Bulk Solder Research Articles

Lifetime prediction of a viscoplastic lead-free solder in power electronics modules under passive temperature cycling

It is well-established now that solder joint in power electronics undergoes a thermal-mechanical fatigue in service. In order to provide a predictive tool to assess fatigue lifetime of solder joint subjected to passive temperature cycling, a cohesive zone method (CZM) based fatigue model, accounting for mixed-mode loadings, is presented in this paper. A cohesive zone model with a specific fatigue traction-separation law is used in conjunction with the Anand viscoplastic behavior for the bulk solder for modelling the interfacial fatigue crack growth. Temperature dependence of both stiffness and fracture energy is incorporated in the CZM based fatigue model. The CZM based fatigue model as well as the Anand viscoplastic constitutive law is implemented in the Abaqus finite element code. Numerical simulations are carried out under passive temperature cycling. A particular attention is given to information in the region ahead of the crack tip such as stress concentration, process zone size, damage distribution. The evolution of the crack extension is used to estimate the number of cycles to failure at which the device becomes faulty.

Board Level Drop Reliability of Epoxy-Containing Sn-58 mass% Bi Solder Joints with Various Surface Finishes

Board-level drop reliability testing under JEDEC (Joint Electron Device Engineering Council) drop conditions was conducted for low-temperature Sn-58 mass%Bi solder joints. The effects of surface finish and the number of reflow processes (one, three, or five) on the drop reliability of the Sn-58Bi joints were evaluated. Three types of surface finishes were used including electroless nickel immersion gold (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG), and organic solderability preservatives (OSP). We successfully fabricated six types of drop test specimens with various surface finishes and bonding materials, and then performed the board-level drop tests. The board-level drop reliability was improved when using the composite Sn-58Bi solder with epoxy as compared to Sn-58Bi solder without epoxy. In addition, the reliability of the Sn-58Bi epoxy solder/OSP joint was better than both the ENIG and ENEPIG surface finishes. When the number of reflow processes increased, the drop reliability showed two different behaviors; specifically, the reliability of the ENIG and ENEPIG surface finishes decreased while the OSP surface finish improved. The existence of epoxy in the solder paste improved the drop reliability of the solder joints. After drop testing, cracks propagated differently with different surface finishes, namely between Ni3Sn4 IMCs and the Ni-P layer for the ENIG surface finish, between Ni3Sn4 IMCs and the solder bulk for the ENEPIG surface finish, and within the solder bulk for the OSP surface finish. These data show that, when it is necessary to perform repeated reflow processes for electronic components, solder joints with an OSP surface finish provide better drop test reliability results.

Correlation between microstructure and mechanical properties of Sn–Bi–X solders

A comparative analysis between the microstructure and the tensile properties of eutectic Sn–Bi, (Sn–Bi)–0.5In, and (Sn–Bi)–0.5Ni solder alloys was conducted, and the shear strengths of the alloys as Cu/solder/Cu joints were evaluated. Tensile tests were performed on bulk solder alloys, followed by post-test fracture surface analysis. The cross-sectional microstructure and shear strength of the Cu/solder/Cu joints were investigated, and the fracture surface of the joints was examined after shear tests. The results of the tensile tests indicated that the In-bearing solder alloys had the highest elongation property and the lowest ultimate tensile strength among all the solder alloys tested in this study. The shear-test results also indicated the advantage of In addition for the suppression of both the excessive growth of solder/substrate interfacial intermetallic compounds and Bi coarsening within the solder bulk, which resulted in superior shear strength in both as-reflowed and thermally aged solder joints. In addition, the dimple-like structure on the fracture surfaces of the tensile- and shear-tested In-bearing solder alloys and joints indicated the ductile property of the alloys.

Solderability of SnBi-nano Cu solder pastes and microstructure of the solder joints

The solderability of the Sn58Bi (SnBi)-nano Cu solder pastes and the microstructure of the solder joints were investigated. Experimental results indicated that the addition of the nano Cu particles in the SnBi solder paste shows limited effect on the solidus. The liquidus of the SnBi-3nano Cu solder paste was 1 °C higher than the SnBi solder paste. Solid Cu6Sn5 intermetallic particles formed in the SnBi-3nano Cu solder paste during the heating process. The Cu6Sn5 intermetallic particles decreased the mobility and wettability of the molten solder. Meanwhile, the Cu6Sn5 nano particles worked as nucleation sites for the formation of Bi grains and Sn–Bi eutectic phase during the cooling process and led to the grain refinement of the solder bulk. The SnBi-1nano Cu solder paste showed the smallest grain size in this research. Additionally, the SnBi-3nano Cu/Cu solder joint showed a eutectic microstructure of Sn–Bi system at the center of the solder bulk but a hypereutectic microstructure with polygon Bi grains near the margin in the solder bulk.

