A Hermetic Seal Using Composite Thin-Film In/Sn Solder as an Intermediate Layer and Its Interdiffusion Reaction with Cu

A bonding joint between Cu metallization and evaporated In/Sn composite solder is produced at a temperature lower than 200°C in air. The effects of bonding temperature and duration on the interfacial bonding strength are studied herein. Cross sections of bonding joints processed at different bonding conditions were examined by scanning electron microscopy (SEM). The optimal condition, i.e., bonding temperature of 180°C for 20 min, was chosen because it gave rise to the highest average bonding strength of 6.5 MPa, and a uniform bonding interface with minimum voids or cracks. Good bond formation was also evidenced by scanning acoustic imaging. For bonding couples of patterned dies, a helium leak rate of 5.8 × 10⁻⁹ atm cc/s was measured, indicating a hermetic seal. The interfacial reaction between Cu and In/Sn was also studied. Intermetallic compounds (IMCs) such as AuIn₂, Cu₆Sn₅, and Cu₁₁In₉ were detected by means of x-ray diffraction analysis (XRD), and transmission electron microscopy (TEM) accompanied by energy-dispersive x-ray (EDX) spectroscopy. Chemical composition analysis also revealed that solder interlayers, Sn, and In were completely converted into IMCs by reaction with Cu. All the IMCs formed in the joints have remelting temperatures above 300°C according to the Cu-In, Cu-Sn, and Au-In phase diagrams. Therefore, the joint is able to sustain high service temperatures due to the presence of these IMCs.     

Intermetallic reactions in a Sn-20In-2.8Ag solder ball-grid-array package with Au/Ni/Cu pads

The intermetallic compounds formed during the reflow and aging of Sn-20In-2.8Ag ball-grid-array (BGA) packages are investigated. After reflow, a large number of cubic-shaped AuIn₂ intermetallics accompanied by Ag₂In precipitates appear in the solder matrix, while a Ni(Sn_0.72Ni_0.28)₂ intermetallic layer is formed at the solder/pad interface. With further aging at 100°C, many voids can be observed in the solder matrix and at the solder/pad interface. The continuous distribution of voids at the interface of specimens after prolonged aging at 100°C causes their bonding strength to decrease from 5.03 N (as reflowed) to about 3.50 N. Aging at 150°C induces many column-shaped (Cu_0.74Ni_0.26)₆(Sn_0.92In_0.08)₅ intermetallic compounds to grow rapidly and expand from the solder/pad interface into the solder matrix. The high microhardness of these intermetallic columns causes the bonding strength of the Sn-20In-2.8Ag BGA solder joints to increase to 5.68 N after aging at 150°C for 500 h.     

Nucleation Control and Thermal Aging Resistance of Near-Eutectic Sn-Ag-Cu-X Solder Joints by Alloy Design

Elemental (X) additions to Sn-3.5Ag-0.95Cu (SAC3595) solder were developed with minimal (<0.25 wt.%) concentration to avoid pro-eutectic Ag₃Sn blades by reducing undercooling (ΔT) and to eliminate thermal-aging-induced embrittlement. Calorimetry and microstructure results on simple Cu/Cu joints identified 0.21Zn, 0.10Mn, and 0.05Al as sufficient to reduce undercooling below that for SAC3595 and to eliminate Ag₃Sn blades. A 211°C melting onset for the X = Mn alloys also suggested the discovery of a new quaternary eutectic. Shear testing and microstructure analysis of larger joints showed that 0.05Al and 0.21Zn additions resulted in reduced as-soldered strength (30 MPa), like Sn-0.95Cu, but all joints showed ductile failure at about 30 MPa after 1000 h at 150°C.     

