Interfacial Intermetallic Compound Grain Research Articles

Stable SnAgCu solder joints reinforced by nickel-coated carbon fiber for electronic packaging

Today’s 3D electronic packaging is characterized by smaller size and higher functional density, which generally require multiple reflow cycles. However, due to the Ostwald ripening process of the interfacial intermetallic compound (IMC), the solder joint’s strength would decrease as the number of reflow cycles increases. To address this, we developed a novel composite solder by incorporating nickel-coated carbon fiber (Ni@CF) into the Sn–3.0Ag–0.5Cu (SAC305) solder connection. The study showed that Ni@CF effectively enhanced the stability and reliability of solder joints during multiple reflow processes. Compared with Ni@CF-free equivalents, Ni@CFs promoted the nucleation of interfacial IMC during the solid–liquid reaction and inhibited Ostwald ripening, leading to a refinement of the interfacial IMC grains. As a result, the interface was strengthened, and the shift of shear fracture to the brittle interface seen in the SAC305 solder joint with multiple reflows did not occur in the composite solder joint. Furthermore, carbon fiber strengthened the solder matrix through the load transfer and shear-off mechanisms, depending on its inclination angle with the shear plane. Consequently, the Ni@CFs-reinforced solder joints maintained a shear strength of over 42 MPa throughout ten reflow cycles, while the pristine SAC305 solder joint exhibited decreased shear strength to 33 MPa. This study provides new insights into composite solder joints’ stability and reinforcement characteristics when subjected to multiple reflows.

Microstructure and orientation evolution of β-Sn and interfacial Cu6Sn5 IMC grains in SAC105 solder joints modified by Si3N4 nanowires

In this study, Si3N4 nanowires (NWs) with ceramic properties were incorporated into Sn1.0Ag0.5Cu (SAC105) solder to enhance its overall performance. The thermal properties, spreading behavior, microstructure, interface, and mechanical properties of SAC105-xSi3N4 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 wt%) solders were systematically investigated. The research revealed that doping Si3N4 NWs into SAC105 solder could expand its melting range and decrease undercooling. Notably, the solder alloy containing 0.6 wt% Si3N4 NWs had the lowest thermal expansion coefficient (CTE), which improved the CTE matching of SAC105 solder with Cu substrate. Although the solder with 0.4 wt% Si3N4 NWs exhibited superior wetting properties, it was less effective in refining the microstructure and interfacial intermetallic compounds (IMC) than those containing 0.6 wt% Si3N4 NWs. Meanwhile, when the β-Sn phase in the matrix and the Cu6Sn5 IMC phase at the interface were analyzed by electron back scatter diffraction (EBSD), their grain orientation was found to be optimized by the doping of 0.6 wt% Si3N4 NWs. The <001> direction of the β-Sn grains of SAC105-0.6Si3N4 was perpendicular to the Cu substrate, showing a texture structure. The grain orientation of the interfacial Cu6Sn5 IMC made it well-adapted for three-dimensional (3D) packaging. Finally, the mechanical properties enhancement of the solder joints were analyzed in detail by combining the fracture morphology and the EBSD results of the Sn matrix.

Study on the dual inhibition behavior of interfacial IMCs in Cu/SAC105/Cu joint by adopting SiC nanowires and nanocrystalline Cu substrate

To obtain high-strength joints suitable for three-dimensional (3D) packaging, SiC nanowires (NWs) were doped to the Sn1.0Ag0.5Cu (SAC105) solder, and the conventional Cu substrate was replaced with nanocrystalline (NC) Cu to form Cu/solder/NC Cu solder joints. It was found that the application of SiC NWs and NC Cu had an essential effect on the evolution and growth kinetics of the solder joint interface. The formation of intermetallic compounds (IMC) bridging and full-IMC joints at the interface was delayed. The absorption effect of SiC NWs and numerous nucleation sites provided by NC Cu resulted in a notable reduction in interfacial IMC grains size and interfacial Cu3Sn IMC layer thickness. In addition, the Cu/SAC105-SiC/NC Cu solder joint exhibited the highest shear strength. On the one hand, SiC NWs were incorporated into Cu6Sn5 IMC grains to form the reinforced concrete structure. On the other hand, NC Cu provided more fine Cu6Sn5 IMC grains for the interface. Combined with the analysis of the fracture morphology, it can be concluded that the combination of SiC NWs and NC Cu substrate substantially improved the mechanical properties of the solder joints.

