Intermetallic Compound Thickness Research Articles

Friction stir spot welding of AA6061-T6 and Cu with preheating: Strength and failure behavior at different test temperatures

In the current study, the influence of test temperatures was investigated on lap shear strength and fracture behavior of dissimilar and similar friction stir spot-welded (FSSW) joints between Cu and AA6061-T6 sheets prepared without and with employing additional heating. Formation and distribution of intermetallic compounds (IMCs) in Cu-AA6061 FSSW joints was further appraised. An increase in maximum lap shear force was observed in Cu-AA6061 and Cu-Cu joints when preheating was employed. Lap shear force decreased with increase in test temperature for Cu-AA6061 joints prepared without preheating; however, for joints prepared employing preheating, the lap strength increased from 25 to 50 °C following a slight drop at 100 °C test temperature. For Cu-Cu joints, lap shear force also increased with test temperature for the range studied in the present work. X-ray diffraction, and energy dispersive spectroscopy analysis illustrates the formation of IMCs in the form of Al2Cu and Al4Cu9 in Cu-AA6061 and oxides in Cu-Cu joints. The interface failure from IMC rich regions in dissimilar joints was observed whereas similar joints failed under plug failure. Intermittently, thick IMC layer was observed in dissimilar Cu-AA6061 joint prepared with preheating. Presence of tearing/rupture from oxide dominated intermetallic region observed in Cu-AA6061 FSSW joints whereas dimples and micro-voids were noted in Cu-Cu joints.

Microstructure and Interfacial Evolution of Sintered NdFeB Permanent Magnet/Steel Joint Soldered with Zn-Sn Alloy

Sintered NdFeB permanent magnets and steel were soldered successfully using Zn-Sn alloy. The effects of Sn content on the microstructure and mechanical properties of soldered joints were investigated. The results showed that the typical interfacial microstructure represented NdFeB/NdFe5.5Zn(B) + ξ-FeZn13/Zn + β-Sn/ξ-FeZn13 + δ-FeZn10 + Γ-Fe3Zn10/steel. With the addition of Sn, the thickness of the NdFe5.5Zn(B) phase increased, while more β-Sn was observed in the soldering seam. The maximum shear strength reached 45.3 MPa with a Sn proportion of 6 wt.%, and excessive growth of the NdFe5.5Zn(B) phase in the joints deteriorated the mechanical properties. The fracture morphology exhibited a ductile-to-brittle transition with the occurrence of interfacial fractures due to thick intermetallic compounds.

Microstructural and shear strength properties of RHA-reinforced Sn–0.7Cu composite solder joints on bare Cu and ENIAg surface finish

In this study, the joint effect of rice husk ash (RHA) reinforcement as an alternative silica source and electroless nickel immersion silver (ENIAg) surface finish on the intermetallic compound (IMC) formation and shear strength of the Sn–0.7Cu solder system was investigated. A series of plain and composite lead-free solder systems (Sn–0.7Cu−xRHA; x = 0, 0.01, 0.05 and 0.1 wt%) was successfully developed and subjected to reflow soldering on bare Cu and ENIAg surface finish. After conducting a comprehensive microstructural study using the scanning electron microscopy and energy dispersive spectroscopy techniques, the Cu6Sn5 and Cu3Sn intermetallic compound (IMC) phases were observed at the interface of the Sn–0.7Cu−xRHA/Cu composite solder joints. On the other hand, the (Cu,Ni)6Sn5 and Ni3Sn4 IMC phases dominated the interface of the Sn–0.7Cu−xRHA/ENIAg counterparts. Given the promising potential of the ENIAg surface finish, the Sn–0.7Cu−xRHA/ENIAg exhibited IMC thickness values within a range of 3.81–4.74 µm as compared to the 6.13–9.3 µm range exhibited by the Sn–0.7Cu−xRHA/Cu counterpart. More so, the ENIAg surface finish was effective in improving the shear strength of the plain solder joint, with the Sn–0.7Cu/ENIAg exhibiting 13.44 MPa relative to the 11.5 MPa exhibited by the Sn–0.7Cu/Cu counterpart. Overall, the strengthening effect of the RHA reinforcement was well marked in the Sn–0.7Cu−xRHA/Cu composite solder joints with the composite having 0.1 wt% RHA exhibiting the highest shear strength (14.6 MPa) across the board.

