X.Q. Shi

Effect of Temperature and Strain Rate on Mechanical Properties of 63Sn/37Pb Solder Alloy

In this study, tensile tests of 63Sn/37Pb solder were carried out at various strain rates from 10 s to 10 s over a wide temperature range from – 40 C to 125 C to study the effect of strain rate and testing temperature on the mechanical properties in a systematic manner. Based on these experimental data, a set of empirical formulae was derived by a statistical method to describe the effect of temperature and strain rate in a quantitative manner and explain the variation in the mechanical properties published in other reports. It is concluded that the empirical formulae can be used to characterize the mechanical properties of 63Sn/37Pb over a wide range of temperatures and strain rates.

A modified energy-based low cycle fatigue model for eutectic solder alloy

Gintic Institute of Manufacturing Technology, Nanyang Drive, Singapore 638075*School of Mechanical and Production Engineering, Nanyang Technological University,Nanyang Avenue, Singapore 639798(Received April 21, 1999)(Accepted May 7, 1999)1. IntroductionSurface mount technology (SMT) is increasingly used in microelectronics to mount components bysoldering onto the printed circuit board (PCB). The solder alloys are used as the electrical andmechanical connections between the component and the board. Fatigue failure of solder joints isrecognized as a major cause of failure in electronic devices. An approach to this problem is to determinethe fatigue behaviors of solder alloy by accelerated fatigue testing at different temperatures. In thepresent research, smooth specimens made entirely of a 63Sn/37Pb solder alloy were tested over a widetemperature range at various low frequencies to study its low cycle fatigue properties.Strain-based models, notably the Coffin-Manson model (1–2), have been widely used to characterizelow cycle fatigue behaviors of engineering materials including solder alloys (3–5). However, it ispractically very difficult to obtain a single plastic strain value in a solder joint because of the complexstress state (6). In contrast, it is much easier to calculate energy density from the low cycle hysteresisloops for any types of solder joints under test (7). Therefore, in recent years, energy-based low cyclefatigue models have been increasingly used for solder alloys (7–9). These models are not so wellestablished as the strain-based ones, and some researchers (10) even expressed doubt on whether energydensity is a true parameter governing fatigue life of solder alloy. Therefore, in the study special effortwas made to examine the existing energy-based models. In the end, a flow stress modified energy-basedmodel is proposed based upon the examination and the experimental results. It is demonstrated in thepaper that the new fatigue model can be used to predict the fatigue life of solder alloy at differentfrequencies or temperatures.2. Experimental ProceduresThe material used in the study was a eutectic alloy 63Sn/37Pb. This material is widely used as a solderin SMT. The chemical composition of the solder is as follows (in wt.%): 63.2 Sn, 0.006 Sb, 0.002 Cu,0.004 Bi, 0.001 Zn, 0.002 Fe, 0.001 Al, 0.01 As, 0.001 Cd, and remainder Pb. Rods of the solder wereproduced by casting and then machined into cylindrical fatigue specimens of 90 mm long. The fatiguespecimens have a diameter of 12 mm at the two ends and a central diameter of 6 mm with a radius ofcurvature of 105 mm in the gauge section to prevent any stress concentration due to sharp corners. After

A New Creep Constitutive Model for Eutectic Solder Alloy

In this study, a large number of creep tests were carried out to study the effect of stress level and testing temperature on the creep behavior of 63 Sn/37Pb solder in a systematic manner. Based on the dislocation controlled creep mechanism and Gibbs’ free-energy theory, a new creep constitutive model was proposed. The model was found to describe accurately the creep flow of the solder and to be capable of explaining the issues of stress and temperature dependent stress exponent and activation energy in the Arrhenius powerlaw creep model. Furthermore, the model was employed to predict accurately the longterm reliability of solder joints in a PBGA assembly. @DOI: 10.1115/1.1462624#

Thermal cycling aging effects on microstructural and mechanical properties of a single PBGA solder joint specimen

This paper reports on an experimental study on how thermal cycling aging exposure changes the solder joint microstructure, intermetallic layer thickness and the residual shear strength and fatigue life in a single plastic ball grid array (PBGA) solder joint specimen. The single BGA solder joint specimen was specially designed to evaluate the microstructure and mechanical properties of three different batches of solder joint after subjected to 0, 500, 1000, and 2000 cycles of thermal cycling aging (-40/spl deg/C to 125/spl deg/C). It is important to relate the effects of thermal cycling aging on the changes of the microstructural and intermetallic layer thickness to the residual shear strength and fatigue life of solder joints subjected to thermal cycling aging exposure. The results of this study shows that the microstructural and intermetallic development due to thermal cycling aging has a major impact on the residual mechanical and fatigue strength of the solder joint. It was noted that the solder joint shear strength and residual fatigue life degrades with exposure to thermal cycling aging.

