Temperature Effect on Intermetallic Compound Growth Kinetics of Cu Pillar/Sn Bumps

The in situ intermetallic compound (IMC) growth in Cu pillar/Sn bumps was investigated by isothermal annealing at 120°C, 150°C, and 180°C using an in situ scanning electron microscope. Only the Cu₆Sn₅ phase formed at the interface between the Cu pillar and Sn during the reflow process. The Cu₃Sn phase formed and grew at the interfaces between the Cu pillar and Cu₆Sn₅ with increased annealing time. Total (Cu₆Sn₅ + Cu₃Sn) IMC thickness increased linearly with the square root of annealing time. The growth slopes of total IMC decreased after 240 h at 150°C and 60 h at 180°C, due to the fact that the Cu₆Sn₅ phase transforms to the Cu₃Sn phase when all of the remaining Sn phase in the Cu pillar bump is completely exhausted. The complete consumption time of the Sn phase at 180°C was shorter than that at 150°C. The apparent activation energy for total IMC growth was determined to be 0.57 eV.     

Effects of Surface Finishes and Current Stressing on Interfacial Reaction Characteristics of Sn-3.0Ag-0.5Cu Solder Bumps

The effects of surface finishes on the in situ interfacial reaction characteristics of ball grid array (BGA) Sn-3.0Ag-0.5Cu lead-free solder bumps were investigated under annealing and electromigration (EM) test conditions of 130°C to 175°C with 5.0 × 10³ A/cm². During reflow and annealing, (Cu,Ni)₆Sn₅ intermetallic compound (IMC) formed at the interface of electroless nickel immersion gold (ENIG) finish. In the case of both immersion Sn and organic solderability preservative (OSP) finishes, Cu₆Sn₅ and Cu₃Sn IMCs formed. Overall, the IMC growth velocity of ENIG was much lower than that of the other finishes. The activation energies of total IMCs were found to be 0.52 eV for ENIG, 0.78 eV for immersion Sn, and 0.72 eV for OSP. The ENIG finish appeared to present an effective diffusion barrier between the Cu substrate and the solder, which leads to better EM reliability in comparison with Cu-based pad systems. The failure mechanisms were explored in detail via in situ EM tests.     

Electromigration-Induced Interfacial Reactions in Cu/Sn/Electroless Ni-P Solder Interconnects

The effect of electromigration (EM) on the interfacial reaction in a line-type Cu/Sn/Ni-P/Al/Ni-P/Sn/Cu interconnect was investigated at 150°C under 5.0 × 10³ A/cm². When Cu atoms were under downwind diffusion, EM enhanced the cross-solder diffusion of Cu atoms to the opposite Ni-P/Sn (anode) interface compared with the aging case, resulting in the transformation of interfacial intermetallic compound (IMC) from Ni₃Sn₄ into (Cu,Ni)₆Sn₅. However, at the Sn/Cu (cathode) interface, the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.2 wt.% Ni) and Cu₃Sn. When Ni atoms were under downwind diffusion, only a very small quantity of Ni atoms diffused to the opposite Cu/Sn (anode) interface and the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.6 wt.% Ni) and Cu₃Sn. EM significantly accelerated the dissolution of Ni atoms from the Ni-P and the interfacial Ni₃Sn₄ compared with the aging case, resulting in fast growth of Ni₃P and Ni₂SnP, disappearance of interfacial Ni₃Sn₄, and congregation of large (Ni,Cu)₃Sn₄ particles in the Sn solder matrix. The growth kinetics of Ni₃P and Ni₂SnP were significantly accelerated after the interfacial Ni₃Sn₄ IMC completely dissolved into the solder, but still followed the t ^1/2 law.     

