Interfacial Interfacial Intermetallic Compound Layer Research Articles

Effect of Si3N4 nanowires doping on microstructure and properties of Sn58Bi solder for Cu bonding

PurposeThe purpose of this study is to delve into the mechanism of Si3N4 nanowires (NWs) in Sn-based solder, thereby furnishing a theoretical foundation for the expeditious design and practical implementation of innovative lead-free solder materials in the electronic packaging industry.Design/methodology/approachThis study investigates the effect of adding Si3N4 NWs to Sn58Bi solder in various mass fractions (0, 0.1, 0.2, 0.4, 0.6 and 0.8 Wt.%) for modifying the solder and joining the Cu substrate. Meanwhile, the melting characteristics and wettability of solder, as well as the microstructure, interfacial intermetallic compound (IMC) and mechanical properties of joint were evaluated.FindingsThe crystal plane spacing and lattice constant of Sn and Bi phase increase slightly. A minor variation in the Sn58Bi solder melting point was caused, while it does not impact its functionality. An appropriate Si3N4 NWs content (0.2∼0.4 Wt.%) significantly improves its wettability, and modifies the microstructure and interfacial IMC layer. The shear strength increases by up to 10.74% when adding 0.4 Wt.% Si3N4 NWs, and the failure mode observed is brittle fracture mainly. However, excessive Si3N4 will cause aggregation at the junction between the solder matrix and IMC layer, this will be detrimental to the joint.Originality/valueThe Si3N4 NWs were first used for the modification of lead-free solder materials. The relative properties of composite solder and joints were evaluated from different aspects, and the optimal ratio was obtained.

Microstructure and Properties of Electromigration of Sn58Bi/Cu Solder Joints with Different Joule Thermal Properties

Electromigration is one of the most important research issues affecting the reliability of solder joints. Current-induced Joule heating affects the electromigration behavior of solder joints. Solder joints with different cross-sectional areas were designed to obtain different Joule heating properties. The effects of the interfacial intermetallic compound (IMC) and mechanical properties of Sn58Bi/Cu solder joints were studied for different Joule heating properties. The results showed that as the cross-sectional area of the Sn58Bi/Cu solder joints increased, the Joule heating of the joint increased. The anode IMC thickness of the joint thickened and transformed into a planar shape. The Bi migrated to the anode region to form a Bi-rich layer and gradually increased in thickness. The cathode IMC thickness first increased, then decreased, and gradually dissolved. The Sn-rich layer formed near the solder side and gradually increased in thickness, with microcracks occurring when the cross-sectional area of the joint increased to 0.75 mm2. The joint shear fracture path moved from the soldering zone near the cathode IMC layer to the interfacial IMC layer. The fracture mechanism of the joint changed from a mixed brittle/tough fracture, dominated by deconstruction and secondary cracking, to a brittle fracture dominated by deconstruction. The joint shear strength was reduced by 60.9% compared to that in the absence of electromigration.

Effects of Multiple Reflow on the Formation of Primary Crystals in Sn-3.5Ag and Solder Joint Strength: Experimental and Finite Element Analysis.

The growth and formation of primary intermetallics formed in Sn-3.5Ag soldered on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish after multiple reflows were systematically investigated. Real-time synchrotron imaging was used to investigate the microstructure, focusing on the in situ growth behavior of primary intermetallics during the solid-liquid-solid interactions. The high-speed shear test was conducted to observe the correlation of microstructure formation to the solder joint strength. Subsequently, the experimental results were correlated with the numerical Finite Element (FE) modeling using ANSYS software to investigate the effects of primary intermetallics on the reliability of solder joints. In the Sn-3.5Ag/Cu-OSP solder joint, the well-known Cu6Sn5 interfacial intermetallic compounds (IMCs) layer was observed in each reflow, where the thickness of the IMC layer increases with an increasing number of reflows due to the Cu diffusion from the substrate. Meanwhile, for the Sn-3.5Ag/ENIG solder joints, the Ni3Sn4 interfacial IMC layer was formed first, followed by the (Cu, Ni)6Sn5 IMC layer, where the formation was detected after five cycles of reflow. The results obtained from real-time imaging prove that the Ni layer from the ENIG surface finish possessed an effective barrier to suppress and control the Cu dissolution from the substrates, as there is no sizeable primary phase observed up to four cycles of reflow. Thus, this resulted in a thinner IMC layer and smaller primary intermetallics, producing a stronger solder joint for Sn-3.5Ag/ENIG even after the repeated reflow process relative to the Sn-3.5Ag/Cu-OSP joints.

