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Zn4Sb3 has received interest for thermoelectric (TE) applications due to its low cost and low environmental impact. In this study, the Zn4Sb3 TE material was diffusion-bonded with multiple electrode materials of Ni, Ag, Cu, Pd and Ti for evaluation of the manufacturing of TE modules. Intermetallic compounds formed at the interfaces of the Zn4Sb3 diffusion couples, with Ni, Ag, Cu, and Pd exhibiting the selective reaction effect with zinc due to its high chemical affinity and low formation energies. Optimized bonding strengths of 6.6 to 8.2 MPa were achieved in Zn4Sb3 bonded with Ni, Ag and Cu materials at 350 °C. The Pd material could only bond at a temperature above 300 °C, and cracks presented after longer bonding times. As for the titanium, Zn4Sb3/Ti was successfully joined only at temperatures between 400 °C and 500 °C. It was observed that a rather thin TiSb2 intermetallic compound formed during the interfacial reactions between Zn4Sb3 and Ti, which grew significantly more slowly than those formed at the interfaces of Zn4Sb3 diffusion-coupled with Ni, Ag, Cu, and Pd metallic materials. Our results provide a reference for evaluating these common materials as suitable electrode and barrier layer candidates for utilizing the Zn4Sb3 TE material. [ABSTRACT FROM AUTHOR]

The wettability and growth of intermetallic compounds (IMCs) of stud bump materials Ag, Ag-4Pd, Cu, and Au with Sn-3Ag-0.5Cu (SAC305) solder have been investigated. Stud bumps produced using wire bonding techniques are widely employed in flip chip assembly, especially in 3D integrated circuit packaging. The material of stud bumps is a key factor in the reliability of the bonds. The wettability of IMCs as studied in this study decreased in the order Au > Ag ≈ Ag-4Pd > Cu, indicating that Ag and Ag-alloy bumps were only slightly inferior to those of Au but superior to those of Cu. Moreover, the morphologies and growth behaviors of the IMCs were evaluated by aging at 100°C to 200°C for 100 h to 1000 h. Without the gold embrittlement effect found in Au or the Kirkendall void issue of Cu, IMCs contained mainly Ag3Sn at the interface after long-term aging tests, and thickened IMCs with Ag3Sn and PdSn4 were observed in the case of Ag-4Pd after prolonged aging for 1000 h at 200°C. The growth kinetics of the IMCs in Ag and Ag-4Pd stud bumps with SAC305 solder followed the Arrhenius equation, with activation energies of 56.45 kJ/mol and 57.37 kJ/mol, respectively. The formation mechanisms of the IMCs in Ag and Ag-4Pd stud bumps were found to be diffusion controlled. The results indicated that Ag and Ag-4Pd are better than Au and Cu for application as stud bump materials. [ABSTRACT FROM AUTHOR]

Stud bumping using a traditional wire bonding process followed by the severing of the wire material from the ball bond has been widely applied in flip chip packages. Traditionally, Au stud bumps were employed because of the physical properties of gold, but the mediocre reliability of Au with Al pads and the cost of gold make this material unfavorable for industrial needs. Copper stud bumps were also considered, but the intrinsic features of Cu, such as its high hardness and ease of oxidation, present no advantages over gold stud bumps. However, Ag-alloy stud bumps exhibit satisfactory bondability and have been proposed as the optimal substitute for Au stud bumps to ensure better reliability in soldering and better intermetallic growth kinetics with Al bond pads. In this article, the intermetallic growth of Ag stud bumps alloyed with 2%, 10%, and 12% Pd after high-temperature storage tests (HTSTs) is investigated. The results indicated that the Pd atoms remained immobile in the bump-side matrix without diffusing into the aluminum pad side. Alloying with higher Pd content enhanced the formation of the favorable Ag2Al intermetallic compound (IMC), whereas lower Pd content resulted in severe cracking due to the formation of Ag3Al IMC. Compositional analysis showed that continuous layers of Ag–Al IMCs doped with large amounts of Pd formed at the interfaces close to the Ag stud bumps, but no Pd was detected on the Al pad side. The bonding strength increased after aging at 150 °C and 200 °C for 200 h, but it decayed drastically after aging at 250 °C for 200 h. The relationship between intermetallic growth and failure mode was established. [ABSTRACT FROM AUTHOR]

