Cemal Basaran

Thermomigration in Pb–Sn solder joints under joule heating during electric current stressing

Electromigration of solder joint under high dc current density is known as a reliability concern for the future high-density flip chip packaging and power packaging. Biased mass diffusion within solder joint from cathode to anode under high dc current density is observed in these experiments. In this letter, the experiments on flip chip solder joints under dc current stressing are conducted and thermomigration due to the thermal gradient in the solder joint caused by joule heating is reported. A three-dimensional coupled electric thermal finite-element (FE) simulation of a realistic flip chip module shows the existence of thermal gradient in the solder joint which is high enough to trigger thermomigration.

A Thermodynamic Framework for Damage Mechanics of Solder Joints

Damage mechanics describes the degradation process that takes place in materials and structures. Traditionally, Coffin–Manson type empirical curves are used to determine the fatigue life. Damage mechanics allows us to determine the fatigue life without the need for empirical curves. The main problem in damage mechanics has always been a lack of universally agreed upon definition of a damage metric. In this paper a damage metric based on the second law of thermodynamics and statistical mechanics is presented. The proposed thermodynamic framework treats a solid body as a thermodynamic system and requires that the entropy production be nonnegative. Verification of the damage model has been performed by extensive comparisons with laboratory test data of low cycle fatigue of Pb40/Sn60 solder alloy.

Failure modes and FEM analysis of power electronic packaging

The development of power electronics technology is driven by the insatiate demand to control electrical power. The new power electronics devices reduce the volume of the converters by three to four orders of magnitude compared to their mercury arc predecessor. And the turn-on and turn-o0 time has decreased from milliseconds to the microseconds and even nanoseconds, depending on power level. The power range commanded by converters now extends from micro-VA to several hundreds of mega-MVA. Among the new power devices, insulated gate bipolar transistor (IGBT) devices are being more accepted and increasingly used in traction application such as locomotive, elevator, tram and subway. Thus the long-term reliability of IGBT is highly demanded. In this paper the failure modes of power electronics devices especially IGBTs are reviewed. A FEM analysis of a multilayered IGBT packaging module under cyclic thermal loading is presented.? 2001 Elsevier Science B.V. All rights reserved.

An Irreversible Thermodynamics Theory for Damage Mechanics of Solids

The entropy production is a non-negative quantity based on irreversible thermodynamics and thus serves as a basis for the systematic description of the irreversible processes occurring in a solid. In this paper, a thermodynamic framework has been presented for damage mechanics of solid materials, where entropy production is used as the sole measure of damage evolution in the system. As a result, there is no need for physically meaningless empirical parameters to define a phenomenological damage potential surface or a Weibull function to trace damage evolution in solid continuum. In order to validate the model, predictions are compared with experimental results, which indicates that entropy production can be used as a damage evolution metric. The theory is founded on the basic premise that a solid continuum obeys the first and the second laws of thermodynamics.

Moiré interferogram phase extraction: a ridge detection algorithm for continuous wavelet transforms

We present a procedure using continuous wavelet transforms (CWTs) to extract the phase information from moiré interferograms. The relationship between precise ridge detection of the two-dimensional CWT magnitude map and accurate phase extraction is detailed. A cost function is introduced for the adaptive selection of the ridge, and a computationally inexpensive implementation of the cost function ridge detection algorithm is explored with dynamic programming optimization. The results of the proposed ridge detection algorithm on actual interferograms are illustrated. Moreover, the resulting extracted phase is demonstrated to be smooth and accurate. As a result, the sensitivity of the moiré interferometry method is improved to obtain a pixel-by-pixel in-plane strain distribution map.

Thermomechanical behavior of micron scale solder joints under dynamic loads

Recent trends in reliability and fatigue life analysis of electronic devices have involved developing structural integrity models for predicting the operating lifetime under vibratory and thermal environmental exposure. Solder joint reliability is the most critical issue for the structural integrity of surface mounted electronics. Extensive research has been done on thermal behavior of solder joints, however, dynamic loading effects to solder joint fatigue life have not been thoroughly investigated. The physics of solder joint failure under vibration is still not very clear. This paper presents a test program which was performed to study inelastic behavior of solder joints of BGA packages. A concurrent loading unit is used which consists of a thermal environmental chamber and an electrodynamic shaker. Laser Moire Interferometry was used to measure the whole deformation field of the prepared specimen surface. The corresponding inelastic strain field is then calculated. It is found that at elevated temperature, vibration and shock can cause the accumulation of inelastic strains and damage in solder joints. In this paper, contrary to the popular belief that all vibration-induced strains are elastic, it is shown that vibration can cause significant inelastic strains.