Effects of Temperature–Humidity Treatment on Bending Reliability of Epoxy Sn–58Bi Solder with Electroless Nickel Immersion Gold (ENIG) and Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Surface Finishes

An epoxy Sn–58wt.%Bi solder joint was evaluated by a three-point bending test with electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG) surface finishes aged at 85 °C and 85% relative humidity. Scanning electron microscopy and electron probe microanalysis were carried out to study intermetallic compound variation. The morphology, total thickness, and chemical composition of intermetallic compound in epoxy Sn58Bi solder joints were the same as those of Sn–58wt.%Bi solder joints with each surface finish. The average number of bending-to-failure cycles for the epoxy Sn–58wt.%Bi solder/ENIG joints and epoxy Sn–58wt.%Bi solder/ENEPIG was more than 4000 and 5000, respectively. The average number of bending-to-failure cycles of the epoxy Sn–58wt.%Bi solder joint decreased with increasing age. Three-point bending reliability of epoxy Sn–58wt.%Bi solder joints was higher than that of Sn–58wt.%Bi solder with both surface finishes. Cracking of all solder joints subjected to as-reflowed was propagated through the solder matrix. However, after aging for 1000 h, cracking occurred primarily between intermetallic compound layers.

Influences of Ni addition into Cu–xNi alloy on the microstructure evolution and mechanical property of Sn–58Bi/Cu–xNi solder joint

In the current investigation, various mass fractions (0, 0.5, 1.5, 5 and 10 wt%) of Ni were doped into Cu substrate, and influences of Ni addition on the microstructure evolution and mechanical property of the Sn–58Bi/Cu–xNi solder joint were investigated. Results showed that the growth rate of intermetallic compound (IMC) would increase with the Ni addition added from 0 to 5 wt% and remarkably decreased for Sn–58Bi/Cu–10Ni joint system. It indicated that the growth rate of IMC layer would decrease when 10 wt% Ni was added into the alloy. Besides, the tensile strength reduced with increase in the IMC thickness, which was under the synergistic effect of increasing liquid-state reaction time and the content of Ni addition (0–5 wt%). Moreover, the fracture mechanism gradually transformed from ductile fracture mode to brittle fracture mode with the decrease in joint strength.

Application of pulse plasma MIG welding process to Al/steel dissimilar joining

Arc welding of aluminium and steel joins are commonly inundated by the formation of brittle intermetallic compounds (IMC) at the aluminium-steel interface. As the formation of IMC is closely related to the heat input into the base metals, it is hypothesised that the ability to regulate the heat input would enable the control of the thickness of IMC. This paper proposes the use of a pulse plasma MIGW system to regulate the heat input during the welding process. A shadowgraph method was used to compare the weld droplet stability of the pulse plasma MIGW with a pulse MIGW system. It was found that the plasma MIGW generated a regular heat input into the base metal due to the smoothness of the droplet detachment and resulted in the formation of IMC with uniform thickness. Al/steel joining was then conducted using both pulsed plasma MIGW and pulse MIGW systems, and the resulting IMC formations and thicknesses were evaluated. It was found that the formation and thickness of IMC in Al/steel dissimilar welding can be controlled by using pulse plasma MIGW. Its lower droplet temperature and decreased cooling rate during welding would suppress the formation of Fe2Al5 and FeAl3, and thus decreasing the overall thickness of IMC at the joining interface.

Optimal Hot-Dipped Tinning Process Routine for the Fabrication of Solderable Sn Coatings on Circuit Lead Frames.

