Growth Of IMC Layer Research Articles

Microstructure Evolution of Sn-3Ag-3Bi-3In/Cu Lead-free Solder Joints during Interval Aging

The microstructure evolution of Sn-3Ag-3Bi-3In/Cu lead free solder joints was investigated over interval aging mode to simulate the real application environment. The solder joints were aged at 150 °C for 48h, 96h, 144h, 196h and 240h, respectively. Results demonstrate that the IMC layer of Sn-3Ag-3Bi-3In/Cu solder joints evolves from scallop shape to layer shape and becomes thicker simultaneously with increasing aging time. It is noteworthy that the IMC layer is composed of only Cu6 (Sn, In)5 phase without Cu3 (Sn, In) phase during interval aging for various aging time. As a result, the reliability of Sn-3Ag-3Bi-3In/Cu solder joints can be improved due to the lack of Cu3 (Sn, in) phase which is detrimental to reliability over interval aging mode. Moreover, it is observed that diffusion control mechanism is fit for the growth of IMC layer of all solder joints over both interval and continious aging mode. Further, the growth formula of IMC layer of Sn-3Ag-3Bi-3In/Cu solder joints is revealed as X = 2.56 + 0.062t1/2, in addition, the diffusion coefficient of is obtained as 0.02491 μm2/h.

Comparative studies on microelectronic reliability issue of Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments

In this study, comparative studies on Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments were conducted to investigate factors like ambient temperature, oxygen level, and 3.5wt% NaCl solution on whisker growth. The experimental results revealed that ambient temperature and oxygen level are two important factors that could determine the oxidation rate of PrSn3 phase, thus indirectly affecting the growth rate of Sn whiskers. In addition, mechanisms of whisker growth under these three environments were established from the perspective of atom diffusion based on the “compressive stress-induced” theory. Although whiskers under different environments were all squeezed out from Pr oxides (hydroxides), the forms of their driving forces were different. For whiskers squeezed out in air whether at room temperature or 150°C, the driving force is the compressive stress produced by lattice expansion due to the oxidation of PrSn3 phase. The representative example was whiskers' growth at 150°C, which could be simplified as three stages: (1) squeezing out, (2) cracking and (3) bursting out. For whisker growth in 3.5wt% NaCl solution, the driving force for much fewer whiskers' growth was proposed to come from lateral stress provided by interfacial IMC layer growth. Moreover, Sn nanoparticles and their agglomerations were also found to form under the driving force of the potential difference between Sn atoms and Sn crystals. Their morphologies could also be affected by factors of ambient temperature, oxygen level and Cl− ions in corrosive liquid.

Fracture mechanisms of Al/steel resistance spot welds in coach peel and cross tension testing

In Al/steel resistance spot welding, increasing welding current helped to reduce generation of dendritic grains and shrinkage, and increasing welding time tended to reduce shrinkage but promoted the transition of grains from preferable columnar to dendritic. Iron phase, existing between the Fe2Al5 fingers of a thin IMC layer facing the steel side, was observed to serve as crack propagation inhibitor and improve load carrying capacity of Al/steel interface. The amount of iron phase decreased as the IMC thickness grew, so did the load bearing capacity. Three fracture modes were observed in the cross tension test: interfacial fracture in the IMC layer, partial thickness fracture within the Al nugget next to the IMC layer, and partial button pullout fracture. All fractured coach peel test samples displayed consistent partial button pullout fracture mode following large deformation of the Al sheet. The crack in the cross tension test always initiated in the IMC layer while the crack in the coach peel test often started in the Al fusion zone. Two characteristic peak points were observed in the load-displacement curves of the coach peel test. The key point for improving mechanical performance under coach peel loading was to increase the first peak point by reducing Al thinning, increasing the Al nugget size, and limiting the IMC layer growth.