Intermetallic compounds formed at the interface between liquid indium and copper substrates

The interfacial reactions between liquid In and Cu substrates at temperatures ranging from 175°C to 400°C are investigated for the applications in bonding recycled sputtering targets to their backing plates. Experimental results show that a scallop-shaped Cu₁₆In₉ intermetallic compound is found at the Cu/In interface after solder reactions at temperatures above 300°C. A double-layer structure of intermetallic compounds containing scallop-shaped Cu₁₁In₉ and continuous CuIn is observed after the Cu/In interfacial reaction at temperatures below 300°C. The growth of all these intermetallic compounds follows the parabolic law, which implies that the growth is diffusion-controlled. The activation energies for the growth of Cu₁₆In₉, Cu₁₁In₉, and CuIn intermetallic compounds calculated from the Arrhenius plot of growth reaction constants are 59.5, 16.9, and 23.5 kJ/mole, respectively.     

<Emphasis Type="Italic">In Situ</Emphasis> Study of Reduction Process of CuO Paste and Its Effect on Bondability of Cu-to-Cu Joints

A bonding method utilizing redox reactions of metallic oxide microparticles achieves metal-to-metal bonding in air, which can be alternative to lead-rich high-melting point solder. However, it is known that the degree of the reduction of metallic oxide microparticles have an influence on the joint strength using this bonding method. In this paper, the reduction behavior of CuO paste and its effect on Cu-to-Cu joints were investigated through simultaneous microstructure-related x-ray diffraction and differential scanning calorimetry measurements. The CuO microparticles in the paste were gradually reduced to submicron Cu₂O particles at 210–250°C. Subsequently, Cu nanoparticles were generated instantaneously at 300–315°C. There was a marked difference in the strengths of the joints formed at 300°C and 350°C. Thus, the Cu nanoparticles play a critical role in sintering-based bonding using CuO paste. Furthermore, once the Cu nanoparticles have formed, the joint strength increases with higher bonding temperature (from 350°C to 500°C) and pressure (5–15 MPa), which can exceed the strength of Pb-5Sn solder at higher temperature and pressure.     

High cycle fatigue behaviour and generalized fatigue model development of lead-free solder alloy based on local stress approach

This paper gives an insight into high cycle fatigue (HCF) behaviour of a Pb-free solder alloy in the region between 10⁴ up to 10⁹ fatigue cycles using fatigue specimen. By means of a local stress approach, the method can be translated into solder joint fatigue evaluation in an application. The effect of temperatures (35 °C, 80 °C, 125 °C) on the fatigue property of Pb-free solder alloy is considered in this work to understand the possible fracture mechanisms and micro structural changes in a solder alloy at elevated temperature. Experiments are performed for different interaction factors under mean stresses (R = 0, − 1, − 3), stress concentration (notched, un-notched) and surface roughness. SN (stress-life) diagrams presented in this work will compare the fatigue performance of Sn_3.8Ag_0.7Cu solder alloy for different conditions. Furthermore, mathematical fatigue model based on FKM guideline (in German “Fachkuratorium Maschinenbau) is extracted out of the experiments under all these external effects. The models can be exported later for lifetime evaluation purposes on applications. The paper thereby proposes the use of FKM guideline in the field of microelectronics.     

Fluxless Bonding of Large Silicon Chips to Ceramic Packages Using Electroplated Eutectic Au/Sn/Au Structures

A fluxless process of bonding large silicon chips to ceramic packages has been developed using a Au-Sn eutectic solder. The solder was initially electroplated in the form of a Au/Sn/Au multilayer structure on a ceramic package and reflowed at 430°C for 10 min to achieve a uniform eutectic 80Au-20Sn composition. A 9 mm × 9 mm silicon chip deposited with Cr/Au dual layers was then bonded to the ceramic package at 320°C for 3 min. The reflow and bonding processes were performed in a 50-mTorr vacuum to suppress oxidation. Therefore, no flux was used. Even without any flux, high-quality joints were produced. Microstructure and composition of the joints were studied using scanning electron microscopy with energy-dispersive x-ray spectro- scopy. Scanning acoustic microscopy was used to verify the joint quality over the entire bonding area. To employ the x-ray diffraction method, samples were made by reflowing the Au/Sn/Au structure plated on a package. This was followed by a bonding process, without a Si chip, so that x-rays could scan the solder surface. Joints exhibited a typical eutectic structure and consisted of (Au,Ni)Sn and (Au,Ni)₅Sn phases. This novel fluxless bonding method can be applied to packaging of a variety of devices on ceramic packages. Its fluxless nature is particularly valuable for packaging devices that cannot be exposed to flux such as sensors, optical devices, medical devices, and laser diodes.     

