Solder Joint Interface Research Articles

Intermetallic growth study at Sn–3.0Ag–0.5Cu/Cu solder joint interface during different thermal conditions

The morphology and growth behaviors of intermetallic compounds (IMCs) of Sn–3.0Ag–0.5Cu/Cu solder joints were investigated during isothermal aging, thermal cycling and thermal shock. The Sn–3.0Ag–0.5Cu/Cu solder joints were isothermal aged at 150 °C, while the non-isothermal aging was performed within the temperature range from −40 to +150 °C. It was observed that the Cu6Sn5 layer initially formed between the copper substrate and solder, followed by the creation of a Cu3Sn layer for aging times greater than 200 h or 200 thermal cycles. However, the Cu3Sn could not be found in solder joints until thermal shock 800 cycles. The results showed that the initial scallop-like IMCs layer changed into a level duplex structure with the aging time (cycling numbers) increasing. The growth rate of Cu6Sn5 was lower than that of Cu3Sn during isothermal aging. While for thermal cycling, it was Cu6Sn5 contributed more to the growth of IMCs than the Cu3Sn. The value of the time exponent n under isothermal aging, thermal cycling and thermal shock were about 0.58, 0.64, 0.66 respectively.

Effects of coupled stressing and solid-state aging on the mechanical properties of Sn–58Bi–0.7Zn solder joint

The mechanical properties of coupled and aged Sn–58Bi–0.7Zn solder joint were respectively investigated in this study. During tensile tests, the ultimate tensile strength of electromigration and thermal aging coupled solder joint was ascribed to the weakening of the anode interface between intermetallic compounds and Bi layer. It was due to the Bi segregation which led to the formation of vacancies and stress concentration at the Cu–Sn–Zn/Cu–Zn interface in the aged solder joint. Nanoindentation results revealed that the creep resistance of coupled solder joint was higher than that of the aged solder joint, which was because the Bi layer could play a role of load transfer at the solder joint interface in coupled sample.

Effect of aluminium additions on wettability and intermetallic compound (IMC) growth of lead free Sn (2 wt. % Ag, 5 wt. % Bi) soldered joints

The effect of a trace Al addition (0, 0.01, 0.05 and 0.1 wt. %) in the Sn-2Ag-5Bi solder alloy on wettability and intermetallic compound (IMC) formation of the alloy was investigated. The interface between the solder and a Cu(17 μm)/Ni(4 μm)/Au (0.02 μm) under bump metallized (UBM) substrate was studied. The microstructure of the bulk solder and the interface of the soldered joints was observed in a scanning electron microscope (SEM), and the thickness of the interface reaction layers was estimated. Various IMC phases were identified by energy dispersive spectroscopy (EDS) and by the electron probe micro analyzer (EPMA). The experimental results indicated that the addition of 0.01 wt. % Al in the Sn-2Ag-5Bi solder alloy significantly improved the wettability of the solder more than the other Al additions did. The IMC layer between the bulk Sn-2Ag5Bi-0.01Al solder and the Cu/Ni/Au UBM substrate was almost uniform and thinner than those between the solders containing 0, 0.05, and 0.1 wt. % Al and their respective Cu/Ni/Au UBM substrates. Furthermore, the growth rate of the IMC layer between the Sn-2Ag-5Bi-0.01Al solder and Cu/Ni/Au UBM after 1 to 10 reflow times was lower than that of the IMC layer between the Sn-2Ag-5Bi solder and Cu/Ni/Au UBM. The IMCs in the solder joint interface (e.g., Ni3Sn4) of the Sn-2Ag-5Bi-0.01Al solder were well distributed near the Bi and fine Ag3Sn. The addition of 0.01 wt. % Al in the Sn-2Ag-5Bi solder yielded the best wetting properties for the solder and the minimum growth rate of the IMCs because it increased the nucleation rate of Ag3Sn and uniformly segregated the Bi phase.