Previous studies merely focus on the hot dipping properties of lead frame materials used in electronic industry. Yet, the environmentally friendly and cost-efficient traits of hot-dipped tinning process make it a possible promising surface modification technique compared with electroplating. As a result, the optimal hot-dipped tinning process routine is proposed in this paper. The hot-dipped tinning process of four different types of copper foils (C11000, C19400, C19210, and C70250), pretreatment parameters, mechanical properties of Cu substrates, thickness of IMC (intermetallic compound) layers and coatings, and microstructure of coatings were investigated to determine the copper substrate suitable for hot-dipped tinning and the optimized tinning procedures. The results indicate that a proper increase in alloying elements (e.g., Cu-Fe-P series alloys) towards Cu substrate leads to a decrease in hot dipping performance. The proper process routine is determined as alkaline cleaning→water scrubbing→accelerant solvent dipping→drying→hot-dipped tinning→cooling. The appropriate dipping temperature range is 260 to 280 °C, which assists to maintain acceptable micro hardness (i.e., maintaining at least 95% of the original hardness). The optimal dipping time should be set as 6–10 s. The proposed hot-dipped tinning process routine may present a guideline for the fabrication of tin coating in electronic industry.

Principle for obtaining high joint quality in dissimilar friction welding of Ti-6Al-4V alloy and SUS316L stainless steel

Ti-6Al-4V alloy (Ti64) and SUS316L stainless steel rods were dissimilarly friction welded. Especially focusing on the detailed observation of interface microstructural evolution during the friction welding (FW), the relationship between the processing conditions, weld interface microstructure, and mechanical properties of the obtained joints were systematically investigated to elucidate the principle for obtaining a high joint quality in the FW of Ti64 and SUS316L. A higher friction pressure produced a lower welding temperature in the FW, hence suppressing the thick intermetallic compound layer formation. However, hard and brittle Ti64/SUS316L mechanically mixed layers generally formed especially at the weld interface periphery due to the high temperature increasing rate, high rotation linear velocity and high outward flow velocity of the Ti64. These harmful layers tended to induce the cracks/voids formation at the weld interfaces hence deteriorating the joints’ mechanical properties. The rotation speed reduction and liquid CO2 cooling during the entire processing decreased the temperature increasing rate, rotation linear velocity and outward flow velocity of the Ti64 at the weld interface periphery. Therefore, they suppressed the formation of the harmful mechanically mixed layers, facilitated the homogeneous and sound interface microstructure generation, and finally produced a high-quality dissimilar joint in the FW of Ti64 and SUS316L.

Diffusion Bonding of Al 6061 and Cu by Hot Isostatic Pressing

Diffusion bonding between Al and Cu was successfully performed by hot isostatic pressing (HIP). To improve the strength of diffusion bonding joint, pure nickel foils with different thickness were used as intermediate layer. Microstructure of the interface between Al and Cu was investigated by X-ray diffraction (XRD) technique, secondary electron microscopy (SEM), and nano-indentation tests. When the temperature was 500 °C and held for 3 h with a processing pressure of 50 MPa, Al and Cu could be bonded with its interface formed by several diffusion layers. With the addition of Ni interlayer, the diffusion of aluminum atoms was effectively hindered, and the interface became smoother. The tensile strength of bonded joints increases with increasing the thickness of Ni interlayer, which contributes to a reduction in the thickness of intermetallic compounds (IMCs) and well bonding quality of Al-Cu joints.

Effects of oscillating laser offset on microstructure and properties of dissimilar Al/steel butt-joint

The key challenge in laser butt-welding of dissimilar Al/steel metals, i.e., unevenly-distributed brittle intermetallic compounds (IMCs) along the interface, was solved via the beam oscillation under optimized laser offset (∆D). The IMCs thickness variance from top to bottom (δ) was adopted to quantitatively evaluate the uniformity of IMCs layer. The smaller the δ, the better the uniformity. The δ decreased from 105.14 to 0.0022 as the ∆D increased from 0.4 to 0.8 mm, indicating the uniformity increased by 4.7 × 104 times. The uniformity enhancement nearly doubled the ultimate tensile strength of the joint, which increased from 77 to 160 MPa. The formation of the IMCs layer and the fracture failure process of the joint were discussed in terms of the melt flow in the molten pool and its effects on the element distribution at the interface.

