Micro-solder Joints Research Articles

Thermal Fatigue Failure of Micro-Solder Joints in Electronic Packaging Devices: A Review

In electronic packaging products in the service process, the solder joints experience thermal fatigue due to temperature cycles, which have a significant influence on the performance of electronic products and the reliability of solder joints. In this paper, the thermal fatigue failure mechanism of solder joints in microelectronic packages, the microstructure changes of the thermal fatigue process, the influence factors on the joint fatigue life, and the simulation analysis and forecasting of thermal fatigue life are reviewed. The results show that the solder joints are heterogeneously coarsened, and this leads to fatigue cracks occurring under the elevated high-temperature phase of alternating temperature cycles. However, the thickness of the solder and the hold time in the high-temperature phase do not significantly influence the thermal fatigue. The coarsened region and the IMC layer thicken with the number of cycles, and the cracks initiate and propagate along the interface between the intermetallic compound (IMC) layer and coarsened region, eventually leading to solder joint failure. For lead-containing and lead-free solders, the lead-containing solder shows a faster fatigue crack growth rate and propagates by transgranular mode. Temperature and frequency affect the thermal fatigue life of solder joints to different degrees, and the fatigue lifetime of solder joints can be predicted through a variety of methods and simulated crack trajectories, but also through the use of a unified constitutive model and finite element analysis for prediction.

All-electrochemical synthesis of SnBi/SnAgCu structures for low-temperature formation of high-reliability solder microjoints

There is a growing interest in low temperature soldering, yet the use of low-melting point alloys for interconnection is often hindered by reliability concerns. Eutectic tin-bismuth (SnBi) alloy is being considered for soldering to Sn3Ag0.5Cu (SAC305) bumps at lower temperatures (150 °C), but the resulting hybrid structures are not very reliable. We have recently shown that superior reliability can be achieved by a post-soldering anneal to distribute up to 6 % Bi throughout the joint. However, the new approach is impractical for solder joints taller than 70 µm and the manufacturing feasibility in such cases warrants both SnBi and SAC305 to be electroplated. The present study utilizes electrochemistry to broaden the applicability of the aforementioned approach by the development of optimized routines for the separate electroplating of both structural alloys. The synthesis of the SAC305 alloy employs elemental co-deposition under strict thickness control, and the SnBi eutectic mixture may be produced likewise and deposited atop to enable low-temperature soldering. If the SnBi is deposited on separate pads, better composition control may be achieved alternatively by the successive plating of both metals, followed by reflow to facilitate their mixing. The established plating routines are further fine-tuned for the processing of an array of pillars.

Phase field simulation of interface evolution behavior of copper–tin micro-solder joints under thermal–mechanical–electrical coupling

The thermal–mechanical–electrical-diffusion strongly coupled equations are established, and the phase field method is employed to investigate the dynamic simulation of intermetallic compound (IMC) evolution at the Cu/Sn/Cu micro-solder joint interface. By comparing with experimental results on IMC evolution in micro-solder joints, this study explores the evolution law of IMCs under the influence of current, temperature, and applied load coupling. Both simulation and experimental findings indicate that higher current density promotes IMC formation, with the largest average thickness observed at a current density of 5000 A/m2. When the analog current density is 5000 A/m2, the anode IMC has a rapid growth period at 62 h, and the corresponding cathode has a rapid consumption period; when the analog current density is 4000 A/m2, the anode has the same phenomenon in 95 h; when the analog current density is 3000 A/m2, this phenomenon is not evident. The observed phenomena are consistent with the evolutionary trend of IMC from the fourth to the fifth day of the experiment, considering it may be affected by ‘electronic wind stress’ which makes the cathode IMC migrate to the anode. The average thickness of IMCs with an analog ambient temperature of 323–353 K is the largest, which is most suitable for the growth of IMCs. The experimental results also indicate that IMC growth is the fastest at 353 K. The anode IMC was inhibited by tensile stress and shear stress, but the effect on cathode was not obvious.

