Effect of intermetallic compound thickness on mechanical fatigue properties of copper pillar micro-bumps
Introduction
With the development of electronic products in the direction of miniaturization, high density and multi-function, device integration continues to increase, and the interconnection distance between chips is shrinking. Nowadays the conventional wire bonding (WB) and tape automated bonding (TAB) fail to meet the harsh demands of high integrated circuit, this is where flip chip comes into play and currently it becomes the most widely-used technology in which the chip is directly connected to the substrate, and is favored owing to its small package profile, short interconnection distance, lower inductance effect and more input/output (I/O) ports [1,2]. Furthermore, due to the continuous thrust of miniaturization and very high integration scale, fine pitch even ultra-fine pitch of interconnects are extensively adopted for electronic packaging. Among all the different types of flip chip bonding, Cu pillar micro bump interconnect technology becomes the main stream due to the compact interconnect pitch and excellent heat dissipation [3,4]. However, with the reduction of interconnect pitch, many challenging problems subsequently arise and remain to be solved.
On one hand, the interconnect joints suffer complex stresses and continuous fatigue creep [5,6]. Especially, the mismatch of coefficient of thermal expansion (CTE) among the interconnect joints makes the interconnect structures constantly subject to shear loads and tensile stress during working hours of devices. Consequently, solder joints are the most vulnerable parts of interconnect and electronic packaging, thereby affecting the integrity and reliability of interconnects significantly [[7], [8], [9]].
On the other hand, due to the reduction of the interconnect distance, the solder joint size along with the solder volume will also decrease. As a result, the volume ratio of the brittle intermetallic compound (IMC) which is generated during the process of bonding, will increase remarkably [10], which is expected to form a reliable joint that bonds the solder to the substrate and withstand certain stress [11]. It is now widely acknowledged that the IMC layer plays a critical role in the interconnect joints integrity and reliability.
Previous work show that with the decrease of solder joint size, the thickness, composition and morphology of IMC have all accordingly changed. Compared to the IMCs in larger size solder, IMCs in small size solder are found to contain larger and rougher grains. Furthermore, owing to the geometry and size effect of small solder joint, the solder is very likely to diminish and be taken place by full IMC, whose influences are still under investigation. At this scale, thinner or thicker IMC both have the potential to undermine the reliability and quality of interconnects [[12], [13], [14], [15]]. In a word, the impact of IMC on solder joints require more investigations, particularly the effect on the fatigue behavior and failure mechanism of solder joints.
Up to date, numerous researchers have studied on thermal fatigue of solder joints by thermal cycling test which has proven to be effective and reasonable in analyzing the reliability of assembled chip packages in actual use condition [[16], [17], [18], [19], [20]]. However, thermal cycling test has longer experimental period which often takes months and its results can be affected by other types of loads such as shock and vibration. Therefore, an increasing number of researchers turn to mechanical fatigue methods, among which the most popular one is shear fatigue test. The working process of shear fatigue test is monotonous with a shear force exerted on the chip, displacing the chip along the direction parallel to the substrate, and some major fatigue parameters, such as shear fatigue strain and strain rate, are controllable. During shear fatigue test, the load can be conducted at high frequency, as a result the reliability test of the interconnection structure can be completed in days, even in hours [[21], [22], [23]]. Additionally, it was also reported that the cyclic shear fatigue test is capable of revealing the fatigue properties of solder joints as well as fitting its results well to the known fatigue model [24].
In this paper, shear fatigue test combined with finite element analysis (FEA) via software ANSYS were applied to systematically investigate the effect of IMC thickness on the failure mechanism as well as fatigue life of Cu pillar micro-bumps. It was found that the IMC thickness affects significantly the fracture toughness of interconnect structures that further corresponding to fatigue failure mode and life of micro-bumps, which is useful to improve the integrity and reliability of interaction bumps in 3D integration.
Section snippets
Shear fatigue testing samples
The samples used in the shear fatigue test consist of two chips, which are called the upper chip and the lower chip, respectively. As shown in Fig. 1 (a) and (b), the upper chip's size is 6 × 6 × 0.5 mm and it consists of a stacking of 45-μm-thick Cu and 20-μm-thick Sn. As comparison, the lower chip's size is 12 × 12 × 0.5 mm and it consists of a stacking of 15-μm-thick Cu and 2-μm-thick Sn. Samples used in this article were provided by National Center for Advanced Packaging Co., Ltd. (NCAP
Effect of IMC thickness on static loading of micro-bumps
When the slope load is applied on the chip sample, the micro-bumps are subjected to static loading. For the purpose of better understanding the performance of micro-bumps under slope load, a finite element analysis (FEA) is carried out firstly based on the real size of chip samples and experimental conditions, as shown in Fig. 5 (a). The properties of Sn solder material are described by ANAND viscoelastic constitutive model in ANSYS, and all other materials are treated with linear elasticity.
Conclusions
The mechanical fatigue performance and fatigue life of Cu pillar micro-bumps with different IMC thickness under varied shear load were investigated by both experimental method and finite element analysis. (1) With the increase of IMC thickness, the ability of micro-bumps to resist static loading reduced. In the 2.3 μm-thick IMC micro-bumps, the fracture caused by static loading occurs at the interior of Sn solder, while in the 5.2 μm-thick IMC micro-bumps, the fracture is located at the Sn-IMC
CRediT authorship contribution statement
Wenhui Zhu:Conceptualization, Methodology.Lei Shi:Data curation, Writing - original draft.Liulu Jiang:Data curation, Visualization.Hu He:Conceptualization, Methodology, Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare there are no conflict of interest.
Acknowledgements
This work was supported by Natural Science Foundation of Hunan Province (2020JJ5728), State Key Laboratory of High Performance Complex Manufacturing (ZZYJKT2019-05), Innovation-Driven Project of Central South University (2020CX05) and National Natural Science Foundation of China (51605497).
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