Nanoscale Silver Paste Research Articles

13.1: The design and preparation of flexible MiniLED plate with nano‐Ag paste

The development and preparation process of a flexible MiniLED line subgrade board on a white reflective sheet was carried out in this paper using silk‐printing nanoscale silver paste assisted bonding technology. The design of the line conductivity parameter ‐ nanoscale silver paste parameter model, printing plate selection, and process verification were systematically completed, as well as the preparation and reliability confirmation of flexible MiniLED lamp board. The flexible MiniLED lamp board with disposable sintered nano silver slurry lines was successfully prepared, and the 65" backlight module was constructed.

Constitutive modelling on the whole-life uniaxial ratcheting behavior of sintered nano-scale silver paste at room and high temperatures

Uniaxial ratcheting and fatigue behaviors of low-temperature sintered nano-scale silver paste at both room and high temperatures were studied in the authors' previous work. The deterioration of sintered silver pastes under cyclic loading which manifest itself as decrease in elastic modulus is addressed by introducing a damage variable as represented by θ function. Then, the damage variable is coupled into a visco-plastic constitutive model, based on the Ohno-Wang and Armstrong-Frederick (OW-AF) nonlinear kinematic hardening rule. In addition, the temperature-dependent ratcheting behavior are also captured by incorporating the Arrhenius power-law model into the flow rule. The whole-life ratcheting behaviors of low-temperature sintered nano-scale silver paste are simulated and are in well agreement with experimental data.

Low-Temperature Sintering of Nanosilver Paste on a Gold Film Surface

AbstractNanosilver paste with a low sintering temperature is a high-demand interconnection material for electronic devices and components due to its superior electrical, thermal, and thermomechanical properties, as well as the fact that it is lead free. This study presents a nanoscale silver paste that can be sintered at 300 °C using 60-nm silver particles and an ethyl cellulose solution. The rheological and thixotropic behaviors of the typical paste were characterized, and the paste's screen printing ability was analyzed in detail. The paste demonstrates typical shear thinning rheological behavior and thixotropy of pseudoplastic fluids. Thermal characteristics of the 60 nm silver particles were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to obtain a proper sintering temperature schedule. In this report, we also discussed the solid content, thermodynamic driving force for densification, and sintering process of the nanosilver particles. A 83 wt% of silver solid content at a heating rate of 20 °C/min was favorable for achieving a proper sintering densification.

Small-chip Attachment on Copper Leadframe with Sintered Nanosilver Paste

Sintered nanoscale silver paste provides a low-temperature alternative to solder for die attachment. Unlike solder, the sintered attachment does not melt upon reaching the original attachment temperature and therefore may be used at higher temperatures. Higher electrical and thermal conductivities mean less Joule heating and better heat dissipation characteristics and the porous microstructure imparts low elastic modulus for lower thermomechanical stress and enhanced reliability. The state of the technology has reached a point where it is now possible to obtain die-shear strengths comparable to solder at sintering temperatures between 250°C and 280°C with little or no applied pressure, depending on the chip size. In addition to attachments on silver or gold-coated surfaces, it is possible to form bonds on bare copper if done under inert or slightly reducing atmosphere. Because attainment of a strong bond depends on the paste being in contact with a clean (oxide-free or untarnished) surface, a study was made to determine if the attachment process will work on copper leadframes with either an anti-tarnish or anti-EBO (epoxy bleed-out) coating. Small mechanical silicon chips less that 3 mm × 3 mm in size were attached without pressure at temperatures as low as 260°C. The sintering atmosphere in the chamber was varied from pure nitrogen, to nitrogen + 4% hydrogen and to nitrogen + 1% oxygen. Attachments sintered in pure nitrogen or nitrogen with hydrogen produced die-shear strengths of at least 30 MPa and were just as strong as those bonded on bare copper. Sintering in nitrogen + 1 % oxygen caused the die-shear strength to drop below 30 MPa but still above 20 MPa. In the presence of oxygen, the binder removal is due to oxidative combustion but the low level of oxygen caused incomplete binder burnout that interfered with the sintering while also causing some oxidation of the copper. On the other hand, the addition of hydrogen appeared to enhance the sintered microstructure accompanied by a slight increase in die shear strength. Sheared attachments that exposed the copper surface showed patches of silver still attached indicating formation of strong bond with the copper.