Residual Stress Testing and Simulation Analysis of Crystal Structures of Electronic Device Materials

In this paper, we analyze the residual stress of different components of the crystal structures of electronic device materials following exposure to elevated temperatures using a combination of experimental tests and finite element simulations. X-ray diffraction (XRD) and LXRD micro-area residual stress analyzer were employed to determine the residual strain and stress of the CBGA sample encapsulation cover and solder joints. Subsequently, the experimental data were utilized to verify the accuracy of the simulation. The discrepancy between experimental measurements and simulation outcomes of the residual stress following reflow soldering of CBGA-assembled micro-solder joints is below 14%. The analysis also included thermal warping deformation of the CBGA encapsulation cover and how the residual stress was influenced by the diameter, spacing, and height of the solder joints. The study reveals that the residual stress following reflow soldering of BGA solder joints is non-uniformly distributed within the array. Within a single solder joint, residual stress gradually increases in distribution from its middle to the point where it make contact with the PCB and chip, with the highest level of residual stress observed where the solder joint contacts the chip. The variation in material parameters, such as the coefficient of thermal expansion, is the primary cause of thermal warping deformation on the surface of CBGA encapsulation covers. Three primary factors significantly impact the residual stress on BGA solder joints: solder joint diameter, spacing, and height. The maximum value is inversely proportional to the height of the solder joints and the residual stress. Conversely, the diameter and spacing of the joints are positively proportional to the highest value. When the diameter of the solder joint is increased from 0.55 mm to 0.75 mm, the maximum residual stress in the BGA solder joint increases from 37.243 MPa to 36.835 MPa. Conversely, increasing the height of the solder joint from 0.36 mm to 0.44 mm reduces the stress from 39.776 MPa to 36.835 MPa.

Study on Cu6Sn5 morphology and grain orientation transition at the interface of (111) nt-Cu and liquid Sn

Nanotwinned copper (nt-Cu) is considered to be very desirable choice for a pad material in system-in-package (SiP) devices due to its excellent anti-electromigration and anti-element migration properties. However, complex diffusion reactions exist between nt-Cu and Sn-based solder. Especially, morphology and grain orientation transition of intermetallic compounds (IMCs) appeared after multiple reflow processes. There is a lack of deep understanding of the mechanisms of the above processes. In this paper, nt-Cu was obtained by direct-current electroplating as a pad material with (111) preferred orientation, and a two-step soldering process including dip-soldering and reflow was designed to investigate the influence of temperature on IMCs morphology. The thermodynamic and molecular dynamic simulations were used to analyze the evolution mechanism of the above process. During dip-soldering process, the size of the atomic clusters was influenced by temperature, resulting in the formation of scalloped and roof-like Cu6Sn5 grains at the solidification stage. The roof-like Cu6Sn5 solder joints had a 10.7% higher shear strength and a 15% slower growth rate compared with the scalloped Cu6Sn5 solder joints. Furthermore, roof-like Cu6Sn5 had a (-12-10) preferred orientation. However, the orientation of Cu6Sn5 grains transitioned to (0001) during reflow process. Molecular dynamics simulations showed that (0001)-oriented Cu6Sn5 grains had the lowest surface energy. The decrease in surface energy was the driving force for the grain orientation transition. This work shows that the influence of temperature on interfacial IMCs could be used to form nt-Cu/Sn/nt-Cu micro-solder joints with controlled Cu6Sn5 morphology and orientation, which can significantly enhance their shear strength and reliability.

