Takeshi Takayanagi

Evaluation of Microstructural Evolution and Thermal Fatigue Crack Initiation in Sn-Ag-Cu Solder Joints

Thermal fatigue damage in solder joints is believed to be closely related to microstructural evolution. In this study, a microstructural approach will be taken to evaluate the lifetime of Sn-3.0Ag-0.5Cu lead-free solder joints before the initiation of thermal fatigue cracks. In order to clarify the relation between the microstructural change and the initiation of thermal fatigue cracks, a series of thermal cycle tests were performed using fabricated PCBs on which various chip resistors were mounted. The following results were obtained via SEM observations. First, both the β-Sn and the Ag3 Sn phases grow as the number of cycles N increases. This phase growth is characterized by phase growth parameters Ss and SA in the β-Sn phase and the Ag3 Sn phase, respectively, which are defined as the average phase size to the 4th power. The phase growth proceeds such that the parameters increase proportionally to N. Furthermore, simple relations exist between the average number of cycles leading to the initiation of thermal fatigue cracks Ni and the average increase in the parameters per cycle ΔSs or ΔSA . That is, power law relations ΔSs = C2 Ni −β and ΔSA = C3 Ni −γ are determined. These relationships will enable us to evaluate the lifetime of the solder joints before the initiation of thermal cracks based on observations of microstructural evolution.Copyright

Nondestructive Evaluation of Thermal Phase Growth in Solder Ball Microjoints by Synchrotron Radiation X-Ray Microtomography

In high-density packaging technology, one of the most important issues is the reliability of the microjoints connecting large scale integrated circuit chips to printed circuit boards electrically and mechanically. The development of nondestructive testing methods with high spatial resolution is expected to enhance reliability. An X-ray microtomography system called SP-μCT has been developed in Super Photon ring-8 GeV (SPring-8), the largest synchrotron radiation facility in Japan. In this work, SP-μCT was applied in the nondestructive evaluation of microstructure evolution, that is, the phase growth due to thermal cyclic loading in solder ball microjoints. Simulating solder microjoints used in a flip chip, specimens were fabricated by joining a Sn-Pb eutectic solder ball 100 μm in diameter to a steel pin in the usual reflow soldering process. The phase growth process was determined by observing the computed tomography (CT) images obtained consecutively at the fixed point of the target joining. In the reconstructed CT images, the distribution of the constituent phases in the Sn-Pb eutectic solder was identified based on the estimation value of the X-ray linear attenuation coefficient. Consequently, the microstructure images obtained nondestructively by SP-μCT provided us with the following useful information for evaluating the reliability of the solder microjoints. First, each phase involves not dispersing particles but a three-dimensional monolithic structure like a sponge. Second, the phase growth proceeds in such a way that the average phase size to the fourth power increases proportionally to the number of cycles. Finally, in the vicinity of the joining interface, more rapid phase growth occurs compared to the other regions because local thermal strain due to the mismatch of thermal expansion leads to a remarkable phase growth.

Nondestructive Evaluation of Thermal Phase Growth in Solder Ball Micro-Joints by Synchrotron Radiation X-Ray Micro-Tomography

In high-density packaging technology, one of the most important problems is reliability of the micro-joints connecting LSI (Large Scale Integrated Circuit) chips to PCBs (Printed Circuit Boards) electrically and mechanically. Development of nondestructive testing methods with high spatial resolution is expected to enhance the reliability. Our research group has developed an X-ray micro-tomography system called SP-μCT at the beamline BL47XU in SPring-8, the largest synchrotron radiation facility in Japan. In this work, SP-μCT was applied to three-dimensional evaluation of microstructure evolution; that is, phase growth due to thermal cyclic loading in solder ball micro-joints. Simulating solder micro-joints used in a flip chip, specimens were fabricated by joining a Sn-Pb eutectic solder ball 100 μm in diameter to a steel pin in the usual reflow soldering process. The phase growth process was determined by observation of the CT images obtained consecutively at the fixed point of the target joining. In the reconstructed CT images, the distribution of the constituent phases in Sn-Pb eutectic solder was identified based on the estimation value of the X-ray linear attenuation coefficient. The following results were obtained. First, each phase involves not dispersing particles but a three-dimensionally monolithic structure just like a sponge. Second, the phase growth proceeds in such a way that the average phase size to the 4th power increases proportionally to the number of cycles. Finally, in the vicinity of the joining interface, more rapid phase growth occurs in comparison to the other regions because local thermal strain due to the mismatch of thermal expansion leads to remarkable phase growth. Consequently, the microstructure images obtained by SP-μCT bring us useful information to evaluate the reliability of micro-joints.© 2005 ASME

