Shin-Bok Lee

Effect of Bias Voltage on the Electrochemical Migration Behaviors of Sn and Pb

Electronic devices are becoming more miniaturized, and their operating environment is becoming more severe. In a condition of high humidity, high temperature, and high voltage applied, metals in electronics can cause insulation failure by making conductive filaments, which is called electrochemical migration. To understand this phenomenon, the water-drop test (WDT) and the anodic-polarization test on Sn and Pb electrodes are conducted, and their results are compared. Under constant voltage conditions for WDT, initial current of Sn rapidly decreased, but that of Pb did not decrease due to the passivity formation of Sn. In the case of Sn, two zones are distinguishable by level of biased voltage. In the first zone, the current value of Sn is lower than that of Pb by about two orders of magnitude. In the second zone, the current value of Sn is lower than that of Pb by about one order of magnitude. This is due to the corrosion behavior of Sn: passivity formation in the first zone and the pitting corrosion occurrence in the second zone. It was shown that the higher the voltage is, the shorter the time to failure becomes. The total amount of electric charge for the insulation failure of Sn is smaller than that of Pb, which can be explained by the difference in microstructure of dendrites of each material caused by the different corrosion behavior.

Relationship between edge drift and atomic migration during electromigration of eutectic SnPb lines

The existence of an incubation stage before edge drift occurs was found by examining the electromigration characteristics of eutectic SnPb solder in an edge drift structure using in situ scanning electron microscopy and the interruptive test method. During this incubation stage, the depletion of Pb was observed at the cathode end. From the change in resistance, the activation energies for the incubation and edge movement stages were calculated to be 0.88 and 1.02eV, respectively. Based on a comparison of the activation energies for each stage with the previously reported values, it is believed that, during the incubation stage, Pb migrated before Sn and that the edge movement resulted from the migration of Sn. These results suggest that Pb depletion is a prerequisite for electromigration-induced void nucleation in eutectic SnPb solder.

Quantum transport in low-dimensional organic nanostructures

We have studied three low-dimensional systems with sub-micron dimensions: a single polyacetylene (PA) nanofiber; a single-walled carbon nanotube (SWNT)-rope; and a lithographically prepared stripe of poly(2-methoxy-5-(2-ethyl hexyloxy)-p-phenylene vinylene) (MEH-PPV). In each case, the sample was contacted to four-probe electrodes, with 100-nm spacing and various electronic transport properties such as the I-V characteristics, the temperature dependence of resistivity and the gate voltage dependence of the transport current were measured. The PA nanofiber was found to be non-ohmic with a room temperature conductivity of ∼ 0.1 S/cm. Its carriers were found to be hole-like with charge carrier mobility of μ = 7.76 × 10 -2 cm 2 /Vs. For the SWNT-rope, the temperature-dependence of resistivity exhibited signatures of a Luttinger liquid for temperatures below 30 K. With varying gate voltage, periodic peaks were seen in the nanotube current which would normally be attributed to the effects of Coulomb blockade. Interestingly, these peaks show three-way splitting, similar to observations in triple quantum dot experiments. The MEH-PPV stripe, which was produced using electron beam lithography, had I-V characteristics similar to that of a large band-gap semiconductor. In the high field region, these characteristics could be explained in terms of a single carrier device model which considers the field-dependent mobility along with space charge limited conduction (SCLC). All three samples can be considered as field-effect transistors (FETs), with potential use in future high density integrated electronic devices.

Orientation dependence of magneto-resistance behaviour in a carbon nanotube rope

The orientation dependence of magneto-resistance behaviour for a single-walled carbon nanotube (SWCN) rope is reported.A clear delineation of behaviours is observed between applying a magnetic field perpendicular or parallel to the rope axis.For a perpendicular field, monotonic negative magneto-resistance is observed due to two-dimensional weak localisation within the rope. By contrast, for a parallel field, complicated oscillatory behaviour is observed due to the Altshuler–Aronov–Spivak effect around closed electron trajectories on discrete cylinders within the SWCN rope.A dominant oscillatory mode can be identified which corresponds to closed paths around the outer circumference of the SWCN rope.However, due to the composite filamentary nature of the rope, the overall oscillatory behaviour is rather complicated and can be classified as universal conductance fluctuations. With a backgate voltage applied to the sample, Coulomb blockade peaks are observed in the transport current with additional peak structure superimposed due to resonant tunnelling.We find an interesting effect whereby these peaks are suppressed in the presence of a magnetic field. 2002 Elsevier Science B.V. All rights reserved.