This study investigated the mechanism of the interfacial reaction of Ag-alloy wires in the gold bond pad of an LED die after a wire bonding process and their reliability. The results showed that in the as-bonded condition, no intermetallic compounds (IMCs) formed between the Ag-alloy wire and the gold bond pad and that the interfacial reaction mechanism was diffusional solid solution reaction. After 168 h of thermal aging at 200 °C, it was found that adding Pd to the Ag-alloy wire could improve the mechanical properties of the wire, and the bonding strength between the Ag-alloy wires and gold bond pad showed an increasing trend. It was speculated that increasing the thermal aging period allowed sufficient time for the diffusion reaction between the silver alloy and gold bond pad. The gold ball residual area difference indicated that adding Pd to Ag-alloy wire was more helpful than adding Au for boosting bond shear test (BST) strength in the as-bonded condition, while adding Au to a ternary Ag-alloy wire helped the bonding strength in terms of thermal stability. BST and wire pull test (WPT) specifications are suggested for Ag-alloy wire joints in LED packaging products based on tests of the critical quality parameters of Cpk. [ABSTRACT FROM AUTHOR]

Silver-based bonding wires such as Ag-4Pd and Ag-8Au-3Pd have drawn remarkable attention in the packaging industry because they are cheaper and more conductive than Au- and Cu-based wires, respectively. This study aimed to investigate the intermetallic compound (IMC) formation and growth at the bonding interface between Ag-4Pd wire and Al-pads and between Ag-8Au-3Pd wire and Al-pads. The as-bonded and reliability-tested Ag-4Pd/Al and Ag-8Au-3Pd/Al specimens were then investigated by transmission electron microscopy (TEM) and scanning transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (STEM-EDS). The bonding properties were examined by ball shear and wire pull tests. In the as-bonded state, hexagonal close-packed (HCP) (Ag, Pd)2Al and HCP Ag2Al were formed at the Ag-4Pd/Al interfaces, whereas dual phase consisting of face-centered cubic Ag alloy with HCP precipitates (Ag, Au, Pd)2Al crystals and HCP Ag2Al layers were observed at the Ag-8Au-3Pd/Al interfaces. The IMCs showed significant growth and oxidation during reliability tests at 130 °C and 85 pct relative humidity for 192 hours. Alloying Au in Ag-4Pd wires promoted the growth of the IMC layer and it also enhanced the mechanical properties in the as-bonded material. By contrast, overgrowth of the IMCs in the Ag-8Au-3Pd/Al system induced microcrack formation in bonding and thus degraded the reliability of the material. [ABSTRACT FROM AUTHOR]

Wire bonding on Al pad packages with an innovative annealing twinned Ag-8Au-3Pd alloy wire results in a sufficient interfacial intermetallic layer at the initial as-bonded stage, while its growth during the further temperature cycling test and pressure cooker test is very slow. Even after prolonged high-temperature storage at 150^\circC for 500 h, the thickness of its intermetallics is only around 1.7 \mum. In contrast, a very thin CuAl2 intermetallic layer appears at the interface of the Pd-coated Cu wire bonded package, and a thick layer of Au8Al3 intermetallic compounds forms in the Au wire package, which grows to a thickness of around 4.0 \mum after high-temperature storage at 150^\circC for 500 h. Energy dispersive X-ray spectrometry analyses indicate that AgAl2, Au8Al3, and CuAl2 are the main intermetallic phases formed in the packages bonded with Ag-8Au-3Pd, and Au and Pd-coated Cu wires, respectively. However, an additional Pd-containing Ag2Al layer found at the AgAl2/Al interface of Ag-8Au-3Pd wire bonded package can interrupt its intermetallic reaction, and the annealing twins can further slow the intermetallic growth. [ABSTRACT FROM PUBLISHER]