Multi-phase field simulation of Al[formula omitted]Ni[formula omitted] intermetallic growth at liquid Al/solid Ni interface using MD computed interfacial energies

Considering its application in developing Raney-type Ni catalysts and in metal surface coatings, the study on the growth behavior of Al3Ni2 intermetallic compound (IMC) at the Al/Ni material interface is of utmost importance. The present work integrates nanoscale molecular dynamics (MD) calculation with mesoscale phase field model for studying the interfacial phenomena associated with Al3Ni2 growth in Al/Ni interface at 1173.15 K. The interfacial energies computed from MD are in the range 0.9–1.2 J/m2 with FCC/IMC featuring as the interface with the largest value and IMC/LIQUID as the one with the lowest value. Phase field model parameters characterizing a varying interface energy formulation are established to simulate the 2D growth of interfacial IMC grains. With the help of an atomistically informed phase field model, it has been revealed that the phase areas and morphology are obviously sensitive to the interfacial properties. The methodologies and results of these multiscale simulations for IMC interfaced between Al and Ni microstructures offer the complementary and accelerated design route of in-silico studies for materials systems experimented at high temperature.

Interfacial reaction and shear strength of ultrasonically-assisted Sn-Ag-Cu solder joint using composite flux

Cu modified carbon nanotubes (Cu-CNTs) doped flux were prepared in this paper, the influence of the composite flux on the evolution of microstructure and shear strength of Sn-3.0Ag-0.5Cu (SAC305) solder joints were investigated, the ultrasonic vibration (USV) was applied for the solder joint during reflowing. Experiments were designed to modify the CNTs with Cu nanoparticles through electroless modification process. Transmission electron microscopy (TEM) was utilized to obtain the graph of modified CNTs. Using X-ray photoelectron spectroscopy (XPS) spectra to prove that the CNTs were successfully modified with Cu nanoparticles. Compared with the solder joint without USV, the application of USV on the solder joint would alter the morphologies of interfacial IMC layer and grains, reduce the IMC thickness and grain size by 16.6 % and 65.6 %, respectively. Reflow process was implemented to evaluate the microstructure evolution and interfacial reaction of SAC305 solder joint with different content of Cu-CNTs doped flux with USV. It was found that the presence of Cu-CNTs could reduce the thickness and grain size of interfacial intermetallic compound (IMC) within solder joint by 3.1 % and 5.25 %, respectively. After addition of the Cu-CNTs in the flux, the thickening of IMC layer and the coarsen of interfacial IMC grain were suppressed, the optimal inhibition effect was gained when the content of Cu-CNTs was 0.1 wt.%. The shear test was conducted for the Cu/SAC305/Cu solder joint with flux containing Cu-CNTs under different soldering condition, the results indicated that as the content of the Cu-CNTs in the flux increased, the shear strength was significantly enhanced by up to 16.8 %. Furthermore, the application of USV could enhance the shear strength by up to 14.2 % as well, The fracture type of solder joints without USV treatment also changed from ductile-brittle mixed type to ductile type as the content of Cu-CNTs in the flux increased. While, the fracture type of all the solder joints with USV treatment was ductile type.

Interfacial IMC Growth and Nanomechanical Characterizations of Solder in Sn-16Sb/Cu Joints during Solid-state Aging

The effects of aging time and temperature on the formation and growth behavior of interfacial intermetallic compound (IMC) of Sn-16Sb/Cu(wt%) solder joints prepared by using dip soldering were investigated. The results show that the major IMCs between Sn-16Sb solder and Cu substrate after thermal aging are Cu3Sn and Cu6Sn5. The thickness of the interfacial IMC in Sn-16Sb/Cu is linearly against the square root of aging time, which indicates that the growth of IMC is mainly controlled by diffusion between Cu and Sn atoms. By using linear regression method, the growth rate constants of interfacial IMC layers are 1.254×10-18, 8.821×10-18 and 1.22×10-17 m2s-1 for Sn-16Sb/Cu joints aged at 120, 150 and 170 °C, respectively. Besides, the activation energy of the interfacial IMC growth was also calculated to be 68.27 kJ/mol. The IMC grain diameters after aging treatment increase with the increasing aging time, with i e, d = 0.492t0.106, d = 0.543t0.143 and d = 0.290t0.263 for aging temperatures of 120, 150 and 170 °C, respectively. Besides, by using nanoindentation, the softening of Sn-16Sb solder was found during aging treatment. Moreover, the U-shape evolution of the values in hardness and Young's moduli was found in this work.

