Academic literature on the topic 'Cu-based Intermetallics'

The present study investigated the effect of isothermal aging up to 2000 hours on the intermetallics formed between Sn-4Ag-0.5Cu lead free solder on electroless nickel electroless palladium immersion gold surface finish (Ni-Pd-Au). For all parameters, aging have an effect of changing the intermetallic morphology to coarser and dense structure. The intermetallic compound formed for the interconnection of the lead free solder changes with increased aging time from (Cu,Ni)6Sn5 compound to (Ni,Cu)3Sn4. At the end of the 2000 hours aging time, it changes to Ni3Sn4. This is the effect of Cu element availability during the intermetallics growth process. Starting from as reflow process, (Pd, Ni)Sn4 intermetallics formed near the interface of the solder joint. The formation of the (Pd, Ni)Sn4 intermetallics act like a diffusion barrier to slow down the growth of interface intermetallics. Lastly, Au element was detected in the Pd-Sn based intermetallic after aging more than 1000 hours.

Due to environmental concerns, lead-free solders were introduced to replace the lead-based solders in microelectronics devices technology. Although there are many lead-free solders available, the Sn-Ag-Cu solders are considered the best replacement due to their good wettability and joint strength. Although the Sn-Ag-Cu solders are accepted widely, but there are still some room for improvement. In this study, 1wt% Zn, which can be considered high percentage for a dopant, was added into the solder via powder metallurgy route. The effects of adding this dopant into the Sn-3.5Ag-1.0Cu solder on the interface intermetallic and thickness were investigated. The intermetallics phases formed were observed under Scanning Electron Microscope (SEM) and their thicknesses were measured. The SEM results showed the presence of Cu6Sn5, Cu3Sn and (Cu,Zn)6Sn5 intermetallics. It can be concluded that Zn behaved as retarding agent and significantly retarded the growth of Cu-Sn intermetallics.

A standard three-step surface pretreatment employed in the aerospace sector for Al alloys have been investigated prior to the generation of cerium conversion coatings (CeCC) on aluminium-copper alloy 2024. Two pretreatments were analysed, one without final acid etching (Pretreatment 1) and another with this step (Pretreatment 2). Both pretreatments affect the alloy intermetallic phases, playing a key role in the development of the CeCC, and also in the susceptibility to localised corrosion in NaCl medium. Scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX) revealed that after Pretreatment 2, Al(Cu,Mg) phases were partially or totally removed through dealloying with their subsequent copper enrichment. Conversely, none of these intermetallic phases were affected when the final acid step was not employed (Pretreatment 1). Meanwhile, Al-Cu-Fe-Mn-(Si) phases, the other major Al–Cu alloys intermetallics, suffers minor changes through the whole pretreatments chain. The protective efficiency of CeCC was evaluated using electrochemical techniques based on linear polarisation (LP) and electrochemical impedance spectroscopy (EIS). Samples with CeCC deposited after the Pretreatment 1 gave higher polarisation resistance and impedance module than CeCC deposited after Pretreatment 2. SEM-EDX and X-ray photoelectron spectroscopy analysis (XPS) indicate that the main factors explaining the corrosion resistance of the coatings is the existence of Al(Cu,Mg) intermetallics in the surface of the alloy, which promote the deposition of a cerium-based coating rich in Ce4+ compounds. These Al(Cu,Mg) intermetallics were kept in the 2024 alloy when acid etching was not employed (Pretreatment 1).

Two atomized alloy powders, those chemical compositions are Al-10Si-5Fe-1Zr and Al- 10Si-5Fe-4Cu-2Mg-1Zr, were pre-compacted by cold pressing with 350MPa and subsequently hot forged at temperatures ranging from 653K to 845K and at an initial strain rate of 10-2/s in order to produce bulk cylindrical type alloys with the diameter of 10 mm. The addition of Cu and Mg into the present alloy causes a decrease in the eutectic reaction temperature of Al-10Si-5Fe-1Zr alloy from 841K to 786K and results in a decrease of flow stress at the given forging temperature. TEM observation revealed that in addition to Al-Fe based intermetallics, Al2Cu and Al2CuMg intermetallics appeared to display the alloying effect additionally. The volume fraction of intermetallic dispersiods increased by the addition of Cu and Mg. Compressive strength of the present alloys was closely related to the volume fraction of intermetallic dispersoids.

A new processing route for intermetallics starting from elemental metal powders and using laser beam heating has been investigated, by which three Al3Ti-based ternary intermetallic systems alloyed with Fe, Ni, or Cu, respectively, have been produced. Structures and compositions of five phases found in the three systems have been analyzed using optical metallography, transmission electron diffraction and microscopy, and thin foil energy dispersive x-ray spectrometry. The structure of three matrix phases in the three systems was identified as L12 type, which is consistent with previous work. The space group of the second phases in the two systems alloyed with Fe or Ni was reconfirmed or newly determined as Fm3m, using convergent beam electron diffraction. The lattice parameter measurements of these five found phases indicated atomic size-effect on the formation of intermetallics.