Microstructure Evolution and Shear Strength of Tin-Indium-xCu/Cu Joints

The low melting temperature In-48Sn alloy is a promising candidate for flexible devices. However, the joint strength of the In-48Sn alloy on the Cu substrate was low due to the rapid diffusion of Cu into the In-rich alloy. In this study, the effect of the addition of xCu (x = 2.0 and 8.0 wt.%) on wettability, interfacial reaction, and mechanical strength of the In-Sn-xCu/Cu joint is analyzed. The results demonstrate that both the In-48Sn and In-Sn-xCu alloys exhibit good wettability on the Cu substrate and that the contact angle increases with an increase in the Cu content. Furthermore, fine grains are observed in the alloy matrix of the In-Sn-xCu/Cu joint and the interfacial intermetallic compound (IMC) comprising the Cu-rich Cu6(In,Sn)5 near the Cu substrate and the Cu-deficient Cu(In,Sn)2 near the solder side. The In-Sn-2.0Cu/Cu joint with fine microstructure and a small amount of IMC in the alloy matrix shows the highest average shear strength of 16.5 MPa. Although the In-Sn-8.0Cu/Cu joint also exhibits fine grains, the presence of large number of voids and rough interfacial IMC layer causes the formation of additional stress concentration points, thereby reducing the average shear strength of the joint.

Microstructure and shear properties of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints under thermal cycling

Through ultrasonic wave assisted Sn2.5Ag0.7Cu0.1RExNi/Cu (x = 0, 0.05, 0.1) soldering test and − 40 to 125 °C thermal shock test, the microstructure and shear properties of Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints under thermal cycling were studied by the SEM, EDS and XRD. The results show that the Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints with high quality and high reliability can be obtained by ultrasonic assistance. When the ultrasonic vibration power is 88 W, the ultrasonic-assisted Sn2.5Ag0.7Cu0.1RE0.05Ni/Cu solder joints exhibits the optimized performance. During the thermal cycling process, the shear strength of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints had a linear relationship with the thickness of interfacial intermetallic compound (IMC). Under the thermal cycling, the interfacial IMC layer of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints consisted of (Cu,Ni)6Sn5 and Cu3Sn. The thickness of interfacial IMC of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints was linearly related to the square root of equivalent time. The growth of interfacial IMC of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints had an incubation period, and the growth of IMC was slow within 300 cycles. And after 300 cycles, the IMC grew rapidly, the granular IMC began to merge, and the thickness and roughness of IMC increased obviously, which led to a sharp decrease in the shear strength of the solder joints. The 0.05 wt% Ni could inhibit the excessive growth of IMC, improve the shear strength of solder joints and improve the reliability of solder joints. The fracture mechanism of ultrasonic-assisted Sn2.5Ag0.7Cu0.1RExNi/Cu solder joints changed from the ductile–brittle mixed fracture in the solder/IMC transition zone to the brittle fracture in the interfacial IMC.

Study on the microstructure and mechanical property of Cu-foam modified Sn3.0Ag0.5Cu solder joints by ultrasonic-assisted soldering

Cu-foam sheets with the thicknesses of 0.15 mm and 0.5 mm were utilized as strengthening structure to improve the performances of the Sn3.0Ag0.5Cu (SAC305)/Cu solder joints. The ultrasonic-assisted soldering was applied to manufacture the solder joints. The effects of Cu-foam and ultrasound-assisted soldering time on microstructure and mechanical property of solder joints were researched. Experimental results showed that excellent metallurgical bonding was generated between the copper substrate and the Cu-foam/SAC305 composite solder layer. The average thickness of interfacial intermetallic compound (IMC) layer decreased with the increase of ultrasonically soldering time, and a thinner interfacial IMC layer was found in the joints with ultrasonic treatment in narrow channels. With the ultrasonic soldering time prolonged, the Cu 6 Sn 5 grains became more refined. Some round-shape Cu 6 Sn 5 grains were transformed into hexagonal prismatic with the increase of the ultrasonic soldering time. The shear test was conducted for the solder joints under different soldering conditions. It was found that the shear strength of the Cu-foam/SAC305 composite solder joints were higher than that of SAC305 solder joints. The grain refinement caused by ultrasonic cavitation significantly improved the shear strength of the solder joints. Besides, the fracture behavior of solder joints without ultrasonic vibration was classified to be ductile-brittle mixed type while that with ultrasonic vibration was changed to be ductile type.