Interfacial Reaction in Twin-Roll Cast AA1100/409L Clad Sheet During Different Sequence of Cold Rolling and Annealing

Besides cold rolling reduction and annealing temperature and time, sequence of cold rolling and annealing also affects interfacial reaction in twin roll cast (TRC) clad sheet. This is because of short contacting time between Al melt and steel and small rolling reduction in as-cast TRC sheet, which results in small and uneven distribution of metallurgical bonding in the sheet. In as-cast sheet, no intermetallic compound (IMC) appears at the bonding interface after 510 °C × 1.5 h annealing and the sheet has a high peeling strength of 21.5 N/mm. However, incubation period of IMC is shortened in the 40% cold rolled sheet. A 2.2 μm thick IMC layer appears in the 40% sheet after 510 °C × 1.5 h annealing and the peeling strength decreases to 15.5 N/mm. 25% reduction cannot reduce the incubation period. This is because the bonding interface becomes tight enough and its energy storage increases after 40% cold rolling and as a result, the inter-diffusion of Al and Fe atoms is enhanced during annealing.

The effect of Ag on the growth of intermetallics at the interface of Sn5Zn/Cu interconnects

The reliability of electronic packages is primarily affected by the growth of the intermetallic compound (IMC) between the solder and the copper interconnection. It is therefore necessary to study the effect of element addition on the growth of the IMC interface with respect to an improvement in the package reliability. In this study, the influence of Ag on the growth of intermetallic compounds in the Sn5Zn solder/copper interconnection interface was investigated. The results revealed that in comparison with Sn5Zn/Cu, the interfacial IMC thickness decreased initially and then increased in accordance with an increase in the doping amount of alloying elements in the solder. Further, it was observed that the doping of the alloying element (Ag) can significantly reduce the IMC growth rate at the interface. Compared with the Cu5Zn8 unit cell formation energy (-0.632eV), one dopant atom and two dopant atoms of Ag can reduce the formation energy of a Cu5Zn8-based unit cell to the minimum values of -0.636 eV and –0.640 eV, respectively; consequently, the stability is increased. The maximum diffusion activation energies of Cu atoms and Zn atoms in a Cu5Zn8-based unit cell are 3.419 eV and 1.692 eV, which increase to 3.512 eV and 1.971 eV, respectively, after doping with Ag.

Comparison and mechanism of electromigration reliability between Cu wire and Au wire bonding in molding state

Electromigration (EM) reliability and failure analysis of Cu wire and Au wire bonding processes on Al substrates after molding were investigated and compared systematically. According to the resistance as a function of EM time trend, there were four stages during the EM test for wire bonding that can tentatively be referred to as the fluctuating resistance stage, stable resistance stage, rapidly increasing resistance stage and quick failure stage of the sample. When the applied current was 1.8 A on a wire with a diameter of 20 μm (at a current density of 5.7 × 105 A/cm2), the lifetimes of the Cu wire and Au wire bonding processes were 1032 h and 119 h, respectively, according to Weibull analyses. Two kinds of failure modes were observed: wire fracture and first bond fracture. Almost 90% of the failed samples belonged to wire fracture category. After the EM test, the thickness of intermetallic compounds (IMCs) at the anode interface was similar to that at the cathode interface for both the Cu wire and Au wire bonding cases, which means that the normal polarity effect during electromigration did not occur here. However, the thickness of the IMCs at the Au/Al bonding interface was thicker than that at the Cu/Al interface for both the aged and EM samples. Voids and cracks were observed within the Au wire-bonding interface. It is suspected that the Cu wire bonding process had a slower thermal diffusion and smaller EM flux than those of the Au wire bonding process. Due to a decrease in vacancy flux within the Cu wire bonding region, the cracks were also suppressed, which greatly extended the lifetime of the Cu wire bonds.