Interfacial Reactions Between Pb-Free Solders and In/Ni/Cu Multilayer Substrates

This study investigates the interfacial reactions between Sn-3.0wt.% Ag-0.5wt.%Cu (SAC) and Sn-0.7wt.%Cu (SC) on In/Ni/Cu multilayer substrates using the solid–liquid interdiffusion bonding technique. Samples were reflowed first at 160°C, 180°C, and 200°C for various periods, and then aged at 100°C for 100 h to 500 h. The scalloped Cu₆Sn₅ phase was formed at the SAC/In/Ni/Cu and SC/In/Ni/Cu interfaces. When the reflowing temperatures were 160°C and 180°C, a ternary Ni-In-Sn intermetallic compound (IMC) was formed when the samples were further aged at 100°C. This ternary Ni-In-Sn IMC could be the binary Ni₃Sn₄ phase with extensive Cu and In solubilities, or the ternary Sn-In-Ni compound with Cu solubility, or even a quaternary compound. As the reflow temperature was increased to 200°C, only one Cu₆Sn₅ phase was formed at the solder/substrate interface with the heat treatment at 100°C for 500 h. Mechanical test results indicated that the formation of the Ni-In-Sn ternary IMC weakened the mechanical strength of the solder joints. Furthermore, the solid–liquid interdiffusion (SLID) technique in this work effectively reduced the reflow temperature.     

Interfacial reaction between multicomponent lead-free solders and Ag,Cu, Ni,and Pd substrates

The interfacial reaction between two prototype multicomponent lead-free solders, Sn-3.4Ag-1Bi-0.7Cu-4In and Sn-3.4Ag-3Bi-0.7Cu-4In (mass%), and Ag, Cu, Ni, and Pd substrates are studied at 250°C and 150°C. The microstructural characterization of the solder bumps is carried out by scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis. Ambient temperature, isotropic elastic properties (bulk, shear, and Young’s moduli and Poisson’s ratio) of these solders along with eutectic Sn-Ag, Sn-Bi, and Sn-Zn solders are measured. The isotropic elastic moduli of multicomponent solders are very similar to the eutectic Sn-Ag solder. The measured solubility of the base metal in liquid solders at 250°C agrees very well with the solubility limits reported in assessed Sn-X (X=Ag, Cu, Ni, Pd) phase diagrams. The measured contact angles were generally less than 15° on Cu and Pd substrates, while they were between 25° and 30° on Ag and Ni substrates. The observed intermediate phases in Ag/solder couples were Ag₃Sn after reflow at 250°C and Ag₃Sn and ζ (Ag-Sn) after solid-state aging at 150°C. In Cu/solder and Ni/solder couples, the interfacial phases were Cu₆Sn₅ and (Cu,Ni)₆Sn₅, respectively. In Pd/solder couples, only PdSn₄ after 60-sec reflow, while both PdSn₄ and PdSn₃ after 300-sec reflow, were observed.     

Conductive adhesive with transient liquid‐phase sintering technology for high‐power device applications

A highly reliable conductive adhesive obtained by transient liquid‐phase sintering (TLPS) technologies is studied for use in high‐power device packaging. TLPS involves the low‐temperature reaction of a low‐melting metal or alloy with a high‐melting metal or alloy to form a reacted metal matrix. For a TLPS material (consisting of Ag‐coated Cu, a Sn96.5‐Ag3.0‐Cu0.5 solder, and a volatile fluxing resin) used herein, the melting temperature of the metal matrix exceeds the bonding temperature. After bonding of the TLPS material, a unique melting peak of TLPS is observed at 356 °C, consistent with the transient behavior of Ag₃Sn + Cu₆Sn₅ → liquid + Cu₃Sn reported by the National Institute of Standards and Technology. The TLPS material shows superior thermal conductivity as compared with other commercially available Ag pastes under the same specimen preparation conditions. In conclusion, the TLPS material can be a promising candidate for a highly reliable conductive adhesive in power device packaging because remelting of the SAC305 solder, which is widely used in conventional power modules, is not observed.     

