Die-attach Technique Research Articles

High-temperature Resistant Interconnections Formed by Ni Nanoparticles/Al Micro-particles Composite Paste for SiC Power Device Application

Sintering bonding using metal nanoparticles is considered a promising die-attach technique for high-temperature operating power devices, such as SiC. However, the thermal stress produced by the difference in coefficient of thermal expansion (CTE) between the chip and substrate, often affects the reliability of this technique. To address this limitation, we analyzed a sintering bonding using Ni nanoparticles and Al microparticles composite paste. The results revealed that the Al microparticles significantly reduced the voids and cracks in the bonding layer formed by Ni nanoparticles paste. Three chips with different sizes were bonded to Cu substrates, and the bonding strength was measured. The result revealed that the bonding strengths in the samples were higher than those of the samples bonded using conventional metal nanoparticles. In addition, the fracture observation after the shear test revealed that the Al particles suffered plastic deformation. Furthermore, the high-temperature storage test at 250°C confirmed the long-term bonding reliability.

Tens-of-seconds solid-state sinter-bonding technique in air using in situ reduction of surface oxide layers on easily bendable dendritic Cu particles

Solid-state sinter bonding using metal particles, such as Ag and Cu, are emerging for the development of the next-generation die-attachment technique of a semiconductor chip that operates at high temperature or generates huge heat, thereby requiring a significant decrease in bonding time for industrial applications. However, this process still requires at least several minutes, even when using pastes containing expensive Ag nanoparticles that are limited in terms of handling and mixing. Here, we present an easily applicable and ultrafast sinter-bonding method that can be applied in air, using a combination of a micron-scale surface-area-enhanced Cu particles and an effective reducing solvent. Using uniquely-shaped semi-dendritic particles of 2.89 μm in D50 size, the bonding under a 5-MPa compression at 300 °C forms a Cu bondline that exhibits a sufficient shear strength of 23.7 MPa after heating for only 10 s to reach 300 °C. Subsequently, the strength increased to 29.0 MPa with the bondline microstructure of near-full density after 60 s. This strategy will provide enhanced sustainability at temperatures above 200 °C, long-term mechanical reliability, and significantly higher thermal conductivity than those in solder joints.

Brief review of silver sinter-bonding processing for packaging high-temperature power devices

Silver sintering is receiving increasing attention due to its novel die-attach technique for high-temperature power electronics. Excellent thermal conductivity, high melting point/remelting temperature and low-temperature sintering behaviors of the silver sintered attachment meet the requirements of high-temperature applications for power devices, specifically SiC devices. The merits and demerits of the existing pressure-assisted sintering and pressure-less sintering techniques using nano-scale, micro-scale and micro-nano-scale hybrid silver sintered materials are separately presented. The emerging rapid sintering approaches, such as the electric-assisted approach, are briefly introduced and the technical outlook is provided. In addition, the study highlights the importance of creating a brief resource guide on using the correct sintering methods.

Correction of Nonuniform Attenuation of Voltage-Controlled Attenuator Under Harsh Environment by Process Improvement

This paper describes the assembly process improvement of die attachment technique of an open-loop multistep voltage-controlled attenuator (VCA) in detail. The VCA is made up of p-i-n diodes (dies). Die p-i-n diodes suffer from attachment issues done by single or binary epoxy followed by controlled temperature treatment. It results in improper ground impedance at microwave and millimeter-wave applications. A thorough analysis is carried out on these epoxy attachment issues. A suitable technique for the attachment of dies on the metallic standoff is adopted to overcome these issues for the mass production scenario. Multiple modules have been made and tested over extensive thermal cycling from − 30 °C to + 70 °C, and random vibrations in three axes to establish the analysis.

Novel silver die-attach technology on silver pre-sintered DBA substrates for high temperature applications

Currently, Ag die-attach techniques, using nano-silver particles, are of high interest for manufacturing of wide-band-gap (WBG) power module due to their high-temperature operation capability. However, the high cost of silver and complicated processing requirements are the main driving force in the search for simpler and more cost-effective attached technologies. In this study, a new die-attach technique based on silver die-attach, without conventional Ag-paste, for high-temperature applications is developed. Glass containing Ag paste was pre-sintered on the DBA substrates, and later on, semiconductor dies were simply placed on this pre-sintered Ag layer and attached under heat and pressure. The samples were tested under shear and thermal cycling loadings (−45 °C/250 °C) to evaluate the quality and reliability. Destructive and non-destructive analysis methods, such as Scanning Acoustic Tomography and cross-section observation, were used to identify fracture modes. The samples demonstrated sufficient shear strength and high thermal reliability. Furthermore, the effects of Ag recrystallization, grain growth and rearrangement of the voids are considered to be the main fracture factor of conventional Ag die-attach joints based on sample's cross-sections.

