Excessive Intermetallic Compound Research Articles

Photothermal effects on low-temperature hybrid solder joints and its superior drop reliability

Hybrid solder joints incorporating the Sn-3.0Ag-0.5Cu/Sn–58Bi structure have been studied to tackle challenges such as warpage, thermal degradation, and excessive intermetallic compound (IMC) growth in heterogeneous integrated packages. However, the presence of Bi diminishes drop impact reliability, and conventional reflow processes aimed at increasing the Bi mixing ratio exacerbate IMC growth. This study investigates the feasibility of intense pulsed light (IPL) soldering for hybrid solder joints on electroless nickel immersion gold (ENIG) and (electroless nickel electroless palladium immersion gold (ENEPIG) PCB substrates, with the goal of advancing reliable and sustainable electronic packaging technology. Microstructural evolution and drop impact reliability of IPL-soldered joints were compared with those of conventionally reflow-soldered joints. Specifically, IPL soldering exhibited approximately sixfold higher drop impact reliability in ENIG and threefold in ENEPIG compared to reflow soldering under the lowest temperature conditions. This underscores IPL's effectiveness in suppressing interfacial reactions, thereby promoting crack propagation in the SnBi phases. Furthermore, under optimized IPL conditions on the ENEPIG substrate, drop reliability was approximately threefold higher than that achieved under the lowest temperature conditions, despite lower power consumption of 13.36 kWh. These findings demonstrate the potential feasibility of IPL soldering as a reliable and sustainable technology for electronic packages.

Microstructure modification and mechanical reinforcement in sub-10 μm scale Cu/Sn–Ag/Cu microbump joints via Co-addition of Zn and Ni in Cu substrates

The trend towards miniaturization of solder joints addresses the need for high-density interconnections in microelectronics. Excessive intermetallic compounds (IMC) and phase transformations cause internal stresses, leading to voiding and cracking in conventional microbumps. Therefore, advanced microbump design must focus on IMC suppression and phase stabilization. This study explores Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni microbumps, comparing them to conventional Cu/Sn-3.5Ag/Cu under various reflow times. High-field emission electron probe micro-analysis and transmission electron microscopy reveal improved microstructures. Electron backscatter diffraction confirms the (Cu,Ni)6(Sn,Zn)5 grain size of several hundreds of nanometers. Fracture modes were established based on the correlation between shear testing and fracture surfaces. Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni microbumps enhanced shear strength (19.6 %–97.8 %) and failure toughness (10.6 %–132.1 %) compared to Cu/Sn-3.5Ag/Cu. The solid solution strengthened and refined (Cu,Ni)6(Sn,Zn)5 combined with tough bulk (Cu,Ag)5(Sn,Zn)8 in Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni microbumps, demonstrate a tough microstructure. These findings provide insights into the design of microbumps with a bonding height in sub-10micron scales.

Strengthening synergies of various interlayers in NiTi toTi6Al4V dissimilar laser joints

A sound NiTi-Ti6Al4V dissimilar joint featuring NiTi's superelasticity is crucial for merging both material benefits in aerospace and biomedical fields. Yet, the solidification of excessive intermetallic compounds (IMCs) for dissimilar par, notably Ti2Ni, poses a considerable risk of severe brittleness. Altering the chemistry of the fusion zone (FZ) could prevent embrittlement and enhance the mechanical performance of the joints. In this regard, understanding the impact of filler materials on the solidification behavior is vital. This study unveils the synergistic impact of employing these diverse interlayers, characterized by varying melting points, metallurgical compatibilities, and reaction spontaneity, on solidification behavior, microstructure evolution, and mechanical properties and presents a viable approach to enhance the microstructure and performance of NiTi and Ti6Al4V joints. A notable improvement in properties was observed in the Pd-interlayered joint, where the strong reaction spontaneity in the Ti-Pd system facilitated the preferential solidification of the TiPd phase in the FZ. This significantly reduced the amount of detrimental Ti2Ni IMC, ultimately decreasing joint embrittlement. Mechanical testing data demonstrated a progression in performance in the following sequence: interlayer-free, Co-interlayered, Zr-interlayered, and Pd-interlayered. Using interlayer materials that form lower-energy compounds with Ti and offer good mechanical properties, such as TiPd in this study, is presented as an optimal approach to enhance joint mechanical properties. The present work advances the metallurgical knowledge associated with non-equilibrium processing of NiTi and Ti6Al4V alloys.

