Study About the Corrosion of SAC Solder Joints Under Various Conditions of Temperature and Salinity

Because failures in lead-free solder joints occur at locations other than the most highly shear-strained regions, reliability prediction is challenging. To gain physical understanding of this phenomenon, physically based understanding of how elastic and plastic deformation anisotropy affect microstructural evolution during thermomechanical cycling is necessary. Upon solidification, SAC305 (Sn-3.0Ag-0.5Cu) solder joints are usually single or tricrystals. The evolution of microstructures and properties is characterized statistically using optical and orientation imaging microscopy. In situ synchrotron x-ray measurements during thermal cycling are used to examine how crystal orientation and thermal cycling history change strain history. Extensive characterization of a low-stress plastic ball grid array (PBGA) package design at different stages of cycling history is compared with preliminary experiments using higher-stress package designs. With time and thermal history, microstructural evolution occurs mostly from continuous recrystallization and particle coarsening that is unique to each joint, because of the specific interaction between local thermal and displacement boundary conditions and the strong anisotropic elastic, plastic, expansion, and diffusional properties of Sn crystals. The rate of development of recrystallized microstructures is a strong function of strain and aging. Cracks form at recrystallized (random) boundaries, and then percolate through recrystallized regions. Complications arising from electromigration and corrosion are also considered.

In SMT, the investigation on solder joint's failure is always very important. Thermal fatigue is the main failure form for solder joint in SMT. In this paper the failure process of solder joint in SMT was investigated by both electrical resistance measurement method and crack observation method together. The characteristics of electrical resistance value variation of lead-tin and lead-free solder (SAC305) joint during thermal fatigue test were obtained. And at the same time the crack propagation in solder joint was observed. According to this, the failure rules of lead-tin and lead-free solder joint were compared. And by FEM, the relationship between electrical resistance value variation and crack propagation of solder joint during thermal fatigue test was studied, through which an criterion based on electrical resistance value variation for solder joint's failure during thermal fatigue test could be obtained from experience. It was shown from experimental results that the lead-free solder (SAC305) joint had a higher resistibility from thermal fatigue than the traditional lead-tin eutectic solder joint. And the criterion based on electrical resistance value variation was built up from the experimental and simulation results.

Lead free solder alloys have been developed which contain small concentrations of rare earth additives such as cerium and lanthanum. It is believed that rare earth element additives refine the microstructure of the solder and improve the mechanical durability and thermomechanical integrity of lead free solder joints. This paper presents the results of a comparative analysis of solder joints with the commonly used SAC 305 alloy (Sn 3.0% Ag 0.5% Cu) and a SACC alloy (Sn 3.0% Ag 0.5% Cu 0.019% Ce), which contains a small concentration of a Cerium additive. The influence of cerium on the microstructural refinement of the bulk solder; the metallurgy of the intermetallic compounds; and the damage evolution of the solder during thermal aging tests are investigated. Mechanical die shear tests and lead pull strength tests have been conducted to compare the mechanical strength and the morphology of the fracture surfaces of SAC and SACC solder joints. Lead pull testing of samples with the cerium doped, SACC alloy exhibit more ductile fracture surfaces and PCB pad lifts, rather than in SAC alloy samples, which exhibit brittle interfacial failures. This change in the fracture surface morphology suggests that cerium additives improve the mechanical integrity of the bulk solder, and correlates with previous materials testing on bulk solder samples of SACC and SAC, which show that SACC solder has a higher Young's modulus (higher stiffness), higher yield stress, and higher strength over a wide range of strain rates and temperature conditions. On the other hand, in the hot storage tests, the SACC samples exhibit thicker intermetallic formations at the bulk solder to component interface than in SAC samples, which suggests that SACC alloy solder joints could be less mechanically robust than SAC solder joints under dynamic loading conditions such as drop testing.