Microstructures and mechanical property of 5052 aluminum alloy/Q235 steel butt joint achieved by laser beam joining with Sn-Zn filler wire

Butt joining of 5052 aluminum alloy and Q235 low carbon steel was achieved using laser beam welding with Sn-30Zn filler wire. The weld formation, microstructures, and mechanical property were investigated with varied laser power. The results show that joining of 5052 Al/Q235 steel dissimilar metals in a butt configuration by laser beam welding with Sn-30Zn filler wire is feasible. The solid solution interface formed at the aluminum side and the reaction layer formed at the steel side. With increasing laser power, the joining mode transformed from brazing to welding-brazing due to the melt of aluminum alloy. The thickness of the interfacial intermetallic compounds (IMCs) layer was also increased with laser power. However, when the laser power was 1200 W, the thickness of the interfacial IMCs layer sharply decreased due to the detachment of the IMCs layer. The fracture of the joint occurred at the steel interface and partially through the weld seam on the top surface. The fracture surface was characterized by both brittle and ductile fracture. When the joints were obtained at the laser power of 800 W and 1200 W, the tensile strength reached 79 MPa and 78 MPa, respectively. Furthermore, the formation mechanism of microstructures and its influence on mechanical property was discussed.

Performance of Sn-3.0Ag-0.5Cu Composite Solder with Kaolin Geopolymer Ceramic Reinforcement on Microstructure and Mechanical Properties under Isothermal Ageing.

This paper elucidates the effect of isothermal ageing at temperature of 85 °C, 125 °C and 150 °C for 100, 500 and 1000 h on Sn-3.0Ag-0.5Cu (SAC305) lead-free solder with the addition of 1 wt% kaolin geopolymer ceramic (KGC) reinforcement particles. SAC305-KGC composite solders were fabricated through powder metallurgy using a hybrid microwave sintering method and reflowed on copper substrate printed circuit board with an organic solderability preservative surface finish. The results revealed that, the addition of KGC was beneficial in improving the total thickness of interfacial intermetallic compound (IMC) layer. At higher isothermal ageing of 150 °C and 1000 h, the IMC layer in SAC305-KGC composite solder was towards a planar-type morphology. Moreover, the growth of total interfacial IMC layer and Cu3Sn layer during isothermal ageing was found to be controlled by bulk diffusion and grain-boundary process, respectively. The activation energy possessed by SAC305-KGC composite solder for total interfacial IMC layer and Cu3Sn IMC was 74 kJ/mol and 104 kJ/mol, respectively. Based on a lap shear test, the shear strength of SAC305-KGC composite solder exhibited higher shear strength than non-reinforced SAC305 solder. Meanwhile, the solder joints failure mode after shear testing was a combination of brittle and ductile modes at higher ageing temperature and time for SAC305-KGC composite solder.

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.

Microstructural evolution and micromechanical properties of SAC305/CNT/CU solder joint under blast wave condition

Purpose The purpose of this paper is to investigate the effect of carbon nanotube (CNT) addition on microstructure, interfacial intermetallic compound (IMC) layer and micromechanical properties of Sn-3.0Ag-0.5Cu (SAC305)/CNT/Cu solder joint under blast wave condition. This work is an extension from the previous study of microstructural evolution and hardness properties of Sn-Ag-Cu (SAC) solder under blast wave condition. Design/methodology/approach SAC/CNT solder pastes were manufactured by mixing of SAC solder powder, fluxes and CNT with 0.02 and 0.04 by weight percentage (Wt.%) separately. This solder paste then printed on the printed circuit board (PCB) with the copper surface finish. Printed samples underwent reflow soldering to form the solder joint. Soldered samples then exposed to the open field air blast test with different weight charges of explosives. Microstructure, interfacial IMC layer and micromechanical behavior of SAC/CNT solder joints after blast test were observed and analyzed via optical microscope, field emission scanning microscope and nanoindentation. Findings Exposure to the blast wave induced the microstructure instability of SAC305/Cu and SAC/CNT/Cu solder joint. Interfacial IMC layer thickness and hardness properties increases with increase in explosive weight. The existence of CNT in the SAC305 solder system is increasing the resistance of solder joint to the blast wave. Originality/value Response of micromechanical properties of SAC305/CNT/Cu solder joint has been identified and provided a fundamental understanding of reliability solder joint, especially in extreme conditions such as for military applications.

