Die-attach Material Research Articles

Review of Die-Attach Materials for SiC High-Temperature Packaging

Review of Die-Attach Materials for SiC High-Temperature Packaging

Optimized bimodal silver particle sintering for high thermal conductivity die-attach layers

A bimodal silver system can achieve higher effective thermal conductivity (ETC) than a mono-system for electronic packaging. However, the effect of micron- and submicron-particle compositions on the microstructure and ETC of sintered Ag is unclear. In this study, Ag films with varying particle compositions were fabricated by pulsed laser deposition. The results indicate that the sintered bimodal Ag film with 76 vol% submicron-particles possesses the highest ETC (260 W m−1 K−1). This enhancement is attributed to the decreased porosity, increased pore circularity and pore distribution uniformity. This particle composition modulation strategy can provide a guideline for developing high-performance die-attach materials for electronic packaging.

Microstructure characteristics for improved thermal shock reliability of sintered Ag[sbnd]Al paste in SiC power module

To meet the harsh thermal shock (−50–250 °C) requirements of power device packaging, a novel sintered Ag-based die attach material based on the sustained release effect of Al was proposed. In this study, Al particles with natural core-shell structure covered with Al2O3 layer were selected as the ideal doping-phase, and micron Al particles doped sintered Ag composite paste (AgAl) was prepared. First, the microstructural evolution of sintered AgAl joints during thermal shock was studied. The results showed that Al particles effectively suppressed the cracks generation, which attributed to the sustained release effect of Al element. To clarify sustained release mechanism of Al, the interfaces evolution between Ag and Al particles was investigated. In the initial stage, Al was covered by the Al2O3 layer, preventing inter-diffusion between Ag and Al. As the thermal shock proceeds, the Al2O3 film was fractured, achieving direct contact between metal Al and Ag. Diffusion thus takes place. For the sintered Ag5Al joint, the shear strength was 17.3 MPa after 1000 cycles, which is 1.57 times that of pure sintered Ag joint. These results indicate the successful development of low-cost, high-reliability die attachment materials for use in harsh thermal shock environments.

Degradation in Electrothermal Characteristics and Failure Mechanism of SiC JBS With Different Die Attach Materials Under 300 ∘C Power Cycle Stress

Degradation in Electrothermal Characteristics and Failure Mechanism of SiC JBS With Different Die Attach Materials Under 300 ^∘C Power Cycle Stress

Construction of microstructures on the Cu substrate using ultrafast laser processing to enhance the bonding strength of sintered Ag nanoparticles

Silver nanoparticle (Ag NP) pastes become a potential die-attachment material with the increased electronic power density. However, the weakness of bonding interface between sintered Ag NPs and bare Cu substrate limits the applications of the Ag NPs paste, thereby reducing the shear strength of the sintered joint. In this work, ultrafast laser processing is utilized to enhance the bonding strength of the sintered Ag joint by fabricating a microstructure interface. The microstructure dimensions are tunable by controlling laser parameters, and then high-strength joints could be obtained. Different substrate microstructures were constructed, and the enhanced bonding mechanism was analyzed by characterizing the cross section and fracture surface morphologies of joints. The ultrafast laser processing could increase the surface energy of Cu substrates to form a more reliable connection with Ag NPs and more energy required for crack extension with the increasing connection area, thereby resulting in a significant improvement in the shear strength of the Ag NP joints. The patterned microstructures on the Cu substrate using this technique showed improved surface energy and increased number of connection areas on the substrate, showing potential for the use in third-generation semiconductors for highly reliable packaging.

