Formation Of Interfacial Intermetallic Compounds Research Articles

Formation mechanism for the interface between Cu and Sn formed by magnetic pulse welding

The formation of interfacial intermetallic compounds (IMCs) such as Cu6Sn5, Cu3Sn or platelet-shaped Ag3Sn which are formed in reflow soldering process will directly destroy the soldering reliability. Magnetic pulse welding (MPW), as a solid state welding technique can effectively restrain or even impede the formation of interfacial IMCs. In this paper, Cu and Sn dissimilar metals were welded by MPW under the discharge energy from 17 to 31 kJ. The typical wavy interface and metal jet flow were found in all samples. When the discharge energy reached 31 kJ, a typical gradient nanograined (GNG) interface has been observed to undergo an evolution from a sub-micron secondary solid solution of Cu6.26Sn5 IMCs to a nanoscale secondary solid solution of Cu81Sn22 IMCs, followed by the formation of nanoscale equiaxed α-Cu grains, and ultimately leading to the deformed elongated nanoscale α-Cu grains. This discontinuous distribution of the aforementioned phases occurs specifically at the interface of the 23 kJ_1.5 mm sample. In contrast, when subjected to a discharge energy of 17 kJ, there were no indications of a secondary solid solution of Cu6.26Sn5 IMCs or elongated nanoscale α-Cu grains. However, a discontinuous secondary solid solution of Cu81Sn22 IMCs was observed to form at the Cu/Sn interface under these conditions. Based on these findings, the utilization of microprojection welding (MPW) techniques involving Cu and Sn-based solder in the microscale is anticipated to serve as a highly efficient and automated technology within the field of electronic micropackaging.

Study on Ni-GNSs enhanced Sn2.5Ag0.7Cu0.1RE/Cu solder joints β-Sn grain orientation and interfacial IMC growth kinetics under constant temperature thermomigration

Aiming at the thermomigration problem caused by the large temperature gradient (TG) of micro-soldering joints, a constant temperature thermomigration device with temperature control function was designed and manufactured. This paper studied the polarity phenomenon, crystallographic characteristics, and interfacial intermetallic compound (IMC) growth kinetics of Ni-GNSs reinforced Sn2.5Ag0.7Cu0.1RE/Cu solder joints under thermomigration. The results indicate that Ni-GNSs reinforced Sn2.5Ag0.7Cu0.1RE/Cu solder joints exhibited a significant thermomigration polarity phenomenon under the conditions of TG ≥ 1000 °C/cm and θ ≤ 43.5° between the c-axis and TG of β-Sn grains. At the cold end of the solder joint, the Cu6Sn5 phase at the interface gradually thickens and forms Cu3Sn phase on the substrate side, while microcracks expanded and gradually developed into macrocracks. At the hot end of the interface, the Cu6Sn5 phase gradually dissolved. The growth of the Cu3Sn phase was accompanied by “Kirkendall voids” that formed cracks at the interface between the joint and the solder until a “fully IMC solder joint” was formed. The growth of Cu6Sn5 IMC at the solder joint interface came before that of Cu3Sn IMC. The average temperature and temperature gradient of the solder joint were correlated with the growth of Cu3Sn IMC, leading to the formation of interface Cu3Sn IMC due to the oversaturation of Cu atoms. The addition of 0.05 wt% Ni-GNSs refines the grain structure and increases the activation energy for the growth of Cu6Sn5 and Cu3Sn IMC at the cold end of the solder joint, suppressing the thermomigration polarity phenomenon.

On the formation of interfacial compounds in the 2A14 Al alloy/steel friction welded joint: A comparative study

In this study, the long-discussed question of the effect of interfacial compound layer in inertia friction welded 2 series aluminum (Al) alloy/steel joint is investigated by altering the state of the base metal, including heat treatment of Al alloy and surface modification of steel. It was found out that the precipitate formed in the precipitation process of Al alloy during cooling strongly affected the formation of interfacial intermetallic compounds (IMCs), rather than its original pre-existing precipitates. At low heat input, the incomplete extrusion of oxide layer led to the existence of nanoscale Fe-Al-O amorphous layer together with the nanoscale Al2Cu occasionally precipitated (semi-coherent with Al) at the interface. At high heat input, the enhanced coupling effect of heat and plastic flow of Al alloy resulted in the possible reaction between Fe, Si and Al2Cu to form a Fe4Al13 type IMC, Al67.5Fe20.7Cu6.2Si5.6, with a lower formation enthalpy, but the non-uniform reaction caused the remnant Al2Cu in the submicron-scale IMCs layer, being non-uniformly distributed. Local tensile tests indicated that joint strength was negatively correlated with the thickness of the IMCs layer in which the joint with nanoscale Fe-Al-O amorphous layer showed the highest bonding strength.

