Mechanism and Prevention of Spontaneous Tin Whisker Growth

Abstract

Spontaneous Sn whisker growth on Cu leadframe finished with Pb-free solder is a serious reliability problem in electrical and electronic devices. Recently, Fortune magazine had an article to describe the urgency of the problem. The spontaneous growth is an irreversible process, in which there are two atomic fluxes driven by two kinds of driving force. There are a flux of Cu atoms and a flux of Sn atoms. The Cu atoms diffuse from the leadframe into the solder finish driven by chemical potential gradient to form intermetallic compound of Cu 6 Sn 5 in the grain boundaries of the solder, and the growth of the compound at room temperature generates a compressive stress in the solder. To relieve the stress, a flux of Sn atoms driven by the stress gradient diffuses away to grow a spontaneous Sn whisker which is stress-free. The typical industry solution is to insert a diffusion barrier of Ni between the Cu and solder to prevent the diffusion of Cu into the solder. It is insufficient, because we have to uncouple the irreversible processes and stop both the fluxes of Cu and Sn. A solution is presented here.

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

“…It can be seen that extensive intermetallic formation has occurred in both samples, i.e. the presence of the electrochemical oxide has had no observable effect on intermetallic growth, which is the primary driving force for whisker growth in these samples 8,23,24 . This suggests that the reduced whisker growth observed for the electrochemically oxidised samples results from the thicker electrochemical oxide providing an enhanced physical barrier to the growth of whiskers rather than the driving force for whisker growth being reduced.…”

“…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.…”

Mechanisms for Sn whisker growth in rare earth-containing Pb-free solders

“…This could result in short-circuiting or interference with other devices. Although the precise mechanisms for Sn whisker growth are not well known, most researchers agree that compressive stresses are a necessary condition for Sn whisker formation [4][5][6][7]. In addition, many researchers believe that the location of Sn whiskers is related to ''weak points" in the thin oxide layer on the Sn solder surface [5][6][7][8][9].…”

Effect of Environmental Conditions on Tin (Sn) Whisker Growth

“…Tensile stress is thought to slow the growth, whereas compressive stress to expedite whisker formation [7], although tin whisker protrusion on tensile regions of a tin finish was observed [9]. It is rationalized that bent tin coated substrates tend to form more whiskers at a faster rate, on the inner-curved area rather than on the flat area [18].…”

“…Growth rate portrays the end-to-end span of a whisker changing over a certain time. Tin whiskers are reported to be single crystalline [7] [8] pure tin protrusions, a few micron diameters [9]. Tin whiskers have been found to grow on electroplated matte tin on a brass-alloy substrate when exposed to a condensation induced corrosive environment [10].…”