Effect of IMC Thickness on the Mechanical Properties of Microbumps

With the continuous development of electronic products towards high density and small size, the size of interconnecting solder joints also continues to decrease. Under normal service conditions, micro-solder joints will be subjected to multiple loads at the same time, which in turn will cause many reliability problems in the package structure. This paper focuses on the growth pattern of IMC (Intermetallic compounds) during hot-press bonding and analyses its effect on the shear strength and creep properties of copper pillar micro-bumps. Firstly, the solder joints with different IMC thicknesses were obtained by adjusting the bonding parameters during the thermal compression bonding process, and the growth pattern of IMC during the thermal compression bonding process was analyzed. Secondly, shear experiments were conducted on solder joints with different IMC thicknesses, and the effects of IMC on the shear strength and shear fracture mode of solder joints were analyzed. Finally, creep experiments were performed on cylinder-shaped solder joints with different IMC thicknesses under different stress conditions. The results show that throughout the bonding process, copper atoms migrate under the combined effect of temperature gradient (JTM) and concentration gradient (JCM), and IMC shows asymmetric growth, and the growth rate of IMC gradually decreases with the increase of bonding holding time. Under the same temperature conditions, the shear strength of the solder joint shows a trend of increasing and then decreasing with the increase of holding time. In addition, with the increase of IMC thickness, the shear fracture mode of solder joints changes from ductile fracture in the initial stage to tough-brittle combination fracture and finally evolves to brittle fracture. With the continuous increase of stress, the steady-state creep strain rate of all solder joints increases, and the fracture location tends to be closer to the intersection of the solder and the copper pillar from inside the solder joint. With the increase of IMC thickness, the steady-state creep strain rate of solder joints shows a trend of decreasing and then increasing.

Phase field study on the effect of roughness on interfacial intermetallic compounds of micro-solder joints under multifield coupling

In order to investigate the influence of roughness on the evolution of interfacial intermetallic compounds (IMCs) and stress states at the micro-solder joints under the strongly coupled thermo-mechanical-electric diffusion, the two-dimensional model of the rough interface for the Cu/Sn/Cu sandwich micro-solder joints was established using phase field method. Parameter A as the evaluation parameter of the interface roughness was used to represent the contour arithmetic mean deviation Ra. The evolutions of the average thickness of the interfacial IMCs and stress distribution at the Cu/Sn/Cu micro-solder joints were investigated with the variation of A. The thickness of interfacial IMCs in anode reached its maximum when A was 0, while obtained the minimum when A was 9. In cathode, the average thickness of the interfacial IMCs at A = 0 was thicker than that at A = 1, 3, 5, but thinner than that at A = 7 and 9. In other words, the growth of the interfacial IMCs was inhibited by the interface roughness in anode. While in cathode, the interfacial roughness inhibited the growth of the interfacial IMCs when 0 ≤ A ≤ 5, but promoted the growth of the interfacial IMCs when 7 ≤ A ≤ 9.

Molecular dynamics simulation and experimental study of tin growth in SAC lead-free microsolder joints under thermo-mechanical-electrical coupling

Sn-Ag-Cu system lead-free solders are considered to be promising solder series. The growth of Sn whiskers on the solder surface is an important factor that threatens the reliability of electronic products. In this study, the IMC evolution and whiskers growth of Sn-0.3Ag-0.7Cu (SAC0307) microsolder joints under thermal-mechanical-electrical coupling were investigated by the combination of molecular dynamics simulation and experimental methods. The simulation and experimental results revealed that the electric field intensity had the greatest influence on the atomic diffusion. When the electric field intensity was 0.004 V/Å, the diffusion speed of copper atom to solder reached the maximum, and the intermetallic compounds Cu6Sn5 and Cu3Sn were formed at the interface, which provided the long-term driving force for the growth of tin whiskers at the weak oxide film on the surface. Tin whiskers were inclined to grow at the anode, while holes and cracks were tend to produce at the cathode. When the external pressure stress was 0.03 nN, most of tin atoms were precipitated. When the temperature was 393 K, the whiskers slowly grew to 3.3 µm in length at the growth rate of 0.0182 Å/s. The results can provide the basis for the growth mechanism of tin whisker in real service situations.