The reliability of semiconductor packages is a cornerstone in the advancement of high-performance electronics. As semiconductor components undergo continuous miniaturization and grow in complexity, traditional methods for inspection and analysis, such as serial sectioning and scanning electron microscopy, have become inadequate due their destructive nature, prohibitive time costs, and limited characterization abilities. Furthermore, while evaluating solder joint failures and solder joint reliability by direct current (DC) methods is potentially useful, this technique often does not capture all incipient failures leading to No Fault Found (NFF) scenarios, which occur when the product fails, but upon checking, no fault is detected. High radio frequency (RF) measurements of signal paths are more sensitive to such incipient circuit (or solder joint) failures. RF methods can capture failures due to mechanical changes, which affect the impedance of the devices under test, through return loss, insertion loss or phase angle. Furthermore, RF methods can catch such incipient failures well before complete solder joint failure. In this talk, we compare the fault detection capabilities and detection speeds, of direct current resistance (R-DC) to RF based fault detection measurements and correlate the electrical data with the results from non-destructive 4D X-ray Computed Tomography (XCT).Historically, non-destructive defect characterization techniques such as optical imaging, radiography, and scanning acoustic microscopy (SAM) have been instrumental in assessing defects in semiconductor packages. However, they lack the resolution and capability to accurately characterize defects in dense packages at the micron scale in 3D. Furthermore, the drive for greater device efficiency and performance has led to more complex thermal stress states in semiconductor packages. Therefore, the detection and analysis of defects goes beyond mere quality control and maintenance; they are fundamental for the chip-package-system co-design itself . Thus, 3D X-ray Computed Tomography (XCT) stands out as an ideal tool for the non-destructive assessment of defects in semiconductor packages.. Figure 1 shows the 3D renderings of preliminary work done on the analyses of defect evolution in thermally cycled packages using X-ray Computed Tomography. By capturing spatial and temporal changes, 4D X-ray CT provides unique perspectives into the package behavior under thermal stress. This enables not only the identification and characterization of defects but advancement of our understanding of their origin and development.As expected, the impedance of the solder joints (i.e., S parameter data) changed over time and temperature cycles as the connectors suffer from wear, even when there are clearly no actual changes to the circuit. The capacitance and associated capacitive reactance that formed by a partial crack in a solder joint was found to be so much larger than the very small resistance of an even tiny remaining amount of intact solder joint that the ultimate effect on the circuit is unmeasurable until the crack is fully open (as indicated by 4D XCT). The results presented in this paper should benefit emerging advanced packing concepts, and traditional components manufacturers to determine the reliability of their components on test boards. Figure 1

A common method of failure in electronic packages is the failure of solder joints. Solder joints subjected to thermal cyclic loading usually fail due to fatigue failure. This is brought about by the shear forces introduced by the different Thermal Expansion Coefficients (CTE) of the package components. Previous researches have shown that recrystallization assisted cracking plays a significant role in the fatigue failure of these solder joints. In this paper, the authors seek to observe the changes in the microstructure of SAC305 solder joints subjected to mechanical cyclic loading under various conditions. The test specimens were 3x3 BGA array of SAC305 joints sandwiched in between two FR-4 PCBs (Printed Circuit Boards). The specimens were held in epoxy preforms and polished to expose the solder joints. The samples were then extracted and some of these were aged at 125 °C for 10 days. Both the aged and non-aged specimens were then isothermally mechanically cycled at either room temperature or at 100 °C. The authors then proceed to observe the differences in the microstructure due to mechanical cycling of the joint under different conditions, namely, aging, or high temperature mechanical cycling, or a combination of both. The micrographs before and after the cycling were captured by an optical microscope. The other objective of this study was to also observe if recrystallization played a part in the ultimate failure of the joints when subjected to shear mechanical cycling for each of the conditions.