Effect of doping Ni nanoparticles on microstructure evolution and shear behavior of Sn–3.0Ag–0.5Cu(SAC305)/Cu–2.0Be solder joints during reflowing

The Cu–2.0Be alloy is widely used as the substrate in aircraft electronic device, which has a potential trend to replace the pure Cu substrate. However, few studies have investigated the influence of the doped Ni nanoparticles on interfacial reaction between Cu–2.0Be substrate and the SAC305 solder during reflowing. In this study, the effect of Ni nanoparticles on wettability, microstructure evolution and mechanical behavior of SAC305/Cu–2.0Be solder joint was investigated. The results showed that a certain amount of Ni nanoparticles could increase the spreading area and mechanical property of the SAC305/Cu–2.0Be solder joint. The interfacial intermetallic compound (IMC) layer thickness could be reduced significantly by adding Ni nanoparticles. Moreover, the interfacial IMC layer thickness of SAC305/Cu–2.0Be solder joint was obviously thicker than that of SAC305-0.1Ni/Cu–2.0Be solder joint after different soldering times, which meant that a certain amount of Ni nanoparticles could hinder the IMC growth during soldering process. The growth kinetic of interfacial IMC layer after adding 0.1 wt% Ni nanoparticles showed that the growth coefficient of interfacial IMC layer changed from 1.21 µm/s0.28 to 1.82 µm/s0.32. In addition, the value of the shear strength of SAC305-0.1Ni/Cu–2.0Be solder joint was larger than that of SAC305/Cu–2.0Be solder joint with a typical mixture ductile and brittle fracture mode, which contributed to increasing the shear strength. Therefore, SAC305-0.1Ni solder alloy is a promising potential solder alloy in aircraft electronic device.

Dynamic Observation of Interfacial IMC Evolution and Fracture Mechanism of Sn2.5Ag0.7Cu0.1RE/Cu Lead-Free Solder Joints during Isothermal Aging.

Dynamic observation of the microstructure evolution of Sn2.5Ag0.7Cu0.1RE/Cu solder joints and the relationship between the interfacial intermetallic compound (IMC) and the mechanical properties of the solder joints were investigated during isothermal aging. The results showed that the original single scallop-type Cu6Sn5 IMC gradually evolved into a planar double-layer IMC consisting of Cu6Sn5 and Cu3Sn IMCs with isothermal aging. In particular, the Cu3Sn IMC grew towards the Cu substrate and the solder seam sides; growth toward the Cu substrate side was dominant during the isothermal aging process. The growth of Cu3Sn IMC depended on the accumulated time at a certain temperature, where the growth rate of Cu3Sn was higher than that of Cu6Sn5. Additionally, the growth of the interfacial IMC was mainly controlled by bulk diffusion mechanism, where the activation energies of Cu6Sn5 and Cu3Sn were 74.7 and 86.6 kJ/mol, respectively. The growth rate of Cu3Sn was slightly faster than that of Cu6Sn5 during isothermal aging. With increasing isothermal aging time, the shear strength of the solder joints decreased and showed a linear relationship with the thickness of Cu3Sn. The fracture mechanism of the solder joints changed from ductile fracture to brittle fracture, and the fracture pathway transferred from the solder seam to the interfacial IMC layer.