Effect of Cu6Sn5 nanoparticles size on the properties of Sn0.3Ag0.7Cu nano-composite solders and joints

The effect of different-sized Cu6Sn5 nanoparticles on thermal behavior and interfacial reaction with Cu substrate of Sn–0.3Ag–0.7Cu solder was evaluated. The results showed that 30 nm and 70 nm Cu6Sn5 nanoparticles were successfully synthesized by using the method of different reductants, attributed to the difference of reductive ability. Under condition of the same heating parameters, 30 nm Cu6Sn5 nanoparticle owned lower sintering temperature than that of 70 nm Cu6Sn5 nanoparticle. The thermal behavior was revealed that the composite solders with the addition of different-sized Cu6Sn5 nanoparticles had slightly lower melting point and higher undercooling value, compared with original Sn–0.3Ag–0.7Cu solder alloy. The highest undercooling value occurred with 30 nm Cu6Sn5 nanoparticle addition. Moreover, the interfacial reactions between Cu substrate and Sn–0.3Ag–0.7Cu solders mixed with different-sized Cu6Sn5 nanoparticles have produced after reflowing at 250 °C and aging at 150 °C for different interval time. It was clearly found that comparing to the initial Sn–0.3Ag–0.7Cu/Cu solder joint, the thickness of interfacial intermetallic compound (IMC) layer on solder joint with adding nanoparticle was thinner and the growth rate was slower. The growth of interfacial IMC was suppressed with the addition of Cu6Sn5 nanoparticle, and 30 nm Cu6Sn5 nanoparticle had the strongest inhibition effect on the growth of IMC, as well as the growth of IMC grains.

Effect of flux doped with Cu6Sn5 nanoparticles on the interfacial reaction of lead-free solder joints

In this paper, flux doped with 0.05, 0.1, and 0.2 wt% Cu6Sn5 nanoparticles (NPs) were reasonably believed to affect the morphology and growth rate of the intermetallic compounds (IMC) between Cu substrate and Sn–3.0Ag–0.5Cu solder. Reflowing was performed at 250 °C, then isothermal aging was conducted at 150 °C up to 360 h. The experimental results show that the thickness of the IMC layer increased with the increment of aging time. When the aging time extended to 120 h or more, the Cu3Sn layer appeared on the side of the Cu substrate and also thickened as time increased. The additions of Cu6Sn5 NPs into the flux did not change the type of IMCs while the total thickness of the IMC changed visually. It was calculated that the corresponding growth rate constant of interfacial IMCs in solder joints with flux contained 0, 0.05, 0.1, and 0.2 wt% Cu6Sn5 NPs were 0.14766, 0.14719, 0.14578 and 0.14726 μm/h1/2, respectively. It means that adding Cu6Sn5 NPs into flux could effectively inhibit the growth of IMC layer. The strongest inhibition effect on the growth of IMC layer could be achieved when the content of Cu6Sn5 NPs was 0.1%. Flux with Cu6Sn5 NPs could also effectively inhibit the coarsening of the interfacial IMC grains, and adding 0.1% NPs to the flux has the best inhibition effect.

Formation and growth of interfacial intermetallics in Sn-0.3Ag-0.7Cu-xCeO2/Cu solder joints during the reflow process

The effects of adding CeO2 nanoparticles on the formation and growth of interfacial intermetallic compounds (IMCs) between Sn-0.3Ag-0.7Cu-xCeO2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3 wt%) (SAC0307-xCeO2) solders and Cu substrates during reflow soldering were investigated. Scanning electron microscopy (SEM) was used to observe the microstructural evolution of the solder joints and measure the thickness and grain size of the intermetallic layers. The results show that the thicknesses and grain sizes of the IMCs decrease with CeO2 nanoparticle addition to the SAC0307 solder and that these parameters decrease significantly at a particle concentration of approximately 0.15 wt%. At higher concentrations, the thicknesses and grain sizes of the IMCs increase slightly. The growth exponents for the IMC layers and grains are determined by curve fitting to study the growth kinetics of the IMCs during reflow soldering. The results reveal that the growth exponents of the IMC layers range from 0.335 to 0.374 and those of the IMC grains range from 0.302 to 0.318, thus suggesting that the growth of the IMCs is controlled by the combined kinetic processes of atomic interdiffusion, interfacial reactions, and grain ripening. These data also show that among the tested solders, the SAC0307–0.15CeO2 solder joint has the smallest growth rate and exhibits the most significant effects in IMC growth suppression and grain size refinement. Based on the theories of heterogeneous nucleation and adsorption, a mechanism for reducing the IMC growth rate by the addition of CeO2 nanoparticles to the SAC0307 solder is proposed.

