High strain rate compression behavior for Sn–37Pb eutectic alloy, lead-free Sn–1Ag–0.5Cu and Sn–3Ag–0.5Cu alloys

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Abstract

This paper emphasized on studying the high strain-rate compression behavior of the unleaded Sn–3Ag–0.5Cu (SAC305), Sn–1Ag–0.5Cu (SAC105) solders and the traditional Sn–37Pb eutectic solder. The split Hopkinson pressure bar (SHPB) apparatus was used to conduct high strain rate tests in order to characterize the associated high rate mechanical response of these alloys. Specimens were tested at strain rates ranging from 380 to 3030 s−1 to obtain the dynamic stress–strain relationship for the Sn–37Pb, SAC305 and SAC105 alloys. The tested soft and ductile samples experienced a large amount of elastoplastic deformation due to impact test. In the high strain rate range studied, limited strain rate hardening effect was observed for SAC305, SAC105 and Sn–37Pb alloys studied. The strain rate sensitivity parameter (m) related to the power law creep equation was also calculated for the present solder materials at specific strain values. In addition, the saturation stresses for the leaded and lead-free solders at the strain rate range studied are also reported.

Introduction

The high strain rate behavior of materials has been increasingly important in many applications related to the design of consumer electronic product, high-speed machinery, high-speed transportation vehicles, advanced fiber reinforced composite aircraft/spacecraft and the armor system. More recently environmental legislation, the RoHS (i.e., Restriction of Hazardous Substances) directives in Europe, requires many new electronic circuit boards to be lead-free by 1st July 2006. Therefore, adopting appropriate unleaded solders in the consumer electronic products seems inevitable from now on. As we know that in order to maintain the electronic products in proper service conditions, it would be important to design electronic components capable of sustaining an external mechanical and/or thermal loading. Solder balls are used to connect the microchip and the associated printed circuit board by a proper joining technique such as the surface mount technology (SMT). This means that the solder joints are used as a structural member of the electronic product. Therefore, to study the dynamic stress–strain constitutive relationship and the fracture/failure behavior of the solder material due to an external impact rate loading become important.

We also know that an insight into the thermo-mechanical response of solder joints is critical to the design and deployment for reliable electronic circuit board assemblies. Understanding the mechanical behavior of lead-free soldered assemblies, e.g. the Sn–Ag–Cu (SAC) solder, is essential to the development of accelerated test plans and predictive reliability models and to their use as effective tools for product reliability assessment. The life of SnPb or lead-free solder joints is limited by the impact damage and the fatigue damage that accumulates in solder materials. Many efforts [1], [2], [3], [4], [5], [6] had emphasized on the testing and modeling related to fatigue and creep response of the near-eutectic SnPb solder alloys, and such models are under construction for lead-free soldered assemblies as we understand. Such models are also of use for reliability analysis of circuit boards at the design stage. Moreover, the development of the impact damage prediction models and dynamic material models of SAC solders in general requires a detailed understanding of material high strain rate behavior in order to develop an adequate constitutive model which can describe the transient behavior for the lead-free solders used in the electronic assemblies. Therefore, a detailed lead-free high strain rate database seems needed to describe the dynamic transient response of materials in question.