An investigation was carried out to know the extent of influence of bonding-time on the interface structure and mechanical properties of diffusion bonding (DB) of TiA|Cu|SS. DB of Ti6Al4V (TiA) and 304 stainless steel (SS) using pure copper (Cu) of 200-μm thickness were processed in vacuum using 4-MPa bonding-pressure at 1123 K from 15 to 120 min in steps of 15 min. Preparation of DB was not possible when bonding-time was less than 60 min as the bonding at Cu|SS interface was unsuccessful in spite of effective bonding at TiA|Cu interface; however, successful DB were produced when the bonding-time was 60 min and beyond. DB processed for 60 and 75 min (classified as shorter bonding-time interval) showed distinctive characteristics (structural, mechanical, and fractural) as compared to the DB processed for 90, 105, and 120 min (classified as longer bonding-time interval). DB processed for 60 and 75 min exhibited layer-wise Cu–Ti-based intermetallics at TiA|Cu interface, whereas Cu|SS interface was completely free from reaction products. The layer-wise structure of Cu–Ti-based intermetallics were not observed at TiA|Cu interface in the DB processed for longer bonding-time; however, the Cu|SS interface had layer-wise ternary intermetallic compounds (T1, T2, and T3) of Cu–Fe–Ti-based along with σ phase depending upon the bonding-time chosen. Diffusivity of Ti-atoms in Cu-layer (DTi in Cu-layer) was much greater than the diffusivity of Fe-atoms in Cu-layer (DFe in Cu-layer). Ti-atoms reached Cu|SS interface but Fe-atoms were unable to reach TiA|Cu interface. It was observed that DB fractured at Cu|SS interface when processed for shorter bonding-time interval, whereas the DB processed for longer bonding-time interval fractured apparently at the middle of Cu-foil region predominantly due to the existence of brittle Cu–Fe–Ti-based intermetallics.

Replacing Sn-Pb solder with lead-free solder is a great challenge in the electronics industry. The presented lead-free solder is Sn based and forms two intermetallic species upon reaction with the Cu substrate, namely Cu6Sn5 and Cu3Sn. The growth of Cu6Sn5 and Cu3Sn intermetallics have been investigated with respect to Sn-40Pb/Cu solder joints. The joints were aged under long-term thermal exposure using single shear lap joints and the intermetallics were observed using scanning electron microscopy. As-soldered solder joints exhibit a single Cu6Sn5 phase, however after aging a Cu3Sn layer below the Cu6Sn5 is observed to manifest. The Cu6Sn5 layer develops with a scalloped morphology, whereas the Cu in layer always develops an undulating planar shape in phase with the Cu6Sn5. The Cu6Sn5 layer begins to transform from a scalloped- to a planar-shape as aging progresses in order to minimize the interfacial energy. The intermetallic layers exhibit a linear dependence on the square root of aging time, which corresponds to diffusion-controlled growth. The activation energy for the growth of the Cu6Sn5 intermetallic layer has been determined to be 56.16 kJ/mol.

Replacing Sn-Pb solder with lead-free solder is a great challenge in the electronics industry. The presented lead-free solder is Sn based and forms two intermetallic species upon reaction with the Cu substrate, namely Cu6Sn5 and Cu3Sn. The growth of Cu6Sn5 and Cu3Sn intermetallics have been investigated with respect to Sn-40Pb/Cu solderjoints. The joints were aged under long-term thermal exposure using single shear lap joints and the intermetallics were observed using scanning electron microscopy. As-soldered solder joints exhibit a single Cu6Sn5 phase, however after aging a Cu3Sn layer below the Cu6Sn5 is observed to manifest. The Cu6Sn5 layer develops with a scalloped morphology, whereas the Cu3Sn layer always develops an undulating planar shape in phase with the Cu6Sn5. The Cu6Sn5 layer begins to transform from a scalloped- to a planar-shape as aging progresses in order to minimize the interfacial energy. The intermetallic layers exhibit a linear dependence on the square root ofaging time, which corresponds to diffusion-controlled growth. The activation energy for the growth of the Cu6Sn5, intermetallic layer has been determined to be 56.16 kJ/mol.

Abstract In this work, using the first-principles method, the alloying stability, electronic structure, and elastic properties of Al-based intermetallics were investigated. It was found that these alloys have a strong alloying ability and structural stability due to the negative formation energies and the cohesive energies. The valence bonds of these intermetallic compounds are attributed to the valence electrons of Cu 3δ states for AlCu3, Cu 3δ and Zr 4δ states for AlCu2Zr, and Al 3s, Zr 5s and 4δ states for AlZr3, respectively. Furthermore, the correlation between elastic properties of these intermetallic compounds and their electronic structures was revealed. The results show that structural parameters and elastic properties such as bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and anisotropy agreed well with experimental results.