Interfacial microstructure evolution and shear strength of Sn0.7Cu–xNi/Cu solder joints

The microstructure and growth behaviors of interfacial intermetallic compounds (IMCs) between the Sn–0.7Cu–xNi (x = 0, 0.1, 0.25, 0.5, and 1.0 mass%) solder alloys and copper pad were explored to reveal the influence of the Ni addition into the Sn–0.7Cu based solder on the formation of interfacial IMCs and shear strength of the solder joints in the present study. The impacts of adding different Ni on the growth behavior and total thickness of Cu6Sn5/(Cu, Ni)6Sn5 and Cu3Sn IMCs layers were compared and discussed after reflowing at 533 K for 600 s and aging at 423 K for various aging durations. The thickness of interfacial IMC layers increased drastically with addition of small amount of Ni into the Sn0.7Cu solder alloy, however, the particle sizes of the IMC grains obviously decreased with the addition of Ni into the solder, compared to the Sn0.7Cu/Cu system. The results indicated that, Cu6Sn5 turned into (Cu, Ni)6Sn5 due to the addition of Ni element and would enhance the growth rate of interfacial IMC resulted from more Ni added. It is quite obvious that 1 wt% Ni addition to the Sn–0.7Cu solder alloy extremely enhance growth and formation of the IMC in Sn–0.7Cu–xNi/Cu joints. In addition, the single-lap shear tests were also carried out to evaluate the mechanical properties of the as-reflowed Sn0.7Cu–xNi/Cu solder joints. The results of shear tests indicated that the shear stress of Sn–0.7Cu–xNi/Cu solder joints gradually increased with the increase of Ni concentration, and the maximum shear strength was reached with 1 wt% Ni addition. Besides, doping Ni element into the Sn–0.7Cu solder changed the shear fractures morphology of Sn–0.7Cu/Cu joints, which were revealed to be ductile essentially with the moderate strain rate (3.33 × 10−3 s−1) and mainly occurred in the solder matrix rather than through the IMCs layer.

Interfacial Reaction and IMC Growth of an Ultrasonically Soldered Cu/SAC305/Cu Structure during Isothermal Aging.

In order to accelerate the growth of interfacial intermetallic compound (IMC) layers in a soldering structure, Cu/SAC305/Cu was first ultrasonically spot soldered and then subjected to isothermal aging. Relatively short vibration times, i.e., 400 ms and 800 ms, were used for the ultrasonic soldering. The isothermal aging was conducted at 150 °C for 0, 120, 240, and 360 h. The evolution of microstructure, the IMC layer growth mechanism during aging, and the shear strength of the joints after aging were systemically investigated. Results showed the following. (i) Formation of intermetallic compounds was accelerated by ultrasonic cavitation and streaming effects, the thickness of the interfacial Cu6Sn5 layer increased with aging time, and a thin Cu3Sn layer was identified after aging for 360 h. (ii) The growth of the interfacial IMC layer of the ultrasonically soldered Cu/SAC305/Cu joints followed a linear function of the square root of the aging time, revealing a diffusion-controlled mechanism. (iii) The tensile shear strength of the joint decreased to a small extent with increasing aging time, owing to the combined effects of IMC grain coarsening and the increase of the interfacial IMC. (iv) Finally, although the fracture surfaces and failure locations of the joint soldered with 400 ms and 800 ms vibration times show similar characteristics, they are influenced by the aging time.

Joule heating dominated fracture behavior change in micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu(Ni) joints under electro-thermal coupled loads

Joule heating dominated fracture position transition from the interface between the solder and the intermetallic compound (IMC) layer to the solder matrix occurs in micro-scale Cu/Sn-3.0Ag-0.5Cu/Cu(Ni) joints under electro-thermal coupled loads with a high current density and increasing temperature. The interfacial fracture is triggered by localized-melting of the solder induced by current crowding at grooves of the interfacial IMC grains and driven by the strain mismatch between the solder and the interfacial IMC layer, while fracture in the solder matrix is caused by matrix bulk melting and the constraint effect from the interfacial IMC layer-substrates.

