Atomic migration in eutectic SnBi solder alloys due to current stressing
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Current stressing at densities from 2.9 to 7.3 × 104 A/cm2 has significant effects on the atomic migration of eutectic SnBi solder alloys. At lower density (2.9 × 104 A/cm2), electromigration dominates the migration of both Sn and Bi, and drives Sn and Bi atoms to migrate toward the anode side. While at higher densities (4.4 and 7.3 × 104 A/cm2), the enhanced Bi electromigration induces a back stress, which promotes a reversed migration of Sn toward the cathode side. A large number of Sn atoms accumulate at the cathode side and form lumps there.
I. INTRODUCTION
With the increasing miniaturization of electronic products, the packaging size for solder joints has been reduced greatly. However, this size reduction has given rise to some new reliability problems. One of the most noticeable problems is the electromigration of solder alloys.1–11 The term “electromigration” refers to the phenomenon of atomic migration induced by current stressing.12 The driving force for the atomic migration results from the momentum exchange during the collisions between moving electrons and atoms, so the atoms will migrate in the direction of the anode side. The directional atomic migration can cause microstructural changes, including solder extrusion at the anode side and void formation at the cathode side. Severe microstructural change may lead to failure of the solder joints. Due to the harmful effect of Pb toxicity on environment and human health, finding suitable Pb-free solders to replace the Pb-containing solders is currently an urgent task for the electronic industry. Among many Pbfree solders, eutectic SnBi solder is a likely replacement, especially for processes that are sensitive to temperature or thermal damage, because of its lower melting point and coefficient of thermal expansion.13 Because electromigration has been considered to be an important concern in the reliability evaluation of solders, it is important to understand the effects of electromigration on eutectic SnBi solder. However, only a limited number of reports can be found.7–10 In these reports, the Bi atoms are found to migrate to the anode side under electromigration. Be-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0128 J. Mater. Res., Vol. 23, No. 4, Apr 2008
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cause Bi is the dominant diffusing species, most of the discussion regarding the electromigration of the solder has concentrated on Bi migration. By comparison, Sn migration seems to be less significant, and there has been a lack of discussion about it. In the present study, we find that after Bi migration, Sn atoms can also migrate to the anode side under electromigration. More interestingly, Sn lumps are formed at the cathode side, which indicates the presence of an opposite Sn migration toward the cathode side.
II. EXPERIMENTAL PROCEDURES
Figure 1 shows the schematic diagram of the specimen for the electromigration test. A bilayer of Ti–Cu thin film was deposited on an oxidized Si wafer us
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