Inhibition of intermetallic compounds using pure Ni or Ni–Al2O3 nanocomposite coating during the ultrasonic-assisted soldering of Mg/Al dissimilar metals

The widespread utilization of lightweight Mg and Al alloys underscores the importance of establishing reliable connections between them. However, joining dissimilar Mg/Al alloys presents challenges, primarily due to uncontrolled formation of intermetallic compounds during the process, leading to weakened joint performance. To inhibit its formation, a Ni or Ni–Al2O3 nanocomposite coating served as a barrier layer during ultrasonic-assisted soldering AZ31 Mg/6061 Al using Sn–3.0Ag–0.5Cu solder. By investigating the impacts of Ni/Ni–Al2O3 composite coating on microstructure, intermetallic compounds formation, and joint performance, we found that the Ni coating partially inhibited Mg diffusion into the solder at 260 °C for 20 min, thus limiting excessive Mg2Sn compound formation and increasing shear strength by approximately 110% compared to uncoated joints. However, with an extension of the holding time to 30 min, block-shaped Mg2Sn compound reemerged as the primary cause of joint failure. Using Ni–Al2O3 composite coating further broadened the process window of Mg/Al soldering, eliminating block-shaped Mg2Sn compound in the joint at 260 °C for 40 min. The incorporation of Al2O3 further enhanced joint properties, with the maximum shear strength of the Mg(Ni–Al2O3)/SAC/Al joint reaching approximately 66.6 MPa, an improvement of 18.3% compared to the joint with Ni coating. This enhancement shifted the fracture position from the Ni/SAC interface to the SAC solder. Additionally, due to grain refinement in the Ni matrix, the microhardness of the Ni–Al2O3 composite coating surpassed that of the Ni coating. A model of Ni/Ni–Al2O3 composite coating preventing intermetallic compounds formation of was proposed. This nanocomposite coating-ultrasonic assisted soldering method effectively avoids the issue of excessive intermetallic compounds formation when joining dissimilar metals.

Microstructural-mechanical property evolution of AA7075 welds with different filler during heat treatment

This study investigated the changes in microstructure and mechanical properties of gas tungsten arc welded 7075 aluminum alloy joints using ER5356 and ER7150 filler wire subjected to post-weld heat treatment. The microstructure and mechanical property evolution of the welds was analyzed by both thermodynamics calculations and experimental methods. After welding, alloying elements segregated in the inter-dendritic regions, resulting in the formation of excessive intermetallic compounds. Within the welds of 7075 aluminum alloy with ER5356 filler material, the inter-dendritic region was primarily composed of the T-AlCuMgZn phase. In contrast, in the welds of 7075 aluminum alloy with ER7150 filler material, this region consisted of a eutectic mixture containing α-Al, T-AlCuMgZn, and η-MgZn2 phases. The α-Al matrix experienced limited supersaturation due to the presence of these inter-dendritic intermetallic compounds, which could not dissolve during the post-weld aging treatment. As a comparison, post-weld solutioning and aging treatment significantly improved the ultimate tensile strength of the joints. Among all the samples, the 7075 + ER7150 sample subjected to post-weld solutioning and aging treatment exhibited the highest strength and elongation, primarily attributed to the complete dissolution of the inter-dendritic intermetallic compounds and the formation of fine precipitates.