Purpose The purpose of this study is to investigate the addition of 0.05 Wt.% carbon nanotube (CNT) into the Sn-1.0Ag-0.5Cu (SAC) solder on the intermetallic (IMC) growth. Lead-based solders play an important role in a variety of applications in electronic industries. Due to the toxicity of the lead in the solder, lead-free solders were proposed to replace the lead-based solders. Sn-Ag-Cu solder family is one of the lead-free solders, which are proposed and considered as a potential replacement. Unfortunately, the Sn-Ag-Cu solder faces some reliability problems because of the formation of the thick intermetallic compounds. So the retardation of intermetallic growth is prime important. Design/methodology/approach The solder joint was aged under liquid state aging with soldering time from 1 to 60 min. Findings Two types of intermetallics, which are Cu6Sn5 and Cu3Sn were observed under a scanning electron microscope. The morphology of Cu6Sn5 intermetallic transformed from scallop to planar type as the soldering time increases. The addition of carbon nanotube into the SAC solder has retarded the Cu6Sn5 intermetallic growth rate by increasing its activation energy from 97.86 to 101.45 kJ/mol. Furthermore, the activation energy for the Cu3Sn growth has increased from 102.10 to 104.23 kJ/mol. Originality/value The increase in the activation energy indicates that the growth of the intermetallics was slower. This implies that the addition of carbon nanotube increases the reliability of the solder joint and are suitable for microelectronics applications.

