High cycle fatigue behaviour and generalized fatigue model development of lead-free solder alloy based on local stress approach
Introduction
Applications like automotive electronics in power-train segments often experience vibration together with temperature in service [1]. Studies on other applications like aircraft systems account for 20% of electronic failure due to vibration, as compared to the 55% from thermo-mechanical fatigue failure [1]. For SnPb alloys, plenty of efforts have been made in past to address the issue of fatigue [2], [3], [4]. It is evident that most of the literature on solder reliability refers to performance under relatively low cycle (< 104 cycles) fatigue conditions, whether imposed thermally, mechanically or both [5], [6], [7], [8], [9], [10], [11]. A generalised lifetime prediction tool or methodology is therefore a requirement in industries to predict the fatigue life during the product development phase for solder joints. For SnAgCu (SAC) alloys, HCF experiments were done by researchers and industries [1], [12], [13], [14], [15], [16] directly on surface mounted technology (SMT). Often, experiments were conducted on application level to understand the failure modes or to create failure models. Even though it is a common practise to conduct experiments on application level, it is an expensive process in terms of cost and time. Normally, the whole cycle of application testing has to be repeated, when components/materials are changed.
Another possibility is investigating HCF in a laboratory on specimens (coupon level). One of the few studies that are related to the HCF specimens on coupon level was a stress-controlled fatigue experiment at 2 Hz but only up to 105 cycles on a bulk specimen with interface [17]. It is also reported that there is a transition in crack propagation mechanism, when the strain is up to a critical value for the Sn3.5Ag solder [17]. These mechanisms were not clearly indicated. The effect of cyclic loading is investigated in this paper for SAC alloy. For generic purpose, the bulk solder specimens were investigated under HCF conditions for different temperatures. This enables one to study the possible deformation and fracture mechanisms encountered in solder material under combined fatigue and temperature conditions. It was observed from the vibration experiments on SMT's from Meier et al. [16], Yang et al. [15], that the crack failure initiates mostly from the stress concentration regions of the solder joints. The convex and concave solder joint geometry creates a high-stress gradient region, which could potentially over predict the stress for lifetime prediction. This is the reason a notched and un-notched specimen is replicated in this study to investigate the material dependent stress-gradient corrections.
The overall idea of this work is to generalise the lifetime assessment of SMT component solder joints under vibration loads. This approach can be later used in finite element (FE) tools. This generalisation work revolves around the idea of using the solder specimens on coupon level instead of testing on components directly to assess the lifetime. Fatigue model results based on FKM guidelines-2003 version [18] are used in this study, as the software (used in this investigation: nCode Designlife™) is still based on 2003 version. The FKM guideline is used to transfer the experiment data based on coupon level to the analysis of real components. In the FKM guideline influences like mean stress, roughness, stress gradient are considered. The FKM guideline is well accepted in civil engineering for steel and aluminium. In this paper, the FKM guideline is adapted for solder joint performance evaluation.
The results of the HCF experiments on coupon level can be validated for the failure assessment of solder joints in microelectronics application, using FE approaches coupled with fatigue analysis software. This paper however only discusses the fatigue models and the consequent FKM guideline modifications.
Section snippets
Experimental procedure
Solder tensile bars of Sn3.8Ag0.7Cu (wt.%) alloy from Stannol Corporation, were used in this study. By means of a “re-melting” process in a special heated aluminium die, specimens can be cast with a good surface quality, without macro-cracks and edges. The details of solder specimen processing is described elsewhere [19].
The specimens were manufactured as seen in Fig. 1(a) and the longitudinal sections were compared to that of SMT component solder joints during the preliminary phases to
Experimental results
The result section is incorporated with three major steps. Firstly, fatigue parameters were determined from uniaxial experimental results for various load cases. The load cases are the possible influential parameters on fatigue of solder alloys such as mean stress, stress gradient, temperature, and roughness. The experiments were carried out in the range between 104 up to 108 cycles, with a common frequency of 60 Hz, which was maintained throughout the experiment.
Next, the strain progressions
Conclusion
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High cycle fatigue experiments for SAC alloy are investigated extensively using four different Wohler curves. Basquins fatigue model based on local stress approach, is found to explain the fatigue behaviour of SAC alloy over a range of various temperatures, stress amplitudes, R-ratios, surface roughness and stress gradients.
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The effect of high temperature on the mechanical property of lead free solder alloy here is observed to be associated with microstructural transformations of material such
Acknowledgement
The authors collectively would like to thank Dr.-Ing Mike Roellig and Mr. Rene Metasch from Fraunhofer IKTS-Dresden in specific for the technical support and manufacturing of bulk solder specimens and Dr.-Ing. Olaf Wittler from IZM Frauhnofer Berlin for expert advises. In addition, we give our sincere gratitude to Continental Automotive GmbH, Germany for funding the complete project.
References (28)
- et al.
Vibration fatigue reliability of BGA-IC package with Pb-free solder and Pb–Sn solder
Microelectron. Reliab.
(February 2006) - et al.
Study of thermal cycling and temperature aging on PbSnAg die attach solder joints for high power modules
Microelectron. Reliab.
(September 2014) - et al.
Mechanics of Pb40/Sn60 near-eutectic solder alloys subjected to vibrations
Appl. Math. Model.
(August 1998) - et al.
Fatigue life of 63Sn–37Pb solder related to load drop under uniaxial and torsional loading
Int. J. Fatigue
(July 2006) Low cycle fatigue behavior and mechanisms of a eutectic Sn–Pb solder 63Sn/37Pb
Int. J. Fatigue
(June 2002)- et al.
Experimental study and life prediction on high cycle vibration fatigue in BGA packages
Microelectron. Reliab.
(July 2006) - et al.
Vibration reliability test and finite element analysis for flip chip solder joints
Microelectron. Reliab.
(July 2009) - et al.
Fatigue life prediction of solder joints with consideration of frequency, temperature and cracking energy density
Int. J. Fatigue
(April 2014) - et al.
Fatigue fracture mechanisms of Cu/lead-free solders interfaces
Mater. Sci. Eng. A
(March 2010) - et al.
Fatigue crack growth under compressive loading
Eng. Fract. Mech.
(January 1985)