Role of Pd interlayer on NiTi to Ti6Al4V laser welded joints: Microstructural evolution and strengthening mechanisms

Obtaining a reliable NiTi to Ti6Al4V dissimilar joint exhibiting NiTi's superelasticity can provide design flexibility in aerospace and biomedical fields via integrating distinct material benefits. However, this materials couple is vulnerable to severe embrittlement due to the development of excessive intermetallic compounds (IMCs), namely Ti2Ni. Pd-free and Pd-interlayered NiTi-Ti6Al4V laser joints were evaluated for their microstructure, compositional changes, thermodynamic mechanism, and mechanical properties. The presence of Pd constrained the formation of Ti2Ni IMC, and NiTi-Ti6Al4V joints with excellent mechanical properties demonstrating superelastic behavior were achieved for the first time. The tensile strength and rupture strain of the Pd-added NiTi-Ti6Al4V joint improved more than twofold, reaching 520 MPa and 5.6%, respectively. During cyclic tensile testing, the Pd-added joint demonstrated superelasticity and a comparable irrecoverable strain to NiTi (1 versus 0.65%). Multiscale characterization revealed that the fraction of Ti2Ni decreased from 83 to 10% near the NiTi boundary and 24 to 6% at the weld center compared to the Pd-free joint. The superior thermodynamic formation tendency of Ti-Pd compounds over Ti2Ni IMC favored their development, and thus Ti-Pd and NiTi compounds dominated the fusion zone (FZ) at the expense of Ti2Ni IMC, explaining the improved mechanical performance of the Pd-added joint.

Design and characterization of a novel MgAlZnCuMn low melting point light weight high entropy alloy (LMLW-HEA)

Development of high-entropy alloys (HEAs) is one of the remarkable advancements in the progress of new materials. In recent years, continuous efforts have been made to design light weight high-entropy alloys (LWHEAs) with suitable microstructure and superior properties. This paper reports the preliminary results of a study aimed at design and development of a new low melting point LWHEA (LMLW-HEA) based on AlMgZnCuMn alloy system without excessive intermetallic compounds formation. In this research, first, the initial chemical composition of the alloy was designed according to thermodynamic parameters, and then the alloy was melted in a resistance electric furnace. Since the thermodynamic indicators have been defined based on alloys containing 3d transition elements, formation of binary intermetallic compounds was predicted with the help of Midema model. To investigate the effects of cooling rate and heat treatment on the structure of the designed alloy, the molten alloy was poured into two different molds made of silica sand and steel, and both as cast samples were also heat treated. Microstructural features, mechanical properties and thermal stability of the samples were then examined. Density of the samples was measured to be less than 3.4 gr/cm3 which was very close to the theoretical density of the alloy. Structures of all the samples were complex and multi-phase and contained intermetallic (ordered solid solution) binary and ternary compounds including Mg7Zn3, MgZn2, Cu2Mg, Al2Cu, Mg32 (Al, Zn)49, Mg32Cu7Al47, Al25Mg37.5Zn37.5, and CuMgZn phases. The designed alloy was brittle under all the processing conditions. Compressive strength of the alloy reached to about 95 MPa, and its hardness was in the range of 110–120 HV. Oxidation resistance of the alloy was evaluated at a temperature about 30 °C below its solidus temperature and no weight change was observed in the sample after 14 h. It is suggested that the designed alloy is a suitable candidate for high temperature applications not involving impact or cyclic loading.

Microstructural evolution, metallurgical reactions, and the mechanical properties of pure titanium and carbon steel sheets bonded via hot-roll diffusion with a nickel interlayer

ABSTRACT This study investigated the microstructural evolution and mechanical properties of titanium-carbon steel bi-metallic composite sheets fabricated via hot-rolling diffusion with a nickel interlayer at temperatures of 800, 850, or 900°C. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed layers of NiTi2, NiTi, and Ni3Ti phases at the Ti-Ni interface. We also observed interfacial σ + β Ti and β-Ti phases. Note that the carbon steel-Ni interface revealed no reaction layers or intermetallic compounds under any of the processing temperatures. The highest shear strength (∼470 MPa) was obtained at a processing temperature of 850°C. The highest peel strength (∼21 N/mm) was obtained at a processing temperature of 900°C. Lower bonding temperatures were shown to reduce bond strength by hindering atomic diffusion, whereas higher bonding temperatures led to the formation of excessive intermetallic compounds. Both conditions resulted in joint failure at the Ti-Ni interface and extensive cleavage planes with various alignments on fracture surfaces. Overall, a hot-rolling temperature of 850°C provided the optimal tradeoff between interfacial bonding strength and ductility.

