The Corrosion And Electrochemical Migration Of Tin Under Thin Electrolyte Layers

Under the synergic effect of the continuously changing temperature field, high density electric filed, water vapor as well as contaminants, the degradation of the packaging materials results in their cracking and even detachment from the interface of solder joints. Consequently, an absorbed water film may be formed easily on the surface of solder joints. Moreover, there is multi-materials combination, especially for the metal materials, together with the contaminants existed at the solder joints. In these cases, the solder joint becomes the most susceptible to suffer corrosion in electronics. Therefore, from the corrosion’s point of view, the largest risk of the electronics failure comes from the corrosion and electrochemical migration of solder joint materials.The new lead free solder alloys commonly used for soldering in the electronic industries consists of almost95%tin. Therefore, the atmospheric corrosion and electrochemical migration of tin is the core of the problem in the electronics failure. Both the atmospheric corrosion and electrochemical migration of tin can be considered as the electrochemical reactions under the thin electrolyte layers. So it is very significant and valuable either for the theory or for the practical application to study the corrosion and electrochemical migration of tin under the thin electrolyte layers in the present work. The main works and results are as follows:1. The corrosion of tin under thin electrolyte layers containing chloride ions.The corrosion of tin was investigated by electrochemistry impedance spectroscopy, cathodic polarization curves and surface characterization including scanning electron microscopy and X-ray photoelectron spectroscopy. The results show that the corrosion rate which is controlled by the oxygen diffusion increases as the electrolyte layer thickness deceases at the initial stage. Moreover, the corrosion rate under the same electrolyte layer thickness increases firstly and then decreases with the exposure time. At the later stage, the corrosion rate under thin electrolyte layer with a thickness more than200μm is still controlled by the diffusion of oxygen with relative low corrosion rates. The corrosion rates under thin electrolyte layer of100and50μm decrease since the anodic process is inhibited due to the difficult diffusion of dissolved metal ions. The maximum corrosion rate is obtained under thin electrolyte layer of200μm, which is the critical thin electrolyte layer thickness for the transition from cathodic control to anodic control. The corrosion products formed on tin surface are composed of stannous and stannic oxides and\or hydroxides, which can retard the corrosion development of tin at the later exposure stage.2. The establishment of the novel approach to study the electrochemical migration.There are some drawbacks for the water drop experiment and simulated environment test which are commonly used the study of electrochemical migration, i.e., poor reproducibility of electrochemical result and unsuitable in situ inspection. Herein, we propose a novel approach to study the electrochemical migration:thin electrolyte layer test. The reliability and the suitability of this method were tested in the electrochemical migration behaviors of tin and copper under electrolyte layer containing chloride ions. The results show that a better reproducibility than water drop experiment and simulated environment test can be obtained. This can be attributed to that an identical area of cathode or anode during the ECM test among the parallel samples can be achieved through the TEL method, ensuring the same electrochemical reaction rate, resulting in a good reproducibility of the electrochemical results. Moreover, this method is also very suitable for in situ real time inspection of the electrochemical migration. The horizontal surface of the electrolyte layer formed through the TEL method is good for the in situ investigation of the dendrite growth process and its morphology, precipitates formation, the migration of ions, diffusion, the pH distribution and so on.3. The electrochemical migration of tin under the steady state electric field. The electrochemical migration of tin under thin electrolyte layers containing chloride ions was investigated using in situ electrochemical and optical techniques, as well as ex situ surface characterization. The conclusions are drawn as follows:(1) The dendrite growth rate first decreases and then increases with the electrolyte layer thickness. The minimum value is at100μm. This can be attributed to the local tin ions concentration nearby the electrode. The higher tin ions concentration is, the faster the dendrite growth rate is.(2) It is found that with a low Cl-concentration (below1mM), tin dendrites co-exist with precipitates. With an intermediate Cl-concentration (17mM), only precipitates but no dendrites occur resulting from the accumulation of precipitates which act as a wall-like barrier to prevent the migration of Sn4+and Sn2+to the cathode. With a high chloride concentration (500mM), the intensive cathodic reaction produces large amount of OH-, resulting in the dissolution of the precipitate to form [Sn(OH)6]2-. Then, the tin dendrites grow again by the reduction of [Sn(OH)6]2".(3) The bias voltage evidently affects the ECM process of tin. With low Cl-concentrations, the time to short circuit decreases with the increase of bias voltage. However, with a high Cl-concentration (500mM) the time to short circuit decreases firstly, and then increases slightly with the increase of bias voltage. Two possible reasons can account for this phenomenon. On the one hand, there is a high nucleation rate of tin in high chloride concentration and high bias voltage. Then, tin nucleates at whole edge of cathode and even on the whole cathode surface. Therefore, the growth rate of single dendrite may decrease even though the increase of tin ions concentration in the electrolyte since there are excessive growth sites. On the other hand, the dendrites will be destroyed by the strong disturbance resulting from the evacuation of large amounts of hydrogen bubbles from the cathode surface and dendrites.4. The electrochemical migration of tin under the unsteady state electric field.Electrochemical migration behaviors of tin under unsteady state electric field were investigated via thin electrolyte layer method using optical and electrochemical techniques. Two kinds of unsteady state electric field were employed in the present work, i.e., unipolar and bipolar square wave electric field.(1) Unipolar square waves with various time periods and duty cycles were first used as the unsteady state electric field for electrochemical migration investigation, where a3-V-bias and a0-V-bias were applied across the two electrodes during the ON time and OFF time, respectively. The results show that there is a reverse polarization applied across the two electrodes during the OFF time to ensure the0-V-bias. The presence of reverse polarization will result in ions migration reverse to their original migration direction and the tin ions that migrated back may even be reduced back to metal tin. Therefore, the reverse polarization is adverse to the electrochemical migration process of tin. At the same time period, the rate of dendrites growth increases with the increase of duty cycle. While, for the same duty cycle, the prolongation of time period is favorable for the precipitates formation and/or dendrites growth. The influence mechanisms of time period and duty cycle on electrochemical migration behaviors have also been proposed, respectively.(2) Whether can the short circuit occur under the bipolar square wave electric field depends on the half period and the number of cycles. The mechanism has been discussed in detail.