Effect of electromigration on intermetallic compound formation in Pb-free solder-Cu interfaces

Abstract

The polarity effect of electromigration on intermetallic compound (IMC) formation at the cathode and the anode in solder lines has been investigated. The lines were formed by flowing molten lead-free solder SnAg3.8Cuo.7 into V-grooves etched on (001) silicon wafers, and two copper wires were used as electrodes. The V-groove solder line samples, with width of about lOOpm and length of 600-800p1, were used to study the changes in thickness and morphology of IMC forming at the cathode and the anode in the SnAg3.8Cuo.7/Cu system under different current density and temperature settings. The current densities were in the range of lo3 to lo4 A/cm2 and the temperature settings were 150°C and 180°C. We found that the same IMCs of Cu6Sn5 and Cu3Sn formed at solder/Cu interfaces with or without the passage of electric current. The growth of IMC has been enhanced by electric current at the anode and inhibited at the cathode, comparing with samples without applying current. With a high current density of lo4 A/cm2, the growth of IMC at the anode obeys the parabolic growth rule, which was not predicted previously. We observed that the IMC forming after the initial reflow had a scallop-type morphology, and it transformed into layer-type morphology in current stressing. This is similar to the result of solid state aging without passage of current. However, how it transforms from scallop-type into layer-type morphology is different from the cathode to the anode, which is also different from the way it does in solid state aging. This is because of the polarity effect of electric current and the existence of local current crowding due to the scallop-type morphology of IMC.

Studies on solder bump electromigration in Cu/Sn–3Ag–0.5Cu/Cu system

“…Gan and Tu reported that Cu-Sn IMC growth was enhanced at the anode side and inhibited at the cathode side, as a result of electron flow in the Cu/Sn-3.8Ag-0.7Cu/Cu system in the model solder strip form [21]. Ebersberger et al reported that Cu-Sn IMC growth was enhanced through the entire joint of the thin-Ni/Sn-Ag/ Cu system [31].…”

“…Tu and co-workers reported on the electromigration of Sn-37Pb, Sn-3.5Ag and Sn-3.8Ag-0.7Cu [19][20][21][22] and disclosed the existence of a polarity effect influencing IMC growth at the solder/ Cu interface. Chen and co-workers reported on the IMC formation in several diffusion couples such as Al/Ni, Sn/Cu, Sn/Ni, Sn/Ag, Sn-0.7Cu/Ni and Sn-3.5Ag/Ni, using current density magnitudes of 0.5 and 1 kA/cm 2 [23][24][25][26].…”

Electro-migration Behavior in Eutectic Sn-Bi Flip Chip Solder Joints with Cu-Pillar Electrodes

“…BSE image and phase map determined from EPMA X-ray map of Sn57Bi bump on Ni/Au pad after current stressed by 4.0×10 4 A/cm 2 at 125 degree C for 10 % resistivity changes is shown in .…”

“…The critical current density at flip chip joint is lower than that in LSI pattern which is generally recognized as a case of electro-migration failure. The current density at the micro solder joint is expected to be in the order of 1x10 4 A/cm 2 2-4) .…”

Electromigration on void formation of Sn3Ag1.5Cu FCBGA solder joints

“…According to the work in [10] and JEDEC standard JEP154, both temperature and current density were responsible for the degradation of FC solder bumps. Additionally, the environmental temperature of 150°C was suggested as a critical upper limit for EM tests [6,11], and the current density of 1 Â 10 4 A/cm 2 a stressing threshold [12]. From the survey of literature [5,6,[11][12][13], most EM tests were generally addressed one as main condition, i.e., either current density or temperature, and each condition resulted in a unique failure mode.…”