Dissimilar friction stir welding of AA6061 and Ti6Al4V alloys: A study on microstructure and mechanical properties

Intact Al/Ti joints were fabricated using W-Re pin tool at various welding speeds when the rotating speed was 1000 rpm. The temperature histories were recorded and tool wear was measured, in order to study the welding processes. The microstructure of the welds had a smaller grain size as the welding speed increased. The base materials were intermixed at a welding speed of 40 mm/min, forming the Al/Ti mechanical mixture and hook structure at/beside the interface. However, no obvious Al/Ti intermixing could be found in joints welded at 80 mm/min or more. Distinguishable amount of intermetallic compound (IMC) were depicted from the Al/Ti mixture in light of the SEM and EDS results. TiAl3 was further identified from the interface by TEM. The lowest hardness was tested at the boundary between HAZ and TMAZ, and the highest hardness was found at interface, due to the excessive IMCs in the Al/Ti mixture. The joints reached the maximum shear load of 4206 N at 80 mm/min. The specimens all failed along the interface with the characteristics of both brittle and plastic fracture.

Microstructure and mechanical properties of friction stir lap AA6061-Ti6Al4V welds

The AA6061 and Ti6Al4V plates were joined by atomic diffusion and interfacial reactions. Layers of solid solution and intermetallic compound were both formed at the interface, and the interface thickness increased as the heat input increased. The intermetallic compound layer was found to be mainly composed of TiAl3. The shear strength of the friction stir lap AA6061-Ti6Al4V welds was found determined by the interfacial microstructure. The shear performance was promoted initially with the increase of interface thickness due to the formation of intermetallic compound. The maximum shear load of 4500 N was reached with a 7.5-μm-thick interface. When the thickness of interface further increased, the maximum shear load was found to decrease again because of the exorbitant residual stresses induced by the excessive intermetallic compounds. The fracture of the joints started with the micro-cracks formed at the intermetallic compounds and then leads to break during tensile shear test.

Influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of solderable anisotropic conductive adhesives (SACAs)

In the present work, the influence of multi-walled carbon nanotube (MWCNT) concentration on the thermo-mechanical reliability properties of carbon nanotube (CNT)-filled solderable anisotropic conductive adhesives (SACAs) containing low-melting-point-alloy (LMPA) fillers was investigated. To measure the reliability-related properties of SACA assemblies, six types of SACAs with different MWCNT concentrations (from 0 to 2 wt%) were formulated, and two types of reliability tests, thermal shock (TS) and high-temperature and high-humidity (HTHH) tests, were conducted on these samples. All of the CNT-filled SACA assemblies with different MWCNT concentrations showed proper and stable electrical reliability during each aging test, due to the molten LMPA fillers forming a metallurgical interconnection between the corresponding metallization regions. Although the mechanical strength of the CNT-filled SACA joints degraded after aging tests due to excessive intermetallic compound (IMC) layer growth, the CNT-filled SACAs with an MWCNT concentration below 0.1 wt% exhibited better mechanical reliability properties than those of SACAs without MWCNTs due to the initially achieved high bonding strength of the SACA joints, enabled by the reinforcing effects of the MWCNTs within the polymer composites.

Rotary friction welding on dissimilar metals of aluminum and brass by using pre-heating method

Continuous drive rotary friction welding was conducted on dissimilar metals of aluminum and brass. A pre-heating method was applied at temperature of 330 °C and 600 °C, which aims to soften brass (i.e., the hard side), so that, at the friction interface, both metals could deform accordingly. The deformation rate was calculated by using a constitutive equation. The joint strengths were measured by tensile test. The joint microstructure and chemical compositions were studied by SEM and EDS. According to the results, without pre-heating, the joint preserves a low strength as 35% of the base metal (aluminum). At pre-heating of 330 °C, the joint strength raised to 70% of the base metal (aluminum) corresponding to apparent plastic flow on brass side as calculated and experimented. A narrow intermetallic compound layer was formed indicating metallic bonding formed on the interface. However, at pre-heating of 600 °C, the strength of joint was only 27% of the base metal (aluminum). The microstructure examination shows excessive intermetallic compound that has formed on the interface indicating overheated temperature. The result is prospective as a sound joint of aluminum and brass could be obtained by further regulating both metals into deformation at the interface, in which pre-heating on the hard side was proved applicable.