Effect of Pr addition on properties and Sn whisker growth of Sn–0.3Ag–0.7Cu low-Ag solder for electronic packaging

In this study, trace amount of rare earth Pr was added into Sn–0.3Ag–0.7Cu low-Ag solder to enhance properties of solders. Experimental results indicated that optimal amount of Pr addition (~0.06 wt%) can improve properties of wettability, shear force and ductility of Sn–0.3Ag–0.7Cu low-Ag solder. This is because that solder with optimal Pr addition not only had a refined microstructure but also owned a regular and thin interfacial IMC layer after soldering on Cu substrate. Meanwhile, we have explained the change of morphology and thickness of interfacial IMC layer after Pr addition based on Kim and Tu’s kinetic model of interfacial IMC growth. However, it was found excessive Pr addition led to the formation of PrSn3 phase, which was easy to be oxidized and became a great deterioration on the properties referred above. Besides, the fracture mode of solder joint also gradually changed from ductile fracture to cleavage fracture. Moreover, this oxidized PrSn3 became the birthplace of Sn whisker since it provided abundant supply of Sn sources and compressive stress for Sn whisker growth. By the in-situ observation of Sn whisker growth in 0.5 wt% Pr-doped solder joint, we obtained that the incubation period of Sn whisker is very short and its growth rate may have a great decrease with time extending. Besides, it was found after 1 day at room temperature, spindly and rod-like Sn whiskers also grew in 0.5 wt% Pr-doped solder joint besides dot-shaped Sn whisker that grew in 0.5 wt% Pr-doped solder matrix. We suggest this may be related with the extra compressive stress provided by interfacial IMC layer growth in 0.5 wt% Pr-doped solder joint.

Effect of thin gold/nickel coating on the microstructure, wettability and hardness of lead-free tin–bismuth–silver solder

This paper investigates the coating thickness and surface morphology of gold/nickel (Au/Ni) layer on copper (Cu) substrate. A cross-sectioned SEM analysis confirmed that the Au/Ni coating was uniform. The top Au layer with average thickness of 0.7 µm appeared to have a very smooth surface without any defect such as cracks and delamination. However, the thin Au/Ni coating greatly influenced the interfacial structure and material properties of electronic interconnections. In the reference Cu substrate/Sn–Bi–Ag solder system, an island-shaped Cu6Sn5 IMC layer at the interface could be clearly observed at the initial reaction stage. After a prolong reaction, a very thin Cu3Sn IMC layer was formed with excessive growth of the Cu6Sn5 IMC layer which can deteriorate the electronic interconnection life-span. However, in the Au/Ni coated substrate/solder system, a very thin scallop-shaped ternary Ni3Sn4 IMC layer formed without the Cu3Sn IMC layer, indicating that the Au/Ni coating hindered the growth of the IMC layer, which consequently changed the activation energies and refined the microstructure. Additionally, the overall micro-hardness of the Au/Ni coated substrate/solder system is higher than that of the reference solder system.

Electromigration reliability for Al2O3-reinforced Cu/Sn–58Bi/Cu composite solder joints

The Sn–58Bi based composite solder was fabricated via mechanical mixing 0.5 wt% Al2O3 particle, and the effect of electromigration (EM) behavior on the microstructure and mechanical properties of the Al2O3-reinforced Cu/Sn–58Bi–0.5Al2O3/Cu lead-free solder joints was investigated under the current density of 0.6 × 104 A/cm2 at room temperature. The results indicated that the EM failure behavior occurred at the cathode side as increasing EM time to 280 h for Cu/Sn–58Bi/Cu and 410 h for Cu/Sn–58Bi–0.5Al2O3/Cu, respectively. A continuous white Bi-rich layer was caused by heavy electron wind flow observed among the bulk solder in the two joints. The remarkable interfacial morphologies change was not found in the 0.5 wt% Al2O3 reinforced joint, and the EM-induced IMC layer growth rate of Cu/Sn–58Bi–0.5Al2O3/Cu was slower than that of the non-composite joint. The mechanical properties, including UTS, shear strength and hardness, of the Cu/Sn–58Bi–0.5Al2O3/Cu solder joint were improved slightly by adding 0.5 wt% Al2O3. Consequently, the EM reliability is enhanced for the Cu/Sn–58Bi–0.5Al2O3/Cu solder joint with adding Al2O3 concentration to 0.5 wt%.