Ceramic ball grid array (CBGA) solder joints were considered to crack after a series of environmental tests, including thermal cycling, random vibration and high temperature operational life test. In this paper, the failure analysis on the crack of ceramic ball grid array (CBGA) solder joints is revealed with the aid of 3D X-ray inspection, microsection technique, optical microscope (OM) and scanning electron microscope (SEM).The result of 3D X-ray shows that voids were found in the CBGA and no obvious cracks in joints were detected. By employing micro-section technique, micro-cracks were found in several CBGA solder joints with aid of OM and SEM. The result of micro-section of CBGA solder joints also reveals that the solder balls were made of eutectic tin-lead alloys. The OM and SEM images of cross-section of CBGA solder joints demonstrated that significant coarsening of solder structure was found near the cracks. In the CBGA solder joints, Pb-rich phase was well distributed into the Sn-rich. The size of Pb-rich phase in the crack area was larger than that in other area of the solder joints. As the size of microstructure increased, the interfaces between different phases decreased, which would weaken the mechanical structure of the solder joints, leading to crack failure eventually. It can be inferred that the crack of CBGA solder joints was related to thermal-mechanical fatigue. Generally, the thermal expansion of ceramic is much smaller than that of PCB substrate, which led to large thermal mismatch during the thermal cycling test, causing the thermal mechanical fatigue crack in the CBGA solder joints. The intermetallic compound (IMC) at the soldering interface and the shape of solder joints were also observed by SEM. The IMC showed a continuous morphology with proper thickness, and the solder joints kept a normal shape.In this paper, the cross section of plastic ball grid array (PBGA) solder joints on the same printed circuit board (PCB) were also observed by OM and SEM. However, no thermal mechanical fatigue cracks and coarsening of solder structure were found. It was due to that the coefficient of thermal expansion of PBGA substrate is similar to that of PCB substrate, and that the thermal mismatch between the PBGA and PCB was much smaller than that between the CBGA and PCB. Hence, thermal-mechanical fatigue cracks were found in CBGA solder joints, while the PBGA solder joints remained intact. This paper also reveals that the height of solder joint can significantly influent the reliability of the CBGA joints under the thermal cycling load.

Reliability of microelectronic assemblies is typically limited by the fatigue failure of one of the interconnected solder joints. The fatigue behavior of the lead-free solder joints doped with bismuth under different stress loading and aging conditions is not yet understood. This article investigates the effect of adding bismuth on the mechanical reliability of SAC alloys considering different loading and aging conditions. The fatigue behavior and shear strength of individual SAC305 (Sn-3.0Ag-0.5Cu) and SAC-Q (Sn-3.4Ag-0.5Cu-3.3Bi) solder joints are examined and compared. A unique experiment is designed to test the individual solder joints using a micromechanical testing system. The results show that the fatigue life and shear strength of SAC-Q with Bi are much higher than SAC305 regardless of the aging and stress conditions. It was also found that increasing stress amplitude leads to a decrease in the fatigue life for both alloys. The aging time has a negative effect on the fatigue life and shear strength of both alloys. The impact of aging on SAC-Q solder joints is significantly less than that for SAC305. Microstructure analysis shows a substantial amount of precipitates coarsening with aging for SAC305 compared with SAC-Q. Hysteresis loop analysis shows that increasing the cycling stress amplitude and aging time leads to an increase in the work per cycle and plastic strain. Common fatigue models, such as Morrow energy model and Coffin-Manson model, fail to predict the life considering the effect of aging; aging affects the constants of both models.

In order to discuss the effects of Cu-cored solder with pure Ni coating and joint height on the interfacial microstructure and mechanical properties of joints, Cu/Sn-3.0Ag0.5Cu (SAC305)/Cu and Cu/Cu-cored + SAC305/Cu sandwich solder joints were prepared using a reflow process. The interfacial microstructure of the solder joints was investigated by scanning electron microscope with energy-dispersive x-ray spectroscopy. The results showed that, at as-reflowed joints, an intermetallic compound (IMC) of scallop-like Cu6Sn5 was formed at the Cu wire/solder interface, and a plane-like (Cux, &!nbsp;Ni1−x)6Sn5 IMC was formed at the Cu core/solder interface. Compared with SAC305 solder joints, Cu-cored solder joints showed more interface layers, a thicker Cu6Sn5 IMC layer, and higher tensile strength. With increasing joint height, the thickness of the IMC at the two interfaces decreased, and the tensile strength also decreased for Cu-cored solder joints. Mixed brittle/ductile fracture appeared in SAC305 and Cu-cored solder joints with height of 600 μm, but were suppressed and transformed into ductile fractures with increasing height of the solder joints.

