Manabu Kakino

Dynamic behavior of electronics package and impact reliability of BGA solder joints

Clarifies the dynamic behavior of BGA or CSP packaging subjected to an impact loading, and establishes a simple analytical method of impact reliability assessment for solder joints. In order to take dynamic material properties into account, a high deformation speed tension test and a vibration test were carried out to obtain the strain rate dependence of yield stress and Youngs modulus of solder materials and PCB. A 3-D analytical model of PCB mounted with a BGA chip was used to simulate the impact behavior of BGA packaging, and explicit-based FEM code LS-DYNA was used to carry out the dynamic analysis. It was found that the impact reliability of solder joints is greatly affected by the falling posture. However, it was found that fine meshing of solder joints causes a rapid augmentation of calculation cost. In this study, the authors proposed a new method of transient response analysis by utilizing implicit-based FEM code NASTRAN to drop the calculation cost of impact study. It was shown that the present method can accurately simulate the dynamic behavior of BGA packaging including the time histories of the deformations and stresses, and it can drop the CPU time to about one tenth of that of LS-DYNA analysis.

The examination of the drop impact test method

The purpose of this study is to establish an experimental method of drop impact reliability for solder joints and an assessment method. The drop impact problem is composed by various factors. Such as, a drop impact induced flexural vibration of PCB (Printed Circuit Board), collision attitude, and dynamic fracture of solder joints. There are many methods examine impact reliability of BGA solder joints. One of the methods is a repetitive drop test with a simple equipment proposed by the authors. In a previous report, it was shown that the dynamic deformation of the PCB shows very stable results by attaching a hemisphere on the jig bottom. And as a result, the relation between drop height and cycle to failure of CSP package was obtained. It can achieve very good certainty of the fatigue test. However, several stable fatigue modes occurred according to the drop height. In this study, to obtain the impact reliability of BGA solder joints, the jig has been improved. In this paper, the authors carried out transient response analysis, and examined how the stresses of each device parts are affected by the jigs dimensions. And then, they conducted the repetitive drop test to study how the analytical results will agree with the experimental results. Finally, based upon the analytical and experimental results, they proposed a drop impact reliability assessment approach.

Impact reliability study of BGA solder joints and dropping test method

The purpose of this study is to develop an assessment method for drop impact reliability of BGA solder joints. In this paper, the dynamic deformation behavior of PCB is examined to evaluate the dynamic fracture of solder joints, where a free fall drop-impact test was used, and explicit-based FEM code LSDYNA was used to study the basic behavior of the drop-impact test, and the effects of falling angle on the dynamic behavior of PCB were discussed. As a result, it was confirmed even if collision angle is very small, the dynamic deformation of PCB was drastically affected by the collision angle. In this study, the authors proposed that the dynamic behavior can dramatically be improved by attaching a hemisphere on the jig bottom to obtain stable dynamic deformation of the PCB. Furthermore, in case of that the PCB flexural rigidity was changed, the optimal test jig was designed so that the PCB and the jig for a drop test might not cause resonance vibration. Generally, the crack growing assessment and drop impact strength evaluation for solder joints were studied by a fine meshing FEM model, nevertheless it causes a rapid augmentation of calculation cost. In this study, the authors proposed a new method of transient response analysis by utilizing implicit based FEM analysis code to drop the calculation cost of impact study. It was shown that the present method can accurately simulate the dynamic behavior of BGA packaging including the time histories of the deformations and stresses, and it can drop the CPU time to about one tenth of that of explicit based analysis code.Copyright

Molecular Orbital Study on Interfacial Strength between Oxide Cluster and Polyimide Substrate

Adsorption processes between silica cluster and poly-(p-phenylene pyromellitimide) substrate are investigated by the atomistic calculations combining with the semi-empirical molecular orbital method and the algorisms that can find minimum energy paths of chemical bonding process. Especially, the activation and sorption energies in the reactions are estimated from the optimal reaction path. The Silica thin film-aromatic Polyimide substrate interface is modeled by a silica cluster and polyimide surface which is layered molecular chains with periodic boundary conditions. Conjugate gradient and Quick-Min methods are used to determine atomistic configurations of the system at local minimum and to evaluate physisorption energy. The dimer and the nudged elastic band (NEB) methods, which use only first derivatives of the potential energy, are used in order to search minimum energy paths on the potential energy landscape. The dimer method is started from local minimum on the potential energy landscape and used to search the transition and final states of the system. The NEB method is then carried out to calculate precise path by using final state obtained by the dimer method. From the simulations, we find that the chemical bonding occurs between a silica cluster and polyimide substrate surface, and the sorption energy defined by the sum of physisorption and chemisorption energy are evaluated. Moreover, we discuss the interfacial strength of silica-polyimide substrate using the optimal minimum energy paths.

Guideline for the dropping test of the impact reliability study of solder joints

As a result of the miniaturization of mobile electronic devices in recent years, assessing the drop impact reliability of solder joints has become increasingly important. The purpose of the present paper is to establish an experimental method by which to determine the drop impact reliability of solder joint. The drop impact problem consists of various factors, including impact-induced flexural vibration of PCB, collision attitude, and dynamic fracture of solder joints. Numerous methods have been developed for examining the impact reliability of BGA solder joints. Among these is a repeated drop test proposed by the present authors that uses a simple apparatus. The effects of the drop angle on the dynamic behavior of PCB were discussed. In a previous report, it was confirmed that even if the collision angle is very small, the dynamic deformation of the PCB is greatly affected by the collision angle. In this study, the dynamic behavior was dramatically improved by attaching a hemisphere to the bottom of the holder, and the result was equivalent to stable dynamic deformation of the PCB in horizontal collisions. In addition, the relationship between drop height and cycle to failure of the CSP was obtained. The authors also investigated the interaction of the vibration modes of PCB and the specimen holder and the relationship between the holder geometry and its impact strength. Finally, through repeated drop testing, a holder geometry was obtained that did not adversely affect the PCB upon impact.Copyright