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 Ag3Sn 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 Ag3Sn 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 = C2Ni−β and ΔSA = C3Ni−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.
New nondestructive inspection methods with high spatial resolution are expected to support the evaluation and enhancement of the reliability of microjoints on printed circuit boards. An X-ray microtomography system, the SP-μCT has been developed at the Super Photon ring-8 GeV (SPring-8), the largest synchrotron radiation facility in Japan. In this work, the SP-μCT was first applied to the nondestructive evaluation of thermal fatigue phenomena, namely microstructure evolution (i.e., phase growth) and microcrack propagation, appearing in actual solder microbumps of flip chip interconnects due to thermal cyclic loading. In addition, a refraction-contrast imaging technique was simultaneously applied to visualize the fatigue cracks with an actual opening of less than 100 nm. The observed specimen has a flip chip structure joined by Sn-37wt%Pb eutectic solder bumps 150 μm in diameter. Consequently, the process of phase growth and crack propagation was determined via observation of consecutive computed tomography (CT) images obtained in the same plane of the same specimen. As the thermal cycle proceeded, remarkable phase growth was clearly observed, followed by the appearance of fatigue cracks in the corners of the interfaces between the solder bump and Cu pad. Moreover, the CT images also enabled us to evaluate the fatigue lifetime of the bumps, as follows. The lifetime to fatigue crack initiation was estimated by quantifying the increase in the phase growth. The crack propagation lifetime to failure was then determined by measuring the average crack propagation rate. Such results have not been obtainable at all by X-ray CT systems for industrial use and demonstrate the possibility of nondestructive inspection by a synchrotron radiation X-ray microtomography system.
An X-ray micro-tomography system called SP-μCT, which has a spatial resolution of 1 μm, 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 micro-crack propagation appearing as thermal fatigue damage in lead-free solder joints. The observed specimens include two typical micro-joint structures by Sn-3.0wt%Ag-0.5wt%Cu lead-free solder. The first is an FBGA (Fine pitch Ball Grid Array) joint specimen in which an LSI package is connected to a substrate by solder bumps 360 μm in diameter, while the second is a chip joint specimen in which chip type resistors 1.6 mm in length and 0.8 mm. in width are mounted on a substrate. A thermal cycle test was carried out, and the specimens were picked up at fixed cycle numbers. The same solder joints were observed repeatedly using SP-μCT at beamline BL20XU in SPring-8. An X-ray energy of 29.0 keV was selected to obtain CT (Computed Tomography) 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. In the FBGA type specimens, fatigue cracks appeared at the periphery of the interfaces between the solder and the UBM (Under Bump Metallization) on the LSI package. As the thermal cycle proceeds, the cracks propagate gradually to the inner region of the solder bumps in the vicinity of the interface. On the basis of the three-dimensional crack images, the fatigue crack propagation lifetime was accurately estimated by means of the average crack propagation rate. On the other hand, in the chip joint specimens, fatigue cracks appeared and propagated through the thin solder layer between the chip and substrate. In contrast to the FBGA specimen, many small voids roughly 5 to 10 μm in length were formed in the solder layer. The important observed fact is that these voids deform and connect to each other due to the thermal cyclic loading prior to crack propagation. Consequently, the obtained CT images clearly show the process of crack propagation due to the thermal cyclic loading of the same solder joint. In contrast, such information has not been obtained, whatsoever by industrially employed X-ray CT systems.
Recently, due to the increasing heat density of printed circuit boards (PCBs), thermal fatigue damage in the joints has exerted a more significant influence on the reliability of electronic components. Accordingly, the development of a new nondestructive inspection technology is strongly desired by related industries. The authors have applied a synchrotron radiation X-ray micro-tomography system to the nondestructive observation of micro-cracks. However, the reconstruction of CT images is difficult for planar objects such as PCB substrates, due to insufficient X-ray transmission in the direction parallel to the substrates. In order to solve this problem, a synchrotron radiation laminography system was developed to relax size restrictions on the observation samples, and was applied to the three-dimensional nondestructive evaluation of several kinds of solder joints, which were loaded under accelerated thermal cyclic conditions via thermal shock tests. Moreover, the thermal fatigue crack propagation process that occurs under actual PCB energization loading conditions will differ from that under the usual acceleration test conditions. In this work, the possibility of in-situ monitoring of the thermal fatigue crack propagation process using the laminography system was investigated at die-attached joints subjected to cyclic energization loading, which is close to the actual usage conditions of PCBs. The optical system developed for use in the laminography system was constructed to provide a rotation stage with a tilt from the horizontally incident X-ray beam, and to obtain X-ray projection images via a beam monitor. In this manner, the X-ray beam is sufficiently transmitted through the planar specimen in all projections. The observed specimens included several die-attached joints, in which 3 mm square ceramic dies had been mounted on a 40 mm square FR-4 substrate using Sn-3.0wt%Ag-0.5wt%Cu solder. Consequently, the laminography system was successfully applied to the in-situ monitoring of thermal fatigue cracks that appeared in the solder layer under cyclic energization. This was possible because the laminography images obtained in the energization state have a quality that is equivalent to those obtained in a non-energized state, provided that the temperature distribution of the specimen is stable. In addition, the fatigue crack propagation process can be quantitatively evaluated by measuring the crack surface area and calculating the average crack propagation rate. However, in some cases, the appearance of thermal fatigue cracks was not observed in a solder layer that had been loaded by the accelerated thermal cycle test. This result strongly suggests that delamination occurred at the interface, which indicates that the corresponding fracture mode was significantly influenced by the type of thermal loading.
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