Suppression of Ag dewetting in submicron Cu@Ag particles in paste for improving sinter bondability through surface modification

Operating temperature of a wide-bandgap semiconductor-based power device can reach 300 °C, which requires a next-generation die-attach sinter material. Ag-coated Cu (Cu@Ag) particles have been considered as a low-cost filler material in pastes for sinter bonding, replacing pure Ag particles. However, there are two main problems: out-diffusion of core Cu and Ag dewetting. Although the Ag shell is sufficiently thick, Ag dewetting induces accelerated out-diffusion of the core Cu and the resultant Cu oxidation through the formation of thin Ag coating parts. Accordingly, a surface-modification method using stearic acid was developed to suppress Ag dewetting and improve the sinterability of Cu@Ag particles. Sinter-bonding characteristics were examined using pastes containing surface-modified submicron Cu@Ag particles. Sinter bonding was conducted at 250 °C under 10 MPa for 1–10 min with N2 blowing using eight types of initial and surface-modified submicron Cu@Ag particles with Ag contents of 10, 20, 30, and 40 wt% (A10, A20, A30, and A40). The shear strength of the bondline containing surface-modified A10 exhibited 17.56 MPa when bonded for 3 min, compared with 11.67 MPa of the initial A10. After a bonding time of 10 min, the shear strength increased to 20.41 MPa. Surface-modified A10 (low Ag content), with the highest efficiency in Ag dewetting suppression, exhibited sinter-bonding characteristics similar to those of Cu@Ag particles with higher Ag contents without surface treatment. Surface-modified A20 provided a sufficient strength of 20.83 MPa after 3 min of bonding.

Interface regulation of micro-sized sintered Ag-10Al composite based on in-situ surface modification and enhanced microstructure stability in power electronic packaging

The increasing demand for high-power SiC semiconductors necessitate the development of a die attachment material that combines high-temperature resistance, reliability, and cost-effectiveness. In this study, a novel micro-sized composite, Ag-10Al paste, containing 10 wt% Al particles, was designed. A remarkable phenomenon, the ejection of ultrafine Ag nanoparticles from micron-sized Ag flakes, was observed for the first time. The phenomenon was utilized for the in-situ surface modification of Al. Subsequently, the microstructure and mechanical properties of the sintered Ag-10Al/direct bonded copper (DBC) joints were studied. Results indicated that the Ag-10Al composite exhibited superior microstructure stability compared to sintered Ag. The Ag/Al interface was systematically analyzed, revealing a unique Ag/nano Ag2O/Al2O3 amorphous/Al structure. This structure was formed through the Ag nanoparticle jetting effect of Ag flakes, achieving effective bonding between nano Ag2O and Al2O3 amorphous phases through mutual dissolution at the atomic level. Moreover, the sintered Ag-10Al joint demonstrated enhanced mechanical performance stability over the sintered Ag joint. After 1000 h aging at 300 ℃, the shear strength of the sintered Ag-10Al joint reached 34.1 MPa, meeting the requirements for power semiconductor packaging. In conclusion, the Ag-10Al composite paste was thoughtfully designed, excelling in both performance and cost-effectiveness.

Prognostic Monitoring of Power QFN Packages with Silver Sintered Die-Attach Materials

Prognostic Monitoring of Power QFN Packages with Silver Sintered Die-Attach Materials

Microstructure and Bonding Strength of Low-Temperature Sintered Ag/Nano-Ag Films/Ag Joints

Nano-Ag paste is one of the most widely used die-attachment materials in modern electronic devices, which are gaining continuously increasing application in transportation industries. The nano-Ag film in a pre-formed dimension and free from the use of chemical dispersing agents has been proposed to be a promising alternative to nano-Ag paste for the die-attachment application. Although the bonding mechanisms of Nano-Ag paste have been extensively studied, little is known about the relationship between the microstructure and mechanical properties of low-temperature-sintered Ag/nano-Ag film/Ag joints. In this work, the influences of temperature, pressure, and dwell time at peak temperature on the microstructure and the shear strength of low-temperature-sintered Ag/nano-Ag film/Ag joints were systematically investigated. Mechanical properties tests indicate that both temperature and pressure have pronounced effects on the bonding strength of sintered Ag/nano-Ag film/Ag joints. TEM and hot nanoindentation characterizations further reveal that the sintering temperature plays the most determinant role in the coarsening of nano-Ag film and, hence, the bonding and fracture behaviors of Ag/nano-Ag film/Ag joints sintered at 210–290 °C. The diffusion-induced coarsening of nano-Ag particles can be activated, but remains sluggish at 250 °C, and the mechanical integrity of sintered joints is circumscribed by the interfacial bonding between nano-Ag film and Ag substrate after sintering at 290 °C.

