High Re-melting Temperature Research Articles

Novel transient liquid phase bonding method using In-coated Cu sheet for high-temperature die attach

To meet the increasing demand for thermal stability in electronic packaging required for next-generation power modules, transient liquid phase (TLP) bonding of Cu/In system is concerned owing to its high remelting temperature and high strength. In this study, a novel TLP bonding method using In-coated Cu sheet was designed for high-temperature die attach. The microstructure and mechanical properties of joints bonded for different parameters were investigated. It was found that the In-coated Cu sheet could join electroless Ni/immersion Au (ENIG)-finished Cu disks strongly by forming two intermetallic compound (IMC) bondlines of δ-(Cu, In), Ni28In72, and AuIn2. With increasing bonding temperature and time, AuIn2 was consumed, while δ-(Cu, In) and Ni28In72 coarsened, reducing the defects in the bondline. Therefore, the shear strength of the bonded joint increased from 16.9 MPa to 38.0 MPa. However, the brittle characteristics and location of the fracture remained almost unchanged for the different bonding parameters.

Fabrication of Sn-plated Cu foam for high-efficiency transient-liquid-phase bonding

To solve the problems of long reflowing time and poor high-temperature resistance of traditional transient-liquid-phase (TLP) bonding, a low cost Sn-plated Cu foam (CF@Sn) preform was developed and fabricated for low-temperature die attachment of high-temperature electronic devices. The preform is composed of a strip-shaped Cu skeleton coated with a uniform Sn layer, displaying a low-bonding-temperature and high re-melting temperature. The microstructure evolutions of interconnection layer and the mechanical properties of joints bonded at 280 °C under 2 MPa for different bonding time were investigated. After bonding for 40 min, the preform demonstrated excellent metallurgic bonding to the substrates, and the bond line with only Cu and Cu3Sn phase was obtained. Additionally, the average shear strength of bonded joints was improved to 28.1 MPa, the failures were occurred along the interface between the Cu skeleton and IMCs. The bonded joints presented good thermal reliability without micro-crack interfaces after the high temperature aging tests.

Facile preparation of Cu foam/Sn composite preforms for low-temperature interconnection of high-power devices

A simple and low-cost Cu foam/Sn composite preform was proposed for the low-temperature interconnection of high-power devices. The composite preform was prepared by pressing a Cu foam as the skeleton between two Sn foils as low-melting point metals with different pressures. The composite preform retains the high re-melting temperature superiority of transient liquid phase (TLP) bonding, as well as shortens the reflow time of intermetallic compounds (IMCs). The microstructures of the bondlines and the electrical and mechanical properties of the composite preform pressed at different pressures were studied. After sintering at 260 °C, the interconnection layer becomes denser with the increasing pressures of composite preform, and the bondline is composed of mainly Cu6Sn5, Cu3Sn and Ag3Sn phase. When the pressure of the composite preform increases from 200 to 400 MPa, the electrical resistivity decreases and the shear strength of bonded joints increases. At the pressure of 400 MPa, the electrical resistivity and the shear strength are 8.83 μΩ cm and 36.4 MPa, respectively, which are far better than the traditional Sn performs. Furthermore, two fracture failure models were obtained to analyze the breaking mechanism of the bonding joints under different sintering temperatures.

Wafer-Level Low-Temperature Solid-Liquid Inter-Diffusion Bonding With Thin Au-Sn Layers for MEMS Encapsulation

A novel solid-liquid inter-diffusion (SLID) bonding process is developed allowing to use thin layers of the Au-Sn material in wafer-level microelectromechanical systems (MEMS) packaging while providing a good bonding strength. The bond material layers are designed to have a robust bond material configuration and a metallic bond with a high re-melting temperature, which is an important advantage of SLID bonding or with its alternative name, transient liquid phase (TLP) bonding. The liquid phase in SLID bonding is the gold-rich eutectic liquid of the Au-Sn material system, where the bonding temperature is selected to be 320 °C for a reliable bonding. The average shear strength of the bonds is measured to be 38±1.8 MPa. The hermeticity of the package is tested with the He-Leak test according to MIL-STD 883, which yields a leak value lower than 0.1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> atm.cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> /s. The vacuum inside the package without a getter is calculated as 2.5 mbar after cap wafer thinning. The vacuum level is well preserved after post-processes such as annealing at 400 °C and the dicing process. These results verify that thin layers of Au-Sn materials can be used reliably with the SLID or TLP bonding technique using the new approach proposed in this study.