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

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

Reduced Graphene Oxide Nanosheets Decorated with Copper and Silver Nanoparticles for Achieving Superior Strength and Ductility in Titanium Composites.

Graphene and its derivates are extensively applied to enhance the mechanical properties of metal matrix nanocomposites. However, their high reactivity with a metal matrix such as titanium and thus the limited strengthening effects are major problems for achieving high-performance graphene-based nanocomposites. Herein, reduced graphene oxide nanosheets decorated with copper or silver (i.e., Cu@rGO and Ag@rGO) nanopowders are introduced into Ti matrix composites using multiple processes of one-step chemical coreduction, hydrothermal synthesis, low-energy ball milling, spark plasma sintering, and hot rolling. The Cu@rGO/Ti and Ag@rGO/Ti nanocomposites exhibit significantly enhanced strength with superior elongation to fracture (846 MPa-11.6 and 900 MPa-8.4%, respectively, basically reaching the level of the commercial Ti-6Al-4V titanium alloy), which are much higher than those of the fabricated Ti (670 MPa-7.0%) and rGO/Ti composites (726 MPa-11.3%). Furthermore, fracture toughness values of the M@rGO/Ti composites are all significantly improved, that is, the highest KIC value is 34.4 MPa·m1/2 for 0.5Cu@rGO/Ti composites, which is 20.28 and 51.5% higher than those of monolithic Ti and 0.5rGO/Ti composites, respectively. The outstanding mechanical properties of Ag@rGO/Ti and Cu@rGO/Ti composites are attributed to the effective load transfer of in situ formed TiC nanoparticles and the formation of interfacial intermetallic compounds between the rGO nanosheets and Ti matrix. This study provides new insights and approach for the fabrication of metal-modified graphene/Ti composites with a high performance.

Plasma arc welding-brazing of aluminum to copper with SiO2 nanoparticles strengthening

Dissimilar metal of T2 copper and 1060 aluminum alloys were welded by plasma arc welding-brazing method with a lap joint, and SiO 2 nanoparticles were added in the welding pool to control the interfacial intermetallic compounds (IMC). The interfacial microstructure and phase composition of the weld joint were identified by scanning electron microscope (SEM) and electron probe microanalysis (EPMA). IMC formation mechanism, weld joint tensile strength, and IMC nanohardness was also discussed. The results show that after adding SiO 2 nanoparticles, due to the adsorption of the nanoparticles on the copper surface, the total interface energy between the aluminum liquid and the copper substrate would be reduced, and the wetting angle would be reduced. So that the aluminum melts can have better wetting ability, and the barrier effect of SiO 2 nanoparticles formed prevents the further diffusion and reaction of copper atoms and aluminum atoms, and inhibits the growth of the IMC layer to a certain extent. The weld fracture occurred at the aluminum-based material added with nano-SiO 2 , showing typical dimple morphology. The phase with the highest hardness in the IMC layer is CuAl 2 . SiO 2 nanoparticles exist in the CuAl 2 intermetallic compound layer due to the adsorption of nanoparticles.