Effect of Graphene Nanoplatelets on Wetting, Microstructure, and Tensile Characteristics of Sn-3.0Ag-0.5Cu (SAC) Alloy

The effect of graphene nanoplatelets (GNPs) on the wettability, microstructure, and tensile properties of Sn-3.0Ag-0.5Cu (SAC 305) was studied using melting and casting route. The microstructure of the bulk solder is observed with a scanning electron microscope and transmission electron microscope, and the intermetallic compound (IMC) phases are identified by electron probe micro-analyzer. The solderability of the samples is assessed by spreading and wetting tests on a Cu substrate. The experimental results indicate that an addition of 0.05 wt pct GNPs in Sn-3Ag-0.5Cu solder improves the spreading and wettability significantly compared to monolithic SAC. It is also revealed that the thickness of the Ag3Sn IMCs is reduced as compared to the monolithic SAC alloy. Tensile results show that the composite solder exhibits the 13.9 pct elongation and 17 pct increase in the ultimate tensile strength when 0.05 wt pct GNPs in Sn-3Ag-0.5Cu alloy are added. This may be due to the refinement of the IMCs in composite solders compared to the same in Sn-3Ag-0.5Cu alloy brought about by the uniform dispersion of graphene nanoplatelets. It is suggested in this study that the amount of GNPs in Sn-3Ag-0.5Cu alloy should not exceed 0.05 wt pct as it may degrade the desired properties due to the agglomeration of GNPs.

Electromigration effect on Sn-58 % Bi solder joints with various substrate metallizations under current stress

The electromigration behavior of low-melting temperature Sn-58Bi (in wt%) solder joints was investigated with a high current density between 3 and 4.5 × 103 A/cm2 between 80 and 110 °C. In order to analyze the impact of various substrate metallizations on the electromigration performance of the Sn-58Bi joint, we used representative substrate metallizations including electroless nickel immersion gold (ENIG), electroless nickel electroless palladium immersion gold (ENEPIG), and organic solderability preservatives (OSP). As the applied current density increased, the time to failure (TTF) for electromigration decreased regardless of the temperature or substrate metallizations. In addition, the TTF slightly decreased with increasing temperature. The substrate metallization significantly affected the TTF for the electromigration behavior of the Sn-58Bi solder joints. The substrate metallizations for electromigration performance of the Sn-58Bi solder are ranked in the following order: OSP-Cu, ENEPIG, and ENIG. Due to the polarity effect, current stressing enhanced the fast growth of intermetallic compounds (IMCs) at the anode interface. Cracks occurred at the Ni3Sn4 + Ni3P IMC/Cu interfaces on the cathode sides in the Sn-58Bi/ENIG joint and the Sn-58Bi/ENEPIG joint; this was caused by the complete consumption of the Ni(P) layer. Alternatively, failure occurred via deformation of the bulk solder in the Sn-58Bi/OSP-Cu joint. The experimental results confirmed that the electromigration reliability of the Sn-58Bi/OSP-Cu joint was superior to those of the Sn-58Bi/ENIG or Sn-58Bi/ENEPIG joints.

Sn-Ag-Cu Solder Joints Interconnection Reliability of BGA Package during Thermal Aging and Cycling

This study illustrates test results and comparative literature data on the influence of isothermal aging and thermal cycling associated with Sn-1.0Ag-0.5Cu (SAC105) and Sn-3.0Ag-0.5Cu (SAC305) ball grid array (BGA) solder joints finished with ImAg, ENIG and ENEPIG on board side. The resulting degradation data suggests that ENIG is the best surface finish for applications involving long-term isothermal aging. ENEPIG ranks second, followed by ImAg. SAC305, with a higher relative fraction of Ag3Sn IMC within the solder, performs better than SAC105. SEM and polarized light microscope analysis show most cracks happened at package side, propagated from corner to center or even to solder bulk, which eventually cause fatigue failures. Three factors are discussed: IMC, Grain Structure and Ag3Sn particle. The continuous growth of Cu-Sn intermetallic compounds (IMC) and grains increase the risk of failure, while Ag3Sn particle seems helpful to block the crack propagation.