Interfacial reaction, ball shear strength and fracture surface analysis of lead-free solder joints prepared using cobalt nanoparticle doped flux

In this research study, we investigated the interfacial reaction and fracture modes in BGA solder joints prepared using Co nanoparticles doped flux. Solder joints were formed in a reflow oven at 240 °C for 45s and subsequently aged at 150 °C up to 1008 h. One set of aged solder joints was used in both microstructural characterization to examine the morphology and growth of interfacial IMC layer and nano-mechanical characterization to determine the hardness and Young's modulus of different phases. Another set was experimented in ball shear test to determine the average shear strength and crack paths in different failure modes. Results showed that the growth of Cu6Sn5 enhanced and that of Cu3Sn suppressed due to the profound stability of Cu6Sn5 in the presence of Co atoms. In the case of undoped condition, the direction of crack changed from a ductile failure mode to a brittle failure mode with respect to aging time. No catastrophic brittle failure mode was observed for Co doped solder joints. The mechanisms for the better fracture behavior in Co-doped solder joints are suggested and correlated to Cu6Sn5/Cu3Sn interfacial reaction.

Effect of Ni and Ni-coated Carbon Nanotubes on the interfacial reaction and growth behavior of Sn58Bi/Cu intermetallic compound layers

The interface reactions between Cu and Sn58Bi solder doped with mass fraction of 0.5 % Ni, 1 % Ni, 0.5 % Ni-CNTs and 1 % Ni-CNTs after reflow and solid-state aging were studied. Results show that the Ni and Ni-CNTs restrain the growth of IMC layer during reflow. After 120 and 240 h aging at 100 °C, the growth of total IMC layer and Cu3Sn layer was inhibited by the addition of Ni and Ni-CNTs. During aging, the thickness of (Cu, Ni)6Sn5 layer in Ni-CNTs-doped solder joint is always thinner than that in Ni-doped solder joint because the Ni-CNTs reduce the CTE of SnBi solder more effectively. The reason for the growth of Cu3Sn with Ni doped less than that with Ni-CNTs is that the stability of (Cu, Ni)6Sn5 gets an improvement as the Ni content increases. Besides, the IMC growth rate of composite solder joints decreases with the increasing Ni and Ni-CNTs content.

Effect of welding energy on microstructure and strength of ultrasonic spot welded dissimilar joints of aluminum to steel sheets

Two dissimilar ultrasonic spot welded joints of aluminum to commercial steel sheets at different levels of welding energy were investigated. The tensile lap shear tests were conducted to evaluate the failure strength in relation to microstructural changes. The main intermetallics at the weld interface in both joints was θ (FeAl3), along with ɳ (Fe2Al5) phase in Al-to-AISI 304 stainless steel joint and Fe3Al phase in Al-to-ASTM A36 steel joint, respectively. The welding strength of Al-to-AISI 304 stainless steel weld samples was slightly higher than Al-to-ASTM A36 steel weld samples, whereas the fracture energies of Al-to-AISI 304 stainless steel weld samples were significantly higher as compared with Al-to-ASTM A36 steel weld samples. The welding strength of both Al-to-Steel welds were higher than other reported dissimilar USW joints in literature. The fracture surfaces of both weld joints exhibits the growth of IMC layer with increasing welding energy or time, whose inherent brittleness compromises the integrity of joints. In both cases, the lap shear tensile fracture occurred from the Al/Fe interface at lower energy inputs and the failure mode at higher welding energy inputs became the “transverse through-thickness crack growth” at the edge of the nugget zone on the softer Al side.