Process Optimization of Pressure-Assisted Rapid Ag Sintering Die Attach for 300 °C Applications

Ag sintering is a promising die-attach technique to fabricate digital and analog thick-film modules for 300 °C applications. In this paper, a low-temperature, pressure-assisted rapid sintering process was developed. Process optimization decreased the sintering time to 1 min, which allowed the use of an FC150 thermocompression flip-chip/die bonder for automated die attach instead of a hydraulic press, improving the manufacturability. The porosity of the sintered Ag layer was decreased from 30% (pressureless assembly) to 15% with the application of a low pressure (7.6 MPa) during the 1-min sintering process for large die. The average shear strength for the 3 $\times $ 3-mm2 die was 70.7 MPa, and the 8 $\times8$ -mm2 die could not be sheared off due to the 100-kg force limit of the shear test module. Both Ag and Au metallization (die and substrate) were studied. A new substrate metallization combination was found that was compatible with thick-film Au metallized substrates.

Reliability modeling of Sn–Ag transient liquid phase die-bonds for high-power SiC devices

In this work, a novel foil-based transient liquid phase bonding process has been used to mount the SiC Schottky diodes. The Sn–Ag TLP interlayer material was produced in the form of preforms of multilayer foils, using electrochemical deposition. The foils were designed to keep the overall composition of Ag and Sn about 80% and 20% respectively. The optimized TLP bonding process parameters were used during the assembly process. The die-attachment characterizations revealed that resulting intermetallic compounds (Ag3Sn and ζ) have melting point beyond 480°C. The die-attachment produced low bending stresses, while heated from 30°C to 400°C. The reliability of Sn–Ag TLP bonded samples was studied during passive temperature cycling and during active power cycling. During power cycling, the crack rates were determined by measuring the crack lengths of the TLP bonded joints after failure. The failure criteria were set to be an increase of diode's forward voltage by 10% since the start of the power cycling tests. The thermo-mechanical simulations were performed to determine the damage parameter i.e. strain range amplitude ∆εp. Based on mechanical characterization of the TLP bonded layers, a plastic material model was used. The crack propagation rates were modeled using Paris' Law. Based on comparisons with state-of-the-art silver sintering technique, it can be stated that the TLP bonding is a promising die-attachment technique and its power cycling reliability is similar to silver sintering.

Enhanced pressureless bonding by Tin Doped Silver Paste at low sintering temperature

The nanosilver sintering die-attach technique has been a promising alternative for wide band gap semiconductors. However, it is less preferable in industry because of its high sintering temperature. Recently research has been initiated to develop transient liquid phase sintering (TLPS) solder paste for use in electronics packaging. In this article, in order to lower the sintering temperature of nanosilver paste, we develop a novel tin (up to 10wt%) doped silver paste (TDSP) and a sintering profile with the highest processing temperature of 235°C based on TLPS. Sintered TDSP is Ag/Ag3Sn/Ag–Sn solid solution composites. The composites have a microstructure of Ag matrix grains reinforced by Ag3Sn and Ag–Sn solid solution within the matrix grains. And this microstructure endows the sintered Ag+4%Sn with a pressureless bonding strength of 23MPa. The improved mechanical properties of sintered TDSP are attributed to second-phase strengthening and solid solution strengthening mechanisms. However, the overmuch formation of brittle Ag3Sn phase is the main reason resulting in sharp decrease of bonding strength when the Sn content over 5wt%. The new TDSP technology is expected to be applicable to a wide range of power semiconductors devices, such as organic devices and printed circuit boards. Furthermore, it provides new strategies for low-temperature sintering.

Assembly and Packaging Technologies for High-Temperature and High-Power GaN Devices

This paper gives a detailed analysis on the assembly and packaging technologies for the state-of-the-art GaN-based high-electron-mobility transistors, which are suitable for high-temperature and high-power applications. Silver sintering and transient liquid phase bonding were selected as die-attachment techniques, and gold and palladium were investigated for electrical interconnection materials. Both the die-attachments were characterized for their high-temperature stability up to 450 °C. Systematic electrical characterizations were performed from on-wafer measurements to the final assembly. The thermal and thermomechanical influences of the assembly were assessed. For die-attachments and interconnections, passive temperature shock cycling and active power cycling were performed as an initial attempt to characterize the assembly reliability. Finally, a complete package along with the base plate was proposed, which can survive high temperatures up to 480 °C.