무전해 Ni-P 두께와 Assembly Process가 Solder Ball Joint의 신뢰성에 미치는 영향

The ability of electronic packages and assemblies to resist solder joint failure is becoming a growing concern. This paper reports on a study of high speed shear energy of Sn-4.0wt%Ag-0.5wt%Cu (SAC405) solder with different electroless Ni-P thickness, with <TEX>$HNO_3$</TEX> vapor's status, and with various pre-conditions. A high speed shear testing of solder joints was conducted to find a relationship between the thickness of Ni-P deposit and the brittle fracture in electroless Ni-P deposit/SAC405 solder interconnection. A focused ion beam (FIB) was used to polish the cross sections to reveal details of the microstructure of the fractured pad surface with and without <TEX>$HNO_3$</TEX> vapor treatment. A scanning electron microscopy (SEM) and an energy dispersive x-ray analysis (EDS) confirmed that there were three intermetallic compound (IMC) layers at the SAC405 solder joint interface: <TEX>$(Ni,Cu)_3Sn_4$</TEX> layer, <TEX>$(Ni,Cu)_2SnP$</TEX> layer, and <TEX>$(Ni,Sn)_3P$</TEX> layer. The high speed shear energy of SAC405 solder joint with <TEX>$3{\mu}m$</TEX> Ni-P deposit was found to be lower in pre-condition level#2, compared to that of <TEX>$6{\mu}m$</TEX> Ni-P deposit. Results of focused ion beam and energy dispersive x-ray analysis of the fractured pad surfaces support the suggestion that the brittle fracture of <TEX>$3{\mu}m$</TEX> Ni-P deposit is the result of Ni corrosion in the pre-condition level#2 and the <TEX>$HNO_3$</TEX> vapor treatment.

Interface reaction between an electroless Ni–Co–P metallization and Sn–3.5Ag lead-free solder with improved joint reliability

To address the reliability challenges brought about by the accelerated reaction with the implementation of lead-free solders, an electrolessly plated Ni–Co–P alloy (3–4wt.% P and 9–12wt.% Co) was developed as the solder metallization in this study. Three compounds layers, (Ni,Co)3Sn4, (Ni,Co)3P and (Ni,Co)12P5, are formed at the reaction interface. Nano-sized voids are visible in the (Ni,Co)3P layer under transmission electron microscopy, but no large voids are found under scanning electron microscopy. This is an indication of effective diffusion barrier performance by the Ni–Co–P metallization compared with the binary Ni–P metallization. The influence of interfacial reaction on the solder joint reliability was reported through the evaluation of the tensile strength of solder micro-joints. Upon aging at 180°C for 600h, the tensile strength of Ni–Co–P/Sn–3.5Ag solder joint remains high, and the failure is caused by the bulk solder necking and collapse. As a comparison, the tensile strength of the Ni–P/Sn–3.5Ag solder joint drops significantly after aging for 400h at 180°C, and the fracture mode has shifted from ductile failure in the bulk solder to brittle failure at the solder joint interface. The Ni–Co–P metallization, having a much slower consumption rate and improved resistance to joint strength degradation during long-term aging treatment, is a potential candidate for future microelectronic solder metallization materials.

Precise Cr-marker investigation on the reactive interface in the eutectic SnIn solder joint

To clarify the interfacial reactions in a liquid–solid system like solder joint, the location of reactive interface is very critical for mechanism investigation, which is hard to be determined due to its reactivity. Electroplated Cr-marker experiments were firstly applied to study the intermetallic compounds growth and Kirkendall voids formation in the solder joint between eutectic SnIn and polycrystalline Cu. The original interface between Cu and liquid SnIn solder was revealed being the reactive interface within the duplex structural Cu2(In,Sn) compounds, where the fine-grain sublayer grew into substrate side while the coarse-grain sublayer grew into solder side. This interface was also determined as the Kirkendall void plane between the coarse- and fine-grain Cu2(In,Sn) sublayers, which is different from those between Cu substrate and intermetallic compound layer in other solder joints. This Kirkendall void plane remained stable during solid-state aging with increased void size, which could degrade the mechanical and electrical properties of the solder joint.