Sensitivity study of temperature and strain rate dependent properties on solder joint fatigue life

Tensile tests of 63Sn/37Pb solder were carried out at various strain rates over a wide range of temperatures to study the effect of strain rates and testing temperatures on the mechanical properties of solder. A statistical method incorporating multiple linear regression was also employed successfully and a set of empirical formulae was derived to describe the effects of temperature and strain rate quantitatively. These mechanical properties were applied to finite element analysis to study the sensitivity of these material properties on the fatigue life of solder joints in a plastic ball grid array (PBGA) package. From this study, the strain rate effect on solder response to different ramp rates during thermal cycling or thermal shock reliability tests can be modeled in finite element analysis.

Reliability assessment of PBGA solder joints using the new creep constitutive relationship and modified energy-based life prediction model

In this study, a new creep constitutive relationship and a modified energy-based fatigue model are proposed. Compared to the commonly used hyperbolic-sine and two-regime power law relationships, the new relationship was found to be capable of covering the whole temperature range and stress regime. The modified energy-based fatigue model represents the cycling temperature and frequency effects well, which is not the case in the widely used strain-based Coffin-Manson and energy-based Morrow fatigue life prediction models. Furthermore, the relationship and model are applied to investigate the thermal fatigue reliability of a 256 I/O plastic ball grid array (PBGA) assembly. The simulation result is found to be in agreement with that obtained from accelerated thermal cycling (ATC) test. It is concluded that the new constitutive equation and the life prediction model can be used confidently for the assessment of long-term reliability of solder joints.

Effect of hygrothermal aging on interfacial reliability of flip chip on board (FCOB) assembly

The moisture migrating into underfill decreases the adhesion strength, swells to deform the assembly, and weakens underfill mechanical and thermal properties. In this study, interfacial reliability of FCOB exposed at 85 degC/85%RH was studied using Moire interferometry and mu-DiSC system developed. A thermal aging study was simultaneously performed to understand the time effect on long duration of hygrothermal aging. The results showed that the thermal aging relieved the stresses induced by hygrothermal swelling mismatch between dissimilar materials involved, which indicated the time effect is not negligible in the moisture conditioning. Subsequently, a mu-DiSC technique was applied to measure critical interfacial fracture toughness of Si/UF interface. It is shown that moisture could reduce the interfacial strength significantly due to the breaking of hydrogen bonding. Finally, the morphologies of fracture were studied under the SEM

Determination of Fracture Toughness of Underfill/Chip Interface With Digital Image Speckle Correlation Technique

Layered packages are prone to multi-mode damage and failure when they are subjected to complicated and coupled environmental loading. As a result, fracture toughness is usually used as a fracture criterion to evaluate the reliability of the polymer/inorganic interface. In this study, an in-situ/real-time micro-digital image speckle correlation (/spl mu/-DiSC) system was established and employed to determine the fracture toughness of the underfill/chip interface involved in flip chip assembly. The tests were carried out over a wide range of temperatures and at various loading angles. In order to verify the findings of the /spl mu/-DiSC technique, an interface fracture mechanics based FEM is implemented into ANSYS to calculate the values of CTOD of the underfill/chip joint under different loading configurations. The results obtained from the simulation are found to be in good agreement with those measured by the /spl mu/-DiSC system, indicating that the system can be used as an accurate and effective experimental tool for electronic packages. The fractographs with respect to different temperatures and loading angles are further discussed.

Flip chip interfacial behavior under thermal testing

In this study, the interfacial behavior of a flip chip structure under thermal testing was investigated using high sensitivity moire interferometry. The real-time moire interferometry was used to monitor and measure the deformation of the specimen during the test. Two kinds of specimen were prepared: (1) specimen without a crack and (2) specimen with horizontal crack at the silicon-epoxy interface. The results show that the maximum shear strain occurs at the silicon-epoxy interface. The shear strain variation increases significantly along the interface, with the maximum shear concentration occurring at the edge of the specimen. The creep effect is more dominant in the FR4-epoxy interface. In order to characterize the behavior of the interfacial crack, stress intensity factors K/sub I/ and K/sub II/, and the strain energy release rate in the vicinity of the crack tip were used to conduct a qualitative study. It was observed that a sharp strain gradient occurred at the crack tip. The stress intensity factors K/sub I/ and K/sub II/ were dependent on temperature. The strain energy release rate with respect to temperature was dominated by K/sub I/ for the interfacial crack in the specimen.

On the moduli of viscoelastic materials

The moduli of viscoelastic materials are dependent on temperature and time. Polymeric materials are widely used in electronic packaging as encapsulation, underfill and die attach. The viscoelastic model offers better representation of material performance than the linear elastic model. It is important to provide the viscoelastic properties for polymeric materials in electronic packages for finite element analysis (FEA) modeling. In this paper, different concepts for describing the moduli of viscoelastic materials have been reviewed. Analysis on how to correlate different types of tests for the determination of different moduli of viscoelastic materials was performed. Results obtained from a constant-strain-rate tensile test were employed to illustrate the applicability of the elastic modulus concept. It was found that the tangent modulus or the differential of the stress-strain curve from constant strain rate test is equivalent to the relaxation modulus from a stress relaxation test. The plot of /spl sigma/ / /spl alpha/ versus /spl epsiv/ / /spl alpha/ is no longer dependent on the strain rate for linear viscoelasticity. The difference between the moduli behavior of viscoelastic materials and that of linear elastic material was also discussed.