Analysis and experimental verification of the competing degradation mechanisms for solder joints under electron current stressing
Introduction
The miniaturization of electronic circuits significantly raises the electron current density through flip chip solder joints. The high current density not only contributes to considerable momentum transfer between the conducting electrons and the atoms of the conducting medium, but also results in Joule heat generation. The subsequent electromigration phenomenon and Joule heating effect combine together to pose a detrimental impact on the reliability of flip chip packages.
Several current-induced degradation mechanisms in flip chip solder joints have been addressed and can generally be classified into two types. The first type is the formation of a pancake-type void within the solder, which is induced by the electromigration of Sn away from the cathode side through a self-diffusion process [1], [2], [3]. Tu and co-workers [1], [2], [3] demonstrated that the void first initiated near the current crowding region, then extended along the UBM/solder interface, and eventually resulted in an open circuit failure. The second type is the excessive consumption of the Cu or Ni metallization layer [4], [5], [6]. One main contributing factor of this degradation mechanism is the rapid interstitial diffusion of Cu or Ni through solders. Despite the fact that these two degradation mechanisms were responsible for most failures reported in the literature, the key factor determining which mechanism is dominant in a specific current stressing condition remains unidentified.
The two major degradation mechanisms outlined in the previous paragraph involve the vacancy-mediated self-diffusion of Sn and the interstitial diffusion of certain fast diffusers in solder, such as Cu and Ni. These two types of diffusion exhibit very different activation energies, and in fact the activation energy of Sn diffusion in matrix Sn is three times higher than that of Cu diffusion in Sn [7], [8]. Such a large difference in activation energy suggests that temperature may play a very important role in determining the dominant degradation mechanism. The objective of the present study is to analyze the temperature effects on Cu and Sn electromigration fluxes in Sn. The analysis is then applied to describing the dominant microstructure evolution under current stressing at various temperatures. Experiments are also carried out to verify whether the analysis has captured sufficient features of the system.
Section snippets
Analysis
Fig. 1 shows a schematic representation of electromigration in a simple Cu/Sn/Cu system. It is assumed that the electron current flows evenly upwards, and no current crowding is allowed. A layer of Cu6Sn5 exists on both interfaces. In certain experimental conditions, a layer of Cu3Sn might also exist between Cu and Cu6Sn5, but it will become clear later that the existence or absence of Cu3Sn will not interfere with the following analysis. Consider a small control volume of Sn very close to the
Experimental
To confirm the proposition that temperature is the key factor determining the dominant mechanism, one needs to carry out electromigration tests under both high-temperature and low-temperature conditions, using test samples with the same internal configuration.
Fig. 5 schematically depicts the solder joints used in this study. The solder alloy was composed of Sn–0.7Cu (wt.%). The under-bump metallurgy (UBM) on the chip side employed a Ti/Ni(V)/Cu structure, while the metallization on the
Results and discussion
Fig. 6 shows a micrograph of the flip chip solder joint at the as-assembly condition the before current stressing. A layer of Cu6Sn5 formed at both the chip side and the substrate side. The formation of only this particular compound was consistent with the literature result, and the rationale has been reported elsewhere [18].
Fig. 7a shows the microstructure of a joint that had been stressed at 3.2 A for 185 h. The electrons entered the joint from the lower-right corner and exited through the
Summary
A kinetic analysis of Cu and Sn electromigration flux ratio at different temperatures is presented to explain the competition of the two major degradation mechanisms reported in the literature. The analysis demonstrated that the formation of a pancake-type void is the dominant degradation mechanism at high temperatures, and the metallization consumption is the dominant one at low temperatures. The former mechanism requires the diffusion of Sn, which operates by the vacancy mechanism and
Acknowledgement
This work was supported by National Science Council of Taiwan through Grant NSC-97-2221-E-002-101-MY3.
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