Electromigration in Tin-bismuth Planar Solder Joints

The real-time monitoring of electromigration in ball-grid-array solder joints is limited to measuring the electrical resistance increase of the solder joints. Tracking the electromigration induced microstructural changes in solder balls requires cross sectioning which is a destructive technique. A novel planar solder geometry was invented and described here that allows real-time, non-destructive monitoring of microstructural changes and the rate of elemental segregation at the anode while simultaneously tracking the extent of electromigration by electrical resistance means. Electromigration in planar geometry tin-bismuth eutectic solder was studied by two means, (a) by the rate of Bi segregation at the anode and (b) by the rate of increase of electrical resistance of the solder, as a function of joint length, solder temperature and electrical current density. At low temperature and low electrical current density there was an extended initial period during which the joint resistance decreased before it increased. At higher temperatures and electrical current densities this initial period of decreasing resistance became less pronounced and at much higher temperature and current density stressing it became non-existent. The rate of bismuth segregation at the anode was somewhat proportional to the solder joint length indicating a probable Blech back-stress effect. Electromigration results from the rate of Bi segregation and the rate of increase of solder joint resistance were summarized using Arrhenius plots. The two plots gave similar electromigration activation energies of 0.7 eV from the electrical measurements and 0.75 eV from the Bi segregation measurements. The Arrhenius plot based on resistance rate increase was also used to predict the electromigration life of Sn-Bi solder joints under typical application conditions.