Ecological comparison of soldering and sintering as die-attach technologies in power electronics

In this study the method of life cycle assessment is used to perform a comparison of two die-attach technologies of power modules: silver sintering and common soldering with a tin–silver–copper solder. The goal of the study is to indicate which die-attach technique is more environmentally friendly. This is important in order to design future electronics more environmentally friendly.Processing covers the manufacturing of the respective pastes, application of pastes, and mounting as well as the subsequent sintering or soldering, including cleaning of the soldered parts. The last step in the processing is a wire bonding process. The environmental impacts are expressed in CO2-equivalents and ReCiPe points and different variants are discussed: comparisons are made between considered pastes and between a soldered and a sintered power module, including consideration of lifetime. Concerning sintering, the influence of using recycled silver is analyzed more closely. The types of stages included in this life cycle assessment best fit a cradle-to-gate assessment, because usage, recycling, and disposal are not included. The software used is OpenLCA version 1.3.3, created by GreenDelta, and the database used is Ecoinvent version 2.2. In order to fill data gaps, electronics manufacturing companies were contacted, and our own calculations and measurements were applied.The consideration of the wet film thicknesses of the soldering system and sintering system, respectively, results in very similar CO2-equivalent emissions for sinter paste and solder paste. Comparing the soldered power module to the sintered one shows minor to significant differences between the two systems which could easily be changed when changing circumstances and assumptions. But when considering the respective lifetimes of the joining technologies, sintering technology is clearly preferable to the conventional soldering technology. Using solely recycled silver for manufacturing of silver sinter paste reduces the CO2-equivalent emissions of one sintered power module by roughly 40%.The results clearly show the dependence of outcomes on the initial settings. Consideration of lifetime has an enormous impact on the comparison of soldering technology to sintering technology. But the benefit of this life extension through the use of sintering technology is only useful when a long lifetime is required.

Pressureless Silver Sintering Die-Attach for SiC Power Devices

Pressureless silver sintering is an interesting die-attach technique that could overcome the reliability limitations of the power electronic devices caused by their packaging. In this paper, we study the manufacturing parameters that affect the die attach: atmosphere, drying time, heating ramp rate, sintering temperature and duration. It is found that sintering under air gives better results, but causes the substrates to oxidize. Sintering under nitrogen keeps the surfaces oxide-free, at the cost of a weaker attach.

Comparison of Au-In Transient Liquid Phase Bonding Designs for SiC Power Semiconductor Device Packaging

Transient liquid phase (TLP) bonding is an advanced die-attach technique for wide-bandgap power semiconductor and high-temperature packaging. TLP bonding advances current soldering techniques by raising the melting point to over 500 °C without detrimental high-lead materials. The bond also has greater reliability and rigidity due in part to a bonding temperature of 200 °C that drastically lowers the peak bond stresses. Furthermore, the thermal conductivity is increased 67 % while the bond thickness is substantially reduced, lowering the thermal resistance by an order of magnitude. This work provides an in-depth examination of the TLP fabrication methodology utilizing mechanical and thermal experimental characterization data along with thermal reliability results.

Die Attachment for −120°C to +20°C Thermal Cycling of Microelectronics for Future Mars Rovers—An Overview1

Active thermal control for electronics on Mars rovers imposes a serious penalty in weight, volume, power consumption, and reliability. Thus, we propose that thermal control be eliminated for future rovers. From a functional standpoint there is no reason that the electronics could not operate over the entire temperature range of the Martian environment, which can vary from a low of ≈−90°C to a high of ≈+20°C during the Martian night and day. The upper end of this range is well within that for conventional electronics. Although the lower end is considerably below that for which conventional—even high-reliability—electronics is designed or tested, it is well established that electronic devices can operate to such low temperatures. The primary concern is reliability of the overall electronic system, especially in regard to the numerous daily temperature cycles that it would experience over the duration of a mission on Mars. Accordingly, key reliability issues have been identified for elimination of thermal control on future Mars rovers. One of these is attachment of semiconductor die onto substrates and into packages. Die attachment is critical since it forms a mechanical, thermal, and electrical interface between the electronic device and the substrate or package. This paper summarizes our initial investigation of existing information related to this issue, in order to form an opinion whether die attachment techniques exist, or could be developed with reasonable effort, to withstand the Mars thermal environment for a mission duration of approximately one earth year. Our conclusion, from a review of literature and personal contacts, is that die attachment can be made sufficiently reliable to satisfy the requirements of future Mars rovers. Moreover, it appears that there are several possible techniques from which to choose and that the requirements could be met by judicious selection from existing methods using hard solders, soft solders, or organic adhesives. Thus, die attachment does not appear to be a roadblock to eliminating thermal control for rover electronics. We recommend that this be further investigated and verified for the specific hardware and thermal conditions appropriate to Mars rovers.