Effect of Al addition to bulk microstructure, IMC formation, wetting and mechanical properties of low-Ag SAC solder

The purpose of this work is to investigate the effect of various percentages of Al addition to bulk microstructure, intermetallic compound (IMC) formation, wetting and mechanical properties of Sn–0.3Ag–0.5Cu (SAC0305) solder. SAC0305, SAC-0.5Al, SAC-1Al, SAC-1.5Al and SAC-2Al solders were prepared via casting process. The solders were reflowed onto Cu substrate at 260 °C for 10 s. The composition of each solders were determined using X-Ray Fluorescent. Differential Scanning Calorimetry was used to evaluate the thermal properties while wetting balance test and spreading test were conducted to analyze the wettability. The microstructures of the bulk solder as well as the interfacial IMC layer were observed using Scanning Electron Microscope equipped with Energy Dispersive X-ray. Meanwhile, ball shear test was carried out to assess the reliability of the solder joints. Addition of Al had increased the melting and crystallization temperature of the solders but decreased the degree of undercooling. The wettability of solders decreased with the increasing amount of Al, but still in the acceptable range. Addition of Al had encouraged the formation of Ag–Al and Cu–Al IMC, suppressed the formation of Ag3Sn and Cu6Sn5 IMC and refined β-Sn dendrites. Further addition of Al above 1 wt% resulted in the formation of primary Al particles. The amount of Ag–Al and Cu–Al IMC and primary Al particles increased with increasing amount of Al. The Al-added solders have thinner IMC layer at the solder joint compared to SAC0305 for reflowed samples. The shear strength of Al added solder were higher than SAC0305 at high and low speed shear.

Effect of Intermetallic Compounds on the Thermomechanical Fatigue Life of Three-Dimensional Integrated Circuit Package Microsolder Bumps: Finite Element Analysis and Study

Currently, intermetallics (IMCs) in the solder joint are getting much attention due to their higher volume fraction in the smaller thickness interconnects. They possess different mechanical properties compared to bulk solder. Large volume fraction of IMCs may affect the mechanical behavior, thermomechanical and mechanical fatigue life and reliability of the solder interconnects due to very brittle nature compared to solder material. The question that this study is seeking to answer is how degrading IMCs are to the thermomechanical reliability of the microbumps used in three-dimensional (3D) integrated circuits (ICs) where the microsolder bumps have only a few microns of bond thicknesses. Several factors such as “squeezed out” solder geometry and IMC thickness are studied through a numerical experiment. Fatigue life is calculated using Coffin–Manson model. Results show that, though undesirable because of high likelihood of creating short circuits, squeezed out solder accumulates less inelastic strains under thermomechanical cyclic load and has higher fatigue life. The results show that with the increase of IMCs thickness in each model, the inelastic strains accumulation per cycle increases, thus decreasing the fatigue life. The drop in fatigue life tends to follow an exponential decay path. On the other hand, it was observed that plastic strain range per cycle tends to develop rapidly in Cu region with the increase in IMC thickness which calls for a consideration of Cu fatigue life more closely when the microbump contains a higher volume fraction of the IMCs. Overall, by analyzing the results, it is obvious that the presence of IMCs must be considered for microsolder bump with smaller bond thickness in fatigue life prediction model to generate more reasonable and correct results.

Wettability and Bond Shear Strength of Sn-9Zn Lead-Free Solder Alloy Reflowed on Copper Substrate

Lead-free solders are environment friendly and are in great demand for microelectronic applications. In the present study, Sn-9Zn lead free solder alloy was solidified on Cu substrate for different reflow times from 10 to 1000s. The influence of reflow time on wetting, formation of intermetallic compounds (IMCs) and bond shear strength was studied using dynamic contact angle analyzer, bond tester and scanning electron microscopy. The results indicate that, the wettability of the solder alloy increased with increase in reflow time. Microstructure study revealed the presence of Cu5Zn8 and CuZn5 IMCs at the interface. The thickness of an IMC increased with increase in the reflow time. The mean thickness of about 11μm for Cu5Zn8 IMC layer was observed for the reflow time of 1000s. The thickness of CuZn5 layer increased up to a reflow time of 100s and decreases thereafter. The bond shear strength increased up to 100s and decreased with increase in reflow time. The decrement in shear strength at higher reflow time is mainly due to excessive thickness of Cu5Zn8 IMC layer and diffusion of Sn from bulk solder towards the substrate. The excessive thick IMC layer exhibited pre micro-cracks led to the brittle failure of bond under the influence of shear stress.

