Interfacial study of RE-doped solder alloys with Pb-free finishes and under electromigration

Abstract

Due to the implementation of the legislations to restrict the use of environmental-unfriendly materials, the use of lead (Pb) in electronic products is banned. This includes the use of surface finish materials such as that for integrated circuit (IC) leadframes. As these leadframes with Pb-free surface finishes will have their leads soldered with Pb-free solder alloys, the reaction between the surface finish and the solder needs to be evaluated. Also, as the phenomenon of electromigration has become an important issue when the current density increases, the present study also dealt with the reliability issues relating to electromigration of tin-silver-copper based alloys. A comparison between four types of Pb-free surface finishes of Matte Sn, Bright Sn, Sn-3Bi and Ni/Pd/Au, and a Pb-containing one was made. The wettability performance between the leadframes electroplated with these five surface finishes and three solder alloys was analyzed using a wetting balance to obtain the wetting force and wetting time. The leadframes soldered with the three types of solders were subjected to thermal aging at 150ëC for 150 hours to study the growth of the intermetallic compounds (IMCs). The results showed that the Ni/Pd/Au surface finish provided the best wetting performance among the five types of surface finishes, when a solder alloy doped with rare earth (RE) elements, e.g. Sn-3.5Ag-0.7Cu-0.1RE, was used. It was also found that the growth of IMC layer between Sn-3.5Ag-0.7Cu-0.1RE and Ni/Pd/Au was the slowest. As it was found by previous investigators that the addition of RE elements in Pb-free solder alloys had increased their performance, an electromigration analysis was carried out on the Sn-3.5Ag-0.7Cu solder alloy with and without the addition of RE elements for comparison. The present study considered the effects relating to Joule heating during electromigration at room temperature due to current stressing. Open-circuit failure caused by Kirkendall voids accumulation at cathodes was found to be driven by the electromigration effect. Before the failure, the polarity effect at the anode and cathode was confirmed as a vital phenomenon of electromigration. The threshold current densities of Sn-3.5Ag-0.7Cu and Sn-3.5Ag-0.7Cu-0.25RE were found to be 8.49x103A/cm2 and 1.42x104A/cm2 respectively. In line with the work of past investigations, the present electromigration failure modes were identified as ªstrong modeº and ªweak modeº. Within the strong mode regime, the cathode IMC thickness was found to be approximately proportional to the square of the applied current density. Within the weak mode regime, the failure phenomena of growth and detachment of IMCs and growth of voids at the interface were identified. A study on the diffusion of Sn-3.5Ag-0.7Cu, Sn-3.5Ag-0.7Cu-0.1RE and Sn-3.5Ag-0.7Cu-0.25RE under isothermal aging was conducted. The interdiffusion coefficients of the solder alloys were calculated. The diffusion activation energies of Sn-3.5Ag-0.7Cu, Sn-3.5Ag-0.7Cu-0.1RE and Sn-3.5Ag-0.7Cu-0.25RE were found to be 0.43, 0.44 and 0.52eV respectively. This indicates that by increasing the amount of RE elements, the IMC growth is retarded. This is in agreement with the results of RE-doped Pb-free solders with respect to surface finishes and electromigration. In particular, for Sn-3.5Ag-0.7Cu-0.25RE, the diffusion activation energies of Cu3Sn and Cu6Sn5 are 0.84 and 0.47eV, respectively, indicating that Sn-3.5Ag-0.7Cu-0.25RE has the best retardation effect on growth of IMCs. It was concluded that by adding RE elements, the Sn-3.5Ag-0.7Cu solder alloy performed better. For example, the RE-doped solder interfaces well with Pb-free surface finishes on IC leadframes. It also has higher electromigration resistance. Further, it has larger diffusion activation energies but smaller interdiffusion coefficients. These attributes indicate that this RE-doped alloys will be a very good alternatives to Sn-Pb solder alloys.

Date of Award	2 Oct 2008
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Lawrence WU (Supervisor)