열시효 처리된 무연 솔더 볼 연결부의 충격 전단강도 평가

The present study investigates the impact shear strength of thermal aged Sn-3Ag-0.5Cu lead-free solder joints at impact speeds ranging from 0.5 m/s to 2.5 m/s. The specimens were thermal aged for 24, 100, 250 and 1000 hours at . The experimental results demonstrate that the shear strength of the solder joint decreases with an increase in the load speed and aging time. The shear strength of the solder joint aged averagely decreased by 43% with an increase in the strain rate. For the as-reflowed specimens, the mode II stress intensity factor () of interfacial IMC between Sn-3.0Ag-0.5Cu and a copper substrate also was found to decrease from to in the speed range tested here. The degradations in the shear strength and fracture toughness of the aged solder joints are mainly caused by the growth of IMC layers at the solder/substrate interface.

The effects of Mn powder additions on the microstructures and tensile property of SnAgCu/Cu solder joints

In this paper, the effects of Mn powder on fusion property of Sn3.0Ag0.5Cu solder alloy and microstructures as well as tensile property of the solder joints of Sn3.0Ag0.5Cu/Cu were investigated by differential scanning calorimetry analysis, scanning electron microscopy and tensile tests. The results showed that the addition of Mn dramatically suppressed under cooling of SnAgCu solder alloy. Mn addition contributed to the growth of Cu6Sn5 intermetallic compound layers since it provided nucleation sites for Cu6Sn5 at the solder joints. Moreover, Mn addition increased the hardness of the solder alloys and reduced the tensile strength of SnAgCu/Cu solder joints. During aging, the growth of IMC layers of SnAgCuMn/Cu solder joints was slower than that of SnAgCu/Cu solder joints, and the tensile strength of all the solder joints increased after aging.

Formation of intermetallic compounds in Mg–Ag–Al joints during diffusion bonding

In this study, we conducted the diffusion bonding of Mg and Al alloys using a 30-μm-thick pure silver foil interlayer at median temperatures between 390 and 490 °C. We obtained a multilayered structure across the Mg–Ag–Al joint: Mg/Mg(ss, Ag)/Mg3Ag/MgAg/Ag/Ag(ss, Al)/Ag2Al/Al. The silver diffusion barrier prevented the formation of brittle intermetallics between Mg and Al. Intermetallics identified at the joint interface include the more ductile types between Mg and Ag, e-Mg3Ag and β′-MgAg, and Ag and Al, δ-Ag2Al. As the bonding temperature increased, Ag2Al, followed by MgAg, favored the growth of Mg3Ag IMC layer. The shear strength of the joints increased with the rising bonding temperature to a maximum value of 11.8 MPa at 470 °C. Fracture failure in the joints mainly occurred in the Ag2Al layer. The formation mechanism for interfacial layers in the joints is believed to consist of four stages: (1) solid-solution formation, (2) Mg–Ag IMC formation, (3) Ag–Al IMC formation, and (4) growth of Mg–Ag and Ag–Al IMCs.

Interfacial IMC layer growth and tensile properties of low‐silver Cu/SACBE/Cu solder joints

PurposeIn a previous study, the authors proposed a new low‐silver solder alloy Sn‐ x(1.0, 1.5, 2.0)Ag‐0.3Cu‐3.0Bi‐0.05Er (wt.%) (SACBE) and the purpose of this paper is to provide additional useful information for new solder alloy development. The growth behaviour of the interfacial IMC layers for Cu/SACBE/Cu and Cu/SAC/Cu joints and their bonding strengths after thermal aging at 150°C for 0, 24, 168 and 500 hours are investigated and the effects of adding elemental Bi and Er on the growth of interfacial IMC layers in the joints, and their tensile properties, are characterized and discussed.Design/methodology/approachThe tensile properties of the Cu/Sn‐3.0Ag‐0.5Cu/Cu (Cu/SAC/Cu) and Cu/SACBE/Cu joints during thermal aging at 150°C for 0, 24, 168 and 500 hours were investigated, respectively. The thickness of interfacial IMC layer and the fracture surface of solder joint after isothermal aging were observed and analyzed by means of scanning electron micrograph (SEM) equipped with an energy dispersive spectroscopy X‐ray (EDX) analysis system.FindingsIt was found that the thickness of the IMC layer at the interface of a Cu/SACBE/Cu joint was remarkably thinner than that of a Cu/SAC/Cu joint. The addition of Bi and Er could significantly improve the tensile properties of the solder joint and enhance its resistance to high temperature aging. A mixture of ductile and brittle fracture mode was observed after tensile testing in the Cu/SACBE/Cu joints.Originality/valueThe paper implies that the addition of Bi and Er could complement effectively the effects of Ag, thereby reducing the cost of solder. The low‐silver SACBE solder is a potential alloy for electronic packaging production.

