Effect Of Single-crystalline And Polycrystalline Structure On Electromigration In Solder Interconnects

As the dimensions of electronic devices shrink continuously with the development of miniaturization, the size of solder joints is forced to reduce accordingly. However, the current density in miniaturized solder joints increases significantly required by the performance enhancement and multi-function of electronic devices. When the current density exceeds 104 A/cm2, electromigration will take place in a solder bump. Electromigration can be greatly accelerated by current crowding and joule heating, and it becomes a serious concern for solder joint reliability.Flip-chip Sn3.0Ag0.5Cu(wt%) solder joints were employed for the electromigration tests. The samples were stressed at 5A and 2.5A. It was the Joule heating effect in the solder balls that let the working temperature keep at 60 ℃. Solder joints were fabricated by reflowing at a peak temperature of 240±5 ℃, and the cooling rate was 0.5-2.0 ℃ /s. The samples were polished for cross-sectional observation using Hitachi S-4700 scanning electron microscope(SEM). EBSD technique was used in this test to characterize the orientation and size of the grains before and after electromigration.It was concluded that there was only body diffusion in the single-crystal structure solder joints. The migration direction of compound was only controlled by grain orientation. The diffusion of solute atoms was accelerated if the c-axis of the grain was parallel with the current direction, and IMC was blocked at the grain boundary if the c-axis of the next grain deviated a large angle from the current direction.However, there existed both body diffusion and boundary diffusion in the polycrystalline solder joints. The numerous and tortuous boundaries made the polycrystalline solder joints have better resistance to electromigration failure.Microstructure evolution of IMC was comprehensively observed at the cathode of the bump during electromigration. Cyclic twinning structure in the Sn3.0Ag0.5Cu(wt%)solder bumps was detailed characterized employing EBSD technique. The rule of intermetallic compounds(IMC) migration path in flip-chip(FC) solder bump with cyclic twinning structure under current stressing was obtained. Long grain boundary was formed across the bump because of the twinning structure. When direction of the long grain boundary was nearly aligned to the current direction, IMC migrated alongthe boundary from cathode to anode massively. When direction of the long grain boundary was roughly perpendicular to the current direction, it was the grain orientation on each side of the long grain boundary that determined the migration path of IMC.In situ electromigration experiment, it was found that the entrance and exit were the area of high current density. The neck region of the solder interconnect was depressed and stretched during electrical current stressing. The corresponding mechanism was elaborately discussed. It was caused by the rotation of the grains to reduce the electrical resistance. The reorientation would make the a- or b-axis rotate towards the electron flow direction because the a- or b-axis had the lowest resistance.The reorientation should be accompanied by the growth of grains with low resistivity at the expense of their counterpart with higher resistivity.