In Situ Study of the Electrochemical Migration of Tin under Bipolar Square Wave Voltage

Abstract

The electrochemical migration (ECM) of tin under bipolar square wave voltage with various time periods was in situ studied using optical and electrochemical techniques. The results show that there is no dendrite growth but only precipitate formation for the time periods from 1 to 100 s during the same test time interval. The amount of precipitates increases with the increasing time period from 1 to 100 s. Moreover, the locations where the precipitates appear are time period dependent. As the time period increases to 200 s, both dendrite growth and precipitates formation can be observed during the ECM test and the short circuit occurs at the end of the third semi-cycle. From one semi-cycle to the next semi-cycle, the switch between the voltage and its reverse voltage derived from the bipolar square wave during the ECM of tin results in a periodic switch between cathodic polarization and anodic polarization applied on the same electrode, periodic changes in the migration direction of ions and a circulation between dendrite growth and its re-dissolution. Therefore, it is believed that the ECM processes including ion formation, ion migration, precipitate formation and dendrite growth under bipolar square wave voltage are strongly dependent upon the semi-cycle.Electrochemical migration (ECM) occurs when two oppositely biased and closely spaced electrodes are connected by a continuous electrolyte layer. Metal ions are produced at the anode and then migrate toward the cathode, where they can be reduced to metallic dendrite which then grows toward the anode. 1-10 Once the dendrite reaches the anode, the short circuit occurs and causes catastrophic failure of electronic devices.In electronics, the metallic surfaces that are directly exposed to the environment are mainly the component electrodes, solder joints, and contact/connector areas. Nowadays, hot air solder leveling surface finishes are widely employed with tin based solders in industrial applications. 3,11 Therefore, it can be considered that large part of the exposed areas is made of tin or tin solder alloys. In this case, the ECM of tin and tin solder alloys is extremely important in connection with reliability of electronic devices.Extensive work has been conducted on the ECM of tin and tin solder alloys regarding the migration mechanisms 3,12-14 and the influential factors such as alloy elements, 7,13,15,16 contaminants 5,17-23 and bias voltage. 24-26 In general, most of these studies were carried out under steady state electric field where a DC bias voltage was used. Only two of them were conducted on unsteady state electric field, 25,26 although the unsteady electric field which is generated by pulsed voltage or AC voltage widely exists in the operating environment of electronics.In these two publications, Verdingovas et al. 25 and Zhong et al. 26 respectively investigated the ECM of tin under unipolar square wave voltage in which one time period (T) consists of an ON time (T ON ) and an OFF time (T OFF ). During the ON time, a voltage with fixed amplitude...

Electrochemical migration of Sn and Sn solder alloys: a review

“…Therefore, the AC could also be expected to inhibit the ECM of Sn and Sn solder alloys. Zhong et al 154 also found that the dendrite growth of Sn under bipolar square wave voltage (like AC voltage) was strongly dependent upon the semi-cycle of bipolar square wave voltage. The dendrite growth will not appear if the semicycle is shorter than a critical value.…”

“…49,155,156 According to the ECM mechanisms of Sn and Sn solder alloys and the previous studies regarding the ECM and its inhibition of other materials, 15,[152][153][154][155][156][157][158][159][160][161] the possible strategies to inhibit the ECM of Sn and Sn solder alloys are summarized, including adding alloy elements, inhibitor, and pulsed or AC voltage.…”

“…It was found that varying of pulse width at a xed duty cycle had a minor effect under investigated conditions, whereas increasing duty cycle signi-cantly reduces the time-to-failure due to the dendrite formation and increases the charge transferred between the electrodes over time. Zhong et al 51,154 investigated the ECM of Sn under unipolar and bipolar square waves using thin electrolyte layer method. Under the unipolar square wave, it was found that at a xed time period the rate of dendrite growth rate increases with the increasing duty cycle.…”

“…51 For the bipolar square wave, the switch between the voltage and its reverse voltage results in a periodic switch between cathodic polarization and anodic polarization applied on the same electrode surface, periodic changes in the migration direction of ions and a circulation between dendrite growth and its re-dissolution. 154 …”

Evidence for Ag participating the electrochemical migration of 96.5Sn-3Ag-0.5Cu alloy

“…The same electrolyte was also added into the plexiglass chamber to minimize the evaporation. This setup which is similar to those reported in previous work about atmospheric corrosion or ECM makes it possible to perform in situ electrochemical migration measurements under thin electrolyte layer [13][14][15].…”