Vibration fatigue reliability of BGA-IC package with Pb-free solder and Pb–Sn solder

doi:10.1016/j.microrel.2005.02.003

Microelectronics Reliability

Volume 46, Issues 2–4, February–April 2006, Pages 459-466

https://doi.org/10.1016/j.microrel.2005.02.003 Get rights and content

Abstract

In this paper, a new method is proposed for evaluating the high-cycle fatigue strength of BGA (Ball Grid Array) packages with Pb-free solder and Pb–Sn solder due to vibration. An attached weight induced mixed mode stress in the solder ball of a package was used. To consider the effect of the mixed mode stress that occurred in a solder ball and the frequency to fatigue strength of the solder ball, a test was carried out with the three kinds of weights (σ_n/τ_n = 4, 5, and 6) at various frequencies (10–27 Hz). To clarify the effect of frequency, a nonlinear analysis with a viscoplastic model was carried out within the range of 0.001–3450 Hz. From the continuous observation of the cross-section of the package and finite element method (FEM) analysis results, it was revealed that the maximum principal stress is the driving force to package failure. Although an intermetallic compound in both packages and a Pb-rich region in a Pb–Sn solder based package were confirmed by EDX microprobe analysis, they do not contribute to the initiation of a crack in a solder ball. The fatigue strength of the Pb-free solder and Pb solder was evaluated on the basis of the maximum principal stress calculated by FEM and the experimental results.

Introduction

In many applications microelectronic devices are used in severe vibration environments, such as in military or automotive applications. The US Air Force estimates that vibration and shock cause 20% of the mechanical failures in airplane electronics. Dynamic loading effect to solder joint fatigue life has not been systematically studied. The role of vibration in the life of solder joint has not been recognized sufficiently.

The main factors were frequency and temperature to influence the vibration fatigue of solder in package. Using disturbed state concept (DSC) Basaran [1] computed the damage occurred in Pb40/Sn60 eutectic solder alloy of a leadless ceramic chip carrier (LCCC) at several frequencies. It was pointed out that the frequency of vibration may affect the solder ball life. Zhao [2] researched the vibration characteristic of 63Sn/37Pb eutectic solder balls in a BGA package with Laser Moiré Interferometry system. Temperature influenced the inelastic strain occurred in solder ball. The strain of solder ball remained elastic over 1000 Hz.

Solder is the most common interconnection material for all microelectronic assemblies used today. With the advent of chip scale packaging technologies, the usage of solder connections has increased. The most popular solder is a Pb–Sn eutectic solder. Emerging environmental regulations worldwide, most notably in Europe and Japan, have targeted the elimination of Pb from electronic assemblies, due to the inherent toxicity of Pb. This has made the search for suitable Pb-free (lead-free) solders an important issue for microelectronics assembly. Although there are some studies [3] about the Pb-free solders, the high cycle fatigue properties of Pb-free solders in BGA-IC have not been clarified.

Section snippets

Specimen

The vibration fatigue of package is influenced by the many factors [4] such as the chip carrier type, the size of chip, the position of chip attached to a printed circuit board (PCB), the number of solder joints, the kind of solder, the size of solder ball (in case of BGA package), the dynamic characteristic of PCB [5]. To reduce the number of factors that affect the vibration fatigue of a package and to clarify the vibration fatigue characteristics of Pb-free (Sn–Ag–Cu alloy) solder and Pb

FEM analysis

To confirm whether the Pb-free and Pb–Sn solder balls behave elastically or not within a range of 10–25 Hz frequencies, an FEM analysis was performed. In general, a nonlinear strain may occur in a solder ball due to a high homologous temperature. However, it is very difficult to observe it in an actual solder ball due to its size. In this study, a comparison of the micro FEM results with the macro results of an experiment was carried out. To obtain more accurate results in FEM, a two-step

Results of test and FEM

The maximum Von Mises stress occurred at C8 among the solder balls shown in Fig. 1. The contour of the Von Mises stress in the Pb-free solder ball is shown in Fig. 8. The distribution of the Von Mises stress in the Pb was also similar to that of the Pb-free one. The maximum of the Von Mises stress in a solder ball occurred on the side of the chip as shown in Fig. 8. The equivalent elastic strain and the equivalent nonlinear strain in the Pb-free solder and the Pb solder on the AB cross-section

Conclusions

In this study, the high cycle vibration fatigue life characteristics of Pb-free and Pb packages were investigated under various mixed mode stresses by a new test method and FEM. The conclusion can be summarized as follows:

(1)
The effect of frequency on the high cycle vibration fatigue life of Pb-free and Pb packages can be ignored within the range of 10–25 Hz.
(2)
Using a nominal shear stress, the evaluation of the Pb-free and Pb package fatigue lives was possible without considering the mixed mode

References (8)

C. Basaran et al.
Mechanics of Pb40/Sn60 near-eutectic solder alloy subjected to vibration
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(1998)
Y. Zhao et al.
Thermomechanical behavior of micron scale solder joints under dynamic loads
Mech Mater
(2000)
M. Amagai et al.
Mechanical characterization of Sn–Ag-based lead-free solders
Microelectron Reliab
(2002)
Q.J. Yang et al.
Vibration reliability characterization of PBGA assembilies
Microelectron Reliab
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There are more references available in the full text version of this article.

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