Intermetallic growth study on SnAgCu/Cu solder joint interface during thermal aging

The intermetallic compound (IMC) growth behavior at SnAgCu/Cu solder joint interface under different thermal aging conditions was investigated, in order to develop a framework for correlating IMC layer growth behavior between isothermal and thermomechanical cycling (TMC) effects. Based upon an analysis of displacements for actual flip-chip solder joint during temperature cycling, a special bimetallic loading frame with single joint-shear sample as well as TMC tests were designed and used to research the interfacial IMC growth behavior in SnAgCu/Cu solder joint, with a focus on the influence of stress–strain cycling on the growth kinetics. An equivalent model for IMC growth was derived to describe the interfacial Cu-Sn IMC growth behavior subjected to TMC aging as well as isothermal aging based on the proposed “equivalent aging time” and “effective aging time”. Isothermal aging, thermal cycling (TC) and TMC tests were conducted for parameter determination of the IMC growth model as well as the growth kinetic analysis. The SnAgCu/Cu solder joints were isothermally aged at 125, 150 and 175 °C, while the TC and TMC tests were performed within the temperature range from −40 to 125 °C. The statistical results of IMC layer thickness showed that the IMC growth for TMC was accelerated compared to that of isothermal aging based on the same “effective aging time”. The IMC growth model proposed here is fit for predicting the IMC layer thickness for SnAgCu/Cu solder joint after any isothermal aging time or thermomechanical cycles. In addition, the results of microstructure evolution observation of SnAgCu/Cu solder joint subjected to TMC revealed that the interfacial zone was the weak link of the solder joint, and the interfacial IMC growth had important influence on the thermomechanical fatigue fracture of the solder joint.

Microstructure and Growth Behavior of the Intermetallic Compound for Sn-1.0Ag-0.5Cu-NiB/Cu Solder Joint Interface

This research investigates the microstructure and growth behavior of the intermetallic compound(IMC) of Sn-1.0Ag-0.5Cu,Sn-1.0Ag-0.5Cu-0.05Ni and Sn-1.0Ag-0.5Cu-0.05N-0.02B/Cu solder joint interface. The interfacial reactions between Cu and the solders at 250±1°C were examined. Experimental results indicated that the IMCs of the above alloy systems on the soldering interface were Cu6Sn5 and (Cux, Ni1-x)6Sn5, respectively. The grain size of primary Sn decreased observably with the micro addition of B and a large number of fine reinforcement particles were found in the solder. With the aging time increasing, the (Cux, Ni1-x)6Sn5 micrograph of the Sn-1.0Ag-0.5Cu-0.05N-0.02B solder joint interface was changed from sawtooth-like to shape-layer, but the thickness of IMCs increased unobservably.

Retarding Electromigration in Lead-Free Solder Joints by Alloying and Composite Approaches

The voids induced by electromigration (EM) can trigger serious failure across the entire cathode interface of solder joints. In this study, alloying and composite approaches showed great potential for inhibiting EM in lead-free solder joints. Microsized Ni, Co, and Sb particles were added to the solder matrix. Cu and Sn particles were added to the melting solder to form in situ Cu6Sn5, which formed a barrier layer in the underbump metallization of flip-chip solder joints. The polarity effect induced by EM was observed to be significantly inhibited in the alloyed and composite solder joints. This indicates that the Sn-Ni, Sn-Co, Sn-Sb, and Cu6Sn5 intermetallic compounds may act as barriers to obstruct the movement of the dominant diffusion species along phase boundaries, which in turn improves the resistance to EM. However, Sb particles could induce crack formation and propagation that might lead to joint fracture.