The interfacial bilayer Cu6Sn5 formed in a Sn–Ag–Cu flip-chip solder joint incorporating Au/Pd metallization during solid-state aging

A solid-state reaction between the Sn–Ag–Cu solder and Ni metallization resulted in the formation of interfacial bilayer Cu6Sn5-based intermetallic compounds (IMCs) in a solder joint incorporating Au/Pd metallization. The layer near the Ni metallization was identified as (Cu,Ni,Au)6Sn5 containing 20.3 at.% of Ni and 1.7 at.% of Au, and the layer near the solder matrix was identified as (Cu,Au,Ni,Pd)6Sn5 containing 5.7 at.% of Au, 1.2 at.% of Ni, and 1.0 at.% of Pd. The electron diffraction analysis with high resolution transmission electron microscopy further characterized the interfacial bilayer IMCs as having the same hexagonal crystal structure with different crystal orientations. The kinetics study revealed that the (Cu,Ni,Au)6Sn5 in the interfacial bilayer reaction products was formed during the initial reflow process, while the (Cu,Au,Ni,Pd)6Sn5 layer was formed during subsequent solid-state aging. Predominant growth of the (Cu,Au,Ni,Pd)6Sn5 layer was observed during solid-state aging. In contrast, the growth of the initially formed (Cu,Ni,Au)6Sn5 layer was suppressed. The growth of the (Cu,Au,Ni,Pd)6Sn5 layer was governed by the dissolution of the (Au,Pd)Sn4 IMC that occurred during solid-state aging. The complete dissolution of (Au,Pd)Sn4 resulted in a reactant-limited chemical reaction from the conversion of (Au,Pd)Sn4 to (Cu,Au,Ni,Pd)6Sn5. The development of the interfacial bilayer IMCs in the solder joint incorporating Au/Pd metallization suppressed excessive IMC growth and unfavorable phase transformation during long-term solid-state aging. The detailed mechanism of the formation of the interfacial bilayer Cu6Sn5-based IMCs was investigated in this paper.

Influence of laser power on microstructure and mechanical properties of laser welded-brazed Mg to Ni coated Ti alloys

AZ31B Magnesium (Mg) and Ti-6Al-4V titanium (Ti) alloys with Ni coating were joined by laser welding-brazing process using AZ92 Mg based filler. The influence of laser power on microstructure and mechanical properties were investigated. Ni coating was found to significantly promote good wetting-spreading ability of molten filler on the Ti sheet. Acceptable joints without obvious defects were obtained within a relatively wide processing window. In the process metallurgical bonding was achieved by the formation of Ti3Al phase at direct irradiation zone and Al-Ni phase followed by a layer of Mg-Al-Ni ternary compound adjacent to the fusion zone at the intermediate zone. The thickness of reaction layers increased slowly with the increasing laser power. The tensile-shear test indicated that joints produced at the laser power of 1300W reached 2387N fracture load, representing 88.5% joint efficiency with respect to the Mg base metal. The corresponding failure occurred in the fusion zone of the Mg base metal, while joints fractured at the interface at lower/higher laser power due to the crack or excessive intermetallic compound (IMC) formation along the interface.

Effect of multi-walled carbon nanotube (MWCNT) concentration on thermomechanical reliability of MWCNT-reinforced solderable isotropic polymer nanocomposites

Multi-walled carbon nanotube (MWCNT)-reinforced solderable isotropic polymer nanocomposites (SIPNs) containing low-melting-point alloys (LMPAs) were developed to enhance the interconnection properties of solderable isotropic polymer composites (SIPCs). In this study, the reliability properties of MWCNT-reinforced SIPN assemblies with different MWCNT concentrations were investigated through thermal shock (−55–125 °C, 1000 cycles) and high-temperature and high-humidity (85 °C, 85 %RH, 1000 h) tests. In addition, the interfacial microstructure and fracture mode of the MWCNT-reinforced SIPN joint were investigated. All the MWCNT-reinforced SIPN assemblies, with different MWCNT concentrations, showed stable electrical reliability during reliability tests owing to the formation of a metallurgical interconnection between the corresponding metallization layers by molten LMPA fillers. The mechanical reliability properties of all the MWCNT-reinforced SIPN assemblies degraded owing to excessive intermetallic compound (IMC) layer growth. However, the mechanical reliability properties of MWCNT-reinforced SIPNs with a low MWCNT concentration (<1 wt%) were superior to those of the SIPNs without MWCNTs owing to the initial high bonding strength of the former due to the reinforcing effect of MWCNTs.