Effect of epoxy resin addition on properties and corrosion behavior of sintered joints in power modules serviced offshore

Power modules applied in offshore applications are facing risks of corrosion failures on die-attach materials due to high humidity and H2S exposure. To investigate such corrosion behavior for sintered die-attach materials, we conducted a study with four groups of samples fabricated using copper and silver metal particles under different solvent systems. Such samples were firstly subjected to high-humidity-H2S conditions for 168 h to simulate the harsh offshore environment. After undergoing corrosion, the primary compounds formed were CuO/Cu2O and Ag2S through SEM, XRD, and XPS analysis. Notably, the incorporation of epoxy resin into sintered copper joints resulted in a remarkable reduction in corrosion and a substantial improvement in electrical conductivity after the reaction. In contrast, while the addition of epoxy did not evidently reduce corrosion in silver joints, it did lead to a significant increase in shear strength. Furthermore, to gain further insights into the effect of epoxy resin on corrosion behavior, electrochemical analysis, and molecular dynamics simulations were conducted. Finally, the mechanical reliability of the corroded copper and silver joints was evaluated through thermal shock tests. In summary, sintered copper joints exhibited better anti-corrosion behaviors than sintered silver under high humidity and H2S exposure, especially with the addition of epoxy resin. However, the corrosion products of sintered copper suffered from a sharp decrease in shear strength after thermal shock tests than sintered silver, which is probably due to the coefficient of thermal expansion mismatch.

A Hybrid Pressureless Silver Sintering Technology for High-Power Density Electronics

Aerospace and defense applications present unique challenges for material suppliers. As increasing adoption of advanced semiconductor materials and Diverse Accessible Heterogeneous Integration technologies, power density of defense and aerospace devices increases rapidly. Traditional die-attaching technology is becoming an increasingly limiting factor in microelectronics packaging for the next generation aerospace and defense systems. Metal particle sintering creates a porous metal foam, which can significantly enhance heat transfer within the sintered material. However, the traditional silver sintering requires very high sintering temperatures that cannot be tolerated by typical microelectronics devices. In addition, the sintered metal foam contains open-cell pores that can absorb/entrap moisture and dusts, which poses a reliability risk. This article introduces an advanced hybrid silver sintering material, which incorporates highperformance silver sintering with high reliability and process-friendly of epoxy-based die-attaching technology. This hybrid sintering paste can be processed without applying any pressures in temperature ranges that are normal in microelectronics packaging processes. Preliminary experimental studies, including Scanning Electron Microscopic study, volume resistivity tests, die shear strength tests, and thermal resistance tests, were performed for developing a low temperature sintering schedule that is compatible with normal assembly processes of high-power density electronics devices. The results indicated that the hybrid material could achieve silver sintering at 150 degrees C and offer 36 enhancement on thermal performance comparing with a widely used die-attach material.

Interface enhancing strategy for low-temperature bonding of Ag-based nanoalloy

Bonding interface quality is crucial for power electronic packaging, while the alloyed Ag nanoparticle sintered joint has lower crack resistance at the interface. In this work, a sandwich structure nanoparticle film consisting of an Ag transition layer, nanoalloy layer, and Ag transition layer was developed for interface-enhanced low-temperature bonding. The optimized sandwich structure increased the shear strength of Ag-based nanoalloy sintered joints, reaching 5.6 times for Ag10Cu and 2.0 times for Ag10Pd, respectively. The low-temperature spreadability of the Ag transition layer provided a micro-nano structure and further driving force for nanoalloy sintering on the substrate. Both experimental and molecular dynamics simulation results revealed that these nanoalloy particles spread easier and realized a higher effective bonding area. This interface-enhancing strategy enables the Ag-based nanoalloy a high-reliability die-attach material with low-temperature bondability.