Non-conductive film with Zn-nanoparticles (Zn-NCF) for 40 μm pitch Cu-pillar/Sn–Ag bump interconnection

Non-conductive film with Zn nano-particles (Zn-NCF) is an effective solution for fine-pitch Cu-pillar/Sn–Ag bump interconnection in terms of manufacturing process and interfacial reliability. In this study, NCFs with Zn nano-particles of different acidity, viscosity, and curing speed were formulated and diffused Zn contents in the Cu pillar/Sn–Ag bumps were measured after 3D TSV chip-stack bonding. Amount of Zn diffusion into the Cu pillar/Sn–Ag bumps increased as the acidity of resin increased, as the viscosity of resin decreased, as the curing speed of resin decreased, and as the bonding temperature increased. Diffusion of Zn nano-particles into the Cu pillar/Sn–Ag bumps are maximized when the resin viscosity became lowered and the solder oxide layer was removed. To analyze the effects of Zn-NCF on IMC reduction, IMC height depending on aging time was measured and corresponding activation energies for IMC growth were calculated. For the evaluation of joint reliabilities, test vehicles were bonded using NCFs with 0 wt%, 1 wt%, 5 wt%, and 10 wt% of Zn nano-particles and aged at 150 °C up to 500 h. NCF with 10 wt% Zn nano-particle showed remarkable suppression in Cu₆Sn₅ and (Cu,Ni)₆Sn₅ IMC compared to NCFs with 0 wt%, 1 wt%, and 5 wt% of Zn nano-particles. However, in terms of Cu₃Sn IMC suppression, which is the most critical goal of this experiment NCFs with 1 wt%, 5 wt%, and 10 wt% showed an equal amount of IMC suppression. As a result, it was successfully demonstrated that the suppression of Cu–Sn IMCs was achieved by the addition of Zn nano-particles in the NCFs resulting an enhanced reliability performance in the Cu/Sn–Ag bumps bonding in 3D TSV interconnection.     

Intermetallic Compound Growth and Reliability of Cu Pillar Bumps Under Current Stressing

Fine-pitch Cu pillar bumps have been adopted for flip-chip bonding technology. Intermetallic compound (IMC) growth in Cu pillar bumps was investigated as a function of annealing or current stressing by in situ observation. The effect of IMC growth on the mechanical reliability of the Cu pillar bumps was also investigated. It is noteworthy that Sn exhaustion was observed after 240 h of annealing when current stressing was not applied, and IMC growth rates were changed remarkably. As the applied current densities increased, the time required for complete Sn consumption became shorter. In addition, Kirkendall voids, which would be detrimental to the mechanical reliability of Cu pillar bumps, were observed in both Cu₃Sn/Cu pillars and Cu₃Sn/Cu under-bump metallization interfaces. Die shear force was measured for Cu pillar samples prepared with various annealing times, and degradation of mechanical strength was observed.     

Cu Substrates with Different Grain Sizes

Cu₆Sn₅ and Cu₃Sn are easily formed at the interface between Sn and Cu during reflow and aging processes. Thick Cu-Sn compounds at the interface become brittle, reducing the mechanical strength of solder joints and increasing the consumption of under bump metallization (UBM). It is noted that intermetallic compound (IMC) growth and substrate consumption are affected by factors such as substrate fabrication, substrate orientation, and substrate microstructure. In this study, to determine the effects of substrate grain size on IMC growth and substrate consumption, pure Sn solder was reflowed on annealed Cu substrates with different grain sizes at 250°C for 30 s to 600 s. It was revealed that Cu substrates with smaller grain sizes exhibited reduced IMC growth. In addition, the interdiffusion coefficients of Cu₆Sn₅ and Cu₃Sn were decreased for the Cu substrate with the smaller grain size. The influence of the Cu substrate grain size on IMC growth and substrate consumption is discussed.     

Effects of Cu contents in Pb-free solder alloys on interfacial reactions and bump reliability of Pb-free solder bumps on electroless Ni-P under-bump metallurgy