Comparative study between the Sn–Ag–Cu/ENIG and Sn–Ag–Cu/ENEPIG solder joints under extreme temperature thermal shock

The interfacial reactions and the growth behavior of interfacial intermetallic compounds (IMCs), as well as their effects on the mechanical properties of the Sn-3.0Ag-0.5Cu (SAC305)/electroless nickel-electroless palladium-immersion gold (ENEPIG) solder joints under extreme temperature thermal shock between − 196 °C and + 150 °C were investigated, and compared with those of the SAC305/electroless nickel-immersion gold (ENIG) solder joints. The morphology of (Cu, Ni)6Sn5 IMCs formed at the SAC305/ENIG interface transformed from column-type to polygon-type during extreme temperature thermal shock, while the needle-type and thin-layer-type (Cu, Ni, Pd)6Sn5 IMCs at the SAC305/ENEPIG interface completely transformed to chunk-type. The growth rate of interfacial IMCs in the SAC305/ENEPIG joint was slower in contrast to that in the SAC305/ENIG joint, since the Pd(P) layer in the ENEPIG surface finish inhibited the formation and growth of interfacial IMCs. After thermal shock for 400 cycles, the shear force of the SAC305/ENIG and SAC305/ENEPIG joints decreased about 29.06% and 9.79%, respectively. Moreover, the shear force of the SAC305/ENEPIG joints was consistently higher than that of the SAC305/ENIG joints. The SAC305/ENEPIG joints always fractured in the solder matrix, regardless of thermal shock cycles, while the fracture location of the SAC305/ENIG joints transferred from inside the solder matrix to partially inside the solder matrix and partially at the solder/IMC layer interface with increasing thermal shock cycles.

Effect of nano-Ag3Sn additions on wettability, interfacial intermetallic growth and mechanical properties of Zn–30Sn–1Ge solders on Cu substrates

The influences of different addition (0, 0.2, 0.4, 0.6, 0.8, and 1.0 wt%) of Ag3Sn on the wettability, the morphology of interfacial intermetallic compounds (IMCs), and the mechanical properties of Zn30Sn1Ge (ZSG) solder on Cu substrate were investigated. The Ag3Sn nanoparticles with an average particle size of approximately 30–45 nm were dispersed in the α-Zn phase and IMC layer. The added Ag3Sn nanoparticles can remarkably improve the wettability of ZSG solder and inhibit the formation and growth of the interfacial IMCs between the nanocomposite solders and Cu substrates. The enhancement of wettability and inhibition of interfacial IMC increased with the increase of nano-Ag3Sn addition amount under low concentration. However, the excessive addition of Ag3Sn nanoparticles in ZSG solder will degrade the wettability and inhibition of interfacial IMCs. When the amount of Ag3Sn nanoparticles is 0.6 wt%, the ZSG–0.6Ag3Sn solders possess the best wettability. In addition, the ZSG–0.4Ag3Sn solder exhibits the most prominent effect in retarding the formation and growth of interfacial IMCs. Meanwhile, the addition of Ag3Sn nanoparticles increases the shear strength and Vickers hardness of the solder joint. When the amount of Ag3Sn nanoparticles is 0.4 wt%, the shear strength of the welded joint is increased by 13.3%. Moreover, the macroscopic hardness of ZSG–0.6Ag3Sn solder is 49.507 HV, which is 57.5% higher than that of the solder without added nanoparticles (31.38 HV). Results show that the addition of Ag3Sn nanoparticles plays an important role in the interfacial reaction and mechanical properties between the ZSG solder and Cu substrate.

Formation and Evolution of Cu-Sn Intermetallic Compounds in Ultrasonic-Assisted Soldering

Interfacial intermetallic compounds (IMCs) help determine the reliability of soldered joints; thus, it is necessary to understand their formation and evolution. This study focus on Cu-Sn IMCs formed in ultrasonic-assisted soldering (UAS), wherein the formation of IMCs at the Sn/Cu interface is controlled by changing the ultrasonic action time. After being subjected to ultrasonic vibration, the IMCs at the Cu/Sn solid–liquid interface are continuously crushed, dissolved, and formed, which occurs successively in the Cu6Sn5 and Cu3Sn layers. The relationship between the thickness of the IMC layer and ultrasonic action time in Cu-Sn samples was identified. Simultaneously, the growth pattern of Cu6Sn5 grains in the Sn solder is transformed, and the tin solder (Sn solder) is kept in a dynamic non-equilibrium state with IMCs at the Sn/Cu interface through UAS. More Cu6Sn5 grains formed and were evenly distributed in the joint after cooling, which improves the performance of the joints.

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.

