Minhua Lu

Comparison of electromigration behaviors of SnAg and SnCu solders

Two commonly used Pb-free solders, SnAg and SnCu, are studied for electromigration (EM) reliability. Two major EM failure mechanisms are identified in Sn-based Pb-free solders, which is mainly due to the differences in microstructures and Sn-grain orientation. In general, the EM damage in SnCu solder is driven by the fast interstitial diffusion of Ni and Cu away from solder/UBM interface and leads to early fails; while the damage in SnAg solders is mostly dominated by Sn-self diffusion resulting in longer lifetime. The effective activation energy is 0.95 eV for SnAg solder and 0.54 eV for SnCu solder. The current density power law exponent is 2 for SnAg and 1.2 for SnCu, respectively. Blech effect is observed in the solders with Sn-self diffusion dominated failures. The roles of Ag and Cu on EM performance will be discussed.

Study of Interfacial Reaction and Electromigration Reliability of Pb-Free Solders With Nickel Iron Barrier Layer

A study of NiFe alloy as under bump metallurgy (UBM) for Pb-free interconnect has been performed. Intermetallic growth rate of NiFe and SnAg solder is about 10x slower than that of Ni and SnAg solder. The thin and uniform intermetallic compound FeSn2 showed excellent thermal and electromigration stability. By slowing down or stopping the UBM dissolution, NiFe effectively eliminated the early EM fails that are common with Ni UBM. NiFe is an interesting candidate for the chip and substrate surface metallurgy of Pb-free interconnects for high power applications.Copyright

Effect of Ag and Cu content in Sn based Pb-free solder on electromigration

The effect of Ag and Cu concentrations on the electromigration of Pb-free solder was investigated. A nine cell experiment with Ag concentrations ranging from 1.2% to 2.2% and Cu concentrations ranging from 0.2 to 2.7% was conducted. Although the EM performance was found to be insensitive to the Ag and Cu concentrations in the range of the study, solder with 1.7% Cu showed the least Ag content dependence. EM failures occurred primarily on the substrate side. An effect of Sn grain texture with Ag and Cu concentrations was also observed.

Influence of UBM metallurgy on Sn textures and EM failure

Influence of UBM and substrate metallurgy on electromigraiton is studied. It is found that SnAg interconnects with Cu UBM and CuOSP substrate finish tend to have single or few grains, while the same SnAg interconnect with Ni UBM and NiAu substrate finish tend to have interlaced twining textures. The twining in Ni/Ni metallurgy randomize the grain orientation to a certain degree, so that no fast diffusion paths exists between the cathode and anode, which could lead to the existence of the critical length observed in EM experiments. With a single grain texture, Cu/Cu metallurgy is more prone to electromigration degradation when the c-axis of the Sn grain is aligned with the current direction, which could be an explanation for the lack of an EM critical current. Morphology of EM induced voids sectioned with FIB, showed that voids are formed by out diffusion of fast diffusors such as Ni and Cu from the intermetallic, leaving behind voids and Sn rich material.

Electromigration and thermal migration in Pb-free interconnects

A set of experiments designed to understand the evolution of electromigration damage in Pb-free interconnects was conducted. It is found that the degree of EM damage is higher with increasing stress current and stress duration, and dependent on the closeness of the c-axis of the Sn grain to current direction. Controlling the solder texture is one of the key elements in improving EM lifetime. The effect of thermal gradient on solder reliability is studied by applying heat to the chip side of the solder joint. No thermal gradient induced migration is observed in Sn bumps with temperature gradients as high as 6.8×105 °C/m.