In this paper, thick film chip resistors with two different types of solder alloys namely SnPb and SnAgCu have been evaluated for the effects of the solder alloy elemental composition on the solder joint failures under cyclic temperature loading conditions. The creep properties of both solders have been modelled using the Garofalo equation and the creep strain energy density has been extracted and used as the damage indicator for lifetime prediction. Three thick film chip resistors of different sizes have been modelled and the effect of device size on the failures in the solder joints has been analysed. In addition, both thermal cycling and thermal shock conditions have been modelled in order to simulate effects of extreme harsh conditions and the total damage has been calculated using the Miner's law of linear damage accumulation. Based on the modelling results, the most vulnerable places in the solder joints where the failures may originate and propagate have been identified. Empirical lifetime models have used to predict the life time of the resistor solder joints.

PurposeThe purpose of this study was to investigate the changes in solder joint stress when subjected to mechanical bending. The analytical theory pertaining to the stresses in the solder joint between the components (including the molding compound, the chip and the substrate) was described, and the printed circuit board (PCB) with a discontinuity function when the PCB assembly is subjected to mechanical bending was developed. Thus, the findings reported here may lead to a better understanding of the solder joint failure based on the Physics of Failure model.Design/methodology/approachThis paper discusses the analytical model for calculating the stress in solder joints, as well as presents a simulation model that can be used for calculating the strain energy density of solder joint. First, the multilayer plate theory is used in discussing the composite material for the component, including the molding compound, the silicon chip and the substrate, or the PCB, including the copper layers, the fiber and the epoxy. Finally, the complete structure of the analytical model developed as a part of this current work is presented.FindingsFor the analytical model of multilayer structures in which the interconnection layer is discrete, mechanical bending has been modeled with respect to varying silicon chip length. The analytical model that describes the stress of the outermost solder joint experiences is chosen, as this is the typical solder joint failure. The analytical model can be applied to discrete solder joints, which are evaluated by calculating the matrix form. Owing to its use of the matrix equation, the analytical model can be highly combinatorial and thus more capable of calculating the solution.Research limitations/implicationsThe analytical solution based on a simple concept was presented and validated using the finite element model for the stress experienced by solder joints subjected to mechanical bending. To verify that the simulation represents a real PCB case, the authors use the finite element method (FEM) to compare their case with the multilayer plate theory. Owing to the good agreement between the theory and simulation results, the authors conclude that the multilayer plate theory can be correctly applied in multilayer PCB and be used in an analytical model for the PCB assembly subjected to mechanical bending.Practical implicationsOwing to the good agreement between the theory and simulation results, the authors conclude that the multilayer plate theory can be correctly applied in multilayer PCB and be used in an analytical model for the PCB assembly subjected to mechanical bending.Social implicationsThe analytical model is validated with the FEM model and provides the way to physically examine the solder joint failure mechanism. In this paper, the analytical model is developed as a means to assess the solder joint stress subjected to mechanical bending.Originality/valueThe analytical model treats the solder joint as discrete and has been successfully validated against the finite element model. The complete structure model (the second analytical model) is presented to discuss the effects of varying silicon chip length on the normal stress in solder joints. When the silicon chip length exceeds to 80 per cent of the total package length, the stress of the outermost solder joint increases rapidly. The design analysis findings have suggested that the failure of the outermost solder joint subjected to mechanical bending on the PCB assembly can be reduced by analyzing the analytical model.

