Study On Electrochemical Migration Behavior Of SnAgCu Solder Alloy Under Thin Electrolyte Film And Controlling Of Corrosion Resistance

In the electronic packaging industries,the lead-bearing solders have already been gradually replaced by lead-free solder pastes due to the restrictions on the application of hazardous substance in a lot of countries.Among them,the ternary Sn96.5Ag3.0Cu0.5 alloy(SAC305)alloy becomes the most widely used solder alloy due tothe excellent solderability,wettability and mechanical properties.However,it is noteworthy that the reliability and environmental acceptability of the SAC305 solder alloy are still the enormous challenges in the electronic industry,especially for the solder alloy serviced in the harsh environment,such as marine or air pollutants from industry atmospheric environment.The electrochemical migration(ECM)failure caused by corrosion is one of most important topics for solder alloys,which can commonly cause the occurrence of catastrophic damage and decrease significantly the service life.Thus,it is of great significance to study the ECM failure mechanism of solder alloys and its protection methods for improving the reliability of electronic devices and prolonging their service life.In this work,the relationship between micro-structure,electronic structure of SAC305 alloy and its corrosion behavior is firstly studied and then further elaborates the influence mechanism of intermetallic compounds(IMCs)on corrosion behavior of SAC305 alloy.Secondly,an innovative thin electrolyte film(TEF)measurement device with high accuracy is designed.Then this device is used to conduct the ECM experiments to study the effects of chloride ions and bias voltage on ECM process of SAC305 alloy.The composition of dendrite is also further determined.The electrode potentials of cathode and anode under bias voltage are obtained by electrochemical workstation and analyze the critical bias voltage for the occurrence of ECM process and the corresponding anodic and cathodic reactions.Finally,to improve the ECM sensitivity and enhance the reliability of electronic device,the inorganic coatings and alloy composition control are discussed.The results show that Cu6Sn5 and Ag3Sn dispersed in the eutectic phase do not display corrosion dissolution,while Sn suffers from severe pits corrosion.This phenomenon is mainly related to the electronic structure and the micro-structure.In terms of electronic structure,the Volta potential of IMCs is much higher than it of Sn,which will lead to the formation of micro-galvanic couples between them.Thus the corrosion process of Sn as anode is accelerated due to the galvanic couples effects.Conversely,the corrosion process of IMCs as cathode is restrained.Moreover,the density of state(DOS)result suggests that corrosion dissolution is more likely to occur on the surface of Sn from corrosion kinetics perspective.As for the micro-structure of SAC305,the eutectic phase has smaller grain size and contains higher content of HAGBs,thus the depth of pits in eutectic phase is much larger.The dendrites are short and slim in low chloride ion concentration.The dendrites become much coarser and have longer branches with the increasing of chloride ions concentration.When the NaCl concentration exceeds 200 mM,the primary dendrites and the branches show the thin and long appearances,which is mainly related to the blocking effects of the precipitates between the two metallic stripes.The time to short circuit is the longest for the TEF environment containing 30 mM NaCl.Ag and Cu do not undergo the ECM process and Sn is the only migration element in this experiment condition.In terms of the effects of bias voltage on the ECM process,there is absence of dendrite growth due to the dense passive film covered on the sample surface under 300 μm TEF containing 30 mM NaCl condition when the bias voltage is less than 0.5 V.At 0.75 V bias voltage,SAC305 alloy suffers from slight corrosion and some corrosion products distribute between the two metallic stripes.But the dendrite still does not emerge.As the bias voltage increases to 1 V,SAC305 alloy suffers from pits corrosion and there is presence of dendrites growth phenomenon.In terms of electrode reactions,when the bias voltage is less than 1.5 V,the anodic potential is relative small and the anodic polarization resistance is large.In this situation,the cathodic reaction is the reduction process of oxygen.When the bias voltage increases to 1.5 V,the anodic potential increases and the anodic reaction is not still the control step for the electrochemical process.In the meantime,hydrogen evolution reaction gradually starts at cathode.The Al2O3,TiO2 and ZrO2 coating deposited by magnetron sputtering have dense surface micro-structure and can extend the time to short circuit of SAC305 alloy.Specifically,TiO2 has the most excellent protective properties,followed by ZrO2.The magnetron sputtering coating deposition method has the characteristics of simple operation,economical and good.effect.The earth element Nd significantly decreases the undercooling required in solidification process for the SAC305-xNd solder alloy.The electrochemical measurements results show that Nd enhances the passivation behavior of the solder alloy and the protective performance of the oxide film on the sample surface in the pseudopassivation region.Among them,0.05 wt.%Nd addition(SAC305-0.05Nd)has the most significant improvement effect on the corrosion resistance of solder.alloys.The effects of Nd on the corrosion behavior of solder alloys is mainly reflected in two aspects.Firstly,the earth element Nd decreases the potential difference between the IMCs and Sn,and thus the galvanic corrosion sensitivity is reduced.However,it is noteworthy that when the Nd addition reaches 0.25 wt.%,a great number of NdSn3 particles are precipitated in the alloy.The volta potential of NdSn3 matrix is much lower than that of the Sn,and the potential difference is up to 130 mV.Therefore,the corrosion micro-couple is built between NdSn3 matrix and Sn in the electrolyte solution and will still cause serious local corrosion.On the other hand,compared with SAC305 alloy,the content of SnO2 in the oxide film on the SAC305-xNd solder alloy surface increases from 67.07%(SAC305)to 80.08%(SAC305-0.05Nd),which enhances the stability of the oxide film and further improves the protection of the oxide film on the substrate.