Effect of initial Cu concentration on the IMC size and grain aspect ratio in Sn–xCu solders during multiple reflows

The effect of initial Cu concentration in Sn–xCu solders, on grain size and morphology of interfacial intermetallic compound (IMC) during multiple reflow process, was investigated in this work. It was found that the initial Cu content in solders is closely related to the IMC nucleation and Cu migration behavior, which can govern the IMC crystal growth as well as grain aspect ratio evolution. In general, higher initial Cu in solder would lead to an increase in thickness of IMC, regardless of the reflow number. For Sn, Sn0.5Cu, Sn1.5Cu and Sn2.0Cu, the grain aspect ratios show decreased values with rising initial Cu percentage in solder; but for Sn0.7Cu, it has an even smaller aspect ratio than pure Sn. Moreover, Sn0.5Cu, Sn1.5Cu and Sn2.0Cu present a pronounced suppression of grain aspect ratio with reflow number, whereas this is not observed in context of pure Sn and Sn0.7Cu solders. Fine Cu6Sn5 nanoparticles on the surface of interfacial IMC grains in Sn0.7Cu alloy were observed in both isothermal heating and cooling, which may have served as the reason for its unique behavior of grain aspect ratio evolution. The relationship between Cu addition and IMC grain growth can be utilized in the reliability design of Pb-free solder materials.

Effect of nano-Cu addition on microstructure evolution of Sn0.7Ag0.5Cu-BiNi/Cu solder joint

PurposeThe purpose of this paper is to focus on the fabrication of SnAgCu (SAC) nanocomposites solder and study the effect of Cu nanopowders (nano-Cu) addition on the microstructure evolution of resultant nanocomposite solder after reflow and thermal aging.Design/methodology/approachMechanical mixing is used in this work to incorporate nanoparticles into the solder and produce more homogeneous mixture. Standard metallographic procedures are applied for microstructural analysis of solder joints.FindingsIt is found that nano-Cu doped into Sn0.7Ag0.5Cu-BiNi solder has no appreciable influence on melting temperature of the composite solder. The addition of Cu nanoparticles refines the microstructure of bulk solder and suppresses the growth of interfacial intermetallic compound (IMC) layers. However, interfacial IMC grain size increases slightly after 1.0 per cent nano-Cu added.Originality/valueThe paper demonstrates a method of nano-composite solder paste preparation by means of mechanical mixing and a comparison study of the microstructure evolution of composite solder with the basic SAC solder.

Polarized evolution of interfacial intermetallic compounds (IMCs) in interconnects under electromigration (EM)

In situ observation of electromigration (EM) has been carried out on cross-sectioned copper/tin–copper/copper (Cu/Sn0.7Cu/Cu) line-type interconnects. The surface vertical variation after EM is measured by step profiler, which indicates serious mass migration during EM. The EM rate is calculated using two distinct methods, in situ mark movement and the growth of anode intermetallic compounds (IMCs). Defects and IMCs grains in the bulk solder are used as marks to measure the atomic flux. It is calculated that the atomic flux under a current density of 4.77 × 103 A cm−2 and a temperature of 60 °C is 5.241 × 1012 cm−2 s−1. Calculation based on the growth of the anode IMCs layer thickness determines the atomic flux as 4.114 × 1012 cm−2 s−1. The top morphology of IMCs layers is studied by etching away the bulk solder. It is found that the initial scalloped IMCs in the as-reflowed interconnect evolves towards different directions as a serrated IMCs layer with deep grooves forms at the cathode side, while a thick planar IMCs layer forms at the anode side. This phenomenon is defined as polarized evolution. The mechanisms for the polarized evolution rely on the fast diffusion of Cu atoms along the IMCs grain boundaries. The planar IMCs at the anode is due to the stacking of Cu atoms, which fills the grooves by reacting with Sn to form Cu6Sn5 IMCs. This polarized evolution threatens the reliability of interconnects since a thick planar IMCs layer tends to be brittle, while a thin serrated IMCs may suffer from accumulated voids.

Effects of Bi addition on interfacial reactions and mechanical properties of In–3Ag–xBi/Cu solder joints

In this study, the effects of Bi addition on the interfacial reactions and mechanical properties of In–3Ag–xBi/Cu joints were investigated. Better wettability was obtained with increased addition of Bi (0–2 wt%), but further additions up to 3 wt% decreased the beneficial effects. The intermetallic compounds (IMCs) formed at In–3Ag–xBi/Cu interfaces were (Ag,Cu)In2 in all cases. With the increase of Bi addition, the interfacial IMC grains coarsened and the IMC layers got thicker. The EPMA mapping revealed that Bi addition reacted with neither In or Ag to form intermetallics, it dissolved as solid solution in Indium matrix uniformly. In addition, the shear strength of the soldered joints were improved by Bi addition, with the peak shear strength occurring at 2 wt% Bi addition. The fracture mode also changed from typical ductile fracture mode to mixed ductile–brittle fracture mode with increased amount of Bi addition.