Secondary Al-Si-Cu-Mg based foundry alloys are widely used in automotive industry to particularly produce powertrain cast components mainly due to their good ratio between weight and mechanical properties, and excellent casting characteristics. Presence of impurity elements, such as Fe, Mn, Cr, Ti, V and Zr, in secondary Al-Si alloys is one of the critical issues since these elements tend to reduce alloy mechanical properties. There is an ongoing effort to control the formation of intermetallic phases containing transition metals, during alloy solidification. Although phases formation involving these transition metal impurities in non-grain-refined Al-Si alloys is well documented in the literature, the role of grain refinement in microstructural evolution of secondary Al-Si-Cu-Mg alloys needs further experimental investigations since chemical grain refinement is one of the critical melt treatment operations in foundries. The primary aim of this PhD work is thus defined to characterize the formation of intermetallic phases containing transition metals in secondary Al-7Si-3Cu-0.3Mg alloy before and after grain refinement by different master alloys and contribute to the understanding of the mechanisms underlying the microstructural changes occurring with the addition of grain refiner. Another critical issue related to Al-Si-Cu-Mg alloys is their limited thermal stability at temperatures above 200 oC. The operating temperature in engine combustion chamber is reported to often exceed 200 oC during service. Moreover, a further increase of operating temperature is anticipated due to the expected engine power enhancement in near future, which indicates the necessity for the development of a new creep-resistant Al alloys. Deliberate addition of transition metals is believed to yield a new heat-resistant alloy by promoting the formation of thermally stable dispersoids inside α-Al grains. This study thus also attempted to investigate the effect of adding transition metals Zr, V and Ni on the solidification processing, microstructural evolution and room/high-temperature tensile properties of secondary Al-7Si-3Cu-0.3Mg alloy, one of the most used alloys in automotive engine manufacturing. The influence of transition metal impurities on microstructural evolution of secondary Al-7Si-3Cu-0.3Mg alloy was investigated before and after chemical treatment with different master alloys: Al-10Sr, Al-5Ti-1B, Al-10Ti and Al-5B. The Al-10Zr, Al-10V and Al-25Ni master alloys were used for the experimental investigations of the effects of deliberate additions of transition metals on the solidification path, microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy. Solidification path of the alloys was characterized by the traditional thermal analysis technique and differential scanning calorimetry (DSC). Optical microscope (OM), scanning electron microscope (SEM) equipped with energy-dispersive (EDS), wavelength-dispersive spectrometers (WDS) and electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) equipped with EDS were used to characterize the type, morphology and distribution of the phases precipitated during solidification and heat treatment of the studied alloys. The static tensile properties of the alloys were characterized at room (20 oC) and high temperatures (200 and 300 ºC). Experimental findings indicate that the Sr-modification and grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy with Al-Ti-B can be enough effective despite the presence of transition metal impurities in the material and the variation of pouring temperature. However, the V and Zr (~100 ppm each) available in secondary Al-7Si-3Cu-0.3Mg alloy tended to promote the precipitation of harmful, primary AlSiTi intermetallics during solidification of grain-refined alloy. This implies that more effective optimization of grain refiner addition level in secondary Al foundry alloys can be achieved by considering the role of transition metal impurities, Ti, V and Zr, since the formation of primary AlSiTi particles causes (1) the depletion of Ti needed for effective α-Al grains growth restriction and (2) the formation of casting defects, such as shrinkage, due to their flaky morphology. Iron available in secondary Al-7Si-3Cu-0.3Mg alloy as impurity only formed more desirable α-Al15(FeMn)3Si2 phase in non-grain refined state. After grain refinement by Al-5Ti-1B, Fe was also involved in the formation of more deleterious β-Al5FeSi phase. The TiB2 particles acted as nucleation site for β-Al5FeSi phase. Both higher cooling rate and higher Al-5Ti-1B addition levels tended to promote the formation of deleterious β-Al5FeSi at the expense of α-Al15(FeMn)3Si2 in the alloy refined by Al-5Ti-1B. This implies that rather than the ratio between Mn and Fe, the nucleation kinetics of Fe-rich intermetallics play a decisive role in the selection of competing α-Al15(FeMn)3Si2 and β-Al5FeSi intermetallic phases for the precipitation during alloy solidification. Moreover, grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy by Al-5B showed comparable performance to that of Al-5Ti-1B master alloy, however, without any deleterious influence on the precipitation sequence of Fe-rich phases, i.e. deleterious β-Al5FeSi reaction remained unfavourable during alloy solidification. Experimental findings from the investigations of the effect of deliberate Zr and V addition revealed that Zr and V addition can induce the grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy. While Zr addition yielded the formation of pro-peritectic Zr-rich particles, which are found to nucleate primary α-Al at low undercooling, the effect of adding V can be characterized by the enhancement of the degree of constitutional undercooling. Combined Zr and V addition showed more effective grain refinement level than their individual additions. Majority of both Zr and V added to the alloy were retained inside α-Al matrix during solidification. As a result, limited amounts of Zr and V were rejected to the interdendritic liquid by the growing α-Al dendrites, then forming small-sized and rarely distributed intermetallics. Owing to its low solid solubility in α-Al, nickel available as impurity (~ 200 ppm) or after deliberate addition (0.25 wt.%) in secondary Al-7Si-3Cu-0.3Mg alloy was mainly bound to interdendritic, insoluble intermetallics, such as Al6Cu3Ni and Al9(FeCu)Ni phases. The presence of ~ 200 ppm Ni was sufficient to diminish to a certain extent the precipitation hardening effect of Cu. Interdendritic Zr/V/Ni-rich phases remained undissolved into the α-Al matrix during solution heat treatment. Therefore, the supersaturated transition metals in α-Al solid solution obtained during solidification was only involved in the solid-state precipitation occurring during heat treatment. Unlike Cu/Mg-rich strengthening precipitates that commonly form during aging, the Zr/V-rich precipitates tended to form during solution heat treatment. Other transition metals, such as Mn, Fe, Cr and Ti, which were present as impurities in secondary Al-7Si-3Cu-0.3Mg alloy significantly promoted the formation of nano-sized Zr/V-rich precipitates inside α-Al grains. These thermally more stable precipitates, including novel α-Al(MnVFe)Si, were credited for the enhanced high-temperature strength properties of Al-7Si-3Cu-0.3Mg alloy by ~ 20 %.
Secondary Al-Si-Cu-Mg based foundry alloys are widely used in automotive industry to particularly produce powertrain cast components mainly due to their good ratio between weight and mechanical properties, and excellent casting characteristics. Presence of impurity elements, such as Fe, Mn, Cr, Ti, V and Zr, in secondary Al-Si alloys is one of the critical issues since these elements tend to reduce alloy mechanical properties. There is an ongoing effort to control the formation of intermetallic phases containing transition metals, during alloy solidification. Although phases formation involving these transition metal impurities in non-grain-refined Al-Si alloys is well documented in the literature, the role of grain refinement in microstructural evolution of secondary Al-Si-Cu-Mg alloys needs further experimental investigations since chemical grain refinement is one of the critical melt treatment operations in foundries. The primary aim of this PhD work is thus defined to characterize the formation of intermetallic phases containing transition metals in secondary Al-7Si-3Cu-0.3Mg alloy before and after grain refinement by different master alloys and contribute to the understanding of the mechanisms underlying the microstructural changes occurring with the addition of grain refiner. Another critical issue related to Al-Si-Cu-Mg alloys is their limited thermal stability at temperatures above 200 oC. The operating temperature in engine combustion chamber is reported to often exceed 200 oC during service. Moreover, a further increase of operating temperature is anticipated due to the expected engine power enhancement in near future, which indicates the necessity for the development of a new creep-resistant Al alloys. Deliberate addition of transition metals is believed to yield a new heat-resistant alloy by promoting the formation of thermally stable dispersoids inside α-Al grains. This study thus also attempted to investigate the effect of adding transition metals Zr, V and Ni on the solidification processing, microstructural evolution and room/high-temperature tensile properties of secondary Al-7Si-3Cu-0.3Mg alloy, one of the most used alloys in automotive engine manufacturing. The influence of transition metal impurities on microstructural evolution of secondary Al-7Si-3Cu-0.3Mg alloy was investigated before and after chemical treatment with different master alloys: Al-10Sr, Al-5Ti-1B, Al-10Ti and Al-5B. The Al-10Zr, Al-10V and Al-25Ni master alloys were used for the experimental investigations of the effects of deliberate additions of transition metals on the solidification path, microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy. Solidification path of the alloys was characterized by the traditional thermal analysis technique and differential scanning calorimetry (DSC). Optical microscope (OM), scanning electron microscope (SEM) equipped with energy-dispersive (EDS), wavelength-dispersive spectrometers (WDS) and electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) equipped with EDS were used to characterize the type, morphology and distribution of the phases precipitated during solidification and heat treatment of the studied alloys. The static tensile properties of the alloys were characterized at room (20 oC) and high temperatures (200 and 300 ºC). Experimental findings indicate that the Sr-modification and grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy with Al-Ti-B can be enough effective despite the presence of transition metal impurities in the material and the variation of pouring temperature. However, the V and Zr (~100 ppm each) available in secondary Al-7Si-3Cu-0.3Mg alloy tended to promote the precipitation of harmful, primary AlSiTi intermetallics during solidification of grain-refined alloy. This implies that more effective optimization of grain refiner addition level in secondary Al foundry alloys can be achieved by considering the role of transition metal impurities, Ti, V and Zr, since the formation of primary AlSiTi particles causes (1) the depletion of Ti needed for effective α-Al grains growth restriction and (2) the formation of casting defects, such as shrinkage, due to their flaky morphology. Iron available in secondary Al-7Si-3Cu-0.3Mg alloy as impurity only formed more desirable α-Al15(FeMn)3Si2 phase in non-grain refined state. After grain refinement by Al-5Ti-1B, Fe was also involved in the formation of more deleterious β-Al5FeSi phase. The TiB2 particles acted as nucleation site for β-Al5FeSi phase. Both higher cooling rate and higher Al-5Ti-1B addition levels tended to promote the formation of deleterious β-Al5FeSi at the expense of α-Al15(FeMn)3Si2 in the alloy refined by Al-5Ti-1B. This implies that rather than the ratio between Mn and Fe, the nucleation kinetics of Fe-rich intermetallics play a decisive role in the selection of competing α-Al15(FeMn)3Si2 and β-Al5FeSi intermetallic phases for the precipitation during alloy solidification. Moreover, grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy by Al-5B showed comparable performance to that of Al-5Ti-1B master alloy, however, without any deleterious influence on the precipitation sequence of Fe-rich phases, i.e. deleterious β-Al5FeSi reaction remained unfavourable during alloy solidification. Experimental findings from the investigations of the effect of deliberate Zr and V addition revealed that Zr and V addition can induce the grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy. While Zr addition yielded the formation of pro-peritectic Zr-rich particles, which are found to nucleate primary α-Al at low undercooling, the effect of adding V can be characterized by the enhancement of the degree of constitutional undercooling. Combined Zr and V addition showed more effective grain refinement level than their individual additions. Majority of both Zr and V added to the alloy were retained inside α-Al matrix during solidification. As a result, limited amounts of Zr and V were rejected to the interdendritic liquid by the growing α-Al dendrites, then forming small-sized and rarely distributed intermetallics. Owing to its low solid solubility in α-Al, nickel available as impurity (~ 200 ppm) or after deliberate addition (0.25 wt.%) in secondary Al-7Si-3Cu-0.3Mg alloy was mainly bound to interdendritic, insoluble intermetallics, such as Al6Cu3Ni and Al9(FeCu)Ni phases. The presence of ~ 200 ppm Ni was sufficient to diminish to a certain extent the precipitation hardening effect of Cu. Interdendritic Zr/V/Ni-rich phases remained undissolved into the α-Al matrix during solution heat treatment. Therefore, the supersaturated transition metals in α-Al solid solution obtained during solidification was only involved in the solid-state precipitation occurring during heat treatment. Unlike Cu/Mg-rich strengthening precipitates that commonly form during aging, the Zr/V-rich precipitates tended to form during solution heat treatment. Other transition metals, such as Mn, Fe, Cr and Ti, which were present as impurities in secondary Al-7Si-3Cu-0.3Mg alloy significantly promoted the formation of nano-sized Zr/V-rich precipitates inside α-Al grains. These thermally more stable precipitates, including novel α-Al(MnVFe)Si, were credited for the enhanced high-temperature strength properties of Al-7Si-3Cu-0.3Mg alloy by ~ 20 %.