Thermo-mechanical analysis of TSV and solder interconnects for different Cu pillar bump types

Graphical abstractDisplay Omitted Highlights? FE modelling of Cu pillar bumps of different types. ? Stress distribution in solder interface and TSV interface. ? Reliability of conical frustum type is excellent than other types. The through silicon via (TSV) is a technology for vertical interconnection between multiple chips that employ a hole bored through a Si wafer and stack chips, which is good for securing a number of I/O per unit area reducing the electric resistance. On the other hand, vertical multi-layers are structurally more unstable than a single-layer but are vulnerable to a thermo-mechanical effect. Therefore, in this study, finite element analysis (FEA) of a three-dimensional (3D) multi-layer semiconductor with TSV was carried out to examine the changes in thermo-mechanical stress at the bonding interface with different models of Cu pillar bumps and Sn-3.5Ag solder bumps. A numerical simulation was carried out with different models, such as a plate type, pin type, skirt type, zigzag type and conical frustum type. The height and diameter of each solder bump were 10µm and 30µm, respectively. The distribution of Von Mises stress occurred more at the solder joint interface and TSV joint interface. At the solder joint interface, the stress in the skirt type (100.6MPa) was approximately double that in the conical frustum type (53.1MPa). At the TSV joint interface, the stress in the plate type (17.2MPa) was higher than that in the other types. The stress values were similar in the other types except for plate type Cu pillar bump. Stress in the solder and TSV joint interface was caused partly by the difference in properties between the two materials. In particular, mismatch in the coefficient of thermal expansion (CTE) had the greatest effect on the stress distribution.

Suppressing Sn-Patch Growth in Ti/Ni(V)/Cu Under Bump Metallization with Sn-Ag-Cu Solder After Reflow and Aging

Sputtered Ti/Ni(V)/Cu under bump metallization (UBM) is widely used in flip chip technology because the metals are nonmagnetic and the consumption of the Ni(V) layer is low. It is noted that V does not react with solders and intermetallic compounds (IMC) during reflow and aging; however, a Sn-patch forms in the Ni(V) layer, and the Sn-patch growth may cause the IMCs to detach from the interface of solder joints. In this study, Sn-3.0Ag-0.5Cu solder was reflowed on Ti/Ni(V)/Cu UBM with different Cu thicknesses at 250°C for 60 s, and then aged at 150°C for various periods of time. It was revealed that the Sn-patch growth could be controlled by increasing the Cu thickness in the Ti/Ni(V)/Cu UBM. A feasible approach to suppress Sn-patch formation after reflow and aging is discussed.

The Morphology and Evolution of Cu&lt;sub&gt;6&lt;/sub&gt;Sn&lt;sub&gt;5&lt;/sub&gt; at the Interface of Sn-2.5Ag-0.7Cu-0.1RE/Cu Solder Joint during the Isothermal Aging

The morphology and growing behavior of Cu6Sn5intermetallic compound (IMC) of low Ag content Sn-2.5Ag-0.7Cu-0.1RE/Cu solder joint interface are investigated by adopting the X-ray diffraction, JSM-5610LV scanning electronic microscope and energy spectrum analysis. The results show that the cross-section morphology Cu6Sn5of the solder joint interface is scallop-like and its section morphology is circle-like grain. With the aging time increasing, the cross-section Cu6Sn5morphology of the solder joint interface can be changed from the scallop-like to the shape-layer, and the growing kinetics is coincidence with the law of parabola and its growing behavior is controlled by diffusion. With adding a small amount of rare earth elements in the Sn-2.5Ag-0.7Cu solder alloy, the growing rate of the Cu6Sn5can be reduced.

Nanomechanical responses of intermetallic phase at the solder joint interface – Crystal orientation and metallurgical effects

Abstract In this study, the relationships between crystal structures, metallurgical effects, and mechanical properties of the most common intermetallic compound formed at the interface of solder joints, Cu6Sn5, were investigated using nanoindentation. Experimental results show that the ( 11 2 ¯ 0 ) oriented hexagonal Cu6Sn5 exhibited anisotropic mechanical behavior compared to those with random growth directions. The closest atomic packing density of the ( 11 2 ¯ 0 ) plane in hexagonal Cu6Sn5 resulted in higher hardness and notably, greater stiffness. Subjected to long time aging at 150 °C, hexagonal Cu6Sn5 was transformed into the equilibrium monoclinic structure, resulting in a reduced modulus and thus inferior ability for plasticity. Alloying of Ni, Mn and rare earth elements (La and Ce) had various contributions to the allotropic transition and thus nanoindentation responses. It was found that the differences in atomic radius between the solute elements and Cu affected the kinetics of the allotropic transformation and also the mechanical performance of Cu6Sn5. There exists a critical value for the modulus/hardness ratio (E/H) of about 17.3–17.5, below which the indent morphology showed a brittle characteristic.

