Sn-based Solder Joints Research Articles

Influence of Sn grain orientation on mean-time-to-failure equation for microbumps in 3D IC technology

The mean-time-to-failure (MTTF) associated with microbumps is a pivotal factor influencing the reliability of cutting-edge consumer electronic devices. Within Sn-based solder joints, the orientation of Sn grains significantly impacts the diffusion velocity of Cu and Ni atoms, thereby impacting MTTF. While a large solder joint encompassing numerous Sn grains can be treated as isotropic, the scenario shifts in a small microbump with just one Sn grain, rendering it anisotropic. In such cases, the Sn grain orientation emerges as a dominant factor dictating the microbump's longevity. Herein, we investigated the influence of Sn grain orientation on MTTF of electromigration, thermomigration, and thermal aging. A new parameter, α, was introduced, which signifies the angle between the c-axis of the Sn grain and the direction of external stressing.

Impact of crystalline orientation on Cu–Cu solid-state bonding behavior by molecular dynamics simulations

High-density electronics are hindered by the constraints of Sn-based solder joints, necessitating the exploration of Cu–Cu solid-state bonding. However, current bonding methods are expensive and time-consuming; therefore, understanding the Cu–Cu bonding mechanism is crucial for optimization. This study utilizes molecular dynamics (MD) simulation to elucidate the Cu–Cu solid-state bonding behavior, focusing on interfacial densification and diffusion phenomena. Furthermore, it highlights the influence of crystal orientation on the interfacial bonding behavior. To analyze the impact of crystal orientation, monocrystalline Cu slabs with a simplified periodic surface structure were employed to replicate surface roughness and subsequently bonded at a specific temperature. The results indicate the critical influence of crystalline orientations on the bonding process: identical orientations result in slower densification at the interface, whereas misoriented orientations significantly accelerate it. This effect, attributed to the grain boundary (GB) structures formed owing to misorientation, suggests a central role for GB diffusion in bonding progression. Diffusion coefficients calculated using the mean square displacement (MSD) confirmed these findings and exhibited significantly larger values for misoriented joints. Additionally, the simulations reveal an activation energy for GB diffusion that is lower than conventional values, highlighting the impact of the crystallographic orientation and voids at the bonding interface. Our research elucidates the role of crystalline orientation in diffusion phenomena at bonding interfaces, offering valuable implications for optimizing bonding-based manufacturing processes.

Current-induced solder evolution and mechanical property of Sn-3.0Ag-0.5Cu solder joints under thermal shock condition

Solder joint reliability suffers great challenges due to high current density and miniature solder bump diameter of electronic packages at cryogenic temperature. To tackle these issues, the solder microstructure evolution and the corresponding failure mechanism should be emphasized. In this study, the current-induced microstructure characteristics and mechanical behavior of Sn-3.0Ag-0.5Cu (SAC305) solder joints during thermal shock process was thoroughly addressed over cycles by combining diffusional, electrical, thermal and mechanical features. This result verified that the combining method of thermal shock and electromigration (EM) contributed to the atom diffusion and high thermal stress formation, further causing intermetallic compound (IMC) growth, the repaid dissolution of Cu pad and high thermal stress of the solder joints. High stress induced by either the thermal expansion mismatch of different component, large temperature change (ΔT =346 ℃) and severe lattice distortion became the direct reason for twins and cracks formation of SAC305 solder joints. The combination of crack propagated along the interface and the quick dissolution of Cu substrate at the corner accelerated the eventually failure of the solder joints. Moreover, as the thermal chock cycles was extended, the initiation and propagation of cracks at the cathode side weaken the cathodic shear strength. High stress-induced twin formation at the interface effectively moderated the shear strength degradation due to anode IMC growth after 9 cycles. This study contributed to thoroughly grasp the failure mechanism of the solder joints and design the twin-strengthened Sn-based solder joints under the coupling effects of extreme temperature variation and current stressing.