Migration of Sintered Nanosilver Die-Attach Material on Alumina Substrate Between 250 $^{\circ}\hbox{C}$ and 400 $^{ \circ}\hbox{C}$ in Dry Air

The low-temperature joining of semiconductor chips by sintering of silver paste is emerging as an alternative lead-free solution for power electronics devices and modules working in a high-temperature environment. A promising die-attachment material that would enable the rapid implementation of the sintering process is nanoscale silver paste, which can be sintered at temperatures below 300°C without an external pressure. In this paper, we report our findings on the silver migration in sintered nanosilver electrode-pair patterns on an alumina substrate. The electrode pairs were biased at an electric field ranging from 10 to 100 V/mm and at a temperature between 250°C and 400°C in dry air. The leakage currents across the electrodes were measured as the silver patterns were tested in an oven. Silver dendrites formed across the electrode gap were observed under an optical microscope and analyzed using scanning electron microscopy and energy dispersive spectroscopy (EDS). The silver migration was found in the samples tested at 400°C, 350°C, 300°C, and 250°C. The measurements on the leakage current versus time were characterized by an initial incubation period, called “lifetime,” followed by a sharp rise as the silver dendrites were shorting the electrodes. A simple phenomenological model was derived to account for the observed dependence of lifetime on the electric field and temperature. The EDS mappings revealed the significant presence of oxygen on the positive electrode but the complete absence on the negative electrode. A mechanism involving the oxidation of silver and the dissociation of silver oxide at the anode was suggested. We suggest that the migration of a sintered nanosilver die attachment can be prevented in high-temperature applications through packaging or encapsulation to reduce the partial pressure of oxygen.

Uniaxial ratcheting and fatigue behaviors of low-temperature sintered nano-scale silver paste at room and high temperatures

Uniaxial ratcheting and fatigue behaviors of low-temperature sintered silver films at room and high temperatures were studied experimentally and particular attention was paid to the influence of ratcheting to fatigue. The effects of stress amplitude, mean stress, stress rate, temperature, peak stress holding time and loading history on the ratcheting response of sintered silver films were analyzed, respectively. Firstly, it can be concluded that the ratcheting strain amplitude and ratcheting strain rate of the material increase with increasing stress amplitude or mean stress, correspondingly. Secondly, the experimental results indicate that ratcheting strain increases significantly with the increasing temperature and decreasing stress rate. Thirdly, the material was found having a strong memorization on loading history, the prior stress cycling with low stress amplitude or mean stress can greatly retard the strain accumulation under subsequent loading conditions. Fourthly, the strain accumulation with peak stress holding time is larger than that without hold time at high temperatures, but the ratcheting evolution with and without hold time is almost equal at room temperature. Finally, it was found that the fatigue failure of sintered silver paste is dominated by ratcheting response especially for high temperature and long peak stress holding time. The factors such as stress rate, temperature and peak stress holding time also have a significant influence on fatigue life since they all greatly affected ratcheting behavior of the material.

Low-Temperature Sintering of Nanoscale Silver Paste for Attaching Large-Area $({>}100~{\rm mm}^{2})$ Chips