(Invited) Low Temperature Wafer-Level Cu-in-Sn Solid-Liquid Interdiffusion Bonding

Ubiquitous electronics is the seamless integration of smart systems into our everyday life and a cornerstone of the trillion-sensor vision. Wafer level bonding though 3D-integration could enable multi-sensor fusion with logic in a vertical high-speed package. However, such complex levels of integration require a robust understanding of the manufacturing processes influence on sensitive Micro-Electro Mechanical Systems (MEMS), such as residual stresses and trace impurities.One promising wafer-level bonding method is Solid Liquid Interdiffusion (SLID) bonding, often referred to Transient Liquid-Phase (TLP) bonding. A reliable Cu-Sn SLID bonding at 320ºC has been demonstrated on numerous occasions. However, sub 250ºC bonding temperatures are of significant interest due to low processing temperatures and subsequent low thermally induced Coefficient of Thermal Expansion (CTE) induced global and local residual stresses.This presentation reviews the recent results on Low Temperature (LT) Cu-In-Sn SLID process from Aalto University that exploits the In-Sn eutectic composition enabling low temperature processing, which results in a wafer-level bond with a high re-melting temperature of >600°C.The outcomes of this work demonstrate: (i) an optimised LT-SLID process including processing parameters, (ii) resulting microstructural evolution and (iii) mechanical reliability (pre- and post-thermal annealing), such as tensile measurements and fracture surface analysis, to assess the LT-SLID bond stability. Figure 1

A low-temperature bonding method for high power device packaging based on In-infiltrated nanoporous Cu

With the rapid development of the third-generation semiconductor materials, an appropriate high-temperature-resistant die attach material has become one of the bottlenecks to fully exploit the excellent properties of the third-generation semiconductor power devices. At the same time, a low-bonding temperature is always the pursuit goal of packaging engineers to reduce the thermal residual stress in electronic devices. In this paper, a low-temperature bonding method was proposed to address the above-mentioned issue based on In infiltrating the nanoporous Cu. In, as a low melting point metal, can significantly reduce the bonding temperature, and the nanoporous Cu structure can provide a very large specific surface area, which greatly increases the consumption rate of In. Furthermore, the formed Cu-In IMCs with high-remelting temperature can withstand the high operating temperature. The microstructures of the bondlines before and after bonding were studied in detail. The results show that the bondline can completely consume the low melting point In, within 10 min at 165 °C under a pressure of 0.75 MPa. When the bonding temperature was further increased to 310 °C, the bondline was composed of η-Cu2In and δ-Cu7In3 phases, whose melting points were more than 600 °C. The average electrical resistivity was determined to be 5.53 ± 0.65 μΩ cm, and the thermal conductivities were 144.33 W m−1 K−1, 139.24 W m−1 K−1 and 129.79 W m−1 K−1 at 30 °C, 150 °C and 300 °C, respectively. The average shear strength were 19.09 ± 3.4 MPa, 20.47 ± 4.6 MPa and 30.73 ± 5.2 MPa at 30 °C, 250 °C, 310 °C, respectively. These results indicate that the nanoporous Cu infiltrated with In could meet the requirements of electrical, thermal conduction, and mechanical support as a die attachment for high-power devices.

An interconnection method based on Sn-coated Ni core-shell powder preforms for high-temperature applications

A die-attach material based on an Sn-coated Ni core-shell powder with a high re-melting temperature can be used for high-temperature applications. The outer Sn layers of the Sn-coated Ni particles possess a low melting point of 232 °C and can be completely converted into a Ni3Sn4 intermetallic compound with a high re-melting point of 794.5 °C after reflowing at a low temperature of 250 °C for 40 min. This die-attach material exhibits an excellent high-temperature operational capability, with a high shear strength of 40.2 MPa at 500 °C. The coefficient of thermal expansion (CTE) of the reflowed preform is between those of SiC and Cu, which is highly beneficial for mitigating the stress caused by the CTE mismatch between these materials. In addition, the thermal conductivity of the bondline is sufficiently high for heat dissipation during operation.

Vertical cracking of Cu-Sn solid-liquid interdiffusion bond under thermal shock test

Copper-tin solid-liquid interdiffusion (SLID) bonding technique results hermetic, high re-melting temperature and mechanically strong interconnections for e.g. MEMS applications. However, the long-term reliability is not well-known and Cu-Sn system often suffers from voiding phenomenon. In this communication, the literature related to voiding is reviewed, as well as the effect of voiding level, arising from the copper electroplating bath condition and plating parameters, to the observed vertical cracking is experimentally studied under thermal shock test. In addition, finite element method is applied to reason the direction of crack propagation, and to study the effect of voiding to the relative stress level.