Study of early-stage thermal fatigue behavior of Sn-based Cu-cored BGA solder joints

Study of early-stage thermal fatigue behavior of Sn-based Cu-cored BGA solder joints

Preparation and performance of Sn-based composite solder joints by solid-liquid low-temperature solder bonding technology

In the era of big data, the advantages of 3D packaging are becoming increasingly significant. In addition, the mismatch between the coefficient of thermal expansion (CTE) of the PCB and BGA components leads to dynamic warpage of the system, and the expansion and innovation of low-temperature soldering have been a new trend. We developed a fully mixed solder joint of a SAC305(Sn-3.0Ag-0.5Cu wt.%) solder ball and Sn–58Bi(wt.%) solder paste by solid-liquid low-temperature solder bonding technology. The mixing degree and diffusion process of the two were calculated and simulated by mathematical calculation and COMSOL Multiphysics. The results show that a completely mixed solder joint can be obtained after reflow at 210 °C for 5 min. The average shear strength of the composite solder joints was 63.68 ± 1.12 MPa, which was about twice that of the single SAC305 and Sn58Bi solder joints, and the shear fracture was a composite mode of brittle and ductile fracture, which appeared in the IMC and the solder, respectively. After complete mixing, the solder wettability angle was less than 26.68°, with outstanding wetting properties. No phase transition occurred after aging, the distribution of each phase remained uniform, and no enrichment of the brittle phase was observed at the interface. The shear strength decreased slightly with aging time, and the reduction value was less than 10 MPa, which has good reliability.

Effects of Nanoparticle Addition on the Reliability of Sn-Based Pb-Free Solder Joints Under Various Conditions: A Review

In order to satisfy the needs of electronic industries for high-reliability joints, different kinds of nanoparticles were incorporated into conventional Pb-free solders to enhance the performance of solder joints by researchers for decades. It is reported that doping certain nanoparticles to the solder can promote the wettability and strength of the solder, refine the grain size of solder and intermetallic compounds, and inhibit IMC layer growth. In this paper, the effects and the corresponding mechanism of nanoparticle addition on the reliability of Sn-based Pb-free solder joints in various conditions are comprehensively analyzed and summarized such as under thermal aging, thermal cycling, thermal shock, electromigration and so on. In addition, directions for future study about the reliability of nanoparticles-doped solder joints were discussed, so as to provide theoretical support for developing high-reliability nanocomposite solder joints.

A Review on Phase Field Modeling for Formation of η-Cu6Sn5 Intermetallic

Formation of intermetallic compounds (IMCs) exhibits remarkable microstructural features and provides opportunities for microstructure control of microelectronic interconnects. Excessive formation of brittle IMCs at the Cu/Sn interface such as η-Cu6Sn5 can deteriorate the reliability and in turn lead to solder joint failure in the Pb-free Sn-based solder joints. Phase field method is a versatile tool for prediction of the mesoscopic structure evolution in solders, which does not require tracking interfaces. The relationships between the microstructures, reliability and wettability were widely investigated, and several formation and growth mechanisms were also proposed for η-Cu6Sn5. In this paper, the current research works are reviewed and the prospective of the application of phase field method in the formation of η-Cu6Sn5 are discussed. Combined phase field simulations hold great promise in modeling the formation kinetics of IMCs with complex microstructural and chemical interactions.

Shear deformation behavior and failure mechanisms of graphene reinforced Sn-based solder joints bonded by transient current

Graphene as a reinforcement is expected to improve the shear strength of solder joint. However, little is known about the deformation behavior and mechanisms of composite solder joints. In this study, sandwich joints are prepared using graphene nanosheets-reinforced Sn-Ag-Cu composite solder (GNSs/SAC) by transient current bonding technology in order to obtain dispersed graphene. Interrupted shear tests combined with orientation image microscopy are conducted at different strain to observe the morphology and crystal orientation evolution of solder joints in situ. Some twins rotate, and the orientation begins to spread during shear, while the orientation of large Sn grain shows a minimal change. Cracks tend to initiate and propagate along the boundary. The failure mechanisms of the solder joint is determined. Molecular dynamics results show that the strain concentration at the boundary. Continuous shear tests show that the shear strength was improved by approximately 50.7% with the addition of 0.1 wt% graphene. Based on microstructure analysis and calculation, load transfer is identified as the dominant strengthening mechanism for GNSs/SAC alloys, followed by Orowan system. Graphene does not refine the grain size of solder joints, and dislocation multiplication from thermal mismatch does not strengthen them.

Process dependence and nucleus models of β-Sn grains in SAC305 freestanding solder balls and BGA solder joints

