Masaki Shiratori

Effect of process-induced voids on isothermal fatigue resistance of CSP lead-free solder joints.

Abstract With the progressive miniaturization of electronic devices, process-induced voids in lead-free solder joints affect the assessment of thermal fatigue resistance. Voids appear randomly in a solder joint, making quantitative evaluation of fatigue life difficult. This study examined the effect of process-induced voids on the thermal fatigue resistance of CSP solder joints. CSP specimens were subjected to isothermal mechanical fatigue tests; specifically, the accelerated thermal cycle test. When a void is small, it has no apparent effect on fatigue life. However, when voids having diameters of at least 30% of solder diameter are located along the crack propagation route, fatigue life is shortened. FEA and Miner’s law for estimation of fatigue life suggest that voids affect not only the crack initiation but also crack propagation. Estimated numbers of cycles to failure agree quantitatively with the experimental results. The effects of the size, location, and number of voids can be extracted by FEA. As voids along the crack path become larger, fatigue life decreases. Moreover, when two voids are located near the corner of a solder joint on the crack path, a 30% decrease in life appears. This result agrees with experimental results reported in several literatures.

A Study of Deformation Mechanism During Nanoindentation Creep in Tin-Based Solder Balls

As the shrinkage and integration of devices, the creep behavior of tin-based alloys becomes important with microscales. In this paper, the behavior of creep deformation in solder alloys during a nanoindentation test was examined. Nanoindentation creep test was carried out for tin-based solder balls. Obtained results summarized as follows: (i) The stress exponent for power-law creep estimated can be evaluated from the evolution of hardness. These values obtained in the early stage corresponds with that of bulk within the range of high strain rate. (ii) The stress sensitivity decreases after stress relaxation in nanoindentation creep tests. The saturated value is 1 in three solder balls. (iii) The morphology of indented surface consists of three parts: initial indentation, power-law creep, and granular surface. It suggests that the transition from power-law creep to diffusion creep takes place. (iv) Finite element method analysis reveals stress and strain concentration appears in the vicinity of the tip. Strain field remains self-similar as the indentation proceeds. (v) The gradient of triaxial stresses below the tip in a nanoindentation test accelerates the creep strain rate due to the diffusive flow, relatively.

Stress-Induced Tin Whisker Initiation Under Contact Loading

As electronic components become smaller, tin whiskers have an increasingly adverse effect on the assessment of the reliability of the components. In particular, the contact between components brings about whisker initiation near the contact area. Although a number of spontaneous tin whisker models have been proposed, a model of tin whisker initiation by contact loading has not yet been established. In this paper, the behavior of stress-induced tin whiskering by contact load was examined in a tin plating based on multiaxial creep theory. Creep properties were measured using a nanoindentation creep technique. The creep rate of Sn-10Pb is higher than that of Sn and affects the generation of compressive stresses after stress relaxation. Nonlinear finite-element analysis (FEA) reveals that the multiaxial compressive stress state appears after stress relaxation. The axial compressive stress increases even though the contact stress decreases. When multiaxial stresses are applied, not only axial stress but also the difference between stresses, affects the behavior of whisker formation. The axial compressive stress depends on the structures of the contact bodies, and the contact stress depends on the creep properties of the plating

Low-Cycle Fatigue Reliability Evaluation for Lead-Free Solders in Vehicle Electronics Devices

The changeover from eutectic Sn-Pb solder to lead-free solder (Sn-Ag-Cu) has been driven by environmental concerns in the last few years. In this study, in order to obtain the low-cycle fatigue characteristic of Sn-Ag-Cu lead-free solder joints, an isothermal mechanical fatigue test with a large strain range, which can clarify the crack generation process and shorten the examination time, was carried out. FEM analysis was also performed in order to evaluate the relationship between the inelastic strain range and the low-cycle fatigue life. As a result, compared with fatigue life longer than 1000 cycles, the scatter of the fatigue cycles from 100 to several hundred cycles becomes larger. So, it seems that it is necessary to carefully evaluate the low-cycle fatigue life in the reliability evaluation. Moreover, in large chip components, not only crack initiation, but also crack propagation, affects the failure life. Thus, the crack path was simulated and the failure cycle of the large chip was evaluated based on Miner’s rule, and reliability of including the fatigue crack propagation can be evaluated by the analytical approach.Copyright

The Effect of Voids on Thermal Reliability of BGA Lead Free Solder Joint and Reliability Detecting Standard

Recently, there have been serious debates about whether Pb should be removed from solder joint, in view of environmental problems. These debates have now developed to the extent that a remarkable movement to establish regulations for the removal of Pb has emerged, especially in European countries and Japan. Therefore, many studies have been aggressively undertaken to develop technologies for replacing Sn-Pb solder with lead-free alternative. From the results obtained so far, it has been proven that the fatigue life of lead-free solder joints is almost equivalent to that of Sn-Pb eutectic solder joints. However, a new problem is that voids are easily formed in lead-free solder joints, and the effect of the voids on the fatigue strength of solder joints has attracted attention. In this study, by using mechanical fatigue test, the effect of voids on the fatigue reliability was investigated, and the detailed fatigue behavior in lead-free solder joint with voids was examined. Until now, it was confirmed that quantitative evaluation of fatigue life of solder joints with the voids could be enabled by FEM analysis. And the experimental approach and the effect of the various positions, sizes and number of voids on the fatigue reliability were studied

Easy reliability design approach for solder joint BGA package considering correlation of each design factor