Using the screen-printed solder-bumping technique on the electroless plated Ni-P under-bump metallurgy (UBM) is potentially a good method because of cost effectiveness. As SnAgCu Pb-free solders become popular, demands for understanding of interfacial reactions between electroless Ni-P UBMs and Cu-containing Pb-free solder bumps are increasing. It was found that typical Ni-Sn reactions between the electroless Ni-P UBM and Sn-based solders were substantially changed by adding small amounts of Cu in Sn-based Pb-free solder alloys. In Cu-containing solder bumps, the (Cu,Ni)₆Sn₅ phase formed during initial reflow, followed by (Ni,Cu)₃Sn₄ phase formation during further reflow and aging. The Sn3.5Ag solder bumps showed a much faster electroless Ni-P UBM consumption rate than Cu-containing solder bumps: Sn4.0Ag0.5Cu and Sn0.7Cu. The initial formation of the (Cu,Ni)₆Sn₅ phase in SnAgCu and SnCu solders significantly reduced the consumption of the Ni-P UBM. The more Cu-containing solder showed slower consumption rate of the Ni-P UBM than the less Cu-containing solder below 300°C heat treatments. The growth rate of the (Cu,Ni)₆Sn₅ intermetallic compound (IMC) should be determined by substitution of Ni atoms into the Cu sublattice in the solid (Cu,Ni)₆Sn₅ IMC. The Cu contents in solder alloys only affected the total amount of the (Cu,Ni)₆Sn₅ IMC. More Cu-containing solders were recommended to reduce consumption of the Ni-based UBM. In addition, bump shear strength and failure analysis were performed using bump shear test.     

Development of chip-on-flex bonding using Sn-based bumps and non-conductive adhesive

We developed a reliable and low cost chip-on-flex (COF) bonding technique using Sn-based bumps and a non-conductive adhesive (NCA). Two types of bump materials were used for the bonding process: Sn bumps and Sn–Ag bumps. The bonding process was performed at 180 °C for 10 s using a thermo-compression bonder after dispensing the NCA. Sn-based bumps were easily deformed to contact Cu pads during the bonding process. A thin layer of Cu₆Sn₅ intermetallic compound was observed at the interface between Sn-based bumps and Cu pads. After bonding, electrical measurements showed that all COF joints had very low contact resistance, and there were no failed joints. To evaluate the reliability of COF joints, high temperature storage tests (150 °C, 1000 h), thermal cycling tests (−25 °C/+125 °C, 1000 cycles) and temperature and humidity tests (85 °C/85% RH, 1000 h) were performed. Although contact resistance was slightly increased after the reliability test, all COF joints passed failure criteria. Therefore, the metallurgical bond resulted in good contact and improved the reliability of the joints.     

Microstructure Evolution of Cu-Cored Sn Solder Joints Under High Temperature and High Current Density

This work investigated the microstructure evolution of Cu-cored Sn solder joints under high temperature and high current density. The Cu₆Sn₅ phase formed at both the Cu core/Sn interface and Cu wire/Sn interface right after reflow and grew with increasing annealing time, while the Cu₃Sn phase formed and grew at the Cu/Cu₆Sn₅ interfaces. Intermetallic compound (IMC) growth followed a linear relationship with the square root of annealing time due to a diffusion-controlled mechanism. Under high current density, the thickness of the interfacial IMCs of the Cu core/Sn interface at the cathode side increased and the Cu core/Sn interface at the anode side exhibited an irregular and serrated morphology with prolonged current stressing time. Finite-element simulation was carried out to obtain the distribution of current density in the solder joint. Since Cu has lower resistivity, the electrical current primarily selected the Cu core as its electrical path, resulting in current crowding at the Cu core and the region between the Cu core and Cu wire. Compared with the conventional solder joint, the electromigration (EM) lifetime of the Cu-cored solder joint was much longer.     

Solid-state intermetallic compound layer growth between copper and 95.5Sn-3.9Ag-0.6Cu solder

Long-term, solid-state intermetallic compound (IMC) layer growth was examined in 95.5Sn-3.9Ag-0.6Cu (wt.%)/copper (Cu) couples. Aging temperatures and times ranged from 70°C to 205°C and from 1 day to 400 days, respectively. The IMC layer thicknesses and compositions were compared to those investigated in 96.5Sn-3.5Ag/Cu, 95.5Sn-0.5Ag-4.0Cu/Cu, and 100Sn/Cu couples. The nominal Cu₃Sn and Cu₆Sn₅ stoichiometries were observed. The Cu₃Sn layer accounted for 0.4–0.6 of the total IMC layer thickness. The 95.5Sn-3.9Ag-0.6Cu/Cu couples exhibited porosity development at the Cu₃Sn/Cu interface and in the Cu₃Sn layer as well as localized “plumes” of accelerated Cu₃Sn growth into the Cu substrate when aged at 205°C and t>150 days. An excess of 3–5at.%Cu in the near-interface solder field likely contributed to IMC layer growth. The growth kinetics of the IMC layer in 95.5Sn-3.9Ag-0.6Cu/Cu couples were described by the equation x=x_o+Atⁿexp [−ΔH/RT]. The time exponents, n, were 0.56±0.06, 0.54±0.07, and 0.58±0.07 for the Cu₃Sn layer, the Cu₆Sn₅, and the total layer, respectively, indicating a diffusion-based mechanism. The apparent-activation energies (ΔH) were Cu₃Sn layer: 50±6 kJ/mol; Cu₆Sn₅ layer: 44±4 kJ/mol; and total layer: 50±4 kJ/mol, which suggested a fast-diffusion path along grain boundaries. The kinetics of Cu₃Sn growth were sensitive to the Pb-free solder composition while those of Cu₆Sn₅ layer growth were not so.     

