Active Metal Brazing Research Articles

Direct formation of pure copper layer on aluminum nitride by multibeam laser deposition with blue diode lasers

A demand of power module devices has been increasing for highly energy efficient social system. The insulated heat sink printed circuit board is the key component of power module devices, which is composed of copper and aluminum nitride (AlN) due to high thermal conductivity. The conventional method of the joining between copper and AlN is active metal brazing method, but there is an issue that the high heat treatment of this method generates thermal residual stress on the board and decrease the reliability. For that reason, the development of a direct bonding process between copper and AlN has been demanded. Therefore, we focused on the laser metal deposition (LMD) method, which is a selective heating and microfabrication process. The LMD method is a method in which a powder material is supplied to the processing point and irradiated with a laser to melt the powder and form layer on the surface of the substrate. Then, a blue diode laser with a wavelength of 450 nm, which has a high absorptivity of 60% for copper, is used as a heat source. In addition, the multibeam type LMD system has been installed for the uniform heating of powder. In this study, the laser power density, laser scanning speed, and powder feeding rate were changed to explore the conditions for forming a copper layer on the AlN substrate.

Competitive correlation between the interface layers determines the shear strength in Kovar/AgCuTi/Si3N4 joints

AbstractAs a promising high‐thermal‐conductivity ceramics, the direct bonding of Si3N4 with metals remains challenging. Here, we employed active metal brazing of Si3N4 ceramics and Kovar alloys using AgCuTi metal filler, and found that the shear strength in the prepared Kovar/AgCuTi/Si3N4 joints depends on the competitive correlation between the interface layers. The competitive consumption for active element Ti by Kovar/AgCuTi interfacial reactions reduced AgCuTi/Si3N4 interface layer thickness and deteriorated final shear strength, even when using a high‐Ti‐content filler. Surface analysis indicated that interfacial fracture between Ag‐based solid solution and Ti5Si3 was the primary cause of failure. Density functional theory (DFT) revealed that Ag (111)/Fe2Ti (001) interface had higher ideal work of adhesion (Wad) than Ag (111)/Ti5Si3 (001) interface, confirming the importance of AgCuTi/Si3N4 interface layer. Ultimately, by adjusting interface reaction layer thickness, the joints reached a high shear strength of 154.3 MPa.

Stress–strain behavior of Cu on the AMB‐Si3N4 substrate undergoing thermal cycles via in situ strain measurement

AbstractThe active metal brazing of a Si3N4 substrate with Cu has been evaluated for excellent reliability, demonstrating durability up to 1000 cycles in a cycling test ranging from −40°C to 250°C. While the finite element method (FEM) is commonly used for predicting thermal stress–strain in cyclic tests, experimental data on the measurement of thermal stress–strain on metallized substrates have remained limited. In this study, a digital image correlation (DIC) method was employed for the in situ measurement of thermal strain on a fully Cu‐coated Si3N4 substrate (AMB‐SN substrate) in various consecutive thermal cycles, ranging from 1–2 to 199–200. The thermal strains exhibited hysteresis curves that expanded slightly with cycles. By incorporating the coefficients of thermal expansion (CTE) of plain Cu and Si3N4, both the thermal stress and strain of Si3N4 and Cu on the AMB‐SN substrate were computed. The stress–strain curves of Cu revealed that the yield stress of Cu increased with the number of cycles, attributed to the cyclic hardening of the Cu layer. The Cu stress–strain curve calculated in this work showed a good agreement with the previous results obtained from compression/tension test of Cu at room temperature, which indicates the stress–strain curve of Cu on the composite was not sensitive to the temperature during the thermal cycle.

Enhanced properties in Si3N4/Cu AMB joints through regulating interfacial reaction modes induced by Mo-particle additives