The recent development of electric and electronic devices has been remarkable. The miniaturization of electronic devices and high integration are progressing by advances in mounting technology. As a result, the reliability of fatigue life has been prioritized as an important concern, since the thermal expansion difference between a package and printed circuit board causes thermal fatigue. In addition, because the development time of the product is being intensified, shortening the development time and reducing the cost are important concerns. It is demanded a good quality product which has short development time. However, it is difficult to guarantee quality during the design phase, because it is hard to know which design factors will prolong the fatigue life. The authors have investigated the influence of various design factors on the reliability of soldered joints in BGA model by using response surface method. However, it became clear that this sensitivity analysis was not enough for reliability design because of the effect of design factor correlation. So, it is necessary that the correlations between design factors of BGA package should be clarified. In order to investigate correlations between design factors, the authors have proposed a new approach to emboss efficiently the each design factor by applying cluster analysis. By using this technique, the correlation structure of the all design factor was clarified. Based upon the analytical results, design engineers can rate each factors effect on reliability and assess the reliability of their basic design plan at the concept design stage

Evaluation technique for the failure life scatter of lead-free solder joints in electronic device

Recently, the electronic device equipments using a lot of semiconductors are widespread to all industrial fields. Solder joints are used to mount the electronic chips, such as ceramic resistors and capacitors, on the printed-circuit boards in almost all electronic devices. However, since in many cases the thermal expansion coefficients of electronic parts and PCBs have mismatch, cyclic thermal stress and strain causes solder fatigue. Especially in the power electronic module and car electric module, the evaluation of thermal fatigue life for the chip components is important. It is understood that the fatigue lives of some electronic devices show big scatter in the thermal cycle test, even if their design is the same. The dispersion of main design factors of solder joints is thought as one of these reasons. Moreover, the influence of the dispersion grows when the lead-free solder materials are used in the devices. Therefore, it cannot be bypassed as the main issue for the reliability evaluation in the solder joints. In this study, how the dispersion of design factors and the interacting effect between the design factors influences the failure life in lead-free solder joint was investigated by the analytical approach. Sensitivity analyses were carried out to study the main effect of the dispersion of each factor on solder joints. And then, the interacting effect between the factors on the reliability was studied by considering the structural asymmetry due to the unbalanced solder joints. As a result, a practical evaluating technique for the failure life scatter of solder joints was proposed.

A study on evaluation technique for the fatigue life scatter of lead-free solder joints

The electronic device equipments using a lot of semiconductors are widespread to all industrial fields. Solder joints are used to mount the electronic chips, such as ceramic resistors and capacitors, on the printed-circuit boards in almost all electronic devices. However, since in many cases the thermal expansion coefficients of electronic parts and PCBs have mismatch, cyclic thermal stress and strain causes solder fatigue. Especially in the power electronic module and car electric module, the evaluation of thermal fatigue life for the chip components is important. It is understood that the fatigue lives of some electronic devices show large scatter in the thermal cycle test, even if their design is the same. The dispersion of main design factors of solder joints is thought as one of these reasons. Moreover, the influence of the dispersion grows when the lead-free solder materials are used in the devices. Therefore, it cannot be bypassed as the main issue for the reliability evaluation in the solder joints. In this study, how the dispersion of design factors influences the fatigue life in lead-free solder joint was investigated by the analytical approach. At first, sensitivity analyses were carried out to study the main effect of the dispersion of each factor on solder joints. And then, the interacting effects between the factors on the reliability were studied by considering the structural asymmetry due to the unbalanced solder joints. FEM analyses were carried out, and the fatigue life in solder joints was calculated from the inelastic strain range. As a result, practical evaluating approach for the fatigue life scatter of solder joints was proposed.Copyright

Delamination Evaluation Approach for Bimaterial Structure Considering Interfacial Layer

In recent years, interfacial fracture is one of the most important issues in the assessment of reliability of electronics packaging. In particular, underfill (UF) resin is used to prevent thermal fatigue of solder joints in flip chip packaging. Interfacial fracture between components/substrates and UF resin also affects the reliability of electronic devices. In general, the interfacial strength can be evaluated with the concept of interfacial fracture mechanics. However, as new materials and new processes using in the devices increase, it becomes clear that the fracture concept is difficult to evaluate the interfacial strength quantitatively. Many researches assumed that the interface is bonded perfectly. However, the interface has the micro-scale structure and the bonding may be imperfect. Specially interfaces of the resin have complicated structure. In this study, an alternative approach for evaluating the mechanical fracture of the interfacial structure of resin in electronic components was proposed. The basic mechanical behavior of the new interfacial model with imperfect bonding layer was examined by using finite element analysis. The stress field around the interfacial layer depends not only on the properties of interfacial layer but also on the micro structure of the interfacial layer. In addition, based on the experimental result of the tensile and the shearing test from the reference, the mechanical models of the interfacial structure were constructed. The conditions of delamination were examined by using FEA Furthermore, the new model and approach was confirmed quantitatively. It was found that the basic properties of the interfacial layer can be tuned to the proper values by two different delamination tests, and the new approach could show good agreement with the experimental results from the initial delaminaiton to the instability fracture process qualitatively. The simulation results were in good agreement with the experimental results.Copyright

Evaluation Method of Interaction Relation Between Design Factors and Simple Assessment Approach for BGA Package Reliability

The recent development of electric and electronic devices has been remarkable. The miniaturization of electronic devices and high integration are progressing by advances in mounting technology. As a result, the reliability of fatigue life has been prioritized as an important concern, since the thermal expansion difference between a package and printed circuit board causes thermal fatigue. It is demanded a long-life product which has short development time. However, it is difficult because of the interaction between each design factor. The authors have investigated the influence of design factors on the reliability of soldered joints in BGA model by using response surface method and cluster analysis. By using these methods, the interaction of all design factors was clarified. Based upon these analytical results, design engineers can rate each factor’s effect on reliability and assess the reliability of their basic design plan at the concept design stage.Copyright