Influence of rare earth Ce addition on the microstructure, properties and soldering reaction of pure Sn

The influence of trace rare earth (RE) Ce addition on the microstructure, melting point and wettability of pure Sn as well as on the soldering reactions in Sn-xCe/Cu(Ni) solder joints was investigated. In bulk Sn-xCe solders, large β-Sn grains were observed with the Ce addition less than 0.2 wt%; while the β-Sn grain size decreased markedly when the Ce addition was 0.2 wt%, resulting in a refined microstructure. The addition of trace RE Ce had little effect on the melting temperature of the solders. Smaller wetting angles of Sn-xCe solders on both Cu and Ni substrates were measured when the samples were reflowed at a higher temperature. The Sn-0.2Ce solder owned the best wettability on Cu substrate. Scallop-like Cu6Sn5 intermetallic compound (IMC) grains formed at the Sn-xCe/Cu interfaces, while a continuous Ni3Sn4 IMC layer formed at each Sn-xCe/Ni interface. With the increase of Ce addition, the interfacial IMC grain size and the interfacial IMC layer thickness on both Cu and Ni substrates decreased gradually. The activity of Sn was lowered with the Ce addition, which depressed the growth of the interfacial IMC. In the current study, the Ce addition of 0.2 wt% exhibits the optimized performance.

Effect of Silver Content on Bulk Properties and Interfacial Morphology of Sn-xAg-Cu Solders

The bulk microstructure, melting behavior, mechanical property, and interfacial Intermetallic compound (IMC) morphology were investigated on Sn-Ag-Cu (SAC) lead free solders with different Ag content (0.3,1.0,2.0,3.0,3.8wt%). The result indicates that SAC solders with higher Ag content present finer and denser intermetallic particles in the bulk solder, as a result that the tensile strength of SAC solders increased with the increasing of Ag content, while the ductility decreased. The melting temperature of SAC305 and SAC387 solders are close to eutectic point from the narrow melting range. It was found that the interfacial IMC morphology didnt appear obvious difference regardless of Ag content for as-soldered. Furthermore, the higher Ag contained solders present smaller IMC grain at the interface of aged joints and all aged joints have a tendency of polyhedron morphology.

Drop impact reliability of Sn–1.0Ag–0.5Cu BGA interconnects with different mounting methods

The poor drop-shock resistance of near-eutectic Sn–Ag–Cu (SAC) solder interconnects drives the research and application low-Ag SAC solder alloys, especially for Sn–1.0Ag–0.5Cu (SAC105). In this work, by dynamic four-point bend testing, we investigate the drop impact reliability of SAC105 alloy ball grid array (BGA) interconnects with two different surface mounting methods: near-eutectic solder paste printing and flux dipping. The results indicate that the flux dipping method improves the interconnects failure strain by 44.7% over paste printing. Further mechanism studies show the fine interfacial intermetallic compounds (IMCs) at the printed circuit board side and a reduced Ag content inside solder bulk are the main beneficial factors overcoming other negative factors. The flux dipping SAC105 BGA solder joints possess fine Cu6Sn5 IMCs at the interface of solder/Cu pads, which increases the bonding strength between the solder/IMCs and the fracture resistance of the IMC grains themselves. Short soldering time of flux dipping joints above the solder alloy liquidus mitigates the growth of interfacial IMCs in size. In addition, a reduced Ag content in flux dipping joint bulk causes a low hardness and high compliance, thus increasing fracture resistance under higher-strain rate conditions.

Spalling behavior of interfacial intermetallic compounds in Pb-free solder joints subjected to temperature cycling loading

Morphological evolution of interfacial intermetallic compounds (IMC) subjected to thermal cycling loading was studied numerically and experimentally. In this study, the microstructure of Sn3.5Ag0.7Cu solder joint and its interfacial IMCs were investigated using Tape Ball Grid Array assemblies. Needle-type and hemispherical-type morphologies of interfacial IMCs were formed after reflow. During temperature cycling, the IMC grains ripened into large hemispherical-type and spheroidal-type morphologies and spalled into the solder joint. Finite element analysis showed that the spalling behavior was promoted by the geometrical changes of the IMC structure and the induced cyclic shear stresses during temperature cycling loading. For needle-type grains, the stress was concentrated at the tip of the IMCs, while the maximum shear stress was redistributed to the roots of the spheroidal-type grains.