Reliable and repeatable bonding technology for high temperature automotive power modules for electrified vehicles

This paper presents the feasibility of highly reliable and repeatable copper–tin transient liquid phase (Cu–Sn TLP) bonding as applied to die attachment in high temperature operational power modules. Electrified vehicles are attracting particular interest as eco-friendly vehicles, but their power modules are challenged because of increasing power densities which lead to high temperatures. Such high temperature operation addresses the importance of advanced bonding technology that is highly reliable (for high temperature operation) and repeatable (for fabrication of advanced structures). Cu–Sn TLP bonding is employed herein because of its high remelting temperature and desirable thermal and electrical conductivities. The bonding starts with a stack of Cu–Sn–Cu metal layers that eventually transforms to Cu–Sn alloys. As the alloys have melting temperatures (Cu3Sn: > 600 °C, Cu6Sn5: > 400 °C) significantly higher than the process temperature, the process can be repeated without damaging previously bonded layers. A Cu–Sn TLP bonding process was developed using thin Sn metal sheets inserted between copper layers on silicon die and direct bonded copper substrates, emulating the process used to construct automotive power modules. Bond quality is characterized using (1) proof-of-concept fabrication, (2) material identification using scanning electron microscopy and energy-dispersive x-ray spectroscopy analysis, and (3) optical analysis using optical microscopy and scanning acoustic microscope. The feasibility of multiple-sided Cu–Sn TLP bonding is demonstrated by the absence of bondline damage in multiple test samples fabricated with double- or four-sided bonding using the TLP bonding process.

Highly Reliable Double-sided Bonding used in Double-sided Cooling for High Temperature Power Electronics

This paper demonstrates the feasibility of double-sided die attachment bonding, a key technology for double-sided cooling structures, using copper-tin transient liquid phase (Cu-Sn TLP) bonding. Recently, double-sided cooling has drawn particular interest by providing a notable improvement in thermal management and increasing allowable power density for automotive power electronics. The use of TLP bonding for double-sided attachment avoids a number of complications in the assembly process, enables multiple attachments, and provides a high bonding quality and reliability at high temperature operation (because of its high re-melting temperature). In addition, Cu-Sn TLP facilitates high thermal and electrical conductivities, which exactly correspond to the aim of double-sided cooling. Cu-Sn TLP bonding is developed using silicon dummy dies and DBC (direct bonded copper) substrates. The feasibility of double-sided Cu-Sn TLP bonding is demonstrated by (1) proof-of-concept fabrication, (2) optical analysis using optical microscopy and SAM, and (3) material identification using SEM and EDX analysis.

Microstructural evolution of predeformed SiC p /ZA27 composites during partial remelting

The microstructural evolution process in SiC particle (SiCp)-reinforced ZA27 composites, previously compressed by different lengths at 270 °C, was investigated during partial remelting. The results indicated that, after being compressed, the microstructure of SiCp/ZA27 composites changed gradually from developed primary α dendrites, to coarsened short dendrites, and then to a structure consisting of texture cells with different orientations with increasing compression. During subsequent partial remelting, the microstructure evolution exhibited four stages: initial rapid coarsening, structural separation, spheroidization, and final coarsening. The change of the primary α particle size with the heating time obeyed the law developed by Lifshitz and Slyosov and Wagner (LSW) after the semi-solid system was up to the dynamic solid-liquid equilibrium state. High compression, high remelting temperature, and large size and proper content of SiCps were beneficial to achieving a desirable structure for semisolid forming (SSF). The primary α particle size distribution was significantly affected by coalescence. These phenomena were extensively discussed metallographically and theoretically.

High-temperature non-eutectic indium-tin joints fabricated by a fluxless process

A new alternative solder joint made of a non-eutectic indium–tin (In–Sn) multilayer composite deposited in high vacuum is reported. The unique features of this design are that it is fluxless, oxidation-free, and more importantly the fabricated joint achieves a re-melting temperature significantly higher than the bonding temperature. The In–Sn non-eutectic multilayer structure with a thin gold film evaporated as a cap layer has a predominantly Sn-rich matrix with a composition of 6 wt.% Au, 14 wt.% In and 80 wt.% Sn. The quality of the joints was first evaluated using a combination of X-ray microfocus and scanning acoustic microscopy techniques, and results have shown that the joints are nearly void-free. In addition, analysis by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray (EDX) spectroscope performed on the joint cross-section clearly indicated a uniform joint thickness of 7.6 μm, while AuIn 2 grains embedded in a heavily Sn-rich matrix were clearly detected. Lastly, we set out to determine the re-melting temperatures of these fabricated joints. Temperature values ranging from 175 to 190 °C were found, higher than the bonding temperature of 150 °C. The results clearly show that the joint composition is heavily Sn-rich, leading to the high re-melting temperature The increase in re-melting temperature opens up the post-processing temperature window of devices in manufacturing processes. No flux is needed during the bonding process, making it particularly useful to packaging devices for which the use of flux is strictly prohibited.