An analytical model for thermal stress analysis of multi-layered microelectronic packaging

Compared to numerical methods, analytical solutions can offer a faster and more accurate procedure for obtaining the interfacial stresses in laminated structures. An analytical model for thermal stress analysis of multi-layered thin stacks on a thick substrate under isothermal loading is proposed in this paper. This analytical approach considers each layer as a beam-type plate with orthotropic material properties. Highly sensitive Moire interferometry is used to validate the model. The strain field in the bi-material interfaces is obtained experimentally. The test data is in good agreement with the proposed analytical solution. Finite element analysis results are also compared with the analytical solution and the test data

Measuring intrinsic elastic modulus of Pb/Sn solder alloys

Youngs modulus (E) values published in the literature for the eutectic Pb37/Sn63 and near eutectic Pb40/Sn60 solder alloy vary significantly. One reason for this discrepancy is different testing methods for highly rate sensitive heterogeneous materials, like Pb/Sn alloys, yield different results. In this paper, we study different procedures used to obtain the elastic modulus; analytically, by single crystal elasticity and experimentally by ultrasonic testing and nano-indentation. We compare these procedures and propose a procedure for elastic modulus determination. The deformation kinetics of the Pb/Sn solder alloys is discussed at the grain size level.

Mechanics of Pb40/Sn60 near-eutectic solder alloys subjected to vibrations

Pb40/Sn60 is the most commonly used solder alloy for microelectronics packaging. It is well understood that, heat generated by the circuits when a semiconductor device is on and the coeAcient of thermal expansion mismatch between the soldered layers lead to the thermal fatigue of the solder joints. On the other hand, there is very little research done to understand failure mechanics of solder joints when microelectronics devices are subjected to vibrations. In this study, it is shown that dynamic stresses contribute to the failure mechanism and in certain circumstances they can become the dominant failure cause when semiconductor devices are used in a vibrating environment. The purpose of this study is to understand the mechanics of the vibration induced damage in solder interconnects. A material nonlinear time domain dynamic analysis of a solder joint was performed using a damage mechanics based constitutive model. The analysis was done for isothermal condition and no thermal loads were considered. The study included a wide range of frequency and acceleration combinations. ” 1998 Elsevier Science Inc. All rights reserved.

A Damage Mechanics-Based Fatigue Life Prediction Model for Solder Joints

A thermomechanical fatigue life prediction model based on the theory of damage mechanics is presented. The damage evolution, corresponding to the material degradation under cyclic thermomechanical loading, is quantified thermodynamic framework. The damage, as an internal state variable, is coupled with unified viscoplastic constitutive model to characterize the response of solder alloys. The damage-coupled viscoplastic model with kinematic and isotropic hardening is implemented in ABAQUS finite element package to simulate the cyclic softening behavior of solder joints. Several computational simulations of uniaxial monotonic tensile and cyclic shear tests are conducted to validate the model with experimental results. The behavior of an actual ball grid array (BGA) package under thermal fatigue loading is also simulated and compared with experimental results. @DOI: 10.1115/1.1536171# With the increasing use of surface mount bonding technology in microelectronics industry, the reliability concerns for solder joints are increasing exponentially. Eutectic solder alloys are most commonly used bonding materials in electronic packaging, which provide electrical and thermal interconnection, as well as mechanical support. The temperature fluctuations due to device internal heat dissipation and ambient temperature changes, along with the coefficient of thermal expansion ~CTE! mismatch between the soldered layers, result in thermo-mechanical fatigue of the solder joints. Progressive damage in solder balls eventually leads to device failure. Fatigue life prediction of solder joints is critical to the reliability assessment of electronic packaging. Standard state of practice in the electronic industry for the number of cycles to failure prediction is based on using empirical relations, such as Coffin-Manson approach. Typically, using the CTE differential between the bonded components, the maximum elastic and plastic strain in the solder joint is calculated. Most of the time, using the plastic strain value, Coffin-Manson curves are used to predict the fatigue life of solder joints. Usually this approach yields very conservative results for BGA packages, Zhao et al. @1#. Recently, numerous physics-of failure based models have been developed for the evaluation of reliability of solder alloys under thermo-mechanical fatigue loading, such as Busso