Interfacial Microstructures and Kinetics of Sn2.5Ag0.7Cu/Cu

The interfacial microstructures and kinetics of low Ag content Sn2.5Ag0.7Cu/Cu solder joint were investigated by the X-ray diffraction, scanning electronic microscope and energy spectrum analysis during the isothermal aging. The results show that the interfacial microstructures of the Sn2.5Ag0.7Cu/Cu solder joint are composed of Cu3Sn and Cu6Sn5after soldering. With the aging time increasing, the intermetallic compound (IMC) pattern of the solder joint interface can be changed from the scallop-like to the shape-layer, and the growing dynamics is coincidence with the law of parabola and its growing behavior is controlled by diffusion. The growing activation energies values of Cu3Sn and Cu6Sn5layer at the Sn2.5Ag0.7Cu/Cu solder joint are 82.4kJ/mol and 69.6kJ/mol respectively.

Interfacial microstructure and properties of Sn–0.7Cu–0.05Ni/Cu solder joint with rare earth Nd addition

Interfacial microstructure and properties of Sn–0.7Cu–0.05Ni/Cu solder joints with trace amounts Nd additions have been investigated in this paper. The evolution of interfacial morphology of SCN solder joints with and without the presence of Nd under long-term room ambience was also studied. The greatest improvement to the solderability and tensile strength of SCN- xNd are obtained at 0.05 wt.% Nd. Meanwhile, the morphology and growth of interfacial intermetallic (IMC) layer are greatly improved by adding Nd, and the propensity for IMC spalling from the interface of solder joint is definitely reduced with the presence of rare earth Nd, since the sponge-like structure was completely inhibited in the solder joint containing Nd. In addition, a significant amount of Sn whiskers were present on the surface of NdSn 3 phases in the SCN0.15Nd/Cu solder joints, and the reason for this phenomenon has been briefly discussed.

Interfacial reactions between Sn-3.5 Ag solder and Ni–W alloy films

In this study, interfacial reactions between Ni-W alloy films and Sn-3.8Ag-0.7Cu solder have been investigated. Ni-W alloys films with tungsten content in the range of 5.0 – 18.0 at.% was prepared on copper substrate by electrodeposition in ammonia citrate bath. Solder joints were prepared on the Ni-W coated substrate at a reflow temperature of 250 °C. The solder joint interface was investigated by cross-sectional scanning electron microscopy, energy dispersive X-ray spectroscopy and electron back scatter diffraction. It has been observed that a (Cu,Ni) 6 Sn 5 layer formed on the Ni-W alloy film after reflow. The thickness of the (Cu,Ni) 6 Sn 5 layer was found to decrease with the increase of tungsten content in the Ni-W film. An additional layer with a bright appearance was also found to form below the (Cu,Ni) 6 Sn 5 layer. The bright layer was identified to be a ternary phase containing Sn, Cu, W and Ni. The bright layer is found to be amorphous and is suggested to have formed through solid state amorphization caused by anomalously fast diffusion of Sn into Ni-W film.