Asymmetrical interfacial reactions of Ni/SAC101(NiIn)/Ni solder joint induced by current stressing

An electric current can asymmetrically trigger either atomic migration or interfacial reactions between a cathode and an anode. The present study investigated the dissolution of metallization and formation of an interfacial intermetallic compound (IMC) in the Cu/Ni/Sn1.0Ag0.1Cu0.02Ni0.05In/Ni/Cu solder joint at various current densities in the order of 103 A/cm2 at temperatures ranging from 100 °C to 150 °C. The polarization behavior of Ni dissolution and IMC formation under current stressing were systematically investigated. The asymmetrical interfacial reactions of the solder joint were found to be greatly influenced by ambient temperature. The dissolution of Ni and its effect on interfacial IMC formation were also discussed.

Evolution of microstructure and mechanical properties of Cu/SAC305/Cu solder joints under the influence of low ultrasonic power

Abstract Cu/SAC305/Cu solder joints were fabricated by a modified ultrasonic-assisted reflow soldering technique under ambient atmosphere. Ultrasonic vibration (USV) with low ultrasonic power ranging from 10 to 30 W was used while the evolution of microstructure and mechanical properties of the solder joints under the influence of such USV were investigated in this study. Results showed that ultrasonic power promoted the formation of eutectic phase and the coarsening of β-Sn phase in the solder matrix of the solder joints. When the samples were treated with longer USV time, the β-Sn phase changed from dendritic structure (10 W of USV) to globular structure (20 and 30 W of USV). The observed morphological change of β-Sn and eutectic phases in the solder matrix led to the variation in the solder matrix hardness and the formation of interfacial intermetallic compounds (IMCs) in the samples treated with USV at increasing ultrasonic power and time. Overall, the shear strength of the ultrasonic-treated samples increased with increasing ultrasonic power, regardless of the USV time.

Influence of rapid solidification on Sn–8Zn–3Bi alloy characteristics and microstructural evolution of solder/Cu joints during elevated temperature aging

The effects of rapid solidification on the microstructure and melting behavior of the Sn–8Zn–3Bi alloy were studied. The evolution of the microstructural characteristics of the solder/Cu joint after an isothermal aging at 150 °C was also analyzed to evaluate the interconnect reliability. Results showed that the Bi in Sn–8Zn–3Bi solder alloy completely dissolved in the Sn matrix with a dendritic structure after rapid solidification. Compared with as-solidified Sn–8Zn–3Bi solder alloy, the melting temperature of the rapid solidified alloy rose to close to that of the Sn–Zn eutectic alloy due to the extreme dissolution of Bi in Sn matrix. Meanwhile, the adverse effect on melting behavior due to Bi addition was decreased significantly. The interfacial intermetallic compound (IMC) layer of the solder/Cu joint was more compact and uniform. Rapid solidification process obviously depressed the formation and growth of the interfacial IMC during the high-temperature aging and improved the high-temperature stability of the Sn–8Zn–3Bi solder/Cu joint.

Effect of P and Ge doping on microstructure of Sn-0.3Ag-0.7Cu/Ni-P solder joints

Purpose The purpose of this paper is to comparatively investigate the microstructure and interfacial intermetallic compound (IMC) layer of Cu/SACPG/Ni and Cu/SAC0307/Ni solder joints after thermal aging. Design/methodology/approach The specimens were thermally aged at 150°C for 0, 24, 168 and 500 h. The microstructure and morphology of the interface IMC layer were observed by means of scanning electron microscope. The IMCs and the solder bump surface were analyzed by EDS. Moreover, the thickness of IMC layer was measured by using the image analysis software. Findings The morphology of IMC of Cu/SAC0307/Ni solder joint was consistent with that of the Cu/SACPG/Ni joint, which indicates that the addition of P and Ge had little effect on the IMC formation. The needle-like (Cu,Ni)6Sn5 was formed at the interface of solder/Ni solder joints. Meanwhile, the tiny particles inferred as Ag3Sn phase attached to the surface of (Cu,Ni)6Sn5. The growth rate of IMC layer of the Cu/SACPG/Ni joint was smaller than that of Cu/SAC0307/Ni joint with aging time increasing, which means the addition of trace P and Ge can slightly suppress the diffusion rate of the interfacial IMC. Originality/value There are no previous studies on the formation mechanism of the IMC layer of SAC0307 solder alloys with P and Ge addition.