Comparison of Sn2.8Ag20In and Sn10Bi10In solders for intermediate-step soldering

We chose Sn−2.8Ag−20In and Sn−10Bi−10In (numbers are in weight percentages unless specified otherwise) as Pb-free solder materials for intermediate-step soldering. We then investigated how the two solders reacted with the under bump metallurgy (UBM) of Au/Ni (Au: 1.5 μm and Ni: 3 μm) at 210°C, 220°C, 230°C, and 240°C for up to 4 min. All, of the Au UBM was dissolved into the solder matrix as soon as the interfacial reaction started. The reaction formed Au(In, Sn)₂ in the case of SnAgIn, and it formed Au(Sn, In)₄ and Au(In, Sn)₂ in the case of SnBiIn. The formation mechanism of the intermetallic phases is explained thermodynamically. The exposed Ni layer reacted with the solder and formed Ni₂₈Sn₅₅In₁₇ in case of SnAgIn, and formed Ni₃(Sn, In)₄ in case of SnBiIn, at the solder joint interface. Under the same soldering conditions, the Ni₃(Sn,In)₄ layer in the SnBiIn/UBM is thicker than the Ni₂₈Sn₅₅In₁₇ layer in the SnAgIn/UBM. Because of the thicker intermetallic compound layer, the SnBiIn solder joint has weaker shear strength than the SnAgIn solder joint.     

Growth of intermetallic compounds in the Sn-9Zn/Cu joint

We have studied the microstructure of the Sn-9Zn/Cu joint in soldering at temperatures ranging from 230°C to 270°C to understand the growth of the mechanism of intermetallic compound (IMC) formation. At the interface between the Sn-9Zn solder and Cu, the results show a scallop-type ε-CuZn₄ and a layer-type γ-Cu₅Zn₈, which grow at the interface between the Sn-9Zn solder and Cu. The activation energy of scallop-type ε-CuZn₄ is 31 kJ/mol, and the growth is controlled by ripening. The activation energy of layer-type γ-Cu₅Zn₈ is 26 kJ/mol, and the growth is controlled by the diffusion of Cu and Zn. Furthermore, in the molten Sn-9Zn solder, the results show η-CuZn grains formed in the molten Sn-9Zn solder at 230°C. When the soldering temperature increases to 250°C and 270°C, the phase of IMCs is ε-CuZn₄.     

Intermetallic compounds formed during the interfacial reactions between liquid In-49Sn solder and Ni substrates

The interfacial reactions between liquid In-49Sn solder and Ni substrates at temperatures ranging from 150°C to 450°C for 15 min to 240 min have been investigated. The intermetallic compounds formed at the In-49Sn/Ni interfaces are identified to be a ternary Ni₃₃In₂₀Sn₄₇ phase using electron-probe microanalysis (EPMA) and x-ray diffraction (XRD) analyses. These interfacial intermetallics grow with increasing reaction time by a diffusion-controlled mechanism. The activation energy calculated from the Arrhenius plot of reaction constants is 56.57 kJ/mol.     

Joule Heating Enhanced Phase Coarsening in Sn37Pb and Sn3.5Ag0.5Cu Solder Joints during Current Stressing

This paper investigated the effect of Joule heating on the phase coarsening in Sn37Pb and Sn3.5Ag0.5Cu ball grid array (BGA) solder joints stressed at −5°C and 125°C with a 6.0 × 10² A/cm² electric current. The phase growth under current stressing was also compared with those under aging at 125°C. It was found that the current stressing produced a substantial Joule heating in the solder joints and conductive traces. Hence, the solder joints underwent a considerable temperature rise by 30–35°C when stressed at −5°C and 125°C in this study. Coarsening of Pb-rich and Ag-rich phases was confirmed to be accelerated by the current stressing as a result of enhanced diffusion at elevated temperature and atomic stimulation due to numerous collisions between electrons and atoms. Different controlling kinetics were suggested for the cases stressed or aged at different temperatures.     