Effects of self-aligned electroplating Cu pillar/Sn-xAg bump on dense Al lines for chip-to-package connection

Self-aligned electroplating is applied to form the Cu pillar/Sn-Ag bump for semiconductor device packaging, while passivation SiN cracks are usually observed at the bump edge on the bump of the array (BOA). In this paper, the simulation method was used to investigate the mechanism of SiN cracks and then, the bump process was optimized to improve the mechanical properties of the Cu pillar/Sn-Ag bump. It was found that higher reflow rounds could improve the shear strength due to the large degree of contact between the rugged scallop-like shape of the Cu₆Sn₅ and the Sn-Ag solder. The fracture plane cleaved between the Sn-Ag and Cu₆Sn₅ interface is consistent with the simulation results. The hardness of the Sn‒Ag solder is proportional to the reflow rounds, and the amount of Ag₃Sn phase precipitation within the Sn-Ag solder contributes to the hardness value. In contrast, the disadvantage is that thermal residual stress could deteriorate the SiN crack, especially for a BOA structure The study concludes that an optimal bump process, including Sn-2%Ag solders at 260 °C for 30 s, could obtain a high shear strength and appropriate solder hardness without passivated SiN cracking.     

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.     

An investigation of the reliability of solderable ICA with low-melting-point alloy (LMPA) filler

Conductive adhesives play a major role in the electronic packaging industry as an alternative to solder due to their potential advantages that include mild processing conditions and superior thermo-mechanical performance. In a conductive adhesive interconnection, adequate mechanical and electrical performance and long-term reliability are critical.In this paper, the reliability of solderable isotropic conductive adhesive (ICA) interconnections was investigated. Reliability testing was performed via thermal shock (−55 to 125 °C, 1000 cycles) and high-temperature and high-humidity tests (85 °C, 85% RH, 1000 h). The interfacial microstructure of the solderable ICA was also investigated. Additionally, the fracture mode was investigated via mechanical pull strength testing before and after the reliability test. The electrical resistance of the solderable ICA interconnection showed improved stability compared to conventional ICAs, and similar stability to conventional solder paste (Sn–3Ag–0.5Cu and Sn–58Bi) due to the metallurgical interconnection formed by the molten LMPA fillers between the corresponding metallization layers. After the reliability tests, the grown IMC layer was composed of Cu₆Sn₅ (η-phase) and Cu₃Sn (ε-phase), and the scallop-type IMC transformed into a layer-type IMC. The fracture propagated along the Cu–Sn IMC/SnBi interface and the fracture surface showed a semi-brittle fracture mode mixed with cleavage and ductile tear bands.     

elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). The (Cu_1−y,Ni_y)₆Sn₅ IMC gradually grew with the rate constant of 4.63 × 10⁻⁸ cm/sec^0.5 before 500 h aging had passed. After 500 h aging, the (Cu_1−y,Ni_y)₆Sn₅ IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu₆Sn₅ IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu_1−y,Ni_y)₆Sn₅ IMC was down to 1.74 × 10⁻⁸ cm/sec^0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.     