Effect of Zn concentration on the interfacial reactions between Sn–3.0Ag–0.5Cu solder and electroplated Cu–xZn wetting layers (x = 0–43 wt%)

Cu–xZn wetting layers (x = 0, 20, 30, and 43 wt%) were electroplated in a non-cyanide solution and the interfacial reaction with Sn–3.0Ag–0.5Cu (SAC) solder was investigated after reflow and isothermal aging. After reflow, a Cu6Sn5 intermetallic compound (IMC) formed at the SAC/Cu interface and a small amount of Zn atoms dissolved into the Cu6Sn5 at the SAC/Cu–Zn interface. After aging, Cu3Sn and microvoids were observed between the Cu6Sn5 and Cu layer, whereas such voids were not formed at the SAC/Cu–Zn interface. A dominant reaction product during aging was Cu6(Sn, Zn)5 in the Cu–20Zn and Cu–30Zn samples, which was changed from Cu6(Sn, Zn)5 to Cu(Zn, Sn) in the Cu–43Zn sample. In addition, a thin CuZn IMC was observed beneath the Cu(Zn, Sn) in both the SAC/Cu–30Zn and SAC/Cu–43Zn joints after aging for a long time. X-ray diffraction showed that α-CuZn and β-CuZn peaks were obtained from the Cu–30Zn and Cu–43Zn wetting layers. The crystal structure of Cu(Zn, Sn) and/or CuZn IMCs was a β′-CuZn (ordered phase) with a superlattice peak. As the Zn concentration in the Cu–xZn wetting layers increased to 30 wt%, the IMCs growth rate was gradually reduced, but was increased in the Cu–43Zn sample. The excessive IMC growth of the Cu–43Zn sample was attributed to the fast growth of Cu(Zn, Sn) IMC.

Electrodeposition and Characterisation of Novel Ni-NbOx Composite Coatings as a Diffusion Barrier for Liquid Solder Interconnects - Part II: Diffusion Barrier Performance

The control of interfacial microstructural stability is of utmost importance to the reliability of liquid solder interconnects in high temperature electronic assemblies. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation, which renders conventional barrier metallisations inadequate. With the intention of reducing such excessive IMC growth, electrically conducting, NbOx containing Ni coatings were developed using electrodeposition and were assessed as solder diffusion barrier layers in terms of their electrical conductivity and barrier property. The present work adopts a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, compactness and purity, from non-aqueous glycol-based electrolytes consisting of NiCl2 and NbCl5 as metal precursors. The effects of cathodic current density and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. Increased NaBH4 concentrations were found to elevate the maximum deposit thickness (above 10 µm), although these led to a reduction in the co-deposited Nb content. The composite coatings generally exhibited good electrical conductivity. The reaction between a liquid 52In-48Sn solder and Ni-NbOx, with Nb contents up to 6 at.%, was studied at 200ºC. The results indicate that, Ni-NbOx with sufficient layer thickness and higher Nb content, offered longer service lifetime. Nb enrichment was generally observed at or close to the reaction front after high temperature storage which suggests evident effectiveness of the enhanced diffusion barrier characteristics. A mechanism for the role of Nb as a barrier performance enhancer was postulated.

