Interfacial Reactions Between Cu Substrates and Lead Free Solder for 3D IC Applications
Date Issued
2014
Date
2014
Author(s)
Chen, Meng-Hsin
Abstract
Semiconductor industry has been guided by Moore’s law in the past 50 years. However, smaller size and better functionality of electronic devices is now forcing Moore’s law to face its imminent limitation. As a result, 3-Dimensional Integrated Circuit (3D IC) is regarded as a promising innovation in packaging industry. By means of through silicon via (TSV) technique, shorter interconnect distance is provided, which decreases power consumption of the electronic devices. With higher input/output (I/O) density, the package volume is able to be effectively scaled down. Due to the miniaturized size of the solder joints in 3D IC packages, intermetallic compound (IMC) forming at the interface occupies higher percentage in the solder volume and plays a very important role in the joints, raising the concern about mechanical properties. Therefore, the space confinement effect on the interface should be under careful consideration. Furthermore, research of lead free solder has been under vigorous development since RoHS and WEEE were put into practice. The interfacial reactions between lead free solder and substrates in the micro jointing for 3D IC applications are worthy of investigation.
In this study, interfacial reactions between Cu substrates and lead free solder for 3D IC micro joints are revealed. Three critical issues arising from the space confinement effect are discussed. The first issue is about the morphology of IMC forming at the interface. After thermal compression, Cu/Sn (10 μm)/Cu sandwiches experienced solid-liquid and solid state aging reactions. During solid-liquid reactions, scallop-type Cu6Sn5 grains which originally formed at opposite interfaces were able to impinge on each other after a short time frame. The time-to-impinge of IMC is defined in the case. In contrast, Kirkendall voids formed in the thick Cu3Sn layer after solid state aging process, which threaten mechanical properties of the joints. Moreover, the effect of minor element addition on morphology is also proposed.
The second issue relates to the IMC growth kinetics. In order to investigate whether solder volume affects the growth rate of the IMC on 3D IC scale, diagrams of thickness of IMC versus reaction time was illustrated. Compared to literature, the growth rate was independent of solder volume in the diagrams. That is, the limited solder volume had little impact on the IMC growing mechanism before impinged. When the minor elements were added, the growth rate of Cu3Sn was retarded.
The last issue arises from the merging behavior and grain orientation. After solid-liquid reactions, the vertically contacted Cu6Sn5 grains merged into a larger one and were identified as a single crystalline grain by electron back-scattered diffraction (EBSD) analysis. The pole figure (PF) showed the preferred orientation among the Cu6Sn5 grains after merging. On the other hand, Cu6Sn5 grains did not merge immediately after impingement during solid state aging reactions but after aging for longer time. After merging, the grains had smaller size and the grain boundaries were not as smooth as those in solid-liquid reactions. These behaviors could be attributed to coarsening process of the grains and slower diffusion rate at the lower temperature. The experimental results are reported in this study. Theoretical hypothesis and detailed analysis of above-mentioned issues will also be discussed.
Subjects
3D IC
介金屬化合物
接點空間受限效應
無鉛銲料
介面反應
生長動力學
合併
晶粒方向性
EBSD
極圖
Type
thesis
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