Effect of elevated temperature on PCB responses and solder interconnect reliability under vibration loading

In this paper, vibration tests are conducted to investigate the influence of temperature on PCB responses. A set of combined tests of temperature and vibration is designed to evaluate solder interconnect reliability at 25°C, 65°C and 105°C. Results indicate that temperature significantly affects PCB responses, which leads to remarkable differences in vibration loading intensity. The PCB eigenfrequency shifts from 290Hz to 276Hz with an increase of test temperature from 25°C to 105°C, during which the peak strain amplitude is almost the same.Vibration reliability of solder interconnects is greatly improved with temperature rise from 25°C to 105°C. Mean time to failure (MTTF) of solder joint at 65°C and 105°C is increased by 70% and 174% respectively compared to that of solder joint at 25°C. Temperature dominates crack propagation path of solder joint during vibration test. Crack propagation path is changed from the area between intermetallic compound (IMC) layer and Cu pad to the bulk solder with temperature increase.

The effect of iron and bismuth addition on the microstructural, mechanical, and thermal properties of Sn–1Ag–0.5Cu solder alloy

This work investigates the effect of Fe and Bi addition, 0.05wt.% Fe and 1wt.% or 2wt.% Bi, on the microstructural, mechanical, and thermal properties of the low silver Sn–1Ag–0.5Cu (SAC105) solder alloy. Adding Bi and Fe to SAC105 increased ultimate tensile strength (UTS) and yield strength and decreased the total elongation which is related to solid-solution and precipitation strengthening effects by Bi in the Sn-rich phase. While 0.05wt.% Fe made few FeSn2 in the solder bulk which does not have considerable effect on mechanical properties. Scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) showed that Bi strengthen solder by scattering in the bulk of SAC105-Fe solder alloy, thereby increased β-Sn and degenerated Cu6Sn5 and Ag3Sn into a chain-like arrangement. Surface fracture of SAC–Fe–Bi solder alloys showed brittle fracture because Bi prevented β-Sn to deform and therefore Bi decreased its ductility. Finally, Bi significantly reduces the melting point and undercooling.

Microstructure and Grain Orientation Evolution in Sn-3.0Ag-0.5Cu Solder Interconnects Under Electrical Current Stressing

In situ observation was performed on cross-sections of Sn-3.0Ag-0.5Cu solder interconnects to track the evolution of microstructure and grain orientation under electrical current stressing. Cross-sections of Cu/Ni–Sn-3.0Ag-0.5Cu–Ni/Cu sandwich-structured solder interconnects were prepared by the standard metallographic method and subjected to electrical current stressing for different times. The electron backscatter diffraction technique was adopted to characterize the grain orientation and structure of the solder interconnects. The results show that metallization dissolution and intermetallic compound (IMC) migration have close relationships with the grain orientation and structure of the solder interconnects. Ni metallization dissolution at the cathode interface and IMC migration in the solder bulk can be accelerated when the c-axis of the grain is parallel to the electron flow direction, while no observable change was found when the c-axis of the grain was perpendicular to the electron flow direction. IMC can migrate along or be blocked at the grain boundary, depending on the misorientation between the current flow direction and grain boundary.

Effect of thermal aging on the electrical resistivity of Fe-added SAC105 solder alloys

The ternary Sn–1.0Ag–0.5Cu (SAC105) solders doped with a minor addition of Fe (0.1, 0.3 and 0.5wt.%) have been shown to demonstrate excellent mechanical thermal cyclic reliability. To complement the mechanical performance, the electrical resistivities of SAC105, SAC105–0.1Fe, SAC105–0.3Fe, and SAC105–0.5Fe bulk solder alloys were also characterised after thermal aging. Results showed that the overall resistivity decreased by up to 13% after the thermal aging process. Meanwhile the electrical resistivity for 0.1wt.%, 0.3wt.%, and 0.5wt.% Fe-bearing SAC105 solder alloys decreased by 25%, 11%, and 17% respectively, showing a rather stable reduction in resistivity. This can be correlated to the stabilisation of the microstructure of the Fe-added SAC 105 after thermal aging.

