What is the black technology of sintered silver?

Sintered silver technology: material revolution and application breakthrough in the field of electronic packaging

Inside the motor controller of the new energy vehicle, the silicon carbide chip is running stably at a high temperature of 200°C, and the connection layer between it and the substrate does not use traditional tin-based solder or high-temperature brazing process, but realizes a reliable connection through a special material technology - silver particles with a diameter of 50-100nm are diffused by atoms at 250°C, forming a conductive network with a density of more than 95%, and the shear strength of this connection layer reaches more than 30MPa and can be used at -55°C Stable in temperature cycling at 200°C. This is the sintered silver technology that has attracted much attention in the field of electronic packaging in recent years, which is reshaping the reliability standards of power devices, sensors and other products with the combination of silver's inherent advantages and innovative processes, and has become a key support for the commercialization of third-generation semiconductor technology.

1. Material system and core definition of sintered silver

The essence of sintered silver technology is to realize material connection through low-temperature diffusion of silver particles, but its material morphology and technology classification present a rich diversity, and each type has its own unique application scenarios.

The classification by material composition constitutes the basic framework of sintered silver technology. Semi-sintered silver (such as AS9331 and AS9335) is a composite system of silver powder and organic resin, in which the resin component accounts for 10-15%, which is partially retained during the sintering process to form a reinforced structure, which not only maintains the high conductivity of silver, but also improves the mechanical strength through the bonding effect of the resin. One test data shows that AS9335 has a shear strength of up to 25MPa, which is slightly lower than fully sintered silver, but the pressureless process makes it more cost-effective in large-area packaging. Fully sintered silver (such as AS9376 and AS9373) is completely organic and relies on the diffusion of sterling silver particles to form connections, requiring higher sintering temperatures (typically 250-300°C) but with a conductivity of up to 98% IACS (International Annealed Copper Standard), making it particularly suitable for power modules with extremely high electrical performance requirements.

The size effect of nano-sintered silver has brought revolutionary breakthroughs. When the diameter of silver powder particles shrinks from the micron scale (1-5 μm) to the nanoscale (50-100 nm), the proportion of surface atoms jumps from 0.1% to 20%, a huge surface that allows silver particles to undergo significant atomic diffusion below 200°C, compared to traditional micron silver powders that require more than 300°C to initiate a similar process. Research by Sumitomo Electric of Japan has shown that the sintering activity of 70nm silver powder is 10 times higher than that of 5μm silver powder, and the densification rate is increased by 3 times at the same temperature, which lays the material foundation for low-temperature packaging processes.

The formula design of functionalized silver paste is the key to the implementation of technology. Commercial sintered silver paste typically contains:

60-85% silver powder (choose spherical, flake or nanoparticles depending on the application)

10-30% organic carrier (composed of solvents, binders and surfactants)

0.5-5% functional additives (e.g., burn aids, antioxidants)

The formulation of AS9385 silver paste is a typical example: 80% flake silver powder (diameter-to-thickness ratio 20) provides high thermal conductivity, 15% modified epoxy resin ensures low-temperature bonding, and 5% organosilane improves interfacial penetration, reducing its contact resistance on bare copper substrates to 5×10⁻⁵Ω·cm², breaking through the reliance of traditional silver paste on precious metal plating.

2. Microscopic mechanism of sintered connection: from particle contact to densification

The joining process of sintered silver is a precise dance of the microscopic world, and the silver particles complete the transition from physical contact to metallurgical bonding through atomic diffusion under the action of temperature and pressure, which can be divided into three distinctive stages.

The neck formation at the beginning of sintering is the starting point for the connection. When the temperature rises above 150°C, the atoms on the surface of the silver nanoparticles gain enough energy to begin to diffuse towards the contact points between the particles, forming tiny "sintered necks". High-resolution electron microscopy showed that after 10 minutes of insulation at 180°C for 10 minutes, a neck with a diameter of about 10nm was formed at the contact point, and the particles were mainly connected by van der Waals forces and preliminary atomic bonds, and the overall structure was still relatively loose, with a porosity of up to 30-40%. This stage of diffusion is mainly surface diffusion, with atoms migrating on the surface of the particles rather than penetrating deep inside, so they do not significantly change the overall morphology of the particles.