Nondestructive Observation of Thermal Fatigue Crack Propagation in FBGA and Die Attached Solder Joints by Synchrotron Radiation X-Ray Laminography

The reliability of solder joints on printed circuit boards (PCBs) is significantly affected by thermal fatigue processes due to downsizing and high density packaging in electronic components. Accordingly, there is a strong desire in related industries for development of a new nondestructive inspection technology to detect fatigue cracks appearing in these joints. The authors have applied the SP-μCT, a synchrotron radiation X-ray microtomography system, to the nondestructive observation of such cracks. However, for planar objects such as PCB substrates, reconstruction of CT images is difficult due to insufficient X-ray transmission along the parallel axis of the substrate. In order to solve this problem, a synchrotron radiation X-ray laminography system was developed to overcome the size limits of such specimens. In this work, this system was applied to the three-dimensional, nondestructive observation of thermal fatigue cracks in solder joints, for which X-ray CT inspection has been extremely difficult. The observed specimens included two typical joint structures formed using Sn-3.0Ag-0.5Cu solder: (1) a fine pitch ball grid array (FBGA) joint specimen in which an LSI package is connected to a substrate by solder bumps 360 μm in diameter, and (2) a die-attached specimen in which a 3 mm square ceramic chip is mounted on a substrate. The optical system developed for use in X-ray laminography was constructed to provide a rotation axis with a 30° tilt from the right angle to the X-ray beam, and to obtain X-ray projection images via the beam monitor. The same solder joints were observed successively using the laminography system at beamline BL20XU at SPring-8, the largest synchrotron radiation facility in Japan. In the FBGA type specimen, fatigue cracks were clearly observed to appear at the periphery of the joint interface, and to propagate gradually to the inner regions of the solder bumps as thermal cycling proceeded. In contrast, in the die-attached joint specimen, micro-cracks were observed to appear and propagate through the thin solder layer. An important observation was that these micro-cracks become interconnected prior to propagation of the main fatigue crack. The fatigue crack propagation lifetime was also estimated in both specimens by measuring the crack surface area and calculating the average crack propagation rate through the three-dimensional images. Consequently, the sectional images obtained by the laminography system clearly show the process of crack propagation due to thermal cyclic loading.Copyright

Three Dimensional and Nondestructive Evaluation of Thermal Fatigue Crack Propagation Process in Complex-Shaped Solder Joints by Synchrotron Radiation X-Ray Micro-Tomography

The reliability or lifetime of micro-joints on printed circuit boards (PCBs) is significantly affected by fatigue processes, including fatigue crack initiation and propagation to failure. Accordingly, the industries producing electronic devices and components strongly desire a new nondestructive inspection technology, which detects micro-cracks appearing as thermal fatigue fractures in the joints. In this investigation, we applied a synchrotron radiation X-ray micro-tomography system called the SP-μCT to three-dimensionally and nondestructively evaluate the fatigue crack propagation process in complex-shaped solder joints. The observed specimens have a typical joint structure in which chip type resistors 1.0 mm in length and 0.5 mm in width are mounted on an FR-4 substrate by joining with Sn-3.0Ag-0.5Cu solder. A thermal cycle test was carried out, and specimens were collected at fixed cycle numbers. The same solder joints were observed successively using the SP-μCT at beamline BL20XU at SPring-8, the largest synchrotron radiation facility in Japan. An X-ray energy of 29.0 keV was selected to obtain computed tomography (CT) images with high contrast among some components, and a refraction-contrast imaging technique was also applied to the visualization of fatigue cracks in the solder joints. The following results were obtained. At the early stage in the fatigue process of normal joints, the main fatigue cracks were clearly observed to initiate from the region around the solder joint tip and the vicinity of the chip corner. Additionally, many micro-cracks roughly 5 to 10 μm in length also formed in the thin solder layer between the chip and substrate. The important observed fact is that these micro-cracks deform, grow, and connect to each other due to the thermal cyclic loading, prior to main crack propagation. On the other hand, in case of solder joints which included relatively larger initial voids, the voids deformed, and the fatigue cracks initiated and propagated from the surface of the voids. Furthermore, by employing the three-dimensional crack images, the crack dimensions were quantified straightforwardly by measuring the surface area of the fatigue crack, and the fatigue crack propagation process was also accurately evaluated via the average crack propagation rate. Consequently, the obtained CT images clearly illustrate the process of crack propagation due to the thermal cyclic loading of a solder joint. In contrast, such information has not been obtained in any form by industrially employed X-ray CT systems or finite element analyses.Copyright