Recently, some reliability data has been acquired or is in the process of being gathered by several lead-free consortia. Efforts are also underway to characterize the mechanical behavior of lead-free solders. The derived creep rate equations are an important ingredient in the constitutive models, however, their applications to circuit board assemblies are still the subject of validation studies at the present stage. On February 15, 2001, the NEMI, NIST, NSF and TMS sponsored the Workshop on Modeling and Data Needs for Lead-free Solders [7] at New Orleans, LA, USA. This workshop’s report provides a quantitative review of the thermo-mechanical properties of lead-free solders, with emphasis on SAC alloys and Sn–Ag alloys. SAC solders are the alloys of choice for solder reflow assemblies. Eutectic SnAg is also recommended for wave soldering applications. The microstructure of both alloys consists of a Sn matrix with finely dispersed intermetallic precipitates. They believe that similarities in the constitutive response of SAC and eutectic SnAg solders. This report starts with a review of SnPb properties and lessons learned from the development of constitutive and life prediction models for near-eutectic SnPb assemblies. This understanding of SnPb behavior, although not fundamentally complete, has proven valuable to industry. This report also reviews material properties for SAC and SnAg alloys in details. Gaps in the material properties database are identified and suggestions were offered as to what additional testing and analysis are needed to develop constitutive models for engineering use. The materials testing and the process of developing a material model for Sn–37Pb solder under impact loading are presented in the work by Ong et al. [8]. During split Hopkinson pressure bar (SHPB) tests on cylindrical solder (eutectic) specimens, it was found that the stress to achieve 2% strain increases from 79 to 112 MPa when the strain rate increased from 450 to 2720 s−1. They also showed that there is a transition from ductile to brittle fracture as strain rate is increased as we may expect. The SHPB tests on single solder balls also showed that the stiffness of the solder balls is strongly dependent on rates of deformation. Siviour et al. [9] measured the high strain rate mechanical properties of 63Sn–37Pb, 96.5Sn–3.5Ag and 95.5Sn–3.8Ag–0.7Cu solders using a SHPB apparatus tested at stain rates from 500 to 3000 s−1. A novel method has been designed and used to measure the shear strength of solder joints at low and high rates using an Instron and SHPB, respectively. Wong et al. [10] used a specialized drop-weight test to determine dynamic material properties of eutectic Sn–37Pb and Sn–1Ag–0.1Cu, Sn–3.5Ag, and Sn–3Ag–0.5Cu alloys for strain rates in the ranges from 0.005 s−1 to 300 s−1. The sensitivity of the solders was found to be independent of strain level but to increase with increased strain rate. Portable electronics products such as mobile phones are susceptible to accidental drops and impact during use. Nowadays, more electronic products are used in automotive control and safety modules, such as air bag module, ABS braking system, vehicle stability control (VSC) system and traction control system (TCS). Electronic modules need to function properly under certain impact and crash loading conditions with strain rates from 102 to 103 s−1. As we understand that limited results were reported to describe high strain rate behavior of SAC solders. It is, therefore, the purpose of this work to study the dynamic mechanical behavior of 96.5Sn–3Ag–0.5Cu (SAC305), 98.5Sn–1Ag–0.5Cu (SAC105) lead-free solders and Sn–37Pb eutectic solder tested at strain rates from 102 to 103 s−1.

Section snippets

Test setup and procedures

In order to achieve this level of high strain rate loading condition, the SHPB [11] apparatus with its associated dynamic strain measuring and recording instruments in the Structural Laboratory of the Institute of Aeronautics & Astronautics, National Cheng Kung University was used to conduct experiments. Test setup of the SHPB experiments, containing an input, an output, and a throw-off bar, is shown in Fig. 1. The mechanical properties of input and output bars are summarized in Table 1. Each

Analysis descriptions

In order to analyze the dynamic elastic wave propagation signals in the circular rods of the split Hopkinson pressure bars, high strength titanium alloy or high strength alloying steel was usually adopted experimentally. Therefore, the input and output bars may remain in the elastic state at all times after impact was initiated, the stress and particle velocity can be accurately determined based on the measured strain in question. In case that the thickness of the sample is small enough, the

High strain rate test results for SAC305 and SAC105 solder materials

The mechanical properties of input and output bars made of Ti–6Al–4V titanium alloy for the present SHPB tests are given in Table 1. Because the solder material is much softer than the titanium alloy, no plastic deformation occurs in the input and output bars after tests. The SAC305 samples were tested at an average strain rate ranging from 447 to 1580 s−1. After the equations shown in the previous section were used, the calculated true-stress and true-strain curves for the tests conducted were

Conclusions

The current split Hopkinson pressure bar (SHPB) high strain rate test results reports that the dynamic 0.2% offset yielding stress and saturation (plateau) flow stress of the SAC305, SAC105 and Sn–37Pb solders are near (95 MPa, 200 MPa), (55 MPa, 185 MPa) and (75 MPa, 145 MPa), respectively. Note that these SHPB tests were conducted at strain rates ranged from 380 to 3030 s−1. The bulk saturation flow stress of the SAC305, SAC105 and Sn–37Pb solders studied is approximately three times higher than the

Acknowledgements

The authors like to thank the sponsorship from the ASE, Inc. under Contract No. 95S060 through the NCKU Foundation, and also the National Science Council (NSC), Taiwan, ROC for continuous support to the University.

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Part of this paper was presented at the 2nd International Symposium on Advanced Fluid/Solid Science and Technology in Experimental Mechanics, 23–25 September 2007, Osaka, Japan.

1

Dept. of Engineering Science, National Cheng Kung Univ., Tainan 70101, Taiwan, ROC.

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