Study On Electrochemical Migration Behavior Of Sn-based Lead-free Solder

The miniaturization of electronic products has led to the development of integrated circuits with high integration,high performance,high density and small pitch and reduces the solder joint or wire bond pitch.This not only imposes stringent requirements on the manufacturing process of electronic products,but also imposes more stringent requirements on the reliability of electronic products.The increased density of electronic devices and the reduction in pitch also provide a convenient condition for electrochemical migration（ECM）to cause device insulation failure,making it one of the common causes of current integrated circuit failure.The phenomenon of electrochemical migration refers to a process in which metal ions are formed due to dissolution of one electrode,and metal ions move through the electrolyte to another electrode to form a metal deposit by reduction deposition.Voltage,electrode spacing,electrolyte concentration,etc.are all important factors in ECM.In this paper,the water droplet experiment method was adopted,based on four lead-free solders of Sn,Sn0.7Cu,Sn3Ag and Sn9Zn,the applied voltage of 1.5V,3V,5V,8V,electrode spacing of0.3mm,0.5mm and 0.7mm,0mM/L,1 mM/L,30 mM/L,500 mM/L NaCl solution concentration,the effects of voltage,electrode spacing,NaCl solution concentration on ECM behavior were studied.The main conclusions are as follows:（1）ECM had a gestation period,and anodic dissolution,ion migration and accumulation were completed during the incubation period,and the formation of dendritic ECM products in the cathode（mainly metal deposits）can be observed.When the products grown from cathode to anode,it caused a short-circuit between anode and cathode.When Sn,Sn0.7Cu,Sn3Ag were used as a solder joint or a bonding wire,the products component was Sn and oxide of Sn;When Sn9Zn was used as a solder joint or a bonding wire,the products component contained Zn and ZnO in addition to Sn and oxides of Sn.Sn9Zn had good electrochemical migration resistance under small voltage;Under large voltage,the electrochemical migration resistance of Sn9Zn decreases sharply.（2）The time from the start of experiment to the time when metal deposits were connected to anode and cathode was called ECM short-circuit time.Under the experimental system,the ECM short-circuit time of Sn,Sn0.7Cu,Sn3Ag and Sn9Zn all decreased with voltage increase.When the voltage was less than 1.5V,the ECM phenomenon was basically not observed.When the voltage was greater than 5V,the ECM short-circuit time decreased sharply;And at a small voltage of 1.5V,metal deposits had fewer nucleation sites and grown slower;At a large voltage of 5V,the metal deposits had more nucleation sites and grown faster.At the same time,it was found that the short circuit time of Sn,Sn0.7Cu,Sn3Ag and Sn9Zn ECM increased with the increase of electrode spacing.Based on the short-circuit time data of each solder at different voltages and spacings,a three-dimensional graph between voltage,spacing and short-circuit time was constructed.This graph can estimate the short-circuit time value at any spacing between 0.3mm and 0.7mm and any voltage combination between 1.5V and 8V.（3）When pure Sn is used as a solder joint or a welding wire for electrochemical migration,the formation mechanisms of metal deposits were different at 1mM/L and500mM/L NaCl solution concentrations（under 30mM/L NaCl solution concentration,there were precipitates,but no metal deposits formed）.At the concentration of 1mM/L NaCl solution,the formation of metal deposits was caused by the direct reduction deposition of the dissolved metal ions at the cathode;At the concentration of 500mM/L NaCl solution,the formation of metal deposits was caused by the indirect reduction deposition of[Sn（OH）6]^2-complex ions at cathode due to the dissolution of precipitate layer.