Active Metal Brazing Substrates Research Articles

Quasi-direct Cu–Si3N4 bonding using multi-layered active metal deposition for power-module substrate

The advancement of power modules demands more reliable insulating circuit substrates. Traditional substrates, comprising Cu and Si3N4, are produced using active metal brazing (AMB). However, AMB substrates have reliability concerns owing to electrochemical migration and void formation from brazing filler metals. This study introduces a quasi-direct Cu–Si3N4 bonding technique using a Ti/Al bilayer active metal deposition at the bonding interface. A sputtered Ti/Al bilayer was formed on the Si3N4 surface, then heated and pressurized the sputtered Si3N4 substrate with Cu sheets in vacuum to bond each other without voids or delamination. The Ti/Al layers reacted with Si3N4 and Cu, forming a 300 nm intermediate layer. TEM observations show this layer contains segregated Ti–N and Cu–Al phases, with a good lattice match to Si3N4 and Cu–Al. Temperature-cycling tests on the Cu/Si3N4/Cu substrate revealed delamination caused by increased tensile stress at the periphery of the bonding area due to asymmetrical Cu patterns. This novel quasi-direct Cu–Si3N4 bonding technique addresses issues of electrochemical migration and void formation seen in AMB substrates, offering a reliable bonding interface for power electronic substrates.

Bonding Characteristics of Solder and Sinter Joints on Active-Metal-Brazing Substrates with Nano Sputtered Ag-Cu-Ti Brazing Filler Metal

In this study, to improve the bonding interfacial characteristics of active metal brazing (AMB) substrates, a Ag-Cu-Ti brazing filler metal (BFM) layer was formed on aluminum nitride (AlN) and silicon nitride (Si<sub>3</sub>N<sub>4</sub>) ceramics by nano sputtering. The AMB substrates were manufactured by brazing bonding. The measured peel strengths of the bonding interfaces of the AlN and Si<sub>3</sub>N<sub>4</sub> ceramics were 2.35 kgf/mm and 4.26 kgf/mm, respectively. Fracture surface analysis revealed AlN crack initiation at the ceramic/BFM interface, which progressed into the ceramic interior. Silicon carbide (SiC) devices for a power module were bonded on the AMB substrates by Sn-3.0Ag-0.5Cu soldering and Ag sintering bonding. To compare the deterioration characteristics of the joint interfaces, a thermal shock test was conducted. Microstructural analysis after the thermal shock test showed that no defects occurred at the Si<sub>3</sub>N<sub>4</sub>/BFM interface, whereas delamination occurred at the AlN/BFM interface. Summarizing, in this study, the characteristics of AMB interfaces coated with a Ag-Cu-Ti BFM using the sputtering process were identified and the suitability of a SiC-based power module package was confirmed.

Pressureless Silver Sintering of Silicon-Carbide Power Modules for Electric Vehicles

Pressureless silver (Ag) sintering was optimized at 250°C in vacuum and nitrogen gas atmosphere with silicon carbide (SiC) chips, and silicon nitride active metal-brazed substrates (AMB). A 1200-V/200-A power module was developed using a pressureless Ag-sintered live SiC metal–oxide–semiconductor field-effect transistor (MOSFET) device and a Si3N4 AMB substrate module, and diverse reliability tests were performed. The void content and the bonding layer thickness of the Ag sinter joints were 1.2–3.4% and 68.5 µm. The bonding strength of the Ag sinter joints after thermal cycling testing (TCT) were slightly decreased due to the crack generation. In contrast, high-temperature storage testing (HTST) was carried out for a long time at high temperature, the sintering process occurred continuously, and the shear strength increased by 31% after HTST. The drain–source on-resistance (RDS(ON)) of a SiC MOSFET power module before and after the TCT and power cycle test was similar to the as-sintered state without change.