Si3N4 ceramics are the new package materials for large-power electronics and devices, which are required to be bonded to the high-thermal-and-electrical-conductivity metal, Cu. Among the methods to join Si3N4 and Cu, active metal brazing (AMB) is widely used, but the joint still faces the challenge of interface bonding reliability for harsh services. Here, a novel approach for enhancement of the Si3N4 AMB joint by regulating interfacial products to be a unitary nanograined TiN layer was proposed. To achieve this, the Si3N4/Cu AMB joint was obtained with an Ag–Cu–In–Ti foil whose formation mechanism was investigated, and the detailed morphology of TiN + Ti5Si3 bilayer was observed at brazing temperatures from 740 to 800 °C. Based on the bilayer formation process, adding different Mo contents from 5 to 20 wt % to hinder the formation of Ti5Si3, obtaining a nanograined TiN layer at the interface with hard-to-observed Ti5Si3 particles around. In the seam, Ag and Cu solid solutions and Cu2InTi were refined after adding Mo, but Cu4Ti and Cu3Ti2 were not generated in the seam, only with CuTi distributed dispersedly. Although the fractures both occurred at the Si3N4/filler interface, the joint with Mo additions had higher strength than those with Mo-free. This was because the reduced CTE of the filler alleviated the residual stress during brazing, the refinement in microstructure as a consequence of nucleation-points Mo functioned, and the unitary TiN layer allowed the joints to bear more load particularly. As a result, the maximum strength of joints with Mo additions reached 424 MPa, 115 MPa higher than those brazed with a foil, and significantly more reliable than joints reported by other literature.

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.

Innovative powder-based wettability evaluation of HfB2-ZrB2-SiC-B4C-CNT composite: Effect of surface roughness and ambient conditions

Active metal brazing, the most common method for joining UHTCs, necessitates the wettability of diborides in contact with filler layer, which is evaluated in present work using a powder-based approach. Ni powder is placed on pre-spark-plasma-sintered HfB2-ZrB2-SiC-B4C-CNT (HZSBC) composites with varying surface roughness (0.1 and 0.7 μm) and heated to 1450°C in Ar and Ar+H2 atmosphere. Formation of gas-liquid-Ni interface for rough surface (Ra:0.7 µm) restricted the spreading of molten Ni on HZSBC surface and increased contact angle (by ∼30°) compared to smooth surface (Ra:0.1 µm). Addition of 5% H2 into Ar atmosphere suppressed the oxide formation and improved molten Ni spreading at a contact angle of ∼10°. Cross-sectional analysis of droplets supports dissolutive wetting where enhanced surface roughness has reduced the dissolution depth (by ∼50% for Ar and ∼12% for Ar+H2 atmospheres). Furthermore, brazing of HZSBC composites with Ni interlayer is performed at 1450°C, in Ar and Ar+H2 atmosphere. Lower wettability of Ni with HZSBC in Ar resulted in detachment of composites during polishing, thus, indicating poor interfacial bonding. Successful brazing of these composites in Ar+H2 atmosphere resulted in microstructural inhomogeneity with decreased hardness and Young's modulus by ∼68% and ∼58%, respectively, from HZSBC region to Ni-Si rich filler layer. Lower interfacial residual stresses (∼0.4 GPa) have provided the structural integrity between ceramic and metal layer, thus making them suitable for applications involving complex shapes.

Insert Molding of Aluminum Alloy A7075 and Alumina Plate by Semi-Solid Forging

Industrial, electric railway, and automotive applications of inverters and converters use case-type semiconductor packages called power modules. Power modules have multiple joint points, and among them, from the viewpoint of reliability and bonding strength, many studies on metal-ceramic substrate bonding have been reported. Alumina Al203 or aluminum nitride AIN are mainly used for ceramics, and copper or aluminum are mainly used for metals, although alumina and aluminum have a price advantage. The joining methods are generally brazing and AMB (Active Metal Brazing), but there is a need to improve the joint strength, heat dissipation characteristics, and productivity, i.e., a joining method that does not use brazing material. The purpose of this study is to achieve direct joining of alumina A1203 and aluminum alloy A7075, and semi-solid forging [1, 2, 3, 4, 5, 6] was used as the method. The A7075 used in this study is considered to be suitable for this application because of its high strength and thermal conductivity among aluminum alloys. In this study, solid phase ratio 30 % semi-solid forging was actually performed using a servo press and a die, and the results were evaluated by observing the cross section of the fabricated specimens. The punches and dies were circular in shape to ensure uniform spreading of the molten material, and carbon steel S50C with high thermal conductivity was used to quickly dissipate the heat of the semi-solid slurry during the pressing process. The thickness of 99.6 % Al2O3 alumina plate was 2.5 mm. The semi-automatic stirring device was modified to stir the molten metal by attaching a 1000 mm round bar to the rotating part of a tabletop drilling machine and attaching a stirring blade to the tip of the bar. Debonding between aluminum alloy and alumina plate was not confirmed. There was no gap between the aluminum alloy and the ceramic plate.

