H.L.J. Pang

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.

Low cycle fatigue analysis of temperature and frequency effects in eutectic solder alloy

Low cycle isothermal mechanical fatigue testing of a eutectic alloy 63Sn/37Pb was carried out in a systematic manner over a wide range of frequencies (10 24 ‐1 Hz) and temperatures (240 to 150°C) with the total strain set at different values (1‐50%). The low cycle fatigue behavior of the eutectic solder was found to be strongly dependent on test temperature and frequency. If the Coffin‐Manson model is used to describe such fatigue behavior, the fatigue exponentm and ductility coefficient C in the model are found to be a function of temperature and frequency rather than numerical constants. The plastic flow law was employed to explain the temperature and frequency dependence. The frequency-modified Coffin‐Manson model was tried and found to be able to eliminate the frequency dependence of the numerical “constants” but not the temperature dependence. To have a full description of the temperature- and frequency-dependent fatigue behavior, a set of empirical formulae was derived based on the frequencymodified Coffin‐Manson model.© 2000 Elsevier Science Ltd. All rights reserved.

Microstructure and intermetallic growth effects on shear and fatigue strength of solder joints subjected to thermal cycling aging

Abstract Microstructure development of eutectic solder alloy (63Sn/37Pb) after thermal cycling aging and its impact on the shear and fatigue failure of the solder joint has been investigated. The solder microstructure changes with the reflow process and subsequent thermal cycling environments in solder joint reliability tests from −40 to 125°C. In service, solder joints are subjected to thermal cycling aging, corresponding to power on–off cycling of the electronic equipment or cyclic environmental temperature loading, leading to thermal fatigue failures. Thus, it is important to study the effect of the microstructure changes, mechanical strength and fatigue resistance of solder before and after thermal cycling aging. A new specimen design has been developed to closely resemble the actual electronic packaging assembly condition. The joint is made simply by soldering a solder ball between two FR-4 substrates with copper pads using the reflow process. The study shows that the solder microstructure coarsened and intermetallic compound layers grew after 500, 1000 and 2000 thermal cycles. The shear and fatigue strength of the solder joint decreased with increased exposure to thermal cycling aging effects.

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#

Vibration reliability characterization of PBGA assemblies

Abstract Generally, the low-cycle fatigue induced by thermal cycling is the major concern in the reliability of surface mount technology for electronic packaging, but the high-cycle fatigue induced by vibration can also contribute significant effect, especially for applications in automobile, military, and avionic industries. To assess vibration induced fatigue failures, the dynamic properties of printed circuit board (PCB) assemblies play a very important role. In this paper, the dynamic properties of a plastic ball grid array (PBGA) assembly were characterized by using experimental modal testing and finite element analysis. The bare PCB and PCB assembly with PBGA modules mounted were tested and analyzed separately, so that the influence of PBGA modules on the PCB’s dynamic properties could be identified. It was found that mounting PBGA modules to PCB increased the stiffness of the PCB. Results of constant-amplitude vibration reliability testing of the PBGA assembly are also reported. It was found that the PBGA assembly was vulnerable to vibration, and fatigue failure always occurred at the corner solder balls of the PBGA module.

Investigation of effect of temperature and strain rate on mechanical properties of underfill material by use of microtensile specimens

Abstract In this paper, a testing methodology was developed for microtensile specimens. With the methodology, the effect of testing conditions on the mechanical properties of an underfill material was investigated at various strain rates from 10−5 to 10−1/s over a wide temperature range from −40 to 240 °C. It was found that the testing conditions and, in particular, the testing temperature affect the measured properties greatly. The relationship between the testing temperature and the mechanical properties exhibits a nonlinear characteristic and shows three trends. In particular, the mechanical properties were found to have a sharp change around the glass transition temperature (Tg). In contrast, the mechanical properties were found to have an approximately linear relationship with the logarithmic strain rate. The fracture surfaces of the tested specimens were analyzed by scanning electron microscope (SEM) and the fracture features were employed to explain the experimental observations.

Modeling and simulation for a drop-impact analysis of multi-layered printed circuit boards

Multi-layered printed circuit boards (PCBs) contain a multi-layered structure that is suitable for high-speed and high-frequency applications. Hence, they are used extensively in electronic packaging assemblies for high-density applications. However, numerous composite parts and complex material properties of multi-layer PCBs complicate the reliability simulation of PCB model. This paper deals with a finite element analysis intended to describe numerically the behavior of multi-layered multi-materials PCB model (combination of metallic and composite plies) in the drop-impact performance. Through the comparison of physical drop test results, the fully multi-layered model illustrates higher accuracy if compared with that of the traditional simplified isotropic model and orthotropic model. The effects of material properties for the multi-layer PCB under drop-impact shock have also been investigated.

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.

Investigation of long-term reliability and failure mechanism of solder interconnections with multifunctional micro-moiré interferometry system

Abstract In this study, a multifunctional micro-moire interferometry (M 3 I) system is developed for the measurement of in-situ deformation of microelectronic packages. Firstly, the system is applied to measure the in-situ deformation of solder interconnections in a plastic ball grid array (PBGA) assembly subjected to temperature cycling (TC) loading. The experimental results demonstrate that by combining with fatigue life prediction model the system can be used as a quick and accurate experimental tool for the assessment of long-term reliability of microelectronic packages. Secondly, with a specially designed brazil nut (BN) loading fixture, the system is employed to determine the displacement field around the tip of interfacial crack in a solder joint under different mechanical loading configurations. The effective critical fracture toughness and mode mixity of the solder/copper interface are determined with the system. Those results are further used to explain the failure mode and mechanism of solder joint during the reliability experiments.