Influence of Dopant on Growth of Intermetallic Layers in Sn-Ag-Cu Solder Joints

The interfacial interaction between Cu substrates and Sn-3.5Ag-0.7Cu-xSb (x = 0, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0) solder alloys has been investigated under different isothermal aging temperatures of 100°C, 150°C, and 190°C. Scanning electron microscopy (SEM) was used to measure the thickness of the intermetallic compound (IMC) layer and observe the microstructural evolution of the solder joints. The IMC phases were identified by energy-dispersive x-ray spectroscopy (EDX) and x-ray diffractometry (XRD). The growth of both the Cu₆Sn₅ and Cu₃Sn IMC layers at the interface between the Cu substrate and the solder fits a power-law relationship with the exponent ranging from 0.42 to 0.83, which suggests that the IMC growth is primarily controlled by diffusion but may also be influenced by interface reactions. The activation energies and interdiffusion coefficients of the IMC formation of seven solder alloys were determined. The addition of Sb has a strong influence on the growth of the Cu₆Sn₅ layer, but very little influence on the formation of the Cu₃Sn IMC phase. The thickness of the Cu₃Sn layer rapidly increases with aging time and temperature, whereas the thickness of the Cu₆Sn₅ layer increases slowly. This is probably due to the formation of Cu₃Sn at the interface between two IMC phases, which occurs with consumption of Cu₆Sn₅. Adding antimony to Sn-3.5Ag-0.7Cu solder can evidently increase the activation energy of Cu₆Sn₅ IMC formation, reduce the atomic diffusion rate, and thus inhibit excessive growth of Cu₆Sn₅ IMCs. This study suggests that grain boundary pinning is one of the most important mechanisms for inhibiting the growth of Cu₆Sn₅ IMCs in such solder joints when Sb is added.     

Electro- and thermomigration induced Cu3Sn and Cu6Sn5 formation in SnAg3.0Cu0.5 bumps

Two important trends in the microelectronics business are the development of three dimensional packaging solutions which increase the number electronics components on the same area, and the application of VLSI electronics under harsh environment conditions. Both trends lead to a growing importance of intermetallic compound (IMC) formation in Sn based solder joints. Due to miniaturization a growing part of the solder joint volume is transformed into IMCs and finally the reflow process becomes a transient liquid phase soldering (TLPS) process. For harsh environment applications TLPS enables the transformation of low melting Sn contacts into high melting IMC joints. In both cases a model for the prediction of migration-induced IMC formation is required for the fabrication of IMC joints.For the general prediction of the migration induced IMC formation the related material parameters are needed. Against this background the Cu₃Sn and Cu₆Sn₅ formation was observed during temperature storage tests on Amkor® Package-on-Package packages (12 × 12 mm) with SnAg_3.0Cu_0.5 ball grid arrays. A mathematical model was developed to calculate the average mass flux of Sn and Cu during the stress tests. Based on the mass flux values the activation energies and diffusion constants for Cu and Sn in Cu₃Sn and Cu₆Sn₅ were determined. Afterwards the temperature storage was combined with an AC and a DC current load to investigate thermo- and electromigration-related phenomena. Based on the IMC formation speed during the AC and the DC tests the heat of transport Q^* and the effective charge of the moving ion Z^* were calculated. An interpretation of the material parameters is given in consideration of the high defect density in Cu₃Sn and Cu₆Sn₅.     

Solid-state annealing behavior of two high-Pb solders, 95Pb5Sn and 90Pb10Sn,on Cu under bump metallurgy

The solid-state annealing behavior of two high-lead solders, 95Pb5Sn and 90Pb10Sn (in wt.%), was examined. After reflow, Cu₃Sn intermetallics formed on the Cu under bump metallurgy (UBM) for both solder alloys. However, solidstate annealing produced significantly different reaction morphologies for the two solder compositions. The Cu₃Sn intermetallics spalled off faster at higher temperatures in the 95Pb5Sn solder. In the case of 90Pb10Sn solder, the Cu₃Sn intermetallics continued to grow even after 1500 h at 170°C. The difference was explained by a two-step phenomenon—Sn diffusion from the bulk solder region to the solder/Cu₃Sn interface (J_Sn), and subsequent intermetallic formation (I_Cu3Sn) by interdiffusion of Cu and Sn. For 95Pb5Sn, the relation, J_Sn < I_Cu3Sn was postulated because of insufficient supply of Sn. The relation, J_Sn > I_Cu3Sn was suggested for the continuous intermetallic growth of the 90Pb10Sn solder. Although a small difference was expected between the two quantities in both solder alloys, the difference in the solid-state annealing behavior was dramatic.     