Design to cost Ag free silicon nitride AMB substrate for automotive power module applications

Silver free AMB technology addresses the cost-performance gap between the active metal brazed (AMB) Si3N4 based metal ceramic substrates (MCS) suitable for automotive applications and the cost-efficient direct copper bonded (DCB) Al2O3 substrates suitable for applications with lower requirements. The cost-reduction of the condura®.ultra process is achieved by using a silver free brazing technology combined with an efficient brazing process. Within this report we will demonstrate the thermal cycling capability, the peel strength of the condura®.ultra process combined with a design to cost Si3N4 ceramic substrate. Additionally showing results regarding isolation partial discharge of performance and robustness in thermal resistance measurements.

Interfacial structure between TiN sintered ceramics with and without an Fe-containing grain boundary phase and Ag–Cu eutectic brazing filler material

To focus on the interfacial reaction between the Ag–Cu alloy layer and TiN in active metal brazing, the Ag–Cu brazed interfacial structures between Cu and two types of TiN sintered ceramics fabricated by different methods were examined. No grain boundary phase components consisting of the Ni-containing Fe phase or Mo2C were detected on the TiN grain surfaces of the TiN liquid-phase sintered ceramic bonding surfaces before brazing. The brazed specimens were heated at 850 °C for 0.5 h. No Cu/TiN solid-phase sintered ceramic bonding was obtained. The Ag–Cu alloy layer was bonded onto the TiN grains in the TiN liquid-phase sintered ceramic through an Fe- and Ni-containing segregation layer. This segregation layer was formed by an interfacial reaction between the TiN grains and the Ni-containing Fe in the TiN liquid-phase sintered ceramic dissolved in the Ag–Cu liquid phase.

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.

Low-temperature bonding of Cu on Si3N4 substrate by using Ti/Cu thin films

A low-temperature bonding method of Si3N4/Si3N4 homostructure and Si3N4/Cu heterostructure was developed in this work. The flawless interfaces were obtained by sputtered Ti/Cu thin films after bonding at 250 °C for 30 min under a uniaxial pressure of 20 MPa. The Ti film is able to improve the adhesion of the Cu film to Si3N4 substrate, and the low-temperature bonding was achieved in view of high diffusion rates of Cu nanocrystalline film with (111) crystal plane. This proof-of-concept study on the low-temperature bonding of Si3N4 ceramic to Cu is expected to address residual stress caused by ordinary active metal brazing (AMB) method, which provides insights for the further heterogeneous integrations of ceramics to metals.

Blackening Mechanism and Mechanical Properties Variation of Zirconia Ceramic Induced by Active Metal Brazing

Blackening of zirconia (ZrO2) ceramics often occurs in active metal brazing, however, its discoloration mechanism and induced variation in mechanical properties remain unclear. Herein, ZrO2 ceramic and Ti are successfully brazed by Ag–Cu–Pd filler in vacuum along with the blackening phenomenon. In the brazed joints, the reaction layers of TiO2 + Ti3(Cu, Pd)O are formed adjacent to ZrO2, indicating that partial oxygen in ZrO2 is taken by active Ti in the liquid filler that dissolved from the Ti substrate. Furthermore, the height of the blackened ZrO2 ceramic near the brazing seam is in line with the amount of Ti content added to the filler, which infers that bulk‐sized black ZrO2 can be efficiently obtained by the interfacial reaction between Ti and ZrO2. Additionally, the black ZrO2 is having abundant Zr3+ defects and more tetragonal phase content than the original ZrO2. The formation of Zr3+ defects results in the blackening of ZrO2, while the phase transformation leads to reduced hardness, improved impact toughness, and enhanced wear resistance of black ZrO2. Thus, this blackening phenomenon of ZrO2 ceramics provides a new way to tune the mechanical properties of ZrO2.

Strength optimization of two-step-bonded Ti-6Al-4V/Si3N4 joint with Nb interlayer via transient-liquid-phase bonding and active-metal brazing

A novel two-step bonding of Ti-6Al-4V/Si3N4 joint was developed with Nb interlayer as residual-stress reliever via low-pressure transient-liquid-phase bonding (TLPB) of Ti-6Al-4V/Nb side prior to active-metal brazing of Nb/Si3N4 side. While 1.75 mass% of Ti in a 50-µm-thick CUSIL-ABA® filler was sufficient for sound bonding at Nb/Si3N4 side when brazed at 1103 K for 10 min, one-step-brazed joints with bonding area of 10 × 10 mm2 were prone to failure at the Ti-6Al-4V/Nb side due to brittle Cu-Ti intermetallic compounds (IMCs). Replacing brazing of Ti-6Al-4V/Nb side with TLPB using pure Cu and Ni foils as filler at 1213 K for 180 min eliminated the formation of brittle IMCs via homogenization of (α + β)-Ti; bending strength increased to 193 MPa with residual-stress-induced failure from Si3N4 ceramics. Finally, effectiveness of stress-accommodation via Nb interlayer and filler’s plastic flow was quantitatively verified with reasonable fidelity by finite-element analysis incorporating temperature-dependent elasto-plastic properties.