“Drop-in” Conventional Reflow Oven Sinter-Able Pressure-Less Silver Paste for Die-Attach Assembly Mass Production

Abstract During the experiment, pressure-less silver sintering pastes that can sinter using a conventional reflow oven for die-attach applications were developed. This development makes it possible to produce die-attach assemblies in a continuous and high throughput fashion for mass production. Different reflow profiles such as “ramp-up and peak” and “ramp-up and plateau” types were tested and high shear strength joints with no voids were obtained. For each sample, the sintering time was around twelve minutes or less, which is about one order of the magnitude that is presently used for box oven sintering time. Two sintering pastes, A and B, were developed. Paste A has been used to study the relationship between processing conditions (reflow oven sintering temperature and atmosphere such as air vs nitrogen) and the die-attach structures (with varied substrate materials such as copper or active metal brazing (AMB) and surface metallization such as copper, silver, or gold). Under the same combined conditions of sintering temperature, atmosphere, and substrate type, the Ag metalized substrate gave the strongest joint shear strength. In comparison, Au metalized substrate displayed the weakest strength because of the formation of a “depletion layer” as a result of the diffusion of silver on gold surface, leaving behind a weak connection between sintered silver and Ag-Au layer. For the above noble metal metallized surfaces, sintering in air normally results in stronger joint shear strength than that in a nitrogen atmosphere. On the bare Cu metallized surface with nitrogen sintering, increasing the sintering temperature and changing to paste B are two ways to increase joint shear strength. Sintering on an AMB substrate displays around 10Mpa higher shear strength than that on a Cu substrate, indicating the importance of surface morphology. When sintering at 250°C in air, oxygen can help accelerate the inter-diffusion between Cu substrate and sintered Ag and enhance interfacial adhesion, increasing the shear strength. A higher sintering temperature reduces the adhesion due to the formation of large amounts of copper oxide; therefore, control of the optimum temperature range is key to obtaining a strong joint in air.

Material Characterization and Warpage Modeling for Power Devices Active Metal Brazed Substrates

The metallized insulating substrates work as mechanical supports for the circuitry of Power Module Packages. Due to their specific functions, substrates for power electronics are made by different materials. The conductive metal layers can assume different functions: the top metal serves as power circuitry routing while the bottom metal improves the mechanical robustness and thermal efficiency. Ceramic layer provides excellent electrical insulation. These features play an essential role in the operation of power modules, which are often operated at high voltage and high current density. The substrates, composed by materials with different thermal expansion coefficients, are subjected to cyclic stresses due to temperature variations induced by operative working conditions. The substrate layouts typically include differences in shape and/or thickness between the top and the bottom side; this generates asymmetrical distributions of stress/strain resulting in overall warpage. The variations of this warpage induce mechanical fatigue during lifetime and represent a limiting factor for reliability. The scope of the presented work is the characterization of the out of plane warpage of Active Metal Brazed substrates (AMB) by means of numerical approach. The elastoplastic properties of metal and ceramic have been measured, evaluating the thermal softening of the copper as well. These characteristics are needed to calculate AMB warpage through Finite Element Models (FEM), simulating the warpage induced by a passive temperature cycling. Warpage computed from numerical model have been benchmarked and validated with optical warpage measurements. The validated numerical model has been developed to optimize the substrate warpage variation during cycling improving the whole package reliability.

Thermal Shock Performance of DBA/AMB Substrates Plated by Ni and Ni⁻P Layers for High-Temperature Applications of Power Device Modules.

The thermal cycling life of direct bonded aluminum (DBA) and active metal brazing (AMB) substrates with two types of plating—Ni electroplating and Ni–P electroless plating—was evaluated by thermal shock tests between −50 and 250 °C. AMB substrates with Al2O3 and AlN fractured only after 10 cycles, but with Si3N4 ceramic, they retained good thermal stability even beyond 1000 cycles, regardless of the metallization type. The Ni layer on the surviving AMB substrates with Si3N4 was not damaged, while a crack occurred in the Ni–P layer. For DBA substrates, fracture did not occur up to 1000 cycles for all kind of ceramics. On the other hand, the Ni–P layer was roughened and cracked according to the severe deformation of the aluminum layer, while the Ni layer was not damaged after thermal shock tests. In addition, the deformation mechanism of an Al plate on a ceramic substrate was investigated both by microstructural observation and finite element method (FEM) simulation, which confirmed that grain boundary sliding was a key factor in the severe deformation of the Al layer that resulted in the cracking of the Ni–P layer. The fracture suppression in the Ni layer on DBA/AMB substrates can be attributed to its ductility and higher strength compared with those of Ni–P plating.