Lead-free solder joints based on SnAgCu solders often consist of one single tin grain. Since tin is highly anisotropic, the grain structure in SAC solder joints and the orientation of the tin grain in single-grained solder joints may have a large impact on the fatigue life of the solder joints when exposed to thermomechanical stress.The scope of this study was to evaluate how the grain structure in SAC solder joints to various BGA components is impacted by surface finishes on soldered surfaces, and the volume and composition of the solder. The grain structure was analysed by cross-sectioning of the solder joints and inspected using optical microscopy with cross-polarised light. In addition, some samples were analysed using electron backscatter diffraction in order to determine the orientation of the grains.Four grain structures were observed in the solder joints: single grained, cyclic twin, interlaced twin or mixed cyclic and interlaced twin structure. The surface finish had the largest impact on the grain structure. Solder joints formed between nickel and copper solder finishes had in most cases a high fraction of solder joints with mixed cyclic and interlaced twin grain structure whereas solder joints formed between two nickel finishes had a high fraction of single-grained solder joints.The results from this study indicate that the morphology of the intermetallic layers formed on soldered surfaces is the main factor determining the degree of undercooling during solidification.

This paper investigated the shear strength and failure mechanism of low-Ag SAC-BiNi solder joints in comparison with SAC0705 and SAC305 solder joints. Experimental results demonstrated that the shear strength of SAC0705-BiNi/Cu solder joint was higher than SAC0705/Cu and SAC305/Cu solder joints before and after HTS aging. Moreover, SAC0705-BiNi/Cu solder joint has the smallest IMC grains and the highest resistance for the spread of shear crack in the three kinds of solder joints after reflow. After HTS aging, the interfacial microstructure of the investigated solder joints coarsened and the shear strength of these solder joints decreased. In SAC0705/Cu and SAC305/Cu solder joints, the failure of the solder joints is due to the ductile fracture of the bulk solder of the solder joints and the brittle broken of the individual large IMC grains. Meanwhile, the failure of SAC0705-BiNi/Cu is mostly because of the broken of interfacial IMC and the separation between IMC and the bulk solder of the solder joins.

In this paper, a direct comparison was conducted between SnPb and lead-free solder joints focused on long-term thermal cycling reliability after being exposed to high-temperature aging for a long period of time using two test vehicle designs. Ball Grid Array (BGA) packages with SAC305 (Sn-3%Ag-0.5%Cu) and eutectic SnPb (63%Sn-37%Pb) were compared in this research. After the test, IMC thickness and failure modes of BGA components on the different test vehicles were investigated. The results from the post-test examination were analyzed to explain certain unexpected outcomes of the two test vehicle designs when comparing the reliability results among the two programs of tests. The results showed that for the TC1 test vehicle, SAC305 solder joints show better reliability than the SnPb solder joints; while the reliability of the SnPb solder joints outperformed the SAC305 solder joints after 6 months of high-temperature aging for the TV7 test vehicle. Meanwhile, even on the same test vehicle, the failure modes varied between components with SAC305 and SnPb solder joints.