Microstructure regulation of Sn-3.0Ag-0.5Cu micro-solder joints by hybridizing nano-TiO2

The intermetallic compound (IMC) formation kinetics of Sn-3.0Ag-0.5Cu micro-solder joints were studied under 100 °C, 120 °C, and 150 °C isothermal aging test. The addition of nano-TiO2 has a great influence on interface reaction, significantly reducing the thickness of IMC. The thickness of IMC layer is able to be regulated by controlling the doping concentration, and the minimum value was achieved with 0.1 wt% doping. The activation energy of IMC formation was approved to be raised after the composite of nano-TiO2, which can further decrease the atomic diffusion rate, and eventually block the overgrowth of IMC. Grain boundary pinning mechanism is regarded as the most important mechanism for inhibiting the growth of IMC because of nano-TiO2 addition.

Characteristics of Cracking Failure in Microbump Joints for 3D Chip-on-Chip Interconnections under Drop Impact.

With the rapid development of microelectronics packaging and integration, the failure risk of micro-solder joints in packaging structure caused by impact load has been increasingly concerning. However, the failure mechanism and reliability performance of a Cu-pillar-based microbump joint can use little of the existing research on board-level solder joints as reference, due to the downscaling and joint structure evolution. In this study, to investigate the cracking behavior of microbump joints targeted at chip-on-chip (CoC) stacked interconnections, the CoC test samples were subjected to repeated drop tests to reveal the crack morphology. It was found that the crack causing the microbump failure first initiated at the interface between the intermetallic compound (IMC) layer and the solder, propagated along the interface for a certain length, and then deflected into the solder matrix. To further explore the crack propagation mechanism, stress intensity factor (SIF) of the crack tip at the interface between IMC and solder was calculated by contour integral method, and the effects of solder thickness and crack length were also quantitatively analyzed and combined with the crack deflection criterion. By combining the SIF with the fracture toughness of the solder–Ni interface and the solder matrix, a criterion for crack deflecting from the original propagating path was established, which can be used for prediction of critical crack length and deflection angle for the initiation of crack deflection. Finally, the relationship between solder thickness and critical deflection length and deflection angle of main crack was verified by a board level drop test, and the influence of grain structure in solder matrix on actual failure lifetime was briefly discussed.

Micro Solder Joint Reliability and Warpage Investigations of Extremely Thin Double-Layered Stacked-Chip Packaging

Abstract To overcome the limited operational speed for nanoscaled transistors, scaling electronic devices to small and thin packaging and high-density arrangements have become the technological mainstream in designing versatile packaging architectures. Among these, a promising candidate is a three-dimensional integrated circuit (3D-IC) package due to its excellent capability of heterogeneous integration. However, sequential reliability is a troublesome concern given the complex packaging structure, especially for the assembly of microsolder joints. To address this issue, we propose a double-layered, thin stacked chip package under the application of temperature cycling load. The packaging warpage and creep impact of SnAg microsolder joints on their fatigue lifespan are examined separately. Nonlinear material/geometry finite element analysis (FEA) is used on important designed factors, including the elastic modulus of underfill, chip thickness, and the radius and pitch of through silicon via (TSV). The simulated results indicate that the best fatigue lifetime of SnAg microsolder joint can be achieved at 10 μm of each chip thickness, 230 μm and 5 μm for TSV pitch and radius within the examined designed extent. Moreover, a hard underfill material requires consideration when the mounted chips thicken. Consequently, reliability significantly improves by dispersing thermomechanical stress/strain of the SnAg microjoints to neighboring underfill and related packaging components, especially for large TSV array spacing.

Study of electromigration in Sn-Ag-Cu micro solder joint with Ni interfacial layer

This paper reports the extreme sensitivity of the microstructure evolution in Sn-Ag-Cu (SAC) micro-solder joint, consisting of 15–20 µm thick solder alloy and electrolytic Ni plated Cu electrodes, to electromigration (EM) conditions resulting from a kinetic competition between the growth of Ni3Sn4 intermetallic compound (IMC) and EM of Sn in solder matrix. Microstructural analysis of samples tested at various conditions reveals that the voiding in the solder joint develops to the full extent only when the EM rate of Sn in the solder alloy is sufficiently greater than the growth rate of Ni3Sn4 IMC because the IMC phase is EM-inactive. The decisive evidence for such behavior is found from the samples tested at 160 and 170 °C under 35 kA/cm2. The samples tested at 170 °C show the classic solder joint microstructure that typically develops under the influence of EM, that is the exaggerated growth of Ni3Sn4 at the anode as a result of Ni-EM and voids at the cathode interface of the joint as a result of Sn-EM. Contrarily, such characteristic features are not found in samples tested at 160 °C, instead, the joint is fully converted to Ni3Sn4 without sign of its biased growth and voiding activities. The near absence of biased EM microstructure at 160 °C is believed to result from the stress-gradient developed by Sn EM counteracting the EM force on Ni. No such effect is at work at higher temperatures due to the significantly higher initial Sn EM rate which leads to voids and reduces the stress gradient.