In this research, thousands of 20-\mum pitch microbumps with a diameter of 10 \mum and a structure of a pure Sn cap on a Cu pillar were electroplated on 8-inch wafers, and those wafers were then respectively singularized as a top chip and bottom Si interposer for stacking. Two methods, namely conventional reflow and solid–liquid interdiffusion (SLID) bonding, were adopted to interconnect the microbumps. In the former case, the as-plated Sn caps were fluxed, and the chip was then placed on the Si interposer. Afterward, the Sn caps on the chip and on the Si interposer were melted and interconnected at a peak temperature of 250^\circC. The flux residues were cleaned after reflow, and the microgap between the chip and the Si interposer was fully sealed by a capillary underfill. In the SLID bonding process, the oxides on the as-plated Sn caps were removed by a plasma etcher first, and then the chip was placed on the interposer with a bonder as well, subsequently, the Sn caps were heated to 260^\circC to react with the Cu pillar to form Cu6Sn5. In the final step, the intermetallic microjoints were protected by the same capillary underfill. After assembly, the Joint Electron Devices Engineering Council preconditioning test was used to screen the test vehicles for reliability assessment, and then a temperature cycling test was performed to predict the lifespan of the microjoints. The test results showed that the microjoints formed by SLID bonding provided a superior reliability performance to those assembled by reflow. The fracture of the microjoints was caused by the volume contraction induced by the growth of Cu6Sn5, but the failure mechanisms of those two microjoints were quite different. [ABSTRACT FROM AUTHOR]

Rapid growth of tin whiskers has been observed on the surface of rosette-shaped NdSn intermetallic phase in a Sn-3Ag-0.5Cu-0.5Nd alloy after air storage. It is shown that various cross sections of NdSn rosettes in the solder matrix reveal different morphologies of tin whiskers, which can be classified as four types: long fibers, short fibers, tiny sprouts, and hillocks. The fibrous whiskers and tiny sprouts are found on the surfaces of specimens exposed to air at room temperature and 423 K (150 °C), while hillocks appear only after storage at 423 K (150 °C). In addition, it is observed that, in most cases, each oxidized NdSn intermetallic phase contains only a single whisker at its center. Through metallographic observations and chemical analyses on the cross sections of the oxidized NdSn intermetallics, a 'successive compressive stress model' has been proposed to interpret the tin whisker growth on the surface of a rare earth (RE)-containing solder. [ABSTRACT FROM AUTHOR]

Abstract: Although it has been verified that tin whiskers can be prevented by the addition of 0.5wt.% Zn into a Sn–3Ag–0.5Cu–0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn–3Ag–0.5Cu–0.5Ce–xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn3 intermetallics and Zn atoms, formed in the Sn–3Ag–0.5Cu–0.5Ce–xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu–Sn intermetallics growth in Sn–3Ag–0.5Cu–0.5Ce–xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn–Ag–Cu solder joints is more appropriate than 0.5wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength. [Copyright &y& Elsevier]

Abstract: It has been reported that the rapid growth of tin whiskers occurring on the surface of rare earth containing solders can be prevented by the addition of 0.5wt.% Zn into a Sn–3Ag–0.5Cu–0.5Ce alloy. The present study shows that the tensile strength of Sn–3Ag–0.5Cu–0.5Ce alloy alloyed with Zn increases significantly in comparison to the Zn-free Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce solder. Moreover, the growth of intermetallic compounds at the solder/pad interfaces during the thermal aging at 100°C and 150°C was inhibited by the Zn addition. Nevertheless, the excess of Zn addition to 0.5wt.% causes poor junction in the Sn–3Ag–0.5Cu–0.5Ce packages. An optimized addition amount of Zn in Sn–3Ag–0.5Cu–0.5Ce solders is 0.2wt.% for both inhibiting the growth of tin whiskers and strengthening the solder joints, although its ductility is still lower than that of the original Sn–3Ag–0.5Cu. [Copyright &y& Elsevier]

Abstract: The addition of 0.5wt.% La into a Sn–58Bi alloy leads to the formation of rare earth containing intermetallic plates in the solder matrix and a slight reduction of its eutectic point. After reflowing, Ni3Sn4 intermetallic compounds appear at the solder/pad interfaces of Sn–58Bi and Sn–58Bi–0.5La packages with Au(0.1μm)/Ni(5μm) surface finish, which grow linearly during the aging at 75 and 100°C for various times ranging from 100 to 1000h. It can also be observed that the interfacial intermetallics layers in Sn–58Bi–0.5La solder joints are much thinner than those in undoped Sn–58Bi. However, voids in the Sn–58Bi–0.5La solder balls resulted in lower bonding strengths of their solder joints as compared to those in Sn–58Bi packages. Ball shear tests with a shear rate of 0.4mm/s caused the reflowed and aged solder joints of both packages to rupture through the solder balls with brittle characteristic, which remain unchanged as the shear rate increased to 2000mm/s. [Copyright &y& Elsevier]