碩士
國立臺灣大學
機械工程學研究所
98
Microstructural evolution and bonding strength of infrared brazed Fe3Al and stainless steels(SS) using Au-based and CuMn-based fillers, respectively, are studied. Firstly the wetting experiments using these braze alloys are conducted to make sure the suitable brazing temperature. Al2FeNi, Fe3Al and AlAu2 intermetallics are found in the joint of AuPdNi (Au-8wt.%Pd-22wt.%Ni) braze alloy brazed at 980℃ and all specimens are fractured in brittle. For AuCu(Au-20wt.%Cu) braze alloy, the brazed joint is fractured along the central β-phase in which the fracture exhibits ductile/brittle mode(s) with different brazing conditions. The highest shear strength for AuCu filler is 327MPa for specimen brazed at 880℃×300s. Raising the brazing time or temperature will deteriorate the strength due to the fracture mode transfers to brittleness. For infrared brazed 304, 422 and 440C stainless steels using Cu-33.7%Mn filler, a continuous interfacial (γFe,γMn) phase is found in between the braze and SS which is not well bonded with (Cu,γMn) primary phase in the brazed joint. Thus some cracks are observed in between (γFe,γMn) and (Cu,γMn) due to the thermal expansion mismatch of these two phases. In addition, solidification shrinkage voids or impurities are also observed in the central region of the joint and the fracture mode along this central voids/impurities is ductile with the shear strength of 280MPa for brazing 304 and 422 SS brazed at 875oC×180s. Cu-33.7%Mn filler alloyed with Ni can improve the joint properties, but increase slightly the fillers’ melting points. The Ni alloyed CuMn fillers can effectively wet all SS at appropriate temperatures and the interfacial cracks in between (γFe,γMn) and (Cu,γMn) phase are greatly reduced with increasing the Ni addition.

