Electromigration of SnAg bump with Ni UBM on various substrate pad finishes with SnCu presolder

Low-k dielectric materials have been adopted in advanced Si technologies for electrical performance improvement. Meanwhile, more functionality is integrated into a single chip, which makes the die size larger and I/O density higher that requires finer C4 bump pitch. Accompanied with Pb-free solder bump transition, chip package interaction (CPI) has become a challenging topic. Sn-rich Pb-free solder with Ag and Cu alloying elements has been widely used as flip chip Pb-free solder bump solution. Reducing Ag composition in the solder system has been shown to improve CPI margin and solder fatigue performance. This can be achieved through changing bump solder composition, or replacing SAC305 presolder to SnCu on the substrate pad. However, to our knowledge the electromigration (EM) of the solder bump joint with commercial substrate of various pad finishes have not been well characterized or understood so far. In this paper, the EM of SnAg solder bumps bonded on substrate with Sn3Ag0.5Cu or Sn0.7Cu presolder is studied. Effect of Ag composition in SnAg bump is also verified for a wide range of Ag% deviated from the nominal condition. Substrate pad finish effects are compared on three types, namely, Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), Organic Solderability Preservative (OSP), and Immersion Tin (IT). Experimental results show that SnAg bump with SnCu presolder and IT pad finish have promising EM result; however, ENEPIG pad finish shows poor EM on the UBM cathode side, and much worse than SnAg bump with SAC305 presolder. We also discover that IT pad finish can have superior EM performance than that of OSP on the substrate cathode side. Failure analysis is conducted to understand the mechanism. The activation energy and current exponent in Black's equation have also been characterized for the product maximum current assessment at use condition.