The mid-term construction of the grain boundary network determines the connection strength. As the temperature rises to 200-250°C, the diffusion mechanism shifts from surface diffusion to grain boundary diffusion and volume diffusion, and the sintered neck continues to thicken and the distance between particles continues to decrease. At this time, the lattice of adjacent particles begins to match, forming a continuous grain boundary, and the originally isolated silver particles gradually fuse into a dendritic structure. Cross-sectional analysis of AS9335 silver paste after 30 minutes of insulation at 220°C showed that the diameter of the sintered neck had reached 50% of the original particles, the grain boundary network coverage exceeded 70%, and the porosity was reduced to 10-15%, and most of them were closed pores with uniform distribution. The key to this stage is to control the rate of warming, too fast will cause the gas to escape in time to form bubbles, and too slow will reduce toughness due to overgrowth of grains.

Later densification and performance optimization are at the heart of quality control. At the high temperature stage of 250-280°C, silver atoms migrate over long distances through lattice diffusion, small pores are filled, and large pores gradually spherical and shrink, eventually forming a continuous structure with a density of more than 90%. After 60 minutes of insulation at 280°C, AS9373 fully sintered silver can control the porosity to less than 5%, and most of them are isolated micropores with a diameter of less than 1 μm. At this time, the original contour of the silver particles has basically disappeared, forming a continuous polycrystalline silver structure, and its electrical and thermal conductivity is close to that of bulk silver materials. Studies have shown that for every 1% increase in densification at this stage, thermal conductivity can be increased by 2W/(m・K) and shear strength can be increased by 1.5MPa, so precise control of retention time and temperature uniformity is crucial.

Liquid-assisted sintering is the synergistic mechanism of some systems. In silver pastes containing low melting point additives (such as AS9331), a small amount of liquid phase forms when the temperature reaches the melting point of the additive (typically 180-220°C). This liquid phase acts as a "bridge" for atomic diffusion, dissolving the oxide layer on the surface of silver particles and promoting particle rearrangement through capillary action, increasing the densification rate by 40%. However, the liquid phase must be strictly controlled below 5%, and excessive amount will lead to component segregation, causing brittle fracture of the joint at high temperatures.

3. Material properties: comprehensive advantages beyond traditional solders

The reason why sintered silver can quickly replace traditional solder in high-end electronic packaging is due to its all-round advantages in electrical, thermal, mechanical and environmental adaptability, which synergize with each other to meet the stringent requirements of the new generation of electronic devices.

The rolling advantage of electrical properties is reflected in low resistance and high stability. The volumetric resistivity of sintered silver is usually between 1.8-3.0×10⁻⁶Ω cm, which is only 1/10 of tin-lead solder (15-20×10⁻⁶Ω cm) and sterling silver (1.6×10⁻⁶Ω・). cm) approached. In a 1200V/200A SiC module, the use of an AS9385 sintered silver connection reduces conduction loss by 15%, which translates to an 8-10°C reduction in module temperature under the same operating conditions. What's more, its resistance temperature coefficient (TCR) is only 0.0039/°C, which varies smoothly in the range of -55°C to 200°C, while tin-based solder can rise sharply as it approaches the melting point, leading to the risk of thermal runaway.

The leapfrog improvement of thermal management capabilities solves the heat dissipation problem of high-density packaging. The thermal conductivity of sintered silver can reach 200-250W/(m・K), which is the same as that of traditional tin-silver-copper solder (50-60W/(m・). K)), which is close to the level of thermal conductivity of copper. In the power amplifier of a 5G base station, the device junction temperature was reduced from 125°C to 95°C using AS9376 sintered silver, which not only met long-term reliability requirements but also increased the power density from 3W/mm² to 5W/mm². Tests by a photovoltaic inverter manufacturer showed that the thermal resistance of IGBT modules was reduced from 0.25K/W to 0.12K/W and the power consumption of cooling fans was reduced by 50% after using sintered silver technology.

The essential breakthrough in high-temperature reliability stems from the high melting point properties of silver. The melting point of silver is as high as 961°C, which far exceeds the 183-220°C of tin-based solders, which makes the sintered silver connection layer less than 300°C melting or significant softening. In the accelerated aging test at 175°C, the AS9335 sintered joint retained 90% shear strength after 1000 hours, while the tin, silver, copper solder only maintained 50%. More stringent temperature cycling tests (-55°C to 150°C, 1000 times) showed that the resistance change rate of sintered silver was less than 5%, well below the 30% of solder, and this stability allowed the motor controller life of new energy vehicles to exceed 15 years/300,000 kilometers.