Microstructural evolution and joint strength of Sn-58Bi/Cu joints through minor Zn alloying substrate during isothermal aging

To improve the interfacial microstructure and mechanical behaviors of Sn-58Bi/Cu joint, minor Zn element was alloyed into Cu substrate. The interfacial intermetallic compound (IMC) growth and bending properties of Sn-58Bi/Cu and Sn-58Bi/Cu-2.29Zn were studied with the effect of isothermal liquid and solid aging. Although there was no significant change on the composition, thickness and morphology of interfacial IMC under liquid aging, the depressing of IMC growth at the interface between Sn-58Bi solder and substrate and the avoidance on formation of Cu3Sn IMC, Kirkendall voids and Bi segregation at the IMC/Cu interface were realized with Cu-Zn substrate under isothermal solid aging. Joint strength and fracture behavior were also improved by Cu-Zn substrate. There was no obvious decreasing on the joint strength and the fracture during bending was mainly occurred in the solder matrix with ductile fracture mode or along the solder/IMC interface with partly brittle fracture mode for Cu-Zn joint compared with the dramatically decreasing on joint strength and brittle fracture mode occurred along the interface between IMC and Cu for Sn-Bi/Cu joints after aging.

Effects of Ga Addition on Interfacial Reactions Between Sn-Based Solders and Ni

The use of Ga as a micro-alloying element in Sn-based solders can change the microstructure of solder joints to improve the mechanical properties, and even suppress the interfacial intermetallic compound (IMC) growth. This research investigated the effects of Ga addition (0.2–1 wt.%Ga) on the IMC formation and morphological evolution in the Sn-based solder joints with Ni substrate. In the soldering reaction at 250°C and with less than 0.2 wt.%Ga addition, the formed phase was Ni3Sn4. When the Ga addition increased to 0.5 wt.%, it changed to a thin Ni2Ga3 layer of ∼1 μm thick, which stably existed at the interface in the initial 1-h reaction. Subsequently, the whole Ni2Ga3 layer detached from the Ni substrate and drifted into the molten solder. The Ni3Sn4 phase became dominant in the later stage. Notably, the Ga addition significantly reduced the grain size of Ni3Sn4, resulting in the massive spalling of Ni3Sn4 grains. With 1 wt.%Ga addition, the Ni2Ga3 layer remained very thin with no significant growth, and it stably existed at the interface for more than 10 h. In addition, the solid-state reactions were examined at temperatures of 160°C to 200°C. With addition of 0.5 wt.%Ga, the Ni3Sn4 phase dominated the whole reaction. By contrast, with increasing to 1 wt.%Ga, only a thin Ni2Ga3 layer was found even after aging at 160°C for more than 1200 h. The 1 wt.%Ga addition in solder can effectively inhibit the Ni3Sn4 formation in soldering and the long-term aging process.

Effect of nano-Al addition on properties and microstructure of low-Ag content Sn–1Ag–0.5Cu solders

In this paper, Al nanoparticles were manually mixed into the Sn–1Ag–0.5Cu solder paste to prepare composite solder. The content of Al nanoparticles ranged from 0 to 0.4 wt% in the solders. The influences of Al nanoparticles on the melting characteristics, wettability, mechanical properties, microstructure and interfacial intermetallic growth of low-Ag Sn–1Ag–0.5Cu solders on copper substrate were investigated. Results show that added Al nanoparticles can not obviously change the melting point of the Sn–1Ag–0.5Cu solder, and improve the wettability and mechanical properties of plain Sn–1Ag–0.5Cu solder. However, excessive addition Al nanoparticles will reduce the wettability and mechanical properties of the solder. There is an optimum amount of Al nanoparticles in Sn–1Ag–0.5Cu solder, which is 0.1 wt%, and Sn–1Ag–0.5Cu–0.1Al nano-composite solder exhibit the best comprehensive properties. Based on the microstructure examination, it is found that adding Al nanoparticles can obviously refine the microstructure of Sn–1Ag–0.5Cu–xAl solder. In addition, the interfacial intermetallic compounds (IMC) layer thickness of the Sn–1Ag–0.5Cu–xAl solders are smaller than that of plain Sn–1Ag–0.5Cu solder, which indicates that Al nanoparticles can effectively suppress IMC formation and growth.