A Model for Intermetallic Growth in Thin Sn Joints Between Cu Substrates: Application to Solder Microjoints

Thin Sn-based solder microjoints, typically less than 25 μm thick, attached to Cu bond pads on both sides in three-dimensional electronic packages are often converted completely into intermetallic compounds (IMC) during service or accelerated testing. The rate of IMC growth and the proportions of the IMCs relative to the unreacted Sn in the joint have strong implications on solder joint reliability, and hence on the performance of the entire package. Although IMC growth at the interface between two semi-infinite slabs of different components (e.g., Cu and Sn) has been previously modeled, to date, no analytical treatment of intermetallic growth at the interfaces of a thin joint (Sn) sandwiched between two massive substrates (Cu), with concurrent growth of two different intermetallics (Cu6Sn5 and Cu3Sn) at each interface, has been proposed. In this work, a multicomponent diffusion-based model is developed for IMC growth in a thin joint between two semi-infinite substrates, with each interface possessing a multiphase structure. Because the Sn layer is thin, after prolonged aging, Sn may be completely depleted by reaction with Cu from the substrates, with the entire joint becoming IMC. Considering the joint as a finite source of Sn and the substrates as semi-infinite sources of Cu, general solutions of Fick’s second law of diffusion were applied to obtain the concentration of Cu within each phase. Values of interdiffusion coefficients available in literature were used to solve for the interfacial position parameters in the multiphase system, and thence to determine the instantaneous thicknesses of Cu6Sn5 (η), Cu3Sn (e), and Sn. The predictions of the model were validated against experimental data on the growth of IMCs in thin solder joints at 180°C and 210°C. Finally, the model was used to simulate the effects of aging temperature, joint thickness, and initial IMC thicknesses on the growth kinetics of IMCs during isothermal aging.

Effect of Ni content on the creep properties of Cu/Sn–0.3Ag–0.7Cu/Cu solder micro-joints

The tensile creep properties of line-type Cu/Sn–0.3Ag–0.7Cu(SAC0307)–xNi (x = 0, 0.02, 0.05 and 0.10 wt%)/Cu solder micro-joints were comparatively investigated via dynamic mechanical analysis. The effect of Ni content on the creep behaviour of the solder micro-joints was analysed and discussed. The creep behaviour of the joints at various temperatures and applied stresses was described by a power law. The results showed that the creep lifetime, creep activation energy and creep stress exponent of the Ni-containing solder micro-joints were greater than those of the Ni-free joint. Hence, adding Ni to the SAC0307 solder can enhance the creep resistance of the solder joints. The creep activation energy and the creep stress exponent of the solder micro-joints at 353–398 K and under a tensile stress of 8–15 MPa were 82.44–93.36 kJ/mol and 4.35–4.75, respectively. As the Ni content increased, the creep activation energy and the creep stress exponent first increased and then decreased. The Cu/SAC0307–0.05Ni/Cu solder micro-joint exhibited the strongest creep resistance among the tested joints. The creep deformation mechanisms of all solder micro-joints were mainly controlled by dislocation climbing, and the main creep failure mode was mixed brittle/ductile fracture.

Miniaturization, Triple-Line Effects, and Reactive Wetting of Microsolder Interfaces.