Diffusion Barrier Characteristics of Ni-NbOx Composite Electrodeposits for Liquid In-Sn Solder Interconnects

The control of interfacial microstructural stability is of utmost importance to the reliability of liquid solder interconnects in high temperature electronic assemblies. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation, which renders conventional barrier metallizations inadequate. With the intention of reducing such excessive IMC growth, electrically conducting, NbOx containing Ni coatings were developed using electrodeposition and were assessed as solder diffusion barrier layers in terms of their electrical conductivity and barrier property. The present work adopts a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, compactness and purity, from non-aqueous glycol-based electrolytes consisting of NiCl2 and NbCl5 as metal precursors. The effects of cathodic current density and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. Increased NaBH4 concentrations were found to elevate the maximum deposit thickness (above 10 μm), although these led to a reduction in the co-deposited Nb content. The composite coatings generally exhibited good electrical conductivity. The reaction between a liquid 52In-48Sn solder and Ni-NbOx, with Nb contents up to 6 at.%, was studied at 200°C. The results indicate that, Ni-NbOx with sufficient layer thickness and higher Nb content, offered longer service lifetime. Nb enrichment was generally observed at or close to the reaction front after high temperature storage which suggests evident effectiveness of the enhanced diffusion barrier characteristics. A mechanism for the role of Nb as a barrier performance enhancer was postulated.

Effect of trace platinum additions on the interfacial morphology of Sn–3.8Ag–0.7Cu alloy aged for long hours

While the Sn–Ag–Cu (SAC) family of solders are considered good candidate as lead-free solder replacement materials, their relatively short processing history and application result in a host of materials as well as reliability problems. For good metallurgical bonding and electrical connection, a thin, even layer of intermetallic compound (IMC) is required but excessive growth of the IMC layer will cause various reliability problems. This is especially critical for miniaturized solder pitches in very large scale integration circuits. This work adopts the composite approach of adding 0.15 and 0.30wt.% of Pt into Sn–3.8Ag–0.7Cu alloy to study the effect of these additions to the IMC layer thickness between the solder and substrate. Alloys were isothermally aged at 150°C for up to 1000h to observe contribution of Pt in suppressing excessive IMC growth. It was found that when more Pt was added to the alloy, the IMC layer became more even and continuous. Voids and IMC layer thickness were reduced. This is attributed to the role of Pt in replacing Cu in the solder and thus impeding excessive diffusion.

Electrodeposition and Characterisation of Novel Ni-NbOx Composite Coatings as a Diffusion Barrier for Liquid Solder Interconnects - Part II: Diffusion Barrier Performance

Electronics products are increasingly required to operate at high temperatures in harsh environments, for instance in oil well, geophysical drilling and aerospace applications. Conventional solder interconnects, that have been extensively utilised in consumer electronics, are no longer adequate. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation. With the intention of reducing such excessive IMC growth, electrically conducting niobium sub-oxide containing Ni coating has been produced which acts as a barrier layer between the solder and substrate. This then promises the possibility of developing a new solder barrier material with electrical conductivity and chemical resistance as required.The present study considers a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, smoothness, compactness and purity. The electrolyte is derived from a Type IV eutectic solvent consisting of NbCl5 and propylene glycol (PG). The addition of a small amount of Ni ions (molar ratio: Ni : Nb=1 : 30) was found to enable the co-deposition of Nb species. The introduction of NaBH4 into the electrolyte has elevated the maximum deposit thickness (above 15 µm), although this led to a reduction in the co-deposited Nb content. The effects of cathodic current density, metal precursor and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. Scanning transmission electron microscopy (STEM) revealed a nanocrystalline structure resembling that of a simple FCC structure of Ni, with a slightly fluctuating composition across the coating thickness and with Nb uniformly distributed (See Fig. 1). From X-ray photoelectron spectroscopy (XPS), a 0.7 eV shift to lower binding energy for the Nb (V) species signifies the partial chemical reduction of Nb2O5 due to possible Ni-Nb bonding. It is thus proposed that a Ni-NbOx deposit was formed by a two-step mechanism: the simultaneous electrochemical reduction of adherent Ni metal and inadherent niobium oxides on Cu, followed by nickelation of the NbOx. The composite coating’s electrical conductivity, solderability and diffusion barrier characteristics (the latter preventing excessive IMC growth between molten Sn-In solder and Cu substrate) were assessed and showed promising results. From diffusion studies, a Nb-enriched layer was observed at the solder/substrate interface after ageing at 200°C suggesting evident effectiveness of the diffusion barrier characteristics.Figure 1 - STEM cross sectional image a) of a 95Ni-5Nb coating with corresponding Energy-dispersive X-ray spectroscopy (EDX) maps of b) Ni Kα & c) Nb Lα.