Effect of γ-Fe2O3 nanoparticles size on the properties of Sn-1.0Ag–0.5Cu nano-composite solders and joints

In this study, the Sn1.0Ag0.5Cu–0.5 Fe2O3 (SAC105–0.5 Fe2O3) composite solders reinforced by γ-Fe2O3 nanoparticles with size of 20 nm, 50 nm and 200 nm, respectively, were prepared by alloy melting method. The effect of nanoparticles size on microstructure, melting properties and wettability of the SAC105–0.5 Fe2O3 composite solder alloys, the interfacial intermetallic compounds (IMCs) growth on Cu substrate and the shear strength of the composite solder/Cu joints were investigated. The results indicated that the size of the added γ-Fe2O3 nanoparticles in the composite SAC105–0.5 Fe2O3 solders is critical to the properties of the solder alloys and solder/Cu joints. When the added γ-Fe2O3 nanoparticles size changed from 200 nm to 20 nm, the microstructure of the composite solder refined, the liquidus temperature of the composite solders decreased slightly and the wettability of the composite solder increased. The addition of smaller-size γ-Fe2O3 nanoparticles suppressed the interfacial IMCs formation during reflow and impeded the interfacial IMC growth during thermal aging more effectively. Smaller-size γ-Fe2O3 nanoparticles also improved the shear strength and the ability of plastic deformation of the solder joints more effectively. In addition, the mechanisms of the nanoparticles size effect were discussed.

Role of intermetallics on the mechanical fatigue behavior of Cu–Al ball bond interfaces

The mechanical fatigue behavior of Cu–Al interfaces occurring in thermosonic ball bonds –typically used in microelectronic packages for automotive applications – is investigated by means of a specially designed fatigue test technique. Fully reversed cyclic shear stresses are induced at the bond interface, leading to subsequent fatigue lift off failure and revealing the weakest site of the bond. A special focus is set on the role of interfacial intermetallic compounds (IMC) on the fatigue performance of such interfaces. Therefore fatigue life curves were obtained for three representative microstructural states: The as-bonded state is compared to two annealed states at 200 °C for 200 h and at 200 °C for 2000 h respectively. In the moderately annealed state two IMC layers (Al2Cu, Al4Cu9) could be identified, whereas in the highly aged state the original pad metallization was almost entirely consumed and AlCu is formed as a third IMC. Finally, the crack path is traced back as a function of interfacial microstructure by means of electron microscopy techniques.Whereas conventional static shear tests reveal no significant decrease of the bond shear force with increased IMC formation the fatigue tests prove a clear degradation in the cyclic mechanical performance. It can be concluded that during cycling the crack deflects easily into the formed intermetallics, leading to early failure of the ball bonds due to their brittle nature.

Microstructure, wetting property of Sn–Ag–Cu–Bi–xCe solder and IMC growth at solder/Cu interface during thermal cycling

The effects of adding small amounts of cerium (Ce) to Sn–3.3Ag–0.2Cu–4.7Bi solder on microstructure, wettability characteristic, interfacial morphology and the growth of interfacial intermetallic compound (IMC) during thermal cycling were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and solderability tester. It is found that the β-Sn phase, Ag3Sn phase and Cu6Sn5 phase in the solder are refined and the wetting force increases. Ce is an active element; it more easily accumulates at the solder/flux interface in the molten state, which decreases the interfacial surface energy and reduces the driving force for IMC formation on Cu substrate; therefore, the thickness of IMC at the solder/Cu interface decreases when appropriate Ce was added into the solder. Moreover, the Ce-containing solders have lower growth rate of interfacial IMC than solders without Ce during the thermal cycling between −55 and 125 °C. When the Ce content is 0.03 wt% in the Sn–3.3Ag–0.2Cu–4.7Bi solder, the solder has the best wettability and the minimum growth rate of interfacial IMC layer.