While reactive microsolder joints are of ubiquitous importance in modern electronics, the effects of joint miniaturization on wetting behavior remain largely unexplored. We elucidate this fundamental question of scalability by investigating the wettability of eutectic SnPb solder on Cu and Ni-electrodeposited metallization strips of varying widths. Contact angles are presented in dependence of the metallization width which is varied from 3 mm down to ∼100 μm. The measured angles clearly increase with decreasing metallization width. Based on the measurements and by modifying Young's equation, it is shown that the behavior of the wetting angle can be quantitatively understood with an "effective" triple-line energy of ϵt = (753 ± 31) × 10-9J/m for SnPb on Cu. The interpretation of this energy term is discussed in relation to the forming intermetallic phase and the ensuing surface roughness. A remarkable similarity between the experimentally observed size dependence and the crossed-groove perturbation model of Huh and Mason demonstrates that the rough intermetallic phase induces wetting hysteresis such that it is quantitatively well described by an effective triple-line energy.

Microstructure evolution of Au/SnSb-CuNiAg/(Au)Ni during high temperature aging

PurposeThe purpose of this paper is to investigate the morphology evolution and the composition transformation of Au-Sn intermetallic compounds (IMCs) of the new Au/Sn-5Sb-1Cu-0.1Ni-0.1Ag/(Au)Ni solder joint during the high temperature aging.Design/methodology/approachSn-5Sb-1Cu-0.1Ni-0.1Ag solder balls (500 µm in diameter), heat sink with structure of 7.4 µm Au layer on 5 µm Ni-plated Cu alloy and Si chip with 5.16 µm plated Au were used to fabricate micro-solder joints. The joints were performed in a furnace at 150°C for 150, 250 and 350 h aging. The samples were polished and deep etched before analyzed by metallographic microscope and scanning electron microscopy, respectively. Energy dispersive x-ray spectroscopy was used to identify the composition of the IMCs.Findingsß-(Au,Ni,Cu)10Sn phase is formed during the soldering process. The IMCs evolution has two periods during the aging. The first is theξ-(Au,Ni,Cu)5Sn,ξ-(Au,Cu)5Sn andδ-AuSn were formed and grew to form a full-compound joint after about 150 h aging. The second is the conversion of the full-compound joint. The IMCs converted toξ′ phase when the aging time extends to 250 h, and transformed toε-(Au,Ni,Cu)Sn2andη-(Au,Ni,Cu)Sn4after 350 h aging. The thicker gold layer and thinner solder joint can promote the growth of the IMCs.ß-(Au,Ni,Cu)10Sn emerged in Au/SnSb-CuNiAg/(Au)Ni in this research, which is not usually found.Originality/valueThe results in this study have a significant meaning for the application of the new Sn-5Sb-1Cu-0.1Ni-0.1Ag in harsh conditions.

Structure-induced metastable phase transformation in Cu6Sn5 intermetallics

The retarded η → η′ phase transformation in Cu6Sn5 has become a potential risk to the service reliability of micro-solder joints. In this paper, both Cu6Sn5 crystal and powder samples were fabricated to explore the phase transformation behaviour of Cu6Sn5. Because of the difference in stress accumulation, η′ ↔ ηc ↔ η and η′ ↔ η phase transformations occurred in the Cu6Sn5 core layers and surface layers, respectively. Moreover, formation of the metastable intermediate ηc phase was confirmed to be both stress induced and structure dependent. In addition, by decreasing the crystal size or using a proper isothermal treatment, formation of the metastable phase in Cu6Sn5 can be significantly reduced. Our study can provide an understanding of the phase transformation behaviour of Cu6Sn5 and may provide visual data for solving the risk of retarded Cu6Sn5 phase transformation.