Characterizations of weld defects, intermetallic compounds and mechanical properties of friction stir lap welded dissimilar alloys

This experimental investigation describes the effect of tilt angle and rotational speed on the characteristics of welding defects, intermetallic compounds, weld strength and micro-hardness in friction stir welding of thin sheets of AA6061-T6 and AISI304. Qualitative and quantitative assessments of the welding defects have been made by using X-ray micro computed tomography and metallographic study. Three-dimensional tomography and special distribution of internal voids (pores) have been discussed by correlating it to different values of tool rotational speed and tilt angle. Internal defects were localized in the stir zone. Welding at a tilt angle of 1° reduced both surface and internal defects. Non-uniform thickness of intermetallic compound (FexAly) layer has been found in the weld zone whose thickness varied with the values of process parameters. Higher weld porosity, bigger steel hook defect and increased layer thickness of intermetallic compound caused a significant reduction in the joint strength and elongation.

An analytical model to predict diffusion induced intermetallic compounds growth in Cu-Sn-Cu sandwich structures

A mass diffusion model is developed to describe the growth kinetics of Cu6Sn5 intermetallic compounds (IMC) in the Cu-Sn-Cu sandwich structure. The proposed model is based on the local interfacial mass conversation law where interfacial Cu/Sn reactions and atomic diffusion are considered. Theoretical analysis shows that the IMC thickness growth is proportional to the square root of the product of the diffusion coefficient and time. The proposed model can explain the polarity effect of electromigration on kinetics of IMC growth where all the parameters have clear physical meaning. The theoretical predictions are compared with experimental results and show reasonable accuracy.

Effect of Trace Zn Addition on Interfacial Evolution in Sn-10Bi/Cu Solder Joints during Aging Condition.

Excessive growth of intermetallic compounds (IMCs) during service affects the reliability of solder joints, so how to suppress the growth of IMC thickness at the interface in solder joints becomes a widespread concern. In this work, the interfacial reaction between Sn-10Bi solder and Cu substrate after thermal aging was investigated. Moreover, to depress the IMC growth at the interface, trace amounts of Zn was added into the Sn-10Bi solder, and the interfacial reactions of Sn-10Bi-xZn solders (x = 0.2, 0.5) and Cu substrate after thermal aging were studied in this paper. Compounds such as Cu6(Sn, Zn)5 and Cu5Zn8 were formed at the interface after adding trace amounts of Zn. The addition of 0.2 and 0.5 wt% Zn significantly inhibited the thickness growth of IMCs and the formation of Cu3Sn IMC at the interface of Sn-10Bi-0.2Zn/Cu and Sn-10Bi-0.5Zn/Cu during thermal aging. Therefore, the addition of trace Zn had an obvious effect on the interfacial reaction of Sn-10Bi/Cu solder joint. Interestingly, the evolution of IMC thickness in Sn-10Bi-0.5Zn/Cu solder joints was completely different from that in Sn-10Bi or Sn-10Bi-0.2Zn solder joints, in which the spalling of IMCs occurred. In order to explore the mechanisms on the depressing effect from the addition of trace Zn, the activation energy Q in solder joints during aging was calculated.

GTA Weld Brazing a Joint of Aluminum to Stainless Steel

The relationship between heat input, microstructures, and mechanical properties was studied in the gas tungsten arc (GTA) weld brazing process of aluminum alloy and stain-less steel dissimilar metals with a pure aluminum core wire (ER1100). The mechanisms involved were further revealed by hot-dip aluminizing experiments. The intermetallic compounds (IMCs) of the brazing interface consisted of a thin layer of -Fe2Al5 next to the steel, and a thick layer of -Fe4Al13 adjacent to the Al joint, surrounded by eutectic identified to be Al-FeAl6 and Al-Fe2Al9 distributed uniformly in the weld metal. With increasing heat input, the total thickness of the IMCs decreased first, reaching a minimum value of 3.5 m, and then increased. The decrease in IMC thickness with heat input was observed for the first time and had never been reported before in welding or brazing processes. The joint strength increased to a maximum value of 238 MPa and then declined, suggesting an inverse relationship between IMC thickness and joint strength. The results of the hot-dip aluminizing, regarding the relationship between IMC thickness and temperature, were consistent with the change of IMC thickness in the welds. A possible mechanism in action is that interfacial kinetics and thermodynamics play a role in the dissolution and decomposition of the thick layer of -Fe4Al13 into the FeAl6 and Fe2Al9 phase. Promoting the IMC dissolution or decomposition by adjusting welding procedures is a promising new way to control the IMC growth and improve the joint strength.

