The polarity effect of electromigration on intermetallic compound formation and back stress in v-groove solder lines

The trend of the miniaturization of VLSI and electronic packaging toward higher input/output density, smaller feature size and greater performance makes electromigration a serious reliability concern in flip chip technology. As an integral part of the joint, intermetallic compound (IMC) formation is very important to achieve good joint strength. However, the effect of electromigration on the IMC formation is a subject in which still very little is known.; We utilize solder v-groove samples etched on (001) Si wafer with 100 mum opening to study the polarity effect of electromigration on IMC formation in solder joints. We focus on the interaction between chemical and electrical forces, and the influence of interface morphology on the IMC dissolution. The current densities used are from 103 to 104 A/cm2 and the temperature settings are in the range of 120°C to 180°C.; We have found in both 95.5Sn3.8Ag0.7Cu/Cu and 96.5Sn3.5Ag systems the growth of the IMC has been enhanced by electric current at the anode and inhibited at the cathode. For Ni-Sn compound, kinetic analysis using the motion of the two interfaces gives the general formula of the growth rate as dXdt=aX + b. We have introduced the concept of mean-field theory and the classic model of Zener's precipitation growth into the discussion of the Cu-Sn compound growth under electromigration. A parabolic dependence of the IMC growth on time at the anode is derived as x 2 ≅ &parl0;Cm-Ce&parr0;2 &parl0;Cs-Ce&parr0;2 Dt.; The interaction between chemical and electrical forces brings a dynamic equilibrium in IMC dissolution at the cathode. This has been proved theoretically and experimentally. A new critical product has been derived from this dynamic equilibrium, which can provide us a critical IMC thickness before voids formation at a given current density. Our study shows the dissolution rate of Cu with current density 5x103 A/cm2 at 150°C is about 0.076 mum/hr. We also notice that the interface morphology plays an important role in the IMC dissolution. When the current density reaches 105 A/cm2, a planar interface will become unstable. Furthermore, this morphological instability can induce phase change in the flip chip solder joint, which finally causes a failure at the cathode.; Efforts are also made in the study of back stress in solder joints since the existence of back stress can resist or prevent electromigration from occurring. A multi-solder-segment v-groove sample structure has been developed to obtain the critical products of eutectic SnPb, Sn3.5Ag and Sn3.8Ag0.7Cu solders.