The precise matching of mechanical properties adapts to the requirements of complex working conditions. The shear strength of sintered silver is usually between 20-40MPa, and it can be adjusted to match different application scenarios through recipe adjustment: 25MPa for AS9331 (semi-sintered) is suitable for flexible substrates, and 35MPa for AS9385 (pressure sintered) is suitable for power modules. Its elastic modulus is about 80GPa, which is between a silicon chip (170GPa) and a ceramic substrate (300GPa), which can effectively relieve the stress caused by thermal expansion mismatch. In vibration tests (20-2000Hz, 10g acceleration), sensor leads with sintered silver have a shedding rate of only 1/5 of conventional solder, making them particularly suitable for vibration environments such as automotive electronics.

The environmentally friendly characteristics are in line with the trend of industrial upgrading. Sintered silver does not contain toxic elements such as lead and cadmium, fully meeting environmental regulations such as RoHS and REACH, while traditional tin-lead solder is facing increasingly stringent restrictions. In the production process, the low-temperature sintering process (200-250°C) reduces energy consumption by 60% compared with high-temperature brazing (above 600°C), and silver can be recycled through dissolution and recovery.

4. Process technology: precise control from no pressure to pressure

The process technology of sintered silver shows a diversified development trend, and different process routes are optimized for specific application scenarios, jointly promoting the industrialization of technology.

The pressureless sintering process occupies a large area of the packaging market due to its simplicity. AS9335 silver paste is a typical example of this field, with only the following steps:

Interface cleaning: Treat the surface of the substrate with plasma to remove the oxide layer and improve the surface energy to more than 60mN/m.

Printing and coating: Silver paste is evenly coated through screen printing, with a thickness of 50-100μm and a line width accuracy of ±10μm.

Pre-baking curing: Keep warm at 150°C for 20 minutes to remove solvent and preliminarily cross-link resin;

Sintering molding: Keep warm in an air atmosphere of 220-250°C for 60 minutes without additional pressure.

This process is particularly suitable for large-area substrate (100mm×150mm) packaging of photovoltaic inverters, with a yield of up to 98% in a production line and an equipment investment of only 1/3 of that of pressure sintering. However, voltage-free processes typically have a slightly lower density (85-90%) and are more suitable for low- to medium-power devices.

The pressure sintering process is irreplaceable in the field of high reliability. The pressurized process of AS9385 silver paste promotes particle contact and diffusion through pressure, with typical parameters:

Pre-pressing stage: 0.5-1MPa pressure is applied at 150°C for 10 seconds to ensure good interface contact;

Fundamental pressure stage: 10-30MPa pressure is applied at 250-280°C for 5 minutes to promote densification;

Cooling control: Reduces room temperature at a rate of 5°C/min to reduce thermal stress.

This process increases the density to more than 95% and the shear strength exceeds 30MPa, successfully solving the packaging problem of bare copper substrates - the silver particles form close contact with the copper surface under pressure, and form a stable connection through interdiffusion, and the contact resistance is controlled below 1×10⁻⁴Ω cm². In SiC modules of new energy vehicles, pressure sintering technology enables power cycle life of more than 100,000 times, which is 5 times that of traditional solder.

Process-assisted technology further improves quality stability. Ultrasonic assistance (20-40kHz) can improve the uniformity of silver paste application by 20% and reduce local porosity caused by thickness deviations; Nitrogen protection (oxygen content <100ppm) is used for sintering copper substrates, which can control the thickness of the interface oxide layer below 5nm, which is 80% lower than the air atmosphere. Laser sintering enables selective curing by local heating (spot diameter 50μm), which is particularly suitable for tight-sealed MEMS sensors with a heat-affected zone controlled within 100μm.

5. Application fields: from power devices to intelligent sensors

The application territory of sintered silver technology is expanding with the improvement of performance, showing irreplaceability in many strategic emerging industries, and has become a key supporting technology for high-end electronic manufacturing.