Developing a lead-free solder alloy Sn-Bi-Ag-Cu by mechanical alloying

A new lead free alloy, Sn-6Bi-2Ag-0.5Cu, has been developed by mechanical alloying and has great potential as a lead-free solder system. Initial trials on the manufacture of solder joints with this alloy revealed that a high quality bond with copper could be formed. Its melting range of 193.87°C to 209.88°C is slightly higher than that of eutectic tin-lead solder. Examination of the microstructure of the as-soldered joints revealed that it mainly consists of small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ particles. The Cu-Sn intermetallic compound (IMC) layer formed at solder-copper interface is the η-Cu₆Sn₅ phase with grain size of 2 μm. The shear strength of the solder joint is higher than that of Sn-37Pb or Sn-3.5Ag. Under shear loading, fracture occurred at IMC layer-solder interface as well as in the bulk of solder.     

Interfacial Microstructure Evolution Between Sn-Zn Solders and Ag Substrate During Solid-State Annealing

In this study, solid-state interfacial reactions between Ag and Sn-Zn alloys with varying Zn content (0.1 wt.% to 9 wt.%) were investigated at 170°C. The reaction couples were prepared by electroplating Ag on the Sn-Zn alloy to avoid dissolution of Ag into the molten solder during soldering. The Zn content greatly influenced the reaction products and the interfacial microstructures. When the Zn content was less than 4 wt.%, Ag₃Sn and AgZn layers were simultaneously formed. Notably, Zn could actively diffuse through the Ag₃Sn layer and react with Ag to form the AgZn phase. With the proceeding reaction, small α-Ag particulates were produced within the AgZn phase. With 9 wt.% Zn, the dominant reactions formed Ag₅Zn₈ and AgZn layers. The interfacial microstructure evolved significantly with reaction time. Interface instability due to Zn depletion in the solder resulted in massive spalling of the Ag₅Zn₈ layer. The Ag₃Sn phase was then produced next to the AgZn layer. Moreover, another reaction couple, Sn-9 wt.%Zn/Sn(15 μm)/Ag, was prepared, in which fast interdiffusion between Zn and Ag across the Sn layer was demonstrated due to the strong chemical affinity of Zn.     

Solid Liquid Interdiffusion Bonding of (Pb,Sn)Te Thermoelectric Modules with Cu Electrodes Using a Thin-Film Sn Interlayer

A (Pb, Sn)Te thermoelectric element plated with a Ni barrier layer and a Ag reaction layer has been joined with a Cu electrode coated with Ag and Sn thin films using a solid–liquid interdiffusion bonding method. This method allows the interfacial reaction between Ag and Sn such that Ag₃Sn intermetallic compounds form at low temperature and are stable at high temperature. In this study, the bonding strength was about 6.6 MPa, and the specimens fractured along the interface between the (Pb, Sn)Te thermoelectric element and the Ni barrier layer. Pre-electroplating a film of Sn with a thickness of about 1 μm on the thermoelectric element and pre-heating at 250°C for 3 min ensures the adhesion between the thermoelectric material and the Ni barrier layer. The bonding strength is thus increased to a maximal value of 12.2 MPa, and most of the fractures occur inside the thermoelectric material. During the bonding process, not only the Ag₃Sn intermetallics but also Cu₆Sn₅ forms at the Ag₃Sn/Cu interface, which transforms into Cu₃Sn with increases in the bonding temperature or bonding time.     