Nondestructive observation of fatigue crack propagation process in some solder joints by synchrotron radiation X-ray micro-tomography

A synchrotron radiation X-ray micro-tomography system was applied to nondestructive visualization and evaluation of micro-crack propagation appearing as thermal fatigue damage in some solder joints. The fatigue crack propagation lifetime was accurately estimated on the basis of the three-dimensional crack images.

Application of Synchrotron Radiation X-Ray Laminography to Nondestructive Evaluation of the Fatigue Crack Propagation Process in Flip Chip Solder Joints

The reliability and lifetime of micro-joints on printed circuit boards (PCBs) is significantly affected by fatigue processes, including fatigue crack initiation and propagation to failure. Accordingly, the industries producing electronic devices and components strongly desire the development of a new nondestructive inspection technology, which detects micro-cracks appearing as thermal fatigue fractures in these joints. Accordingly, the authors have demonstrated that the micro-cracks in the micro-solder joints can be observed using the SP-μCT synchrotron X-ray micro tomography system. However, in order for such solder joint micro-cracks to be observable by SP-μCT, the observation object must have a diameter of less than roughly 1 mm. In this investigation, we applied a synchrotron radiation X-ray laminography system to three-dimensionally and nondestructively evaluate the fatigue crack propagation process in flip chip solder micro-joints. X-ray laminography is a technique for nondestructively observing planar objects. The optical system developed for use in X-ray laminography was constructed to provide the rotation stage with a 20° tilt from the horizontally incident X-ray beam. For this reason, X-rays were sufficiently transmitted through the planar object, in all directions. The observed specimens had a flip chip structure, in which a 10.04 mm square LSI chip is connected to a 52.55 mm (length) × 30.0 mm (width) FR-4 substrate by 120 μm diameter Sn-3.0wt%Ag-0.5wt%Cu lead-free solder bumps. A thermal cycle test was carried out, and specimens were collected at fixed cycle numbers. The same solder joints were observed successively using the synchrotron radiation X-ray laminography system at beamline BL20XU at SPring-8, the largest synchrotron radiation facility in Japan. An X-ray beam energy of 29.0 keV was selected to obtain laminography images with high contrast among component. The obtained laminography images clearly show the evolution of cracks, voids, and the Ag3Sn phase due to the thermal cyclic loading of the solder joints. In addition, the surface area of the same fatigue cracks was also measured, to quantify the crack propagation process. However, the surface area change measured by laminography differed from the crack propagation results obtained by standard SP-μCT. This difference may be due to an inability to observe some micro-cracks, due to crack closure to beneath than the detection limit of synchrotron radiation X-ray laminography. Consequently, these results demonstrate the possibility that nondestructive observation of fatigue cracks in the solder bumps on a large size electronic substrate by synchrotron radiation X-ray laminography, although its detection ability for narrow cracks may be limited, compared to SP-μCT.Copyright

Application of Synchrotron Radiation X-Ray Micro-Tomography to Nondestructive Evaluation of Thermal Fatigue Damage in Flip Chip Interconnects