Effect of Intermetallic Content on Shear Deformation of Thin Sn-3.0Ag-0.5Cu Solder Micro-joints Between Copper Substrates

In 3D electronic packages, stacked dies are connected vertically using through-silicon vias and solder micro-bumps, which are typically between 1 μm and 50 μm thick. Solder micro-joints undergo significant shear deformation due to various loading conditions, which can occur during usage of microelectronic devices, such as thermal cycling, mechanical bending, and drop impact. A limited amount of work has been done in shear deformation and failure mechanism of these joints. To explore this, 25-μm-thick joints of SAC305 solder between two Cu substrates were tested, containing three different Cu6Sn5-to-Cu3Sn ratios, in shear at strain rates from 1 s−1 to 100 s−1. The joint shear strength is correlated with observed failure mechanisms such as Sn, Cu6Sn5, Cu3Sn and Cu6Sn5/Cu3Sn interface failure. The growth kinetics of intermetallic compounds (IMCs) in thin Sn-3Ag-0.5Cu joints attached to Cu substrates have been analyzed, and empirical kinetic laws for the growth of Cu6Sn5 and Cu3Sn in thin joints are reported. By combining the shear deformation results, we infer that increased IMC content due to heat treatment deteriorates the mechanical properties of the joint due to the presence of disconnected incipient micro-cracks. Deformation and damage are controlled by the intermetallics, and not the strain-rate sensitive solder for the aged samples. Both Cu6Sn5 and the Cu6Sn5/Cu3Sn interface fracture are the dominant mechanisms with increasing aging under the shear deformation.

In Situ Electromigration in Cu-Sn and Ni-Sn Critical Solder Length for Three-Dimensional Integrated Circuits

An in situ electromigration study has been conducted on U-groove Cu/Sn-3.5Ag/Cu and Ni/Sn-3.5Ag/Ni sandwich structures; the results were used to simulate microsolder joints passing current density of 1 × 104 A/cm2 at 150°C. The solder gap was only 15 μm, shorter than the critical length of Sn-3.5Ag solder. Backstress was proved to exist at critical solder lengths and to influence the electromigration mechanism. Theoretical calculations of the diffusivity of Cu and Ni in Sn solder indicated that the degree to which the dominant diffusion species (Cu or Ni atoms) diffused through the solder line is retarded by the backstress effect. The morphologies of intermetallic compounds (IMCs) were observed, and the grain boundaries in Sn solder were measured using electron backscatter diffraction to determine the kinetics of intermetallic growth. The results reveal that the unique electromigration characteristics of microbump joints, including the diffusivity, morphology, and backstress, can be determined. The retardation of atomic migration improves the reliability against electromigration.

Effect of Au-Sn IMCs’ formation and morphologies on shear properties of laser reflowed micro-solder joints

Purpose – The purpose of this paper is to investigate the effect of typical morphologies of Au-Sn IMCs (intermetallic compounds) at the interfaces of solder and pads on shear properties of laser reflowed micro-solder joints. Design/methodology/approach – Sn-2.0Ag-0.75Cu-3.0Bi (SnAgCuBi) solder balls (120 μm in diameter), pads with 0.1, 0.5, 0.9 or 4.0 μm thickness of Au surface finish, and different laser input energies were utilized to fabricate micro-solder joints with Au-Sn IMCs having different typical morphologies. The joints were performed by a shear test through a DAGE bond test system. Fracture surfaces of the joints were analyzed by scanning electron microscopy and energy-dispersive X-ray spectrometry to identify fracture modes and locations. Findings – Morphologies of Au-Sn IMCs would affect shear properties of the joints remarkably. When needle-like AuSn4 IMCs formed at the interfaces of solder and pads, almost entire surfaces presented the manner of ductile fracture. Moreover, shear forces of this kind of solder joints were higher than those of joints without Au-Sn IMCs or with a nearly continuous/continuous Au-Sn IMCs layer. The reason was that the shear performance of the solder joints with needle-like AuSn4 IMCs was enhanced by an interlocking effect between solder and needle-like AuSn4 IMCs. As a nearly continuous or continuous Au-Sn IMCs layer formed, the fracture surfaces presented more character of brittle than ductile fracture. However, if an Au layer still remained under Au-Sn IMCs, the shear performance of the joints would be enhanced. Originality/value – The results in this study can be used to optimize microstructures and shear properties of laser reflowed micro-solder joints.