Solder alloys doped with rare-earth elements have been reported to show many beneficial effects. However, tin whiskers have been observed to appear on the surface of Sn-3Ag-0.5Cu-0.5Ce solder joints after air exposure for short periods. Such a phenomenon of abnormal whisker growth may significantly degrade the reliability of electronic packaging. The present study shows that the tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn-3Ag-0.5Cu-0.5Ce solder. The inhibition effect on the whisker growth is correlated to the refining of (Ce0.9Zn0.1)Sn3 intermetallics in this Sn-3Ag-0.5Cu-0.5Ce-0.5Zn alloy. [ABSTRACT FROM AUTHOR]

Present developing trends in the miniaturization and power escalation on electronic components have driven research interest in thermal interface materials (TIMs) to solve the fundamental issue of heat dissipation. Among the various TIMs, the In32.5Bi16.5Sn low melting alloy (LMA) has drawn much attention owing to its low melting point and great bondability, which could mitigate the warpage issue and provide very low thermal interface resistance. In this study, the Cu/In32.5Bi16.5Sn/Cu structure was used to simulate the practical conditions of the application of TIM in joining the substrate and heat sink. The thermal properties before and after an aging test at 80 °C were measured with a thermal tester, and the growth kinetics of the intermetallic compound (IMC) at the In32.5Bi16.5Sn/Cu interface were investigated. Without fast dissolution of Cu into the In32.5Bi16.5Sn alloy, the growth kinetics of the IMC at the interface were found to be diffusion-controlled at the temperature studied. It was found that the main change in the Cu/In32.5Bi16.5Sn/Cu system after the aging test was the thickening of the IMC, and the interface was free of cracks. As the aging time increased, the thermal impedance increased to twice that of the initial value, showing parabolic degradation, which is relevant to the growth kinetics of the IMC at the interface. Implications for the degradation of the thermal performance and further research are also discussed. • Thermal properties and performance of In32.5Bi16.5Sn low melting alloy as thermal interface material are evaluated. • Interface reaction of In32.5Bi16.5Sn/Cu and growth kinetics of resultant intermetallic compounds were investigated. • The degradation of the thermal impedance after aging tests of 80 °C is closely related to the growth of IMC. [ABSTRACT FROM AUTHOR]

• Bondability of titanium as barrier layer and electrode for Zn 4 Sb 3 is confirmed by evaluation of Zn 4 Sb 3 /Ti diffusion couple. • Optimized process parameter for avoiding unfavorable pressure-induced phase separation of Zn 4 Sb 3 was suggested. • Growth kinetics of resultant TiSb 2 intermetallic compounds were investigated. • The TiSb 2 IMC was confirmed to be stable at the temperature where the highest ZT value of Zn 4 Sb 3 could be achieved (400 °C). Cheap, environmentally-friendly zinc antimonide is considered more favorable than conventional tellurite-based and skutterudite thermoelectric (TE) materials. Although the highly mobile zinc atoms located in the interstitial sites of the Zn 4 Sb 3 crystalline structure greatly increase the figure of merit (ZT) of TE materials, the high chemical activity of zinc also makes it difficult to find suitable barrier layer and electrode materials. This paper introduces a method of joining titanium with Zn 4 Sb 3 by means of diffusion bonding, in which the critical mutual-interdiffusion phenomenon is prevented. Optimal process parameters for manufacturing TE modules are suggested in this work. The intermetallic compounds (IMCs) that formed at the interface of Zn 4 Sb 3 /Ti diffusion couples were proved to be stable at the temperature with the highest conversion efficiency of Zn 4 Sb 3. The growth kinetics of IMCs were also investigated, and the joint strength could be enhanced to 13.9 MPa after long-term reliability tests of 1000 h without deterioration. The results indicated that titanium is feasible barrier layer and electrode material for Zn 4 Sb 3 TE modules. [ABSTRACT FROM AUTHOR]