碩士
國立臺灣大學
材料科學與工程學研究所
97
Microstructural evolution and bonding strength of infrared brazed Fe3Al using Ag, Cu, and BAg-8 braze alloys have been studied. The joint of the Ag-brazed specimen only contains Ag-rich phase alloyed with Al. The shear strength isn’t affected by the brazing time and reaches 127MPa for specimens brazed at 980oC×300s with the ductile fracture of Ag-rich phase. The microstructure and shear strength of infrared brazed Fe3Al using pure Cu at 1100oC are strongly dependent upon the brazing time in which the brazed joint changes from Cu-rich phase into β1’ (N18R) martensite phase, and causes different fracture mechanism and bonding strength. The highest shear strength reachs 291MPa for the specimen brazed at 1100℃×300s with the mixture of brittle and ductile phases in the joint. The microstructure of the Fe3Al joint brazed by BAg-8 contains Ag-rich and Cu-rich phases eutectic for all brazing conditions. The shear strength increases slightly with increasing the brazing time because the Al is alloyed in the Cu-rich matrix. The highest shear strength of 181MPa is acquired from the 800℃×600s brazed specimen. The analyses of dynamic wetting angles and microstructures of Ag-based braze alloys on 304, 422 and 440C substrates are performed. Experimental results shows that Sn added into Ag-Cu braze alloy can reduce its melting point effectively but can not improve its wettability for stainless steel. The Ag-Cu-Sn braze alloy with adding Mn can reduce the wetting angle on stainless steel. The more the Mn content , the better the wettability it has. The best wettability is the Ag-Cu-Sn-10wt.%Mn alloy and its microstructure after wetting tests consistsd of Ag-rich and copper-rich phases. Meanwhile, increasing the Mn content results in the copper-rich phases in the braze changing from complex phases (Cu3Sn, Cu4MnSn, Cu-rich phases) into single Cu-rich phase with many Ag-rich precipitates.

This dissertation constitutes two major sections. In the first major section, a kinetic analysis was established to investigate the electromigration (EM), enhanced intermetallic compound (IMC) growth and void formation for Sn-based Pb-free solder joints to Cu under bump metallization (UBM). The model takes into account the interfacial intermetallic reaction, Cu-Sn interdiffusion, and current stressing. A new approach was developed to derive atomic diffusivities and effective charge numbers based on Simulated Annealing (SA) in conjunction with the kinetic model. The finite difference (FD) kinetic model based on this approach accurately predicted the intermetallic compound growth when compared to empirical observation. The ultimate electromigration failure of the solder joints was caused by extensive void formation at the intermetallic interface. The void formation mechanism was analyzed by modeling the vacancy transport under electromigration. The effects of current density and Cu diffusivity in Sn solder were also investigated with the kinetic model. The second major section describes the integration of Step and Flash Imprint Lithography (S-FIL®) into an industry standard Cu/low-k dual damascene process. The yield on a Back End Of the Line (BEOL) test vehicle that contains standard test structures such as via chains with 120 nm vias was established by electrical tests. S-FIL shows promise as a cost effective solution to patterning sub 45 nm features and is capable of simultaneously patterning two levels of interconnect structures, which provides a low cost BEOL process. The critical processing step in the integration is the reactive ion etching (RIE) process that transfers the multilevel patterns to the inter-level dielectrics (ILD). An in-situ, multistep etch process was developed that gives excellent pattern structures in two industry standard Chemical Vapor Deposited (CVD) low-k dielectrics. The etch process showed excellent pattern fidelity and a wide process window. Electrical testing was conducted on the test vehicle to show that this process renders high yield and consistent via resistance. Discussions of the failure behaviors that are characteristic to the use of S-FIL are provided.
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Abstract In this chapter, the quantum-based interatomic potentials (QBIPs) developed in Chapters 3–5 for elemental metals are extended to the much larger domain of alloys and intermetallic compounds. The main focus here is on binary systems, but applications to multi-component systems are also considered. Generalized pseudopotential theory (GPT) has been used to develop QBIPs and investigate the trends in cohesion and structure for Mg-Al and transition-metal aluminide (TM-Al) compounds. In this regard, first-principles GPT potentials have been calculated across the entire 3d TM-Al series as a function of TM concentration, together with applications to their binary and ternary phase diagrams in Co-Al, Ni-Al and Co-Cu-Al, as well as a predicted quasicrystal structure in the latter system. Bond-order-potential investigations of 3d transition-metal aluminides have focused on potential development for Ti-Al compounds with applications to dislocation core structure and mobility. An efficient strategy to develop and use model-GPT potentials for pure TM high-entropy alloys is also discussed.