Interfacial Reaction of Sn and Cu-<Emphasis Type="Italic">x</Emphasis>Zn Substrates After Reflow and Thermal Aging

Zn additions to Cu under bump metallurgy (UBM) in solder joints were the subject of this study. An alternative design was implemented to fabricate pure Sn as the solder and Cu-xZn (x = 15 wt.% and 30 wt.%) as the UBM to form the reaction couple. As the Zn content increased from 15 wt.% to 30 wt.% in the Sn/Cu-Zn system, growth of both Cu₃Sn and Cu₆Sn₅ was suppressed. In addition, no Kirkendall voids were observed at the interface in either Sn/Cu-Zn couple during heat treatment. After 40-day aging, different multilayered phases of [Cu₆Sn₅/Cu₃Sn/Cu(Zn)] and [Cu₆Sn₅/Cu(Zn,Sn)/CuZn] formed at the interface of [Sn/Cu-15Zn] and [Sn/Cu-30Zn] couples, respectively. The growth mechanism of intermetallic compounds (IMCs) during aging is discussed on the basis of the composition variation in the joint assembly with the aid of electron-microscopic characterization and the Sn-Cu-Zn ternary phase diagram. According to these analyses of interfacial morphology and IMC formation in the Sn/Cu-Zn system, Cu-Zn is a potential UBM for retarding Cu pad consumption in solder joints.     

EBSD Investigation of Cu-Sn IMC Microstructural Evolution in Cu/Sn-Ag/Cu Microbumps During Isothermal Annealing

The microstructural evolution of Cu/Sn-Ag (~5 μm)/Cu Cu-bump-on-line (CuBOL) joints during isothermal annealing at 180°C was examined using a field-emission scanning electron microscope equipped with an electron backscatter diffraction (EBSD) system. Cu₆Sn₅ and Cu₃Sn were the two key intermetallic compound (IMC) species that appeared in the CuBOL joints. After annealing for 24 h (= t), the solder had completely converted to Cu-Sn IMCs, forming an “IMC” joint with Cu/Cu₃Sn/Cu₆Sn₅/Cu₃Sn/Cu structure. EBSD analyses indicated that the preferred orientation of the hexagonal Cu₆Sn₅ (η) was $ (2\bar{1}\bar{1}3) $ , while the preferred orientation was (100) for the monoclinic Cu₆Sn₅ structure (η′). Upon increasing t to 72 h, Cu₆Sn₅ entirely transformed into Cu₃Sn, and the IMC joint became Cu/Cu₃Sn/Cu accordingly. Interestingly, the grain size and crystallographic orientation of Cu₃Sn displayed location dependence. Detailed EBSD analyses in combination with transmission electron microscopy on Cu₃Sn were performed in the present study. This research offers better understanding of crystallographic details, including crystal structure, grain size, and orientation, for Cu₆Sn₅ and Cu₃Sn in CuBOL joints after various annealing times.     

Kinetics of Intermetallic Compound Formation at the Interface Between Sn-3.0Ag-0.5Cu Solder and Cu-Zn Alloy Substrates

The growth kinetics of an intermetallic compound (IMC) layer formed between Sn-3.5Ag-0.5Cu (SAC) solders and Cu-Zn alloy substrates was investigated for samples aged at different temperatures. Scallop-shaped Cu₆Sn₅ formed after soldering by dipping Cu or Cu-10 wt.%Zn wires into the molten solder at 260°C. Isothermal aging was performed at 120°C, 150°C, and 180°C for up to 2000 h. During the aging process, the morphology of Cu₆Sn₅ changed to a planar type in both specimens. Typical bilayer of Cu₆Sn₅ and Cu₃Sn and numerous microvoids were formed at the SAC/Cu interfaces after aging, while Cu₃Sn and microvoids were not observed at the SAC/Cu-Zn interfaces. IMC growth on the Cu substrate was controlled by volume diffusion in all conditions. In contrast, IMC growth on Cu-Zn specimens was controlled by interfacial reaction for a short aging time and volume diffusion kinetics for a long aging time. The growth rate of IMCs on Cu-Zn substrates was much slower due to the larger activation energy and the lower layer growth coefficient for the growth of Cu-Sn IMCs. This effect was more prominent at higher aging temperatures.