Mechanical Properties of Flip-Chip Solder Joints Effected by Electromigration

A frequent cause of failure of portable and hand-held devices is an accidental drop to the ground. The effect of electromigration on the mechanical properties of solder joints was discussed in this paper. Without current stressing, the samples were broken in the bulk of solder or at the interface of Al interconnect and solder. If the Al-solder interfacial mechanical strength was improved by changed the interfacial structure or optimized the jointing process, the flip chip devices would show the lonely ductile fracture in the bulk of solder. After electromigration the samples were broken abruptly at the interface near the chip side while the bulk of the solder joints maintained the original shape. Due to the interfacial reaction and the polarity effect of electromigration on the interfaces, a ductile solder joint can become a brittle solder joint. The ductile-to-brittle transition is very sensitive to a high speed shear stress applied to the joints. Because solder alloys are ductile by nature, it is of interest to understand how electromigration can influence the mechanical properties of solder joints’ interfaces and change their ductile nature. Owing to the polarity effect of electromigration, vacancies will accumulate to form voids at the cathode interface of solder joints. Besides, much more intermetallic compound formation at the joint interfaces also caused the ductile-to-brittle transition. Thus the interfaces become more and more brittle with time due to IMC formation or vacancy accumulation from electromigration.

MICROSTRUCTURE AND RELIABILITY OF LASER JET SOLDER BALL BONDING SOLDER JOINTS

Laser jet solder ball bonding (LJSBB) technology is a special soldering method developed in cooperation by Fraunhofer Ltd and PacTech Ltd in 1998 to fulfill the specific needs in the packaging of micro-electro-mechanical system (MEMS) and other functional micro-devices. However, the metallurgy characteristics and reliability of the solder joint produced by LJSBB has not been broadly reported. In the present paper, the commonly used Au/Ni/Cu soldering pads were bonded by LJSBB with Sn3.0Ag0.5Cu lead free solder. The effect of laser energy on the interfacial microstructure of solder joint was investigated by SEM and EDS. The reliability of solder joints was also evaluated by thermal cycle treatment and strength test. Results show that due to the limited heat provided by laser beam, the Au layer of the commonly used Au/Ni/Cu soldering pad cannot completely dissolve into solder during LJSBB process, which leads to the formation of multilayered reactive structure of Au+AuSn+AuSn2+AuSn4 in the interface of solder joint. The AuSn4 locates at the most outside of the reactive structure and presents a needle-like shape. In despite of the brittle nature of Au-Sn intermetallics, the as-soldered joints present a comparatively high strength. However, the strength of solder joint decreases significantly after 100 thermal cycle treatment and keeps nearly unchanged after even more thermal cycle treatment. Metallurgy analysis of joint interface reveals that the thermal cycle treatment plays the roles of promoting the dissipation of residual Au in joint interface through solid state inter-diffusion, and leading to the formation of a quaternary phase of (Au, Ni, Cu, Sn) in

Interfacial Reactions and Joint Strengths of Sn-xZn Solders with Immersion Ag UBM

The solder joint microstructures of immersion Ag with Sn-xZn (x = 0 wt.%, 1 wt.%, 5 wt.%, and 9 wt.%) solders were analyzed and correlated with their drop impact reliability. Addition of 1 wt.% Zn to Sn did not change the interface microstructure and was only marginally effective. In comparison, the addition of 5 wt.% or 9 wt.% Zn formed layers of AgZn 3 /Ag 5 Zn 8 at the solder joint interface, which increased drop reliability significantly. Under extensive aging, Ag-Zn intermetallic compounds (IMCs) transformed into Cu 5 Zn 8 and Ag 3 Sn, and the drop impact resistance at the solder joints deteriorated up to a point. The beneficial role of Zn on immersion Ag pads was ascribed to the formation of Ag-Zn IMC layers, which were fairly resistant to the drop impact, and to the suppression of the brittle Cu 6 Sn 5 phase at the joint interface.

Time-dependent deformation behavior of interfacial intermetallic compound layers in electronic solder joints

Using nanoindentation, this study develops the criteria to evaluate the creep performance of the intermetallic compounds (IMCs) formed at the interface of microelectronic solder joints. Regardless of crystal structure and melting point, the creep stress exponent (X), one of the parameters determining creep resistance, is in good agreement with tendencies of the work-hardening exponent (n) and also the ratio of yield stress (Y) to Young's modulus (E), which reveals the ability against plastic deformation.