Simulation study on thermo-fatigue failure behavior of solder joints in package-on-package structure
Introduction
Electric circuits in devices are subjected to various loadings, e.g. mechanical fatigue, thermal fatigue, impact and creep rupture etc. And the failure probabilities caused by the thermal fatigue and mechanical fatigue are about 67% and 13% in their applications, respectively. The ultra-large-scale integrated (ULSI) circuit generally consists of different elements, complex structures and fabrication processes in the micro scale. To assure its failure-free performance and longer service life in the applications, the physical failure models such as the thermal cycling (low cycle fatigue, Nf < 104) model and mechanical cycling (high cycle fatigue, Nf = 104–107) model have been widely investigated by various failure analysis methods [1], [2], [3], [4]. In addition, the increasing requirements for integrated circuit devices with high performance have led to the development of the single-package-stacking multi-die.
Package on package (PoP) is a novel structure consisting of solder joints, top package and bottom package. In recent decade, the technology of logic and memory integration has been matured, which places one package on the top of another to integrate different functions while maintaining compact size. Therefore, it has become the first choice for the industries [4], [5], [6], [7]. To obtain a reliable structure, reliability tests should be conducted to assess the performance of PoP. It has been widely used in packaging field for reliability prediction of the PoP structure in different environments such as drop and shock, bend, thermal cycling [8]. Due to the great mismatch in the coefficient of thermal expansion between different elements, thermal cycling fatigue is a common failure mode during its service process. The thermal fatigue behavior of the PoP structure has been widely focused in recent years. For example, Lai and Wang etc. [9], [10], [11] investigated the thermal characteristics of a board-level PoP under coupled power and thermal cycling test conditions using the numerical method. The results show that the reliability of the PoP stacking assembly is dominated by the critical FBGA solder joint, and the reliability of the PoP is highly related to the range of temperature excursions and the degree of deviation caused by the coupled power and thermal cycling. Yan and Li [12] studied the effects of different Ag contents of solder on the fatigue lifetime of the Fan-in Package on Package (Fi PoP) by finite element analysis, which indicate that a higher Ag content is available for the enhancement of the thermal reliability. In addition, Lee and Hwang [7] discussed the optimal underfill material for PoP to achieve reliable board level performances. It was found that underfills with lower coefficient of thermal expansion & higher glass transition temperature (Tg) are better than other factors for temperature cycle performance.
Upon the PoP structure, the die attach elements, which usually have enough adhesion strength [13], [14], [15], are sandwiched between dies. Compared with the die attach, the weakest element in the PoP structure is the solder joint such as SnPb or SnAgCu solder with low strength' resistance caused by the low melting point. Due to the manufactured defects and the stress/strain mismatch in the interconnections, the crack, accompanied by the primary creep and plastic deformation, is easy to appear at the interface or the surface of solder joint in general. And the failure behaviors (especially thermal-fatigue failure behavior) of solder joints exist great difference in different package structures [15], [16].
In previous literatures [4], [17], [18], [19], [20], [21], [22], the interface fracture/fatigue cracking behaviors of solder joints including the surface-mounted technology (SMT) and PoP structure have been studied by the scanning electron microscope (SEM) in-situ experiment and finite element analysis method. The results indicate that the PoP structure has a strictly hierarchical failure behavior under mechanical loadings. In this paper, the thermal-fatigue stress distribution in PoP structure and the thermal-fatigue failure behavior of SnAgCu solder joints are investigated by using direct thermal-cycle analysis and Coffin-Manson method. A thermal-fatigue failure criterion is defined by analyzing the failure data of solder balls.
Section snippets
Constitutive equation of solder joint in PoP structure
The classical Wiese' model [23], [24] is used in this work as following:
where E = 56.00–0.088 T (GPa) for SnAgCu solder [23], [24], which results in the soften behavior due to the phase coarsening of the intermetallic structure at high temperature T(K). B1, B2 are material's constants of 1.7 × 1012/s, 8.9 × 1024/s, respectively. σ is the stress amplitude (MPa). Q is the activation energy of about 34.60 kJ/mol [24], which was obtained from creep tests on precipitation strengthened
Results and discussions
As the thermal expansion coefficients are different in the PoP structure, the thermal-stress or thermal-strain mismatch occurs when the assembly undergoes the temperature variation and the interconnections are mainly in a shear stress state. The thermal-fatigue damage occurs most possibly in either the small melted resistance element or the large stress/strain element. Therefore, the thermal-stress distributions need to be clearly known in PoP structure at the maximum (125 °C) and minimum (0 °C)
Conclusions
Based on simulating analysis of the thermal-fatigue failure processes of different elements in the actual PoP structure, the thermal-fatigue failure behavior and probability are obtained as following:
- 1.
Based on the direct thermal-cycle analysis and Coffin-Manson method, the research on the thermal-fatigue damage behavior and evolution process of solder balls in PoP structure becomes possible in lower cost of time.
- 2.
Under the temperatures from 0 °C to 125 °C, the maximum value of the accumulating
Acknowledgements
The present research was supported by the National Natural Science Foundation of China (Grants Nos. 11572170, 11272173, 11072124).
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