The influence of Copper and Bismuth element in Sn-Ag solders and ENIMAG substrate

The addition of copper and bismuth elements in Sn-Ag solders on interfacial reaction during reflow soldering and isothermal aging of SA25 and SACB25051 was investigated. The growth of intermetallic compound (IMC) was characterized by using scanning electron microscopy (SEM), optical microscope (OM) and energy dispersive x-ray (EDX). The results show the irregular circle shapes of (Cu,Ni)6Sn5 IMC at the interface of solder joint for SA25 while rod-like shape for SACB25051 during both reflow soldering process and isothermal aging. The SACB25051 solder showed the thinner IMC thickness compared to SA25 solder for both conditions. The Cu and Bi element in Sn-Ag solder promotes a slower growth rate of IMC formation during interaction. The thickness and morphology of the IMC for both solders increase gradually with increasing of aging time.

Effect of external magnetic field on the microstructure and strength of laser-welded aluminum to titanium

An external magnetic field was firstly been applied to laser welding of Al/Ti alloys. Microstructure and tensile strength for Al/Ti laser-welded joint under external magnetic field were investigated. The results indicate that the strength of Al/Ti joint is improved by 44.4% via application of an external vertical magnetic field with an intensity of 120 mT. The enhancement of strength is attributed to the diminishment of intermetallic compounds, that is, the volume of TiAl3 is decreased from 10.69 to 6.44%. It is deemed that, on the one hand, the induced Lorentz force via the interaction between horizontal fluid flow and perpendicular magnetic field suppresses the horizontal convection, leading to the decreased thickness of intermetallic compounds. On the other hand, the thermoelectric magnetic convection contributes to the heat and solute transfer at the Al/Ti interface, leading to the redistribution of elements with disordered and decreased intermetallic compounds.

Novel polarity effect on intermetallic compound thickness changes during electromigration in Cu/Sn-3.0Ag-0.5Cu/Cu solder joints

Growth of an intermetallic compound (IMC) plays a critical role in the reliability of flip-chip solder joints. It has been found that IMC growth is accelerated on the anode and inhibited on the cathode during electromigration (EM), although there are discrepancies in the relevant literature. In this study, Cu/Sn3.0Ag0.5Cu/Cu solder joints were examined by EM under a current density of 1×104A/cm2 at 150 °C; an aging test was conducted at 150 °C to compare the EM results. Cu atoms are the dominant diffusion species that migrate from the cathode to the anode during EM. The increase in IMC thickness at the anode is divided into two stages: during stage 1, the IMC thickness first grows with a gradual linear tendency, followed by retarded IMC growth. During stage 2, the EM driving force dominates IMC growth because of the high IMC thickness after stage 1, inducing a lower Cu concentration gradient and decreasing the driving force of the chemical potential. At the cathode, the IMC thickness fluctuates between two threshold values. This phenomenon is attributed to the competition between the effects of chemical potential and EM on IMC growth. In the initial stage, the chemical potential is sufficiently large to induce considerable Cu flux from the Cu underbump metallization into the Sn solder, inducing IMC growth. When the IMC becomes too thick to significantly reduce the chemical potential, a substantial number of Cu atoms migrate via EM, reducing the IMC thickness at the cathode. However, when IMC thickness decreases, the chemical potential again increases and enhances IMC growth. Therefore, thresholds 1 and 2 may be set based on changes observed in the IMC thickness at the cathode during EM. These findings elucidate the polarity effect in intermetallic compounds, and its effect on the stability of solder joints.