Sn-8Zn-3Bi覆晶組裝製程及其電遷移研究

Abstract

In flip chip packaging technology, using electroplating to develop the flip chip solder bumps of Sn-8Zn-3Bi in this study. Investigating two-steps electroplating method, to plate the eutectic Sn-Zn and Bi in order. The Bi layer could cover the surface of Sn-Zn alloy and avoid the oxidation of the Zn. After the reflow, Bi will melt and dissolve into the eutectic Sn-Zn, and then formed the ternary alloy. This method could overcome the difficulty of the co-plating the ternary alloy at a time. Electromigration of Sn-8Zn-3Bi flip chip solder bumps on Cu has been studied at 120 oC and 150 oC with different current density. Formation of voids and hillocks at the cathode and anode, respectively, has been observed. In addition, a Sn-rich phase has replaced some part of the intermetallic compound of Cu-Zn (γ) which formed between the solder and the Cu pad in the anode side. Due to the polarity effect, the thickness of the intermetallic compound at the anode is thicker than at the cathode. The solder joint fails after 117hrs under 120 oC with an average current density 4.5 x 104 A/cm2 and voids formation at the cathode can clearly be seen after polishing. It is the melting at the edge of the bump that fails the solder. The simulation of the current density distribution indicates that the current density is not uniformly distributed and the current crowding occurs inside the bump. The results indicate that the increase of current density associated with joule heating has affected and enhanced the damage in the solder joint under electromigration.