Abstract
Electromigration tests of SnAg solder bump samples with 15 μm bump height and Cu under-bump-metallization (UBM) were performed. The test conditions were 1.45 × 104 A/cm2 at 185 °C and 1.20 × 104 A/cm2 at 0 °C. A porous Cu3Sn intermetallic compound (IMC) structure was observed to form within the bumps after several hundred hours of current stressing. In direct comparison, annealing alone at 185 °C will take more than 1000 h for porous Cu3Sn to form, and it will not form at 170 °C even after 2000 h. Here we propose a mechanism to explain the formation of this porous structure assisted by electromigration. The results show that the SnAg bump with low bump height will become porous-type Cu3Sn when stressing with high current density and high temperature. Polarity effects on porous Cu3Sn formation is discussed.
Highlights
In the pursuit of higher operation speed and improved performance of microelectronic devices [1], the application of flip chip solder joints has become a key technology for high-density packaging.The solder joints have been used in high power devices, such as central processing unit (CPU) and application processors (AP)
The results demonstrate that the solder joints do not transform transform into porous at current density and transform porous structures structures at an an insufficient insufficient current and temperature
We have carried out a systematic study on an important reliability issue related to high-density packaging of microelectronic devices
Summary
In the pursuit of higher operation speed and improved performance of microelectronic devices [1], the application of flip chip solder joints has become a key technology for high-density packaging. To meet the demand for portable devices, the input/output pin numbers continue to increase while the size of the solder joints continues to shrink This inevitably leads to higher current densities and operating temperatures in the joints. It raises serious reliability issues such as electromigration and thermomigration [2]. UBM (a structure with a thicker Cu) was developed to alleviate both the Joule heating and current crowding effects in flip-chip solder joints under normal device operating conditions, having a longer lifetime [10]. In low-bump-height solder joints, when they are combined with a thick column-type Cu. UBM, the solder reaction on the side wall of the Cu-column becomes one of the new reliability issues of concern. Liang et al found that in reflow tests, the effect of side wall reaction interconnects during due thevoids degradation ofjoints the Cu
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