Improved resistance to thermal fatigue of active metal brazing substrates for silicon carbide power modules using tough silicon nitrides with high thermal conductivity

The effect of temperature cycling from −40 to 250 °C on active metal brazing (AMB) substrates for power modules was investigated using newly developed silicon nitride ceramics with both high thermal conductivity of 140 W m−1 K−1 and superior fracture toughness of 10.5 MPa m1/2. Other types of AMB substrates made of AlN or Si3N4 were also tested for comparison. Both visual inspection and acoustic scanning microscopy (ASM) observation of the new Si3N4-AMB substrates after 1000 cycles revealed almost no cracks. In contrast, the Si3N4-AMB substrates with lower fracture toughness experienced crack initiation beneath the corner of the copper plate. The degradation in the bending strength after 1000 cycles was negligible for the new Si3N4-AMB substrates, whereas the bending strength of the other substrates decreased gradually with each thermal cycle. The endurance of the AMB substrates to thermal fatigue could be improved significantly by employing the new tough Si3N4 with high thermal conductivity.

Effect of high temperature cycling on both crack formation in ceramics and delamination of copper layers in silicon nitride active metal brazing substrates

Crack formation in Si3N4 active metal brazing (AMB) ceramic substrates and delamination of copper layers on the AMB substrates subjected to temperature cycling from −40 to 250°C were investigated to evaluate the reliability of these substrates under harsh environments. Acoustic scanning microscopy (ASM) observation of the Si3N4 substrates with 0.30mm thick Cu layers revealed crack formation beneath the corner of the copper plate after 100 cycles, whereas no cracks were detected on the Si3N4 substrate with a 0.15mm thick Cu layer, even after 1000 cycles. The residual bending strength of the Si3N4 substrates with 0.30mm thick Cu layers was 78% of the as-received substrate after 10 thermal cycles, and gradually decreased with an increase in the number of thermal cycles until ca. 65% of the initial strength after 1000 cycles. The Si3N4 substrates with 0.15mm thick Cu layers exhibited a gentler degradation of residual strength than those with 0.30mm thick Cu layers. In contrast, the residual bending strength of AlN-AMB substrates with 0.15mm or 0.30mm thick Cu layers were reduced by 50% within only 10 thermal cycles. The depth of cracks developed during the thermal cycles was measured from the fractured surface of the Si3N4-AMB and AlN-AMB substrates. The crack-growth rate in the Si3N4-AMB substrates was much slower than that in the AlN-AMB substrates, which could account for the different degradation behavior of the residual bending strength.

Properties and Reliability of Silicon Nitride Substrates with AMB Copper Conductor

This paper focuses on the properties of Si3N4 substrate material with AMB (active metal brazing) copper conductor. A recently developed type of tape casted, gas pressure sintered silicon nitride ceramic with a three times higher thermal conductivity than known from typical standard silicon nitride materials and with good flexural strength was applied. The increase of thermal conductivity is the result of using different species of sintering aids and the optimization of their ratio in the material. The high bending strength allows creating a thinner substrate compared to other standard ceramic materials for power electronics, e.g. aluminum nitride. This reduction in thickness leads to a decrease of the total thermal resistance of the substrate which improves heat dissipation. For the AMB process a silver based active brazing solder composition optimized for Silicon Nitride was used. This optimization could be obtained by an investigation of the physical and chemical interactions between the brazing and the base material. A void free joint without short circuits between adjacent structures could be formed. The copper surface can be coated on demand with Nickel or Nickel/Gold for improved solderability and wire bondability as well as for corrosion protection. The silicon nitride substrate with AMB copper conductor lines and fully covered back side ground shows a higher reliability than comparable substrates made out of common, well known ceramic materials. The heat dissipation is comparable with conventional AMB substrates made of high thermal conductive ceramic such as Aluminum Nitride, but thermal cycling behavior exceeds the limits well known from AlN-AMB or AlN-DCB.

Diamond Composites for Power Electronics Application

In view of power electronics applications, baseplates made from metal diamond composites have been manufactured and characterised. The surface contours of the baseplates were measured during thermal loads up to 180°C starting at room temperature with help of the TherMoiré technique. X-ray analysis investigation was performed to detect porosity and local inhomogeneities of the baseplates. Al- and Cu-based diamond composite baseplates were Ni-plated and used for manufacturing of 3.3 kV IGBT modules. The solder layer between AlN AMB (active metal brazing) substrates and baseplates was investigated by ultrasonic and X-Ray analyses. Thermal resistance of the manufactured IGBT modules was characterised and compared to that of IGBT modules with AlSiC or Cu baseplates. The influence of thermal cycling on the solder layer and thermal resistance of the manufactured module was investigated.