The effect of praseodymium (Pr) additions on shear strength and microstructure of SnAgCu joints during the ageing process

PurposeThe purpose of this paper is to investigate the influence of praseodymium (Pr) additions (0, 0.05 and 0.5 wt%) on the mechanical properties and microstructure of SnAgCu solder joint during aging process. Moreover, the authors aim to indicate that the decreased soldification undercooling of Sn3.8Ag0.7Cu solder can suppress the formation of Ag3Sn plate to some extent.Design/methodology/approachThe shear strength evolution of SAC, SAC‐0.05Pr and SAC‐0.5Pr solder joint were studied under 150°C aging process with STR‐1000. The effect of Pr additions on the solidification behavior of SnAgCu solder was also studied by differential scanning calorimetry. To study the microstructure evolution, the cross‐sections of all specimens were observed by means of scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Meanwhile, the etchant, consisting of 20%HNO3+distilled water was used for deep etching to reveal the interfacial morphology.FindingsThe shear force reduction rate of SAC solder joint during aging was restrained by 0.05%Pr addition but promoted with 0.5% Pr addition. The growth of IMC layer of SnAgCu joint in the aging process was suppressed significantly by different amounts of Pr additions. However, the beneficial effect of Pr addition due to the suppression of IMC layer growth was weakened by the micro‐cracks formed in PrSn3 compounds in SnAgCu‐0.5Pr joint. Pr additions (0.05, 0.5 wt%) decrease the solidification undercooling of SnAgCu solder, which will suppress the formation of Ag3Sn plate to some extent.Research limitations/implicationsFurther studies are necessary for confirmation of the practical application, especially of the manufacturing technology of solder paste containing Pr.Practical implicationsThe appropriate amount of Pr in Sn3.8Ag0.7Cu solder is about 0.05 wt%. It is found that SAC‐0.05Pr solder has an improvement in solder joint reliability in long aging processes. The results suggested the novel solder alloys can meet the requirements of high reliability application.Originality/valueThe paper demonstrates that: Pr additions promote the solidification of SAC solder; shear force reduction rate of SAC solder joint was reduced by 0.05%Pr addition; the IMC layer growth rate of SnAgCu solder joint was suppressed by Pr additions; and micro‐cracks were found in PrSn3 phases after aging.

Effect of TiO2 nanoparticles on the microstructure and bonding strengths of Sn0.7Cu composite solder BGA packages with immersion Sn surface finish

This study investigated the effects of TiO2 nanoparticles on the interfacial microstructures and bonding strength of Sn0.7Cu (SC) composite solder joints in ball grid array packages with immersion Sn surface finishes. The addition of TiO2 nanoparticles to SC solders resulted in significant changes in the interfacial microstructure. The nanoparticle addition suppressed the growth of the Cu6Sn5 IMC layer, significantly improving shear strength and reliability of solder joints after multiple reflows. The fracture surfaces of the SC solder exhibited a semi-brittle fracture mode with a relatively smooth surface, while those of the SC composite solder showed typical ductile failures with very refined dimpled surfaces.

Electromigration-enhanced intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints

This paper presents some experimental observations relative to the influence of elevated current densities on the intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints. Three samples were stressed with different current densities of 104, 1.2 × 104, and 1.4 × 104 A/cm2, respectively, for 80 h. The abnormal polarity effect of electromigration (EM) on chemical reactions at the cathode and the anode was investigated as well as the effect of EM on phase segregation in the two-phase eutectic microstructure. Results indicate that electric current enhances the growth of IMC layer at the cathode and retards it at the anode due to the Bi accumulation acting as a barrier layer with current density of 104 A/cm2. However, when current density increases, the electrical force dissolves the IMC at the cathode into the solder. More and more intermetallic precipitates formed due to the dissolution of Cu into the solder at the cathode side with increased current densities, leading to a very different morphology at the anode and the cathode interfaces, one being planar and the other being very irregular. It can be concluded that the chemical force and the electrical force are the main driving forces contributing to the IMC growth at both interfaces.

