaction may also provide additional flux to form whiskers. 50 This is similar to accelerated growth of tin whiskers in the presence of rare earth elements.…”