The breakthrough in the field of power electronics is the most significant. In wide bandgap semiconductor devices such as silicon carbide (SiC) and gallium nitride (GaN), sintered silver perfectly matches its high-temperature operating characteristics - a 1200V/400A SiC module with AS9385 pressure sintering increases the operating junction temperature from 150°C to 200°C, and the power density exceeds 50W/cm², which is 1x higher than traditional solder solutions. In IGBT modules, sinteredsilver connections reduce thermal resistance by 40% and increase current carrying capacity by 25% under the same heat dissipation conditions, which means that motor controllers for new energy vehicles can be miniaturized and weighed down by 30%.

Reliability needs in automotive electronics are met. In the battery management system (BMS) of new energy vehicles, AS9331 semi-sintered silver is used for the connection of chips to PCBs, which remains stable in temperature cycles from -40°C to 85°C, so that the measurement accuracy of the BMS (±1mV) is not affected by temperature. The failure probability after vibration test (10-2000Hz, 20g) is reduced from 10% to 0.1% after the use of sintered silver in the motor controller, which meets the requirements of ISO 16750 for automotive grade. A real vehicle test of a car company showed that the failure rate of the electronic system using sintered silver technology after driving 100,000 kilometers was only 1/5 of the traditional scheme.

The boundaries of performance in sensor manufacturing are expanded. In high-precision pressure sensors, AS9376 fully sintered silver is used for the connection of sensitive components to bases, and its low stress characteristics reduce the temperature drift of the sensor from 0.1% FS/°C to 0.02% FS/°C, improving measurement accuracy by 5 times. The MEMS gyroscope uses laser sintered silver technology, which reduces the parasitic capacitance of the package by 30% and achieves an angular velocity measurement resolution of 0.001°/h, meeting the navigation requirements of autonomous driving. In medical sensors, the biocompatible sintered silver formulation (with the addition of 1% titanium) enables long-term contact with human tissue, and the lifespan of implantable blood glucose sensors is extended from 3 months to 1 year.

The efficiency of new energy and communication fields has been significantly improved. After the photovoltaic inverter is encapsulated in sintered silver, the heat dissipation efficiency of the power device is improved, and the conversion efficiency is increased from 96.5% to 97.5%, and a 100MW photovoltaic power plant can generate 1.2 million kWh more electricity per year. In the RF front-end module of 5G base stations, AS9335 pressureless sintered silver reduces signal transmission loss by 0.5dB and extends base station coverage by 15% in the 3.5GHz band. A test by a communication equipment manufacturer showed that 5G base stations using sintered silver technology reduced the outage rate by 60% and the operation and maintenance cost by 30% in high-temperature environments.

6. Technology trends: balance between performance improvement and cost optimization

The development of sintered silver technology is advancing along the two main lines of performance breakthrough and cost control, and new material systems and process innovations are emerging, promoting its penetration from high-end fields to the mass market.

Material innovation focuses on particle design and composite systems. A breakthrough has been made in the research and development of nano-silver-coated copper particles (silver layer thickness 5-10nm), which reduces material costs by 40% while maintaining 80% conductivity, and the volumetric resistivity of a sample reaches 3.5×10⁻⁶Ω・cm and shear strength of 22MPa, which has met the needs of medium-power devices. Graphene-reinforced silver-based composites increase thermal conductivity to 300W/(m・K), providing a new solution for heat dissipation in ultra-high frequency devices. The more advanced core-shell structure (silver-nickel) increases corrosion resistance by a factor of 10, expanding applications in harsh environments such as marine engineering.

Process innovation is committed to lowering the barrier to entry and improving efficiency. Room-temperature sintering technology shows potential in wearable devices by adding new castositic aids (such as nano-zinc oxide) to reduce the sintering temperature below 150°C, adapting to flexible PI substrates. Inkjet printing sintered silver technology enables precision patterning of 5μm line widths and increases material utilization from 60% to 90%, making it ideal for low-volume customized production. The development of a continuous sintering furnace increased production cycle times from 300 to 1,000 pieces per hour, reducing energy consumption per unit by 50%, laying the foundation for large-scale applications.

The construction of the standard system accelerates the maturity of the industry. The International Electrotechnical Commission (IEC) has initiated the formulation of specifications for sintered silver materials (IEC 62899-401) and reliability test methods (IEC 60749-41), clarifying key indicators such as silver powder purity (≥99.9%), particle size distribution (D50=50-100nm), and shear strength (≥15MPa). Domestically, the group standard of "General Specification for Nano Silver Sintered Slurry" has been released to promote the performance of domestic materials to benchmark against the international level.