Reliability Comparison of Aged SAC Fine-Pitch Ball Grid Array Packages Versus Surface Finishes

Pb-free solder joints exposed to elevated isothermal temperatures for prolonged periods of time undergo microstructural and mechanical evolution, which degrades the joint electrical performance. We report the effect of isothermal aging on the reliability of Sn–Ag–Cu (SAC) assemblies on three different surface finishes [immersion Ag (ImAg), electroless Ni/immersion Au (ENIG), and electroless Ni/electroless Pd/immersion Au (ENEPIG)]. The characteristic life for SAC alloys in 10- and 15-mm ball grid array packages on ImAg degraded over 40% after an 85 °C/12 months aging and over 50% during a 125 °C/12 months aging. ENIG and ENEPIG outperformed ImAg for all aging treatments. For passive components (2512 resistors) on ImAg, the reliability performance degraded 16.7%/28.1% after a 1-year aging at 85 °C/125 °C. Failure analysis showed dramatic intermetallic binary Cu–Sn and ternary Ni–Cu–Sn film growth at the bottom of the solder joint interfaces for ImAg and ENIG/ENEPIG. For 125 °C-aged samples, the cracks appeared at the corners of both package and board sides of the solder ball and propagated along (near) the intermetallic compound location. For the case of aged fine-pitch packages, the failures tended to start at the component side solder ball corner and then propagate along an angled path downward into the bulk solder.

Study on Board-Level Drop Impact Reliability of Sn–Ag–Cu Solder Joint by Considering Strain Rate Dependent Properties of Solder

Board-level drop reliability test and analysis require dynamic characterization of the high strain-rate properties of the solder, along with solder joint failure tests. Relying on just the drop impact analysis of board-level dynamic response (i.e. G-levels and board bending strains) and an oversimplification of deformation response of solder joints (i.e., assuming elastic stress criteria) can lead to misleading conclusions in the physics-of-failure understanding in drop impact tests. In this paper, impact tests were conducted with a split Hopkinson pressure bar test system to study the dynamic response of bulk solder materials. A finite-element analysis of drop impact was conducted by considering different solder constitutive models such as the elastic and strain rate-dependent plastic model to investigate the effect of the solder constitutive model on the dynamic response of the solder joint. The important finding of this study is that the constitutive model used has a major impact on the dynamic response of solder joint stress and strain results. Theoretically, it is predicted that the strain rate-dependent plastic model gave better correlation results than the simple elastic model. A solder joint reliability characterization using the drop impact test with a clamped–clamped boundary condition was investigated for a plastic ball grid array assembly with Sn–Ag–Cu solder and two board surface finishes. In order to assess drop reliability of solder joint with different surface finishes, Charpy test was conducted to evaluate the dynamic strength of the soldered specimen using Sn–Ag–Cu solder and with or without Ni/Au plating.

Effect of temperature on interface diffusion in micro solder joint under current stressing

The effects of temperature on Cu pad consumption and intermetallic compound (IMC) growth were investigated under current stressing. The Cu/Sn–3.0Ag–0.5Cu (SAC305)/Cu solder joints were used, with a certain current density of 0.76×104A/cm2 at 100, 140, 160 and 180 °C. The constitutive equations of cathode Cu pad consumption and anode interface IMC growth are established, respectively, based on the loading time and sample temperature. The cathode Cu pad consumption (δ) increases linearly with the loading time and the consumption rate shows parabolic curve relationships with sample temperature. The anode interface IMC thickness (δ1) increased is linearly with the square root of loading time and the interface IMC growth coefficient shows parabolic curve relationship with sample temperature. The δ and δ1 have different variation laws under current stressing, due to the current facilitating larger amount of IMC formation in the bulk solder.