Abstract Coating of the Cu bond wire with Pd has been a rather widely accepted method in semiconductor packaging to improve the wire bonding reliability. Based on comparison of a Cu bond wire and a Pd-coated Cu bond wire on AlCu pads that had passed HAST, new insight into the mechanism of the reliability improvement is gained. Our analysis showed the dominant Cu-rich intermetallics (IMC) were Cu3Al2 for the Cu wire, and (CuPdx)Al for the Pd-coated wire. The results have verified the Cu-rich IMC being suppressed by the Pd-coating, which has been extensively reported in literature. Binary phase diagrams of Al, Cu, and Pd indicate that the addition of Pd elevates the melting point and bond strength of (CuPdx)Al compared with CuAl that formed with the bare Cu wire. The improvements are expected to decrease the kinetics of phase transformation toward the more Cu-rich IMC. With the suppression of the Cu-rich IMC, the corrosion resistance of the wire bonding is enhanced and the wire bonding reliability improved. We find that Ni behaves thermodynamically quite similar to Pd in the ternary system of Cu wire bonding, and therefore possesses the potential to improve the corrosion resistance.

Conflicting results in reliability tests for backward compatible and Pb-free soldered assemblies has motivated RoHS-exempted industries to practice reballing. Reballing is the name given to the process of removing Pb-free solder balls from the copper (Cu) pads of the Ball Grid Array (BGA) components received through the supply chain and replacing them with SnPb solder balls. Recent studies on the subject of reballing have shown the possibility that the removed Pb-free solder ball leaves behind some intermetallic remnants of the Pb-free solder alloy and the Cu from the pads. A modeling approach based on physics of failure (PoF) is presented that quantifies the interactions between different thermal cycles applied to reballed Ball Grid Arrays (BGA) with remnants of the Pb-free solder alloy on the Cu pads. These resulting interactions are compared to backward compatible Sn-3.8 Ag-0.7Cu (SAC) balls soldered with eutectic SnPb paste for the same thermal cycles. For the latter, the risk of having improper mixing during the assembly process is also studied. The approach is formulated at the microscale, incorporating physical mechanisms of the intermetallics created with Cu, and at the macroscale, capturing the creep phenomenon of the bulk solder as dominant failure driver. Simulation results show that the reballed cases have higher inelastic energy density per cycle averaged over damage volume near the copper pads and that the inelastic energy density is higher across the bulk of the improperly mixed backward compatible solder balls when compared to properly mixed backward compatible solder balls. The results of this study permit extrapolation of laboratory results to field life predictions and to explore the design of accelerated re-balled or backward compatible BGA tests that relate better to application-specific usage environments.

Abstract Coating-substrate adhesion in cold spray is a paramount property, the mechanisms of which are not yet well elucidated. These mechanisms are governed by metallurgical and morphological phenomena occuring when cold-sprayed particles impinge on the substrate. To go into these mechanisms, due to the intrinsic characteristics of the cold spray process, i.e. the low-temperature and high velocity of the particles, direct observation and control of inflight particles and related phenomena (especially when impinging) cannot be done easily. For this reason, an experimental simulation of the particle-substrate reactions at the particle impingement was developed. This simulation is based on original filter impact experiments from laser shock acceleration of plates/foils (fliers). These were applied to the Cu-Al metallurgically-reactive system to simulate Cu cold-sprayed onto Al. The velocity of the plate was selected in the range of actual cold spray velocities. Relevant Cu-Al interaction phenomena were featured and studied as a function of filter impact conditions, i.e., primarily, shearing, plastic deformation, phase transformation (including rapid melting/solidification and formation of intermetallics). These phenomena were shown to be similar to those involved in cold spray. This was ascertained by a parallel study of cold-sprayed Cu coating of Al using SEM, TEM, EPMA, and an energy balance and diffusion calculations. In addition, this simulation can be used to feed FE modeling of cold spray particle impingement on the substrate. Preliminary results are discussed from modeling using the “RADIOSS®” code. More generally, laser shock flier impact experiments were demonstrated to result in a powerful tool capable of simulating cold spray coating-substrate interface mechanisms. Major assets rest on their high significance, reproducibility, flexibility and potential for substituting for direct laborious cold spray optimization testing.

Oak Ridge National Laboratory (ORNL) and Caterpillar have recently developed a new cast austenitic stainless steel, CF8C-Plus, for a wide range of high-temperature applications, including diesel exhaust components and turbine casings. The creep-rupture life of the new CF8C-Plus is over ten times greater than that of the standard cast CF8C stainless steel, and the creep-strength is about double. Another variant, CF8C-Plus Cu/W has been developed with even more creep strength at 750–850°C. The creep-strength of these new cast austenitic stainless steels is close to that of Ni-based superalloys like 617. CF8C-Plus steel was developed in about 1.5 years using an “engineered microstructure” alloy development approach, which produces creep resistance based on formation of stable nano-carbides (NbC) and prevention of deleterious intermetallics (sigma, Laves). CF8C-Plus steel won a 2003 R&D 100 Award, and to date, over 32,000 lb have been produced in various commercial component trials. The current commercialization status of the alloy is summarized.