Porosity Dependence of Thermal and Electrical Properties in Nano-Silver Paste

Nano-silver paste is considered one of the most promising die-attach materials for high-temperature applications contributing to its excellent thermal and electrical properties and low bonding temperature. However, it is difficult to measure the thermal conductivity and electrical resistivity of nano-silver in practical applications. In this work, we proposed a finite element modeling method to predict the thermal conductivity and electrical resistivity of nano-silver. Nano-silver flake and film with different porosity were prepared by adjusting the sintering temperature, and the thermal and electrical properties were measured. The simulated results are in good agreement with the experimental results under all porosity. The error values between the experimental and simulated temperatures are less than 10%. With the increase of porosity, the thermal conductivity of nano-silver decreases and the electrical resistivity increases. Pores will cause heat flow and current concentration, and increase the possibility of failure of nano-silver in practical applications. The results indicate that the porosity plays a more important role in the thermal and electrical properties of the nano-silver compared with pores location.

Die-attach bonding at 230 °C using micron Sn-coated Zn particles for high-temperature applications

The advancement of wide bandgap semiconductors has posed higher requirements for power chip packaging technology. Nanoparticles (Ag, Cu, and Cu@Ag) sintering was regarded as an innovative low-temperature bonding method that can be serviced at a high temperature. Considering the expensive cost of silver nanoparticles, Zn particles have been considered as an alternative. Herein, a novel type of submicron Sn-coated Zn particles was proposed to overcome the easy oxidation and high sintering temperature problem of Zn. The structure of Zn70Sn30 (wt.%) core/shell was determined by Scheil model calculation to meet the requirements of low-temperature bonding and high-temperature applications. Multiple reducing agents and reduction times were tentatively investigated to prepare Zn70Sn30 (wt.%) particles. The solder joints prepared based on Zn@Sn preform achieved bonding at 230 °C and exhibited an average shear strength of 28.13 MPa at 250 °C. Unlike the Zn–30Sn solder alloy, the lower reflow temperature produced only a single layer of Cu5Zn8 at the interface of the solder joint, which ensured that all fractures occurred primarily in the Zn@Sn preforms. The electrical resistivity and thermal expansion coefficients of Zn@Sn preform were 8.1 μΩ cm and 12.7 × 10−6/K, respectively. Therefore, the Zn@Sn particles were a potential die-attach material for high-temperature power device packaging.

Reliability and Lifetime Prediction Model of Sintered Silver Under High-Temperature Cycling

Although excellent reliability has been reported for sintered silver as a die-attach material under both thermal and power cycling loads in power electronics applications, the promise of this material as a large-area attachment at temperatures beyond 200 °C needs to be investigated. This article presents insights into the thermomechanical behavior and reliability of sintered silver under extreme thermal cycling conditions. In this study, we bonded sintered silver samples and subjected it to a thermal cycling profile of −40 °C to 200 °C with high ramp rates. We periodically monitored samples under thermal cycling to detect the presence of any failure mechanisms using a scanning acoustic microscope. We also included 95Pb5Sn solder in the study to obtain reference data. Results show the occurrence of cracks in sintered silver followed by a rapid rate of crack growth that exceeded the failure criterion in just 50 cycles. The predominant failure mechanism we observed was adhesive failure. As a large-area attachment, solder exhibited a higher reliability than sintered silver but failed within 100 cycles. Finally, we performed thermomechanical modeling to compute strain energy density values and correlated these with the experimentally observed crack growth rates to formulate a lifetime prediction model for sintered silver.