Microstructure Evolution and Shear Behavior of the Solder Joints for Flip-Chip LED on ENIG Substrate

The microstructure evolution and shear behavior of the solder joints for the flip-chip light-emitting diode on the electroless nickel/immersion gold (ENIG) substrate were investigated in this study. The experimental results reveal that the solder joints for the anode and cathode have different microstructures and failure characteristics during the shear test before and after isothermal aging. For the solder joints for the anode, the interfacial intermetallic compound (IMC) is (Au, Ni)Sn4 at the solder/anode interface but dendritic Ni3Sn4 grains at the solder/ENIG interface after reflow. Meanwhile, the dendritic Ni3Sn4 grains are surrounded by (Au, Ni)Sn4, which suppresses the growth of the Ni3Sn4 grains during aging. For the solder joints for the cathode, a nano scaled Au-rich layer can be observed near the cathode/solder layer interface after reflow. And the Au-rich layer moves toward the bulk solder because of the volume expansion by the transformation from Au into (Au, Ni)Sn4 during reflow and isothermal aging. Due to the diffusion of the Au atom from the Au-rich layer into the bulk solder, the Au-rich layer transformed into an interface inside of the solder joint. The average shear force of the solder joints shows a decrease from 380 gf to 250 gf because of the microstructure evolution during the isothermal aging for 1000 h at 85°C. After long time aging, the primary failure mode of the solder joint for the anode changed from the anode broken to the brittle failure of the solder layer. The delamination between the IMC layer and the insulation layer is suggested to be the dominated failure mode of the solder joint for the cathode after aging.

Reflow Optimization Process: Thermal Stress Using Numerical Analysis and Intermetallic Spallation in Backwards Compatibility Solder Joints

This study applies the reflow optimization process to investigate the phenomenon of spalling in aerospace backward compatibility solder joints when utilized with a Ni substrate [electroless nickel immersion gold, printed circuit board surface finish, and ball grid array (BGA) pads]. The backward compatibility assembly comprised a lead-free tin–silver–copper (96.5Sn3.0Ag0.5Cu/SAC 305 alloy) solder ball assembled with tin–lead paste. The soldering of the lead-free BGA was conducted using two reflow temperature profiles and two conveyor speeds under a nitrogen atmosphere in a full convection reflow oven. The optimized reflow profile has peak temperatures ranging from 237.06 to 237.09°C for 68.94–69.36 s. Scanning electron microscope reveals intermetallic compound formation with maximum thicknesses which are lower than 12 μm as per aerospace requirement. Intermetallic compound spalling of solder ball interface components was not observed. However, spalling between printed circuit board and solder bulk was noted. Nevertheless, at both reflow temperature profiles, the composition and phase distribution of the lead-free BGA ball and tin–lead solder paste were fairly uniform across all joints. This work also presents a finite element-based simulation of backward compatibility assembly in reflow process. A growing number of manufacturers are changing their components to lead-free types without notifying customers. If an aerospace production line is still running a tin–lead-based process, understanding how these lead-free components are processed with tin–lead solder becomes essential. This paper will serve as a reference for manufacturing engineers, particularly those involved in surface mount technology application.

Deformation and fracture behaviour of electroplated Sn–Bi/Cu solder joints

This work utilizes the lap shear test to investigate the shear strength and fracture behaviour of electroplated and reflowed Sn–Bi/Cu lead-free solder joints. Particular emphasis is given on the effects of reflow temperature on the interrelationships among the interfacial intermetallic compound (IMC) morphology, shear strength and the fracture mechanism of the solder joints. Single-lap shear specimens are prepared by joining two commercially pure Cu substrates with electroplated Sn–Bi solder of about 50 µm thickness. The geometry of the lap shear specimen is designed to minimize the differences between far-field and actual responses of the solder. Three reflow temperatures (200, 230 and 260 °C) are used to investigate the effects of reflow temperature on the microstructure and shear strength of the solder. The specimens are loaded to failure at a strain rate of 4 × 10−4/s. Elemental mapping of the fracture surface is performed with field emission scanning electron microscope coupled with energy dispersive X-ray spectroscopy. A reflow temperature of 200 °C yields prism-like interfacial IMC morphology, while higher reflow temperatures of 230 and 260 °C yield scallop-like interfacial IMC morphology. The shear strength and elastic energy release, U, of the solder joints increase with increasing reflow temperature. Fractographs of the failed joints suggest that the fracture mechanism is dependent on the interfacial IMC morphology, where solder joints with prism-like interfacial IMC fail within the bulk solder and solder joints with scallop-like interfacial IMC failed with a mixture of bulk and interfacial fracture.