Abstract Cu needs a higher level of ultrasound combined with bonding force to be bonded to the Al pad properly, not just because Cu is harder than Au, but it is also harder to initiate intermetallic compounds (IMC) formation during bonding. This increases the chances of damaging the metal/low k stack under the bondpad. This paper presents a fundamental study of IMC as well as one example of a failure mode of Cu/Al bonded devices, all based on detailed analysis using scanning electron microscopy, scanning transmission electron microscopy, energy dispersive spectrometers, and transmission electron microscopy. It presents a case study showing a corrosion mechanism of Cu/Al ballbond after 168hr UHAST stress. It is observed that all Cu9Al4 was consumed, while very little copper aluminide remained after 168 hours of UHAST stressing.

This paper presents an experimental study of copper dissolution in molten tin and tin-silver (Sn-Ag) solders and the formation and presence of the Cu-Sn intermetallic compound at solder/copper interfaces. During the experiments, copper (99.9% pure) samples, coated with a RMA flux, were dipped vertically in a molten solder for different time periods ranging from 5 seconds to 10 minutes. The molten solder was maintained at temperatures of 232°C, 250°C and 300°C for pure tin and 221°C, 250°C, and 300°C for Sn-3.5%Ag respectively. The samples were then cut, cleaned and cold mounted in epoxy at ambient temperature. Mechanical grinding, finish polishing, etching, and optical metallographic procedures were utilized for examining the microstructures of the polished and etched samples. The average thickness of the intermetallic compound and the amount of copper dissolved was determined. Experimental results indicate the temperature of molten solder to control the rate of dissolution of copper and the formation and presence of intermetallic compounds at the interfaces. At a given temperature of the solder temperature, the rate of dissolution of copper in the solder revealed a rising trend with an increase in dwell time of copper in the solder. For short contact time periods, the dissolution rate is low and the thickness of the intermetallic compound is small. With an increase in dwell time, the dissolution rate of copper rapidly increases and eventually reaches a plateau. Initiation of dissolution of copper causes a layer of the Sn-Cu intermetallic compound to form around the copper substrate. This in turn prevents direct contact of the copper substrate with the molten solder. The rate of formation of the layer of intermetallic compound reveals a similar trend. Based on experimental results, the kinetic parameters involved in governing the growth of the intermetallic were determined for the two solders. The parameters can be used to estimate the kinetics of copper dissolution and intermetallic compound formation during soldering.

This work reports a systematic study that addresses the evolution of the solder and intermetallic bond layer microstructure and resulting properties in microbump based assemblies during reflow and subsequent aging. So far, pure Sn as well as alloys with two different initial concentrations of Ag were considered. Thicknesses down to 5μm were deposited and reflowed on Cu, Ni, and Ni/Au pads of different thicknesses and diameters ranging from 150μm to 11μm. Flip chips with these structures were finally assembled onto different substrate pads using a fine pitch bonder and aged at different elevated temperatures for various lengths of time. Solder and intermetallic layer microstructures were characterized by cross polarizer microscopy and scanning electron microscope (SEM). Different combinations led to size and composition dependent effects including different solder composition and microstructure as well as much faster growth of intermetallic layers, and different ratios of intermetallic layer thicknesses.

The Cu6Sn5 intermetallic compound (IMC) is an important interfacial reactive product in electronic packaging. The properties of Cu6Sn5 have been demonstrated to be crucial to the interface reliability at the solder interconnections. Due to the element inter-diffusion between the packaging side and PCB (printed circuit board) side during soldering process, a ternary Cu6Sn5-based Cu-Ni-Sn intermetallic compound is often generated. This ternary phase exhibits a similar crystal structure as Cu6Sn5 phase, in which the Ni atoms are regarded as the solubility by replacing the Cu atoms. Therefore, this Cu-Ni-Sn ternary phase is labeled as (Cu6−x, Nix)Sn5. It has been found that the Cu6Sn5 unit cell consists of 44 atoms, in which 24 atoms are Cu and 20 atoms are Sn. The 24 Cu atoms occupy 4a, 4e, 8f1 and 8f2 sites, while 20 Sn atoms occupy 4e, 8f1 and 8f2 sites. The reported experimental results are quite sparse and thus a fundamental calculation is required. In this paper, the elastic stiffness of (Cu6−x, Nix)Sn5 crystal structure is calculated based on the first-principle approach within density functional theory. The results indicate that Cu6Sn5 phase show a nearly isotropic elasticity. However for the phase Cu5Ni1Sn5 (x = 1) where Ni atom at 4a space site, the elasticity shows slightly anisotropic. With the Ni solubility increase (x=2), the anisotropic elasticity of Cu4Ni2Sn5 phase becomes profound. The density of states (DOS) and partial density of states (PDOS) from individual element contributions, as well as the hybridization between the element states are simulated herein to reveal the mechanism of the anisotropic elasticity of (Cu6−x, Nix)Sn5 phase due to the occupancy of Ni atoms.