Unique interfacial reaction and so-induced change in mechanical performance of Sn–3.0Ag–0.5Cu/Cu solder joints formed during undercooled and eutectic liquid soldering processes

A novel technique is proposed to prepare Sn–3.0Ag–0.5Cu/Cu joints at three different processing temperatures, which are called undercooled liquid (UL, 213.0 °C), eutectic liquid (EL, 217.0 °C) and normal liquid (NL, 231.0 °C) soldering temperatures correspondingly, and each of them is lower than, equal to and higher than Sn–3.0Ag–0.5Cu solder’s melting point (217.0 °C), respectively. The interfacial reaction, intermetallic compound growth and mechanical performance of Sn–3.0Ag–0.5Cu/Cu joints formed during UL, EL and NL soldering processes were investigated systematically. Results show that for Sn–3.0Ag–0.5Cu solder as the undercooled liquid melt or eutectic liquid melt or normal liquid melt in the formation of joints, the thickness of interfacial intermetallic compound (IMC) layer in the joints increases with prolonging dwelling time. For three different liquid soldering processes, the change tendency of the interfacial IMC layer thickness is distinct. For joints formed at UL and EL temperatures, the excessive growth of primary Cu6Sn5 and interfacial IMC (mainly Cu6Sn5) can be suppressed during isothermally dwelling in the liquid state for more than 5 min. Ball shear test results show that solder joints formed at UL and EL temperatures have lower value of the maximum shear force (MSF) than those formed during the NL soldering process. For UL and EL types of joints, the fracture occurs either by shear slide fracture totally along the shear tool tip movement plane (i.e., in-plane shear slide fracture) or by shear slide fracture initially along the shear tool tip movement plane and afterwards shear deformation fracture in the solder matrix under the plane (i.e., out-of-plane shear slide and deformation mixed-mode fracture), while the fracture takes place in NL joints only by in-plane shear slide fracture mode.

Effect of Revolutionary Pitch on Interface Microstructure and Mechanical Behavior of Friction Stir Lap Welds of AA6082-T6 to Galvanized DP800

Friction stir lap welding of 1.5-mm thick 6082-T6 aluminum alloy to 2-mm thick galvanized DP800 steel (Zn-coated) was carried out. Optimal welding conditions were obtained aiming to defect-free joints with good mechanical properties. The interfacial intermetallic compounds (IMCs) at the stir zone and hook zone were characterized under different revolutionary pitches. With a revolutionary pitch of 1.0 mm/rev, maximum joint strength reached 71% of that of the aluminum alloy. In the meantime, the average thickness of IMC layer is less than 1 μm; Al3.2Fe in the Al-rich side and Al5Fe2 in the Fe-rich side at the interfaces of stir zone while Al6Fe and nanocrystalline close to Al3.2Fe at the interface of the hook zone. At a relatively lower revolutionary pitch (0.5 mm/rev), Zn was found with the aggregation of Si and Mn at the hook-zone interface, leading to the generation of Al-Fe-Si phase thus decreasing the thickness of the IMC layer. In the stir zone, the revolutionary pitch has a significant influence on the interfacial microstructures. The interfacial IMC layer at 1.0 mm/rev is simple and flat, but the one at 0.5 mm/rev becomes thicker and more complex. Stir zone aluminum under different revolutionary pitches is similar in microhardness and tensile behavior. The mechanical response of joints was modeled based on linear mixture law with an iso-strain assumption and neglection of the IMC layer. The modeling results are in good agreement with the experimental ones indicating the resultant interfaces act as good as the good boundaries between dissimilar Al/steel joints.

Investigation on the Interfacial IMCs Layer of Sn-58Bi/Cu Solder Joints

The interfacial intermetallic compounds (IMCs) layer of Sn-58Bi lead-free solder reacting with Cu substrate were explored at aging stage after soldering for 1day, 3days, 6days, 8days and 10days, respectively. Results show that Bi doesn’t react with Cu atoms or Sn atoms. The IMCs layer is composed of Cu6Sn5 phase and Cu3Sn phase. The IMCs layer evolves from scalloped shape to layer shape with addition of aging time, meanwhile, Bi phase in matrix grows bigger. The growth kinetic equation of IMCs layer is obtained and described as X=1.06+0.69t1/2, where 0.69μm/d1/2 is the growth coefficient. Compared with Sn-3Ag-0.5Cu lead-free solder, the growth coefficient of Sn-58Bi solder is much smaller. Accordingly, the growth of IMCs layer of Sn-58Bi solder is more slowly than that of Sn-3Ag-0.5 Cu solder.

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.