• Relationship between β-Sn grain morphology and process parameters was studied systematically. • Two nucleus models were proposed to explain the formation of β-Sn morphologies. • Relationship between spatial position of nucleus models and β-Sn orientation was established. • Large θ angle β-Sn grains were theoretically and experimentally proved to have a formation probability higher than 77 %. Due to the anisotropy characteristic of β-Sn grain, the microstructure and orientation of Sn-based solder joints are closely related to the reliability of the solder joints. In this paper, the effects of cooling rate and solder ball size on β-Sn morphology and orientation in Sn-3.0Ag-0.5Cu (SAC305) freestanding solder balls and SAC305/Cu ball grid array (BGA) solder joints were firstly investigated to reveal the process dependence of β-Sn grain features. Three dominant solidified morphologies, i.e., interlaced & beach ball-like grains with three orientations, multiple twin grains with three orientations and single grain with one orientation, were found in the solder balls/joints. Solder ball size showed more significant impact than cooling rate on the morphologies that interlaced & beach ball-like grains and multiple grains were mainly presented in small sized solder balls/joints (≤ 400 μm) and single grain was mainly presented in large sized solder balls/joints (≥ 700 μm). These three solidified morphologies were nucleated and developed from two nucleus model. That is, the interlaced & beach ball-like grains and multiple twin grains morphologies were attributed to {101} nucleus model, while the single grain morphology was attributed to single grain nucleus model. Regardless of whether the solder balls/joints nucleated and solidified based on the {101} or single grain nucleus model, large θ angle β-Sn grains were proved to have a formation probability higher than 77 % both theoretically and experimentally, and this formation probability had little relationship with interfacial Cu 6 Sn 5 grains, cooling rate and solder ball size.

Controlling the distribution of porosity during transient liquid phase bonding of Sn-based solder joint

The rapid growth of (Cu,Ni)6Sn5 intermetallic compounds (IMCs) when Sn reacts with Cu containing 6 wt%Ni (Cu-6Ni) offers huge potential for resolving the lengthy transient liquid phase (TLP) processing time. However, the uncontrolled formation of voids at the centre of the joint remains a serious issue. Therefore, we have studied the influence of different Ni concentrations (0, 2 and 6 wt%) in the Cu substrate and the relative location of the porosity when reacting with Sn-0.7Cu solder in the sandwich structure of the joint. Through in-situ X-ray microradiography, the changes in the void positions and the reaction time required to form the IMC joints was interpreted with reference to growth kinetics equations. The results indicate that the different growth rates of (Cu,Ni)6Sn5 and Cu6Sn5 IMCs due to the different Ni concentrations of the Cu-Ni substrates can control the location of the porosity and influence the shear strength of the TLP lap joint. This discovery is a potential gateway to developing reliable electronic interconnects for the power device industry.

Revealing the ductile-to-brittle transition mechanism in polycrystalline body-centered tetragonal tin (Sn) for cryogenic electronics

The inherent ductile-to-brittle transition (DBT) of body-centered tetragonal Sn at cryogenic temperatures restricts the use of Sn-based solders in the interconnection of cryogenic electronics, but little is known about the deformation behaviors accompanying with the transition and the underlying transition mechanism. In this work, the deformation features before cryogenic brittle fracture and the DBT mechanism in polycrystalline Sn were studied through uniaxial tensile experiments at different temperatures. Compared to the softening process stimulated by dynamic recovery and dynamic recrystallization before ductile fracture at room temperature (~293 K), a high strain hardening rate (~5% of the shear modulus) is maintained during the linear hardening period preceding brittle fracture at the liquid nitrogen temperature (~77 K) due to the pronounced intersecting of {301} deformation twins. But the irreconcilable velocity difference between dislocation glide (~3 µm/s) and twin thickening (~10 µm/s) at 77 K leads to a premature brittle fracture in the midst of the linear hardening, and indeed the DBT. The suggested specific DBT mechanism is substantiated by the fact that a significant increase in the velocity (~1500 µm/s) with the increasing temperature (123 K) allows the dislocation slip to readily accommodate the shear strains due to {301} twin thickening at the grain boundaries, thereby resulting in ductile fracture rather than brittle fracture. This deep understanding about the DBT in polycrystalline Sn may help forge a new path to design ductile and strong Sn-based solders and solder joints for cryogenic electronics by deformation twinning.

Finite element method modeling of temperature gradient-induced Cu atomic thermomigration in Cu/Sn/Cu micro solder joint

Predicting the growth of interfacial intermetallic compounds (IMCs) under temperature gradient (TG)-induced atomic thermomigration (TM) in micro solder joint is an important aspect for improving the reliability of packaged devices. In this paper, we propose a new model based on finite element method (FEM) to simulate the Cu concentration distribution and the diffusion flux for the cold end IMC formation in Cu/Sn/Cu micro solder joints under TG with different preferred β-Sn grain orientation. It is confirmed that the orientation of β-Sn grains had a great effect on the growth rate of the cold end IMC. Meanwhile, the orientation of β-Sn grains also affected the time when the Cu concentration distribution reached a steady state, but not the final Cu concentration values at the same positions after reaching the steady state. By comparing the simulation results with the experimental data from literature, the proposed FEM model was proved to be reliable for analyzing the atomic concentration field and the diffusion flux for IMC formation in Sn-based micro solder joint under TG.