Tilt- and warpage measurement as inline quality assessment tool

The tilt of semiconductor dies is a common issue during assembly in power electronics as, e.g. die bonding during silver sintering caused by inhomogeneous thicknesses of applied die-attach material, problems with a homogeneous force application, uneven substrates etc. This tilt usually leads to reliability problems later during testing or operation in the field. A weakly sintered layer will be formed with voids larger than 1–2 μm in major numbers and more importantly will lead to insignificantly developed interfaces. None of them can be detected by SoA failure analytical techniques like scanning acoustic microscopy (SAM) or pulsed infrared thermography (PIRT), since both will still provide a sufficient phonon coupling, indistinguishable to well sintered layers. However, the undetected weak interfaces will lead to severe reliability problems and can only be detected indirectly by thickness measurement without a cross section. In this work, we present a scanning system based on confocal polychromatic distance sensors, which can scan a fully loaded tray of bonded dies on the substrate for industry-grade inline integration. We demonstrate the novel system on different bonded power dies on active metal braze substrate (AMB) under industrial production conditions. We achieve this in the required accuracy of about 2–3 μm in an environment with strong vibration influence. The in-line capability has been proven by using a setup, which is independent of mechanical vibrations and uses only optical fibers to transfer measuring signals within future smart production environments.

Tensile characterization and constitutive modeling of sintered nano-silver particles over a range of strain rates and temperatures

Sintered nano-silver die-attach materials have been widely used in high-power electronics packaging because of their high thermal and electrical conductivities. In this study, we characterized the tensile properties of sintered nano-silver particles over a range of strain rates and temperatures, and established the constitutive models. First, 50 nm nano-silver particles were sintered at 275 °C for 50 min as test samples, and their tensile tests were conducted under a dynamic thermomechanical analyzer (DMA Q800) and an IBTC 300SL in-situ mechanical test system respectively with different strain rates and ambient temperatures. Then, both Anand and variable-order fractional models (VoFM) were adopted to analyze the obtained stress-strain data and we studied their fitting accuracy and applicability. The results showed that: (1) The Young's modulus of the sintered nano-silver particles decreased with increasing temperature. In addition, the tensile strengths declined under lower strain rates and higher temperature conditions; (2) both the Anand model and VoFM characterized the tensile stress-strain properties of the sintered nano-silver material well. Compared to the Anand model, the VoFM utilized a simpler formula with fewer parameters and higher precision.

Development of Ag-In Alloy Pastes by Mechanical Alloying for Die Attachment of High-Power Semiconductor Devices.

Sintered silver paste is widely used as the die-attachment material for power semiconductors. However, sintered silver joints encounter problems, such as severe coarsening of sintered pores and oxidation issues, in harsh high-temperature environments. These lead to the deterioration of the die-attachment joints. In this paper, a novel method of sintering silver joints is demonstrated, where silver–indium alloy paste is used to improve the reliability of sintered Ag joints. The silver–indium (Ag–In) alloy paste was fabricated through mechanical alloying using the ball-milling technique. The well-bonded sintered Ag–In alloy joints inhibited pore coarsening better than pure sintered Ag joints and significantly enhanced the mechanical properties at high operating temperatures. Lastly, an oxidation mechanism for the sintered joint was proposed, and strategies to prevent such high-temperature oxidation were discussed.

Long-time reliable direct bonding of silver flake paste on Al substrate for power electronic die-attachment

Silver paste is supposed to be a most prospective die-attach material for power electronics. While, the bonding of silver paste with aluminum substrate cannot be easily achieved due to the native oxidation of aluminum. This work provides a direct bonding of aluminum with silver micro flake-based paste under 250℃ with a shear strength of 15 MPa with a propriate surface treatment method of aluminum, and the shear strength even keeps higher than 15 MPa after storing at 250℃for 1000 h. The bonding mechanism was elucidated with high resolution transmission electron microcopy. The results indicate a new route for the wide application of Al based substrate in power electronic with low cost.

Thermal Fatigue Fracture Behavior of Sintered Ag Nanoparticles under Equibiaxial Thermal Stress

An equibiaxial thermal fatigue test of die-attach materials that reproduces the same stress state as the power cycle was proposed, and the fatigue fracture behavior of Ag sintered die-attach materials under equibiaxial thermal stress was investigated. Thermal fatigue tests under equibiaxial stress showed that vertical cracks were observed throughout the sintered Ag nanoparticles. Thermal fatigue fracture was out-of-phase type, in which crack initiation and opening were driven by equibiaxial tensile stress during temperature drop. It is suggested that the fatigue crack growth in the die-attach joint during power cycling can be predicted by the fatigue crack growth law at high temperatures obtained by mechanical fatigue tests.