Effect of initial anodic dissolution current on the electrochemical migration phenomenon of Sn solder

As electronic packages become smaller in size, the pitch of metal line is getting shorter. Decrease of line pitch in electronic packages and substrate of electronic component causes the electrochemical migration (ECM) more frequently. If the electronic components are exposed to high temperature and high humidity environments with authorized voltage condition, the metals in packages and substrate are easily ionized and form conductive dendrites, leading to insulation failure. ECM process consists of three steps: anodic dissolution, ion migration, and dendritic growth. The first step is the most important because anodic dissolution determines strongly affect the occurrence of remaining steps. In this work, anodic dissolution of Sn solder is characterized by water-drop test (WDT) and anodic polarization teat (APT). Initial anodic dissolution current of Sn solder is measured in 0.001 wt.% NaCl, Na2SO4 solutions and deionized water by WDT, and the results were compared with APT results. It is found that the higher the voltage is, the shorter the time-to-failure (TTF) is. In lower-voltage condition, anodic dissolution current is limited by passivity formation. The pitting corrosion occurred in higher voltage condition with shorter TTF. The anodic dissolution current is dependent on the electrolyte: the highest value in 0.001 wt.% NaCl solution and lowest value in D.I water.

Improvement of Electrochemical Migration Resistance by Cu/Sn Intermetallic Compound Barrier on Cu in Printed Circuit Board

A thin Sn coating was coated on the Cu electrodes on a printed circuit board (PCB) and annealed to form Cu/Sn intermetallic compounds (IMCs). Electrochemical migration (ECM) characteristics were investigated with respect to coating thickness, conductor spacing, and potential bias. From the anodic polarization test, the corrosion current of Cu/Sn IMCs was reduced to nearly two orders of magnitude at 0.217 V (versus saturated calomel electrode) compared to pristine Cu. The corrosion resistance of Cu/Sn IMCs was improved. Water drop test (WDT) results revealed that mean time to failure also increased over 50 times on the sample with the Sn coating (160 nm) at 50- μm spacing compared to the pristine Cu and a thick Sn coating (160 nm) was more effective than thin Sn coating (90 nm) due to the formation rate of IMCs. Passivity behavior and pitting corrosion were observed in both the anodic polarization test and the WDT. The graph of current-density potential in the anodic polarization test and the TTF potential in the WDT showed a quite similar curve shape. Therefore, dissolution step is the rate-determining step in ECM and governs the entire ECM process. Cu/Sn IMCs can be a new alternative surface barrier to improve the ECM resistance of Cu electrodes on PCBs.

Dominant migration element in electrochemical migration of eutectic SnPb solder alloy

In harsher condition with elevated temperature/humidity and complex chemical species, electronic components can be electrochemically ionized and migrate by electric field. The migrated ions form conducting filament between anode and cathode across a nonmetallic medium. The filament can cause a failure of electronic system. This phenomenon is called as the electrochemical migration (ECM). A model test system for water-drop test was devised to assess the ECM characteristics of eutectic SnPb (63Sn-37Pb, wt%). The result for model test system was compared with that for real electronic component on printed circuit borads (PCBs). Although the filament formation mechanism was different, it was found that Pb atoms are more susceptible to polarization than Sn atoms for both cases. This tendency agrees well with the composition distribution of the filament on the model system and PCBs

Effect of SO 4 2- Ion on Corrosion and Electrochemical Migration Characteristics of Eutectic SnPb Solder Alloy

Electrochemical migration phenomenon is correlated with ionization of anode electrode, and ionization of anode metal has similar mechanism with corrosion phenomenon. In this work, in-situ water drop test and evaluation of corrosion characteristics for SnPb solder alloys in solutions were carried out to understand the fundamental electrochemical migration characteristics and to correlate each other. It was revealed that electrochemical migration behavior of SnPb solder alloys was closely related to the corrosion characteristics, and Sn Ivas primarily ionized in solutions. The quality of passive film formed at film surface seems to be critical not only for corrosion resistance but also for electrochemical migration resistance of solder alloys.ጊ吀Ѐ㘹〻Ԁ䭃䑎䴀

Negative differential resistance in single-wall carbon nanotube network

The current-voltage (I-V) characteristic of single-wall carbon nanotube (SWNT) network was investigated at low temperatures. At T∼ 1.6K, S-shaped negative differential resistance (NDR) was observed, which was systematically examined under the variation of temperature, magnetic field in transverse direction and distances between probes. S-shaped NDR was explained by the nondestructive impact ionization of carriers and the resulting rmation of inhomogeneous spatial structures such as high-current filaments. Critical electric field for impact ionization was estimated to be about IV/ cm.

Electrochemical Migration Characteristics of Sn-3.0Ag-0.5Cu Solder Alloy in NaBr and NaF Solutions

Electrochemical migration characteristics of Pb-free solder alloys are quantitatively correlated with corrosion characteristics in harsh environment conditions. In-situ water drop test and corrosion resistance test for Sn-3.0Ag-0.5Cu solder alloys were carried out in NaBr and NaF solutions to obtain the electrochemical migration lifetime and pitting potential, respectively. Sn-3.0Ag-0.5Cu solder alloy shows similar ionization and electrochemical migration behavior with pure Sn because of Ag and Cu do not migrate due to the formation of resistant intermetallic compounds inside solder itself. Electrochemical migration lifetime in NaBr is longer than in NaF, which seems to be closely related to higher pitting potential in NaBr than NaF solution. Therefore, it was revealed that electrochemical migration lifetime of Sn-3.0Ag-0.5Cu solder alloys showed good correlation to the corrosion resistance, and also the initial ionization step at anode side is believed to be the rate-determining step during electrochemical migration of Pb-free solders in these environments.