Research Progresses on Reliability of Sn-based Solder Joints in Extreme Temperature Environment

Research Progresses on Reliability of Sn-based Solder Joints in Extreme Temperature Environment

The Effect of Grain Orientation of β-Sn on Copper Pillar Solder Joints during Electromigration

The grain orientation of Sn-based solder joints on copper pillars under the combined action of electron wind force and temperature gradient greatly affects their electromigration damage. The copper pillars with Sn-1.8Ag lead-free solder on the top was subjected to a current density of 1.5 × 104 A/cm2 at 125 °C to study the electromigration behaviors. The grain orientation was characterized by scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD) detector. Metal dissolution and voids formation in the cathode as well as massive intermetallic compounds(IMC) accumulation in the anode were observed after electromigration. Closer examination of solder joints revealed that the Sn grain whose c-axis perpendicular to electric current may have retarded Cu diffusion to anode and IMC accumulation. In addition, the newly formed Cu6Sn5 exhibited preferred orientation related to the electric current direction.

Effects of size and cooling rate on solidification behavior of freestanding Sn-3.0Ag-0.5Cu solder balls

The diversity of electronic product types needs variable solder joint sizes and processing conditions, which can lead to the complexity and diversity of solidified microstructure in Sn-based solder joints. Due to the significant anisotropy of β-Sn in physical and mechanical properties, the solidified microstructure of the Sn-based solders after soldering should be carefully controlled. In this paper, Sn-3.0Ag-0.5Cu freestanding solder balls were used to understand the effects of size and cooling rate on the solidification behavior and microstructure. Larger undercooling, ∆T , was detected for smaller solder balls or under larger cooling rate. With the solder ball size increased from small level of 100 and 200 μm to medium level of 400 μm and then to large level of 700 μm and 1200 μm, the dominant solidified microstructure transformed from interlaced grain & beach ball to randomness and then to single grain. Correspondingly, three nucleation & growth models, i.e., single grain ( ∆T ≤ 20 K), cyclic twin (20 K ≤ ∆T ≤ 25 K) and interlaced grain & beach ball (25 K ≤ ∆T ), were proposed to explain the solidification behavior of β-Sn. A mapping of the most likely formed β-Sn microstructure regarding solder ball size and cooling rate was established to not only predict the major microstructure of solder balls but also optimize the process parameters for solder ball fabrication. • Effects of size and cooling rate on undercooling, secondary dendrite arms and grain features of β-Sn were studied systematically. • A mapping of the most likely formed β-Sn microstructure regarding different process parameters was established. • Three nucleation & growth models were proposed to explain the solidification behavior and microstructure of freestanding SAC solder balls.

Rapid fabrication of Cu/40-μm thick full Cu3Sn/Cu joints by applying pulsed high frequency electromagnetic field for high power electronics

With the application of wide bandgap semiconductors for high power electronics, the joints are inevitably faced with a trade-off between withstanding high temperature environments and remaining low bonding temperatures. In this study, a full Cu3Sn intermetallic compounds layer with thickness of 40 μm, which can survive at 679 °C, were obtained between Cu plates using pulsed High Frequency Electromagnetic Field (HFEF) within 10 min at ambient temperature. Output power and frequency of the HFEF was 6 kW and 530 kHz, respectively. In contrast to common Sn-based solder joints, the joints prepared through HFEF consisted predominately of Cu3Sn, because both the horizontal and vertical temperature gradients and the eddy currents within the joints strongly promoted the 3D diffusion of copper atoms. With increasing HFEF exposure time, the Cu3Sn changed from an irregular needle-like to smoothly scalloped or columnar morphology. The HFEF approach enabled Cu3Sn joints to achieve a superior shear strength (>56 MPa) and a greater stand-off height (>30 μm), which are useful to mitigate the coefficient of thermal expansion (CTE) mismatch problems; thereby yielding higher reliability. We envision that these full Cu3Sn joints prove useful in a wide range of applications in high-temperature and high-power electronics.