A low-temperature sintering technique enabled by a nanoscale silver paste has been developed for attaching large-area (>100 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) semiconductor chips. This development addresses the need of power device or module manufacturers who face the challenge of replacing lead-based or lead-free solders for high-temperature applications. The solder-reflow technique for attaching large chips in power electronics poses serious concern on reliability at higher junction temperatures above 125°C. Unlike the soldering process that relies on melting and solidification of solder alloys, the low-temperature sintering technique forms the joints by solid-state atomic diffusion at processing temperatures below 275°C with the sintered joints having the melting temperature of silver at 961°C. Recently, we showed that a nanoscale silver paste could be used to bond small chips at temperatures similar to soldering temperatures without any externally applied pressure. In this paper, we extend the use of the nanomaterial to attach large chips by introducing a low pressure up to 5 MPa during the densification stage. Attachment of large chips to substrates with silver, gold, and copper metallization is demonstrated. Analyses of the sintered joints by scanning acoustic imaging and electron microscopy showed that the attachment layer had a uniform microstructure with micrometer-sized porosity with the potential for high reliability under high-temperature applications.

“Migration of Sintered Nanosilver Die-attach Material on Alumina Substrate at High Temperatures”

A nanoscale silver paste that can be sintered at temperatures below 300°C without external pressure is emerging as a promising die-attach material for implementing the low-temperature joining technology in high-temperature packaging. In this paper, we report our findings on silver migration in sintered nanosilver electrode-pair patterns on alumina substrate. The electrode pairs were biased at electric field ranging from 10 to 100 V/mm and at temperature between 250 °C and 400°C in dry air. Leakage currents across the electrodes were measured as the silver patterns were tested in an oven. Silver dendrites formed across the electrode gap were observed under an optical microscope and analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Silver migration was found in samples tested at 400°C, 350°C, and 300°C, and 250°C. The measurements on leakage current vs. time were characterized by an initial incubation period, called “lifetime”, followed by a sharp rise as silver dendrites were shorting the electrodes. A rapid rise in the “lifetime” with decreasing oxygen partial pressure was also found. A simple phenomenological model was derived to account for the observed dependence of “lifetime” on electric field, temperature, and oxygen partial pressure. The reliability of sintered nanosilver die-attachment over silver migration in high temperature applications can be significantly improved through packaging or encapsulation to reduce oxygen exposure.

Applying Anand model to low-temperature sintered nanoscale silver paste chip attachment

Tensile behaviors of low-temperature sintered films of a nanoscale silver paste used for chip-attachment are studied. Stress–strain curves of partially sintered films were measured at different temperatures and strain rates on a dynamic mechanical analyzer (DMA). The tensile strength of the sintered silver films decreased with increasing temperature or with decreasing strain rate. Then the uniaxial tensile inelastic deformation behaviors of the material were simulated by a unified visco-plastic constitutive model, Anand model. Finally, finite element simulation of chip-attachment using this interface material under thermal cycling was done with Anand model via ANSYS software. The simulation results showed that accumulation of plastic strain took place in the silver-bonding layer during thermal cycling, which might lead to the final failure of the chip-attachment.

Tensile Behaviors and Ratcheting Effects of Partially Sintered Chip-Attachment Films of a Nanoscale Silver Paste

The mechanical behaviors of partially sintered thick films of a nanoscale silver paste used for attaching semiconductor chips are studied. The films, about 150 μm thick, were made by repeatedly stencil-printing the paste on a ceramic substrate and sintering by a recommended heating profile suitable for device attachment. The partially sintered films were lifted off the substrate, and their tensile behaviors, i.e., stress–strain curves, were measured at temperatures between −60°C and 300°C using a dynamic mechanical analyzer (DMA). The elastic modulus and tensile strength of the sintered silver films decreased with increasing temperature. Ratcheting behaviors of the films under cyclic tension at 150°C were also tested by using the DMA by examining the effects of loading rate, mean stress, and stress amplitude. The ratcheting strain grew with increasing mean stress or stress amplitude and with decreasing loading rate.