In-situ electromigration study on Sn−Ag−Cu solder joint by digital image speckle analysis

The phenomenon of electromigration in Pb-free Sn-Ag-Cu solder joint specimen subject to high current density was characterized. Electromigration measurements in the direction of the electron flux, via Cu metal lines, ENIG metallization, and Sn-Ag-Cu solder joint is documented. The consequence of this migration phenomenon causes void formation and the significant accumulation of voiding is a serious reliability concern. Digital image speckle analysis (DISA) was used to measure the in-situ micro-deformation of a cross sectioned solder joints, which is subject to electromigration with a current density of 5*10/sup 3/A/cm/sup 2/ under a constant temperature of 150/spl deg/C. The deformation and strain field of the solder joint were analyzed by a digital image correlation software. After 120hrs thermal-current test, higher strain near large voids was detected. The IMC growth behavior with/without current are compared. It was noted that IMC layer growth on anode interface is faster than cathode interface, and both are faster than isothermal aging.

Improvement of microstructure and interface structure of eutectic Sn–0.7Cu solder with small amount of Zn addition

Abstract With small amount of Zn addition, a refined microstructure of bulk Sn–0.7Cu solder alloy has been obtained. Meanwhile, retardation effect has been observed on the growth of IMC layer at the solder/Cu joint interface. The composition variation of the interface IMC layer with the amount of Zn addition is reported and thermodynamically analyzed.

Aging studies of Cu-Sn intermetallic compounds in annealed surface mount solder joints

This is the second in a series of investigations aimed at studying the effect of Cu-Sn intermetallic compound on the reliability of surface mount solder joints. Our previous investigation revealed the formation kinetics and characteristics of Cu-Sn intermetallic compounds in LCCC surface mount solder joints during IR-reflow soldering. The present study focuses on the solid state growth of the interfacial Cu-Sn intermetallic compound in LCCC surface mount solder joints under prolonged annealing at elevated temperature. A thick Cu-Sn IMC layer at the Sn-Pb solder/Cu interface of a surface mount solder joint (which can be achieved by prolonged storage at high temperature or after long term operation of modern SMT electronic assemblies) makes the interface more sensitive to stress and may eventually lead to fatigue failure of all SMT solder joint. The microstructural morphology of the Cu-Sn IMC layer at the solder/Cu pad interface in all annealed LCCC surface mount solder joints is duplex and consists of /spl eta/-phase Cu/sub 6/Sn/sub 5/ and /spl epsi/-phase Cu/sub 3/Sn IMC. Both Cu-Sn IMC phases thicken as the aging time increases. On the other hand, at the interface close to the component metallization, the growth of both the /spl eta/- and /spl epsi/-phase were showed to be suppressed, with more pronounced effect on /spl epsi/-phase, by Ni originating from the metallization. The mean total layer thickness was found to increase linearly with the square root of aging time and the growth was faster for higher annealing temperature. The activation energy for the growth of interfacial Cu-Sn IMC layers and the pre-exponential factor, D/sub o/, for diffusion were calculated to be 1.09 eV and 1.68/spl times/10/sup -4/ m/sup 2//s respectively for the 0805 LCCC surface mount solder joint using eutectic Sn-Pb solder. These values are useful in predicting the thickness of interfacial Cu-Sn IMC layers in real surface mount solder joints in electronic assemblies after prolonged operation and hence the reliability factor of the solder joint attributed to the interfacial Cu-Sn IMC layer. Finally, the pad size and quantity of Sn-Pb solder employed in LCCC surface mount solder joints were shown to have an insignificant effect on the solid state growth rate for the interfacial Cu-Sn IMC layers.