A synchrotron radiation X-ray micro-tomography system called SP-μCT with a spatial resolution of about 1μm has been developed in SPring-8, the largest synchrotron radiation facility in Japan. In this work, SP-μCT was applied to the nondestructive evaluation of microstructure evolution; that is phase growth, and micro-crack propagation appearing as thermal fatigue damage in solder micro-bumps of flip chip interconnects. The observed specimens have a flip chip structure joined by Sn-37wt%Pb eutectic solder bumps 100μm in diameter. A thermal cycle test was carried out, and the specimens were picked up at any number of cycles. The solder bumps were observed by using SP-μCT at the beamlines BL47XU and BL20XU in SPring-8. An X-ray energy of 29.0 keV was selected to obtain absorption images with a high contrast between the Sn-rich and the Pb-rich phases. Additionally, a refraction-contrast imaging technique was applied to visualize fatigue cracks in the solder bumps. The obtained CT (Computed Tomography) images clearly show the process of phase growth and crack propagation due to the thermal cyclic loading of the same solder bump; such information has not been obtained at all by industrially-used X-ray CT systems. In the initial state, the Pb-rich phase was dispersed with characteristic shape, which appears in reflow soldering process. Remarkable phase growth was also observed clearly as the thermal cycle test proceeded. When the loading reached 300 cycles, fatigue cracks appeared in the corners of the interfaces between the solder bump and the Cu pad. The CT images enabled us to evaluate the lifetime of the bumps to the initiation of fatigue cracks by estimating the increase in a phase growth parameter, which corresponds to the accumulation of fatigue damage in the solder joints. The results showed that the estimated lifetime strongly agreed with the average value, which was determined by SEM (Scanning Electron Microscope) destructive observations. As the thermal cycle proceeded, the cracks propagated gradually to the inner region of the solder bump. From the CT images, the average propagation rate was calculated, and the mean of the total fatigue lifetime was estimated to be less than 1800 cycles. These results show the possibility that nondestructive testing by a synchrotron radiation X-ray micro CT system is useful for evaluating the thermal fatigue lifetime in micro-joints.Copyright

Evaluation of Thermal Fatigue Crack Initiation in Sn/Pb Eutectic Solder Joint by Phase Growth Approach.

Thermal fatigue damage of solder joint is considered to be closely related to microstructural phase growth. In order to evaluate the relation between the microstructural change and thermal fatigue crack initiation, thermal cycle tests were carried out by using fabricated PCB on which various chip capacitors were mounted. Following results were obtained through the microstructural observation. The phase growth is characterized by phase growth parameter S which is defined as average phase size to the 4th power d4 and proceeds such that S increases proportional to the number of thermal cycles N. Furthermore, there exists simple relation between average number of cycles to thermai fatigue crack initiation Ni and average increase in parameter Sc per one cycle 〓Sc, where Sc is component of S, represents strain induced phase growth and corresponds to total equivalent creep strain approximately. That is, power law relation 〓Sc=CN-βi is found out. The relation enables us to evaluate the thermal crack initiation of solder joint based on the observation of microstructural evolution.

Evaluation of Phase Growth Process in Sn/Pb Eutectic Solder under Thermal Cyclic Loading

Thermal fatigue damage of solder joint is considered to be closely related to microstructural coarsening. In the present study, phase growth model is devised in order to express the microstructural evolution of Sn/Pb eutectic sobder. The model is based on results of systematic microstructural abservation using lap joint type shear specimen proposed by the authors. And the model is also derived from the vacancy diffusion theory in phase boundary. The phase growth is characterized by parameter S which is defined as average phase size to the 4 th power d4 and increases approximately proportional to time and total equivalent creep strain. Furthermore, the model is applied to evaluation of phase growth process in the solder joint of the specimen under thermal cyclic loading which realizes approximately thermal deformation state of printed circuit board (PCB). Consequently, it is clarified that phase growth proceeds at any temperature to which PCB is subjected and is mainly induced by strain rather than thermal equilibrium. And parameter S increases corresponding to total equivalent creep strain. This suggests that S would be evaluation parameter for thermal fatigue damage of actual solder joint.