Due to aging phenomena, the microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal and/or thermal cycling environments. In our ongoing studies, we are exploring aging phenomena by nano-mechanical testing of SAC lead free solder joints extracted from PBGA assemblies. Using nanoindentation techniques, the stress-strain and creep behavior of the SAC solder materials are being explored at the joint scale for various aging conditions. Mechanical properties characterized as a function of aging include the elastic modulus, hardness, and yield stress. Using a constant force at max indentation, the creep response of the aged and non-aged solder joint materials is also being measured as a function of the applied stress level. With these approaches, aging effects in actual solder joints are being quantified and correlated to the magnitudes of those observed in testing of miniature bulk specimens. In our initial work (ECTC 2013), we explored aging effects in single grain SAC305 solder joints. In the current investigation, we have extended our previous work on nanoindentation of joints to examine a full test matrix of SAC solder alloys. The effects of silver content on SAC solder aging has been evaluated by testing joints from SACN05 (SAC105, SAC205, SAC305, and SAC405) test boards assembled with the same reflow profile. In all cases, the tested joints were extracted from 14 × 14 mm PBGA assemblies (0.8 mm ball pitch, 0.46 mm ball diameter) that are part of the iNEMI Characterization of Pb-Free Alloy Alternatives Project (16 different solder joint alloys available). After extraction, the joints were subjected to various aging conditions (0 to 12 months of aging at T = 125 C), and then tested via nanoindentation techniques to evaluate the stress-strain and creep behavior of the four aged SAC solder alloy materials at the joint scale. The observed aging effects in the SACN05 solder joints have been quantified and correlated with the magnitudes observed in tensile testing of miniature bulk specimens performed in prior studies. The results show that the aging induced degradations of the mechanical properties (modulus, hardness) in the SAC joints were of similar order (30–40%) as those seen previously in the testing of larger “bulk” uniaxial solder specimens. The creep rates of the various tested SACN05 joints were found to increase by 8–50X due to aging. These degradations, while significant, were much less than those observed in larger bulk solder uniaxial tensile specimens with several hundred grains, where the increases ranged from 200X to 10000X for the various SACN05 alloys. Additional testing has been performed on very small tensile specimens with approximately 10 grains, and the aging-induced creep rate degradations found in these specimens were on the same order of magnitude as those observed in the single grain joints. Thus, the lack of the grain boundary sliding creep mechanism in the single grain joints is an important factor in avoiding the extremely large creep rate degradations (up to 10,000X) occurring in larger bulk SAC samples. All of the aging effects observed in the SACN05 joints were found to be exacerbated as the silver content in the alloy was reduced. In addition, the test results for all of the alloys show that the elastic, plastic, and creep properties of the solder joints and their sensitivities to aging are highly dependent on the crystal orientation. The observed mechanical behavior changes in joints are due to evolution in the microstructure and residual strains/stresses in the solder material, and measurements of these evolutions are critical to developing a fundamental understanding of solder joint aging phenomena. As another part of this work, we have performed an initial study of these effects in the same SAC305 solder joints that were tested using nanoindentation. The enhanced x-ray microdiffraction technique at the Advanced Light Source (Synchrotron) at the Lawrence Berkeley National Laboratory was employed to characterize several joints after various aging exposures (0, 1, and 7 days of aging at T = 125 C). For each joint, microdiffraction was used to examine grain growth, grain rotation, sub-grain formation, and residual strain and stress evolution as a function of the aging exposure. The entire joints were scanned using a 10 micron step size, and the results were correlated with changes in the mechanical response of the joint specimens measured by nanoindentation.

Plastic ball grid array (PBGA) packages are one of the widely used electronic packages in semiconductor industry. In PBGA packages, solder joint is the most vulnerable component due to continuous exposure of cycling temperatures in real applications. In our previous study, it was found that cyclic temperature causes the solder joints to evolve their mechanical properties. The continuous shear fatigue on solder joints due to CTE mismatch between die and PCB material causes damage on the solder material which eventually leads to crack propagation and failure. Hence, it is important to understand the change in stress-strain behavior in real solder joints and to identify the critical region of failure during this temperature cycling phenomena. In our previous paper, we have analyzed evolution of mechanical behavior of SAC305 solder joints as a function of cycling using nanoindentation technique.In this study, we have used FEA modeling technique to understand change in stress-strain behavior of as reflowed and thermally cycled solder joints and identify critical failure location under shear loading. A simplified three dimensional model of solder joint was generated and simulated using ANSYS software. Most of the prior studies predicted the stress-strain behavior of solder joints using viscoplastic model based on bulk solder material characterization and not on real joints. In this study, we have utilized nanoindentation technique to extract material properties of SAC305 solder joints to incorporate into ANSYS software. At first, test specimen were prepared by cross sectioning a PBGA package to reveal single grain SAC305 solder joints by surface polishing to facilitate SEM imaging and nanoindentation testing. After preparation, the package sample were thermally cycled from T = -40 to 125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C in an environmental chamber. At various points in the cycling (e.g. after 0, 50, 100, 250, 500,750 and 1000 cycles), the package was taken out from the thermal cycling chamber. Nanoindentation testing was then performed at room temperature (25 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C) on above single grain solder joints to obtain equivalent mechanical behavior. Elastic-viscoplastic material properties were extracted as a function of no. of thermal cycles which was used for FEA analysis.