Effect of aging on mechanical properties of high temperature Pb-rich solder joints

The effect of aging on the evolution of interfacial microstructure and mechanical properties of Pb-rich PbSnAg solder joints with different Sn content was investigated. Tensile samples were prepared by soldering two pieces of Ni or Cu strips resulting in joints with gap sizes of about 250 μm. Multi-layered structures composed of Ni coated Si-chips soldered onto Ni/Cu metallized ceramic substrates were used for fatigue testing. All samples were subjected to thermal aging at 250 °C up to 500 h. Microstructural investigations revealed that independent from the substrate material, increased Sn content of the solder alloy results in improvement of the tensile and fatigue properties of the joints and a higher growth rate of the interfacial intermetallic compound (IMC) layers. It was found that the thickness of the wettable substrate especially in the case of low-Sn alloys, also affects the interfacial properties of high temperature PbSnAg solder joints. In this case, a reduced Sn content in solder joints results in weakening of the interface during the reflow process and subsequent thermal aging. The dominant failure mode of the solder joints subjected to cyclic loading was delamination of the interfacial IMC layer from the substrate. Finite element simulations were conducted by using a strain rate and pressure dependent material model for PbSnAg solder in order to analyse the states of stress and strain during static and cyclic loading.

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.

Growth of Interfacial Intermetallic Compound Layer in Diffusion-Bonded SAC–Cu Solder Joints During Different Types of Thermomechanical Excursion

The effects of mechanical strain on the growth kinetics of interfacial intermetallic compounds (IMCs) sandwiched between Cu substrate and Sn-1.0 wt.%Ag-0.5 wt.%Cu (SAC105) solder have been investigated. Isothermal aging (IA) at 70°C and 125°C, and thermal cycling (TC) as well as thermomechanical cycling (TMC) with shear strain of 12.8% per cycle between −25°C and 125°C were applied to diffusion-bonded solder joints to study the growth behavior of the interfacial IMC layer under various types of thermomechanical excursion (TME). The microstructure of the solder joint tested under each TME was observed at regular intervals. It was observed that the growth rate of the IMC layer was higher in the case of TMC compared with TC or IA. This increased growth rate of the IMC layer in the presence of mechanical strain suggests an additional driving force that enhances the growth kinetics of the IMC. Finite element analysis was performed to gain insight into the effect of TC and TMC on the stress field in the solder joint, especially near the interface between the solder and the substrate. Finally, an analytical model was developed to quantify the effect of strain on the effective diffusivity and express the growth kinetics for all three types of TME using a single expression.

Improvement on interfacial structure and properties of Sn–58Bi/Cu joint using Sn–3.0Ag–0.5Cu solder as barrier

Sn–58Bi solder recently attracts much attention for low temperature soldering, but is also restricted by the degradation on Sn–Bi solder joint from the interfacial Bi embrittlement. This study introduced a thin solder layer with the composition of Sn–3.0Ag–0.5Cu (SAC) on Cu pad to improve the interfacial structure and shear properties of Sn–58Bi solder joints under soldering and the subsequent aging conditions. SAC solder layer with the thickness about 5 μm was coated on Cu pad using Hot Air Soldering Leveling (HASL). During soldering condition, the thickness of interfacial intermetallic compound (IMC) layer in Sn–58Bi joints increased with the soldering times, and there was no obvious difference on the bare and SAC-coated Cu. However, during aging condition, the growth on interfacial IMC layer was effectively retarded by the SAC coating compared to their bare Cu counterparts. SAC-coated samples that were reflowed with varying soldering times and then aged also showed a thinner IMC layer. The retarding effect on IMC growth from SAC-coated Cu pad was attributed to the reason that SAC solder on Cu produced an initial Cu6Sn5 layer with fewer scallops, fewer grain boundaries between scallops, and therefore fewer diffusion pathways for Cu atoms to diffuse into the solder and react to grow the intermetallic layer. Ball shear tests also showed that aged SAC-coated joints displayed higher shear strength and through-solder ductile failure while Sn–58Bi on bare Cu pad displayed lower shear strength and brittle failure through the interfacial IMC layer. Therefore, SAC Pb-free solder can be recommended as the surface finishing on Cu pad to improve the performance of Sn–58Bi joints because it can be easily realized by HASL method in electronic industry.