A Study on the Thermal Reliability of Cu/SnAg Double-Bump Flip-Chip Assemblies on Organic Substrates

The Cu/SnAg double-bump structure is a promising candidate for fine-pitch flip-chip applications. In this study, the interfacial reactions of Cu (60 μm)/SnAg (20 μm) double-bump flip chip assemblies with a 100 μm pitch were investigated. Two types of thermal treatments, multiple reflows and thermal aging, were performed to evaluate the thermal reliability of Cu/SnAg flip-chip assemblies on organic printed circuit boards (PCBs). After these thermal treatments, the resulting intermetallic compounds (IMCs) were identified with scanning electron microscopy (SEM), and the contact resistance was measured using a daisy-chain and a four-point Kelvin structure. Several types of intermetallic compounds form at the Cu column/SnAg solder interface and the SnAg solder/Ni pad interface. In the case of flip-chip samples reflowed at 250°C and 280°C, Cu₆Sn₅ and (Cu, Ni)₆Sn₅ IMCs were found at the Cu/SnAg and SnAg/Ni interfaces, respectively. In addition, an abnormal Ag₃Sn phase was detected inside the SnAg solder. However, no changes were found in the electrical contact resistance in spite of severe IMC formation in the SnAg solder after five reflows. In thermally aged flip-chip samples, Cu₆Sn₅ and Cu₃Sn IMCs were found at the Cu/SnAg interface, and (Cu, Ni)₆Sn₅ IMCs were found at the SnAg/Ni interface. However, Ag₃Sn IMCs were not observed, even for longer aging times and higher temperatures. The growth of Cu₃Sn IMCs at the Cu/SnAg interface was found to lead to the formation of Kirkendall voids inside the Cu₃Sn IMCs and linked voids within the Cu₃Sn/Cu column interfaces. These voids became more evident when the aging time and temperature increased. The contact resistance was found to be nearly unchanged after 2000 h at 125°C, but increases slightly at 150°C, and a number of Cu/SnAg joints failed after 2000 h. This failure was caused by a reduction in the contact area due to the formation of Kirkendall and linked voids at the Cu column/Cu₃Sn IMC interface.     

Electromigration in a Sn-3 wt.%Ag-0.5 wt.%Cu-3 wt.%Bi Solder Stripe Between Two Cu Electrodes Under Current Stressing

The electromigration behavior of a Sn-3 wt.%Ag-0.5 wt.%Cu-3 wt.%Bi solder stripe between two Cu electrodes under current stressing at various densities has been investigated for a current stressing time of 72 h and a temperature of 120°C. After current stressing at a density of 1.0 × 10⁴ A/cm², the solder matrix exhibited a slight microstructural change as well as formation of a distributed Cu₆Sn₅ phase near the anode-side solder/Cu interface. Upon increasing the current density to 3.9 × 10⁴ A/cm² and 5.0 × 10⁴ A/cm², a high density of distributed Cu₆Sn₅ phase was formed across the entire solder stripe, resulting in pronounced microstructural change of the solder. Hillocks were also formed near the anode-side interface due to accumulation of a Sn-rich phase, a Bi-rich phase, and a distributed Cu₆Sn₅ phase, while voids were formed in the solder matrix and at the opposite cathode side. The mechanisms of formation of the distributed Cu₆Sn₅ phase and migration of Bi and Sn are discussed.     

Reflow and burn-in of a Sn-20In-0.8Cu ball grid array package with a Au/Ni/Cu pad

The intermetallic compounds formed after reflow and burn-in testing of a Sn-20In-0.8Cu solder ball grid array (BGA) package are investigated. Along with the formation of the Cu₆(Sn_0.78In_0.22)₅ precipitates (IM1) in the solder matrix, scallop-shaped intermetallic compounds (IM2) with a compositional mixture of Cu₆(Sn_0.87In_0.13)₅ and Ni₃(Sn_0.87In_0.13)₄ appear at the interfaces between the solder balls and Au/Ni/Cu pads. A significant number of intermetallic particles (IM3), with a composition of (Au_0.80Cu_0.20)(In_0.33Sn_0.67)₂, can also be found in the solder matrix. After aging at 115°C for 750 h, an additional intermetallic compound layer (IM4) with a composition of (Ni_0.91Cu_0.09)₃(Sn_0.77In_0.23)₂ is formed at the interface between IM2 and the Ni layer. The ball shear strength of the Sn-20In-0.8Cu BGA solder after reflow is 4.5 N and will rise to maximum values after aging at 75°C and 115°C for 100 h. With a further increase of the aging time at both temperatures, the joint strengths exhibit a tendency to decline linearly at about 1.7×10⁻³ N/h.