Low-Temperature Sintering of Nanoscale Silver Paste for Large-Area Joints in Power Electronics Modules

Today, reflow soldering is a commonly used technique to establish large-area joints in power electronics modules. These joints are needed to attach large-area (&gt;1 cm2) power semiconductor chips to the substrate, e.g., a direct-bond copper substrate, and the multichip module substrate to a copper base plate for heat spreading. Thermal performance, specifically thermal conductivity and thermomechanical reliability, of these large-area joints are critical to the electrical performance and lifetime of the power modules. Soft solder alloys, including the lead-tin eutectic and lead-free alternatives, have low thermal conductivities and are highly susceptible to fatigue failure. As demands mount for higher power density, higher junction temperature, and longer lifetime out of the power modules, reliance on solder-based joining is becoming a barrier for further advancement in power electronics systems. Recently, we successfully demonstrated lowtemperature sintering of nanoscale silver paste as a lead-free solution for achieving highperformance, high-reliability, and high-temperature interconnection of small devices (&lt;0.09 cm2). In this paper, we report the results of our study to extend the low-temperature sintering technique to large-area joints. The study involved redesigning the organic and inorganic components of the nanoscale silver paste, analyzing the burnout kinetics of the various organic species sandwiched between large-area plates, and developing desirable temperature-time profile to improve sintering and bonding strength of the joints.

High-Temperature Operation of SiC Power Devices by Low-Temperature Sintered Silver Die-Attachment

In this paper, we present the realization of high-temperature operation of SiC power semiconductor devices by low-temperature sintering of nanoscale silver paste as a novel die-attachment solution. The silver paste was prepared by mixing nanoscale silver particles with carefully selected organic components which can burn out within the low-temperature firing range. SiC Schottky diodes were placed onto stencil-printed layers of the nanoscale silver paste on Au or Ag metallized direct bonded copper (DBC) substrates for the die-attachment. After heating up to 300degC and dwell for 40 min in air to burn out the organic components in the paste and to sinter the nanoscale silver, the paste consolidated into a strong and uniform die-attach bonding layer with purity >99% and density >80%. Then the die-attached SiC devices were cooled down to room temperature and their top terminals were wire-bonded to achieve the high-temperature power packages. Then the power packages were heated up from room temperature to 300degC for high-temperature operation and characterization. Results of the measurement demonstrate the low-temperature silver sintering as an effective die-attach method for high-temperature electronic packaging. An advanced packaging structure for future SiC transistors with several potential advantages was also proposed based on the low-temperature sintering technology.

Low-Temperature Sintered Nanoscale Silver as a Novel Semiconductor Device-Metallized Substrate Interconnect Material

A nanoscale silver paste containing 30-nm silver particles that can be sintered at 280degC was made for interconnecting semiconductor devices. Sintering of the paste produced a microstructure containing micrometer-size porosity and a relative density of around 80%. Electrical and thermal conductivities of around 2.6times105 (Omegamiddotcm)-1 and 2.4W/K-cm, respectively, were obtained, which are much higher than those of the solder alloys that are currently used for die attachment and/or flip-chip interconnection of power semiconductor devices. The sintered porous silver had an apparent elastic modulus of about 9GPa, which is substantially lower than that of bulk silver, as well as most solder materials. The lower elastic modulus of the porous silver may be beneficial in achieving a more reliable joint between the device and substrate because of increased compliance that can better accommodate stress arising from thermal expansion mismatch

Thermomechanical Reliability of Low-Temperature Sintered Silver Die Attached SiC Power Device Assembly

A thermomechanical reliability study was conducted on a low-temperature sintered silver die attached SiC power device assembly. The silver die attachment was formed by sintering nanoscale silver paste in air at 300 degC to form a strong bond between silver- or gold-coated direct-bond-copper substrates and silver-metallized SiC Schottky diodes. Using the 50% drop in the die-shear strength as the failure criterion, an accelerated thermal cycling experiment between 50 degC and 250 degC showed that the silver die attachment can survive more than 4000 cycles, indicating its high thermomechanical reliability at the interested temperature range. Established mainly by scanning electron microscopy/energy-dispersive spectroscopy, the drop of the die-shear strength during the thermal cycling was attributed to the pile-up of creeping dislocations to form microcavities at the grain boundaries of the sintered silver