Impact of in situ current stressing on Sn-based solder joint shear stability

This paper reports experimental observations showing that a current flow produces an effect of strengthening a solder joint against a shear load. This conclusion is found from a single-joint shear test conducted on Sn–1Ag–0.5Cu wt% (SAC 105) joints with in situ current stressing varied from 700 to 1400 A/cm2 at room temperature. To isolate the current effect from the Joule heat effect, the same type of tests were conducted at room temperature, 40, 50, and 80 °C, without current applied to the joint, thus mimicking the condition of steady-state temperature resulting from Joule heating. Comparative testing was also conducted after aging the samples at 150 °C for 200 h. These tests produced indications suggesting that the current flow causes the maximum shear load to increase, while the rise in temperature by the Joule heat effect results in the opposite effect. Experimentally, as much as a 9.5% increase in the maximum shear load was observed from the isothermally aged sample tested under a current density of 700 A/cm2, while an ~ 25% reduction was estimated to result from a temperature increase by Joule heating. The potential mechanism for these observations is discussed.

Effects of cooling rate and joint size on Sn grain features in Cu/Sn–3.5Ag/Cu solder joints

In Sn-based micro solder joints for fine pitch flip chip or 3D packaging technologies, the number of Sn grains is always limited and sometimes only one Sn grain exists in a joint, which inevitably causes anisotropy in the performance and reliability of micro interconnection. It is known that the parameters of solder joint structure and reflow process, especially cooling stage, have a great significance for regulating the features of Sn grains after soldering. Hence, in the present paper, the Sn grain features of Cu/Sn–3.5Ag/Cu solder joints with different joint sizes (300 μm, 100 μm and 50 μm) solidified under different cooling rates (1.5 °C/s, 0.5 °C/s and 0.1 °C/s) were studied. Generally, the number of Sn grains decreased as the joint size decreased. When the joint size was 50 μm, the solder joints tended to contain only one or two Sn grains irrespective of the cooling rate. Furthermore, cooling rates greatly affected the number and orientation of Sn grains as well as the growth of large Ag3Sn intermetallic plates to varying degrees. The formation of Sn grains and Ag3Sn plates was discussed from the viewpoint of nucleation under different conditions.

Effects of Cooling Rate and Joint Size on Sn Grain Features in Cu/Sn-3.5ag/Cu Solder Joints

In Sn-based micro solder joint s for fine pitch flip chip or 3D packaging technologies , the number of Sn grains is always limited and sometimes only one Sn grain exists in a joint, which inevitably causes anisotropy in the performance of micro interconnection. It is known that the parameters and reliability of solder joint structure and reflow process, especially cooling stage, have a great significance for regulating the features of Sn grain s after soldering. Hence, in the present paper, the Sn grain features of Cu/Sn-3.5Ag/Cu solder joints with different joint size s (300 μm, 100 μm and 50 μm) solidified under different cooling rate s (1.5 °C /s, 0.5 °C /s and 0.1 °C /s) were studied. Generally, the number of Sn grains decrease d as the joint size decrease d . When the joint size was 50 μm, the solder joints tended to contain only one or two Sn grains irrespective of the cooling rate. Furthermore, cooling rates greatly affected the number and orientation of Sn grains as well as the growth of large Ag3Sn intermetallic plates to varying degrees. The formation of Sn grains and Ag 3 Sn plates was discussed from the viewpoint of nucleation under different conditions.

Comparative study on the hourglass-like joint of electroplated Sn-base solder reinforced by adding Ag3Sn nanoparticles and Ag micro-alloying elements

In this work, two types of tin (Sn)-based solder bumps reinforced by Ag3Sn nanoparticles (nano-Ag3Sn), and Ag microalloying elements were successfully deposited on copper-clad laminates (CCL) by electroplating. Hourglass-shaped Sn-based solder joints were formed by precise control of the solder volume and the joint height during the reflowing process in solder bonding. Results show that when shaping the solder joints during solder reflowing, Ag3Sn nanoparticles would uniformly disperse in the Sn-nano-Ag3Sn solder joint, while microalloying Ag preferentially binds to Sn elements enriching at Sn-Ag solder/CCL interfaces and producing large interfacial intermetallic compounds (IMCs). After the solder reflowing, the nano-Ag3Sn will redistribute in the solder joints, where the nano-Ag3Sn adjacent to the solder/CCL interface would advance and adhere to the IMC surface. Compared with the solder joints containing Ag microalloying elements, nano-Ag3Sn can aggregate on the surface IMC surfaces and hinder the interdiffusion of Sn and Cu ions across the solder/CCL interface. Mechanical shear strength tests of solder joints illustrate the enhancement of mechanical robustness of nano- Ag3Sn reinforced solder, which inherently correlates to the absorption of nano- Ag3Sn at the interfaces. Thus, we conclude that the nano-Ag3Sn will inhibit the overgrowth of the interfacial IMC, specifically Cu6Sn5, which ultimately improves the mechanical strength and interconnect reliability of the hourglass-shaped solder joints.