Investigation of Partial Discharges in AlN Substrates under Fast Transient Voltages

The objective of this article is to characterize partial discharge (PD) inception voltages and patterns at sinusoidal and square voltages in insulating ceramic substrates with generic manufacturing defects. PD tests were conducted on active metal brazed (AMB) aluminum nitride (AlN) substrates either with cavities in the polymer coating or sharp protrusions of the metal brazing. The substrates were tested in silicone liquid instead of silicone gel. Light sensitive imaging was used to localize the PD sites on the surface. PDs at square voltage with rise time of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.6 ~\mu \text{s}$ </tex-math></inline-formula> were measured with a high-frequency current transformer and a photomultiplier. In both substrate types, the peak–peak value of the PD inception voltage (PDIV) was higher at sinusoidal voltage compared to peak-peak value of the square wave. This difference was more pronounced in uncoated substrates, where discharges incepted at sharp edges surrounded by silicone liquid. For both types of defects, comparable PDIVs were measured at bipolar and unipolar square waves. Results for unipolar pulses are discussed considering the role of charge memory effect.

Interface Reactions Responsible for Run-Out in Active Brazing: Part 3

This study examined the interface reaction between sessile drops of the Ag-xAl filler metals having x = 0.2, 0.5, and 1.0 wt-% and KovarTM base material as an avenue to understand the run-out phenomenon observed in active filler metal braze joints. The brazing conditions were combinations of 965°C (1769°F) and 995°C (1823°F) temperatures and brazing times of 5 and 20 min. All brazing was performed in a vacuum of 10–7 Torr. Microanalysis confirmed that a reaction layer developed ahead of the filler metal to support spontaneous wetting and spreading activity. However, run-out was not observed with the sessile drops because the additional surface energy created by the sessile drop free surface constrained wetting and spreading. The value of z in the reaction layer composition, (Fe, Ni, Co)yAlz, increased with x of the Ag-xAl sessile drops for both brazing conditions. Generally, the values of z were lower for the more severe brazing conditions. Also, the reaction layer thickness increased with the Al concentration in the filler metal but did not increase with the severity of brazing conditions. These behaviors indicate that the interface reaction was controlled by the chemical potential rather than the rate kinetics of a thermally activated process. The determining metrics were filler metal composition (Ag-xAl) and brazing temperature. The findings of the present study provided several insights toward developing potential mitigation strategies to prevent run-out.

Active Brazing of Alumina and Copper with Multicomponent Ag-Cu-Sn-Zr-Ti Filler

The study was designed to investigate the synergic effect of Ti and Sn in the active metal brazing of Al2O3 ceramic to copper brazed, using the multicomponent Ag-Cu-Zr filler alloy. Numerous fine and hexagonal-shaped rod-like ternary intermetallic (Zr, Ti)5Sn3 phase (L/D = 5.1 ± 0.8, measured in microns) were found dispersed in the Ag-Cu matrix of Ag-18Cu-6Sn-3Zr-1Ti alloy, along with the ternary CuZrSn intermetallic phases. An approximate 15° reduction in contact angle and 3.1 °C reduction in melting point are observed upon the incorporation of Ti and Sn in Ag-18Cu-3Zr filler. Interestingly, the interface microstructure of Al2O3/Cu joints brazed by using Ag-18Cu-6Sn-3Zr-1Ti filler shows a double reaction layer: a discontinuous Ti-rich layer consisting of (Cu, Al)3(Ti, Zr)3O, TiO, and in-situ Cu-(Ti, Zr) precipitates on the Al2O3 side and continuous Zr-rich layer consisting of ZrO2 on the filler side. The shear strength achieved in Al2O3/Cu joints brazed with Ag-18Cu-6Sn-3Zr-1Ti filler is 31% higher, compared to the joints brazed with Ag-18Cu-6Sn-3Zr filler. Failure analysis reveals a composite fracture mode indicating a strong interface bonding in Al2O3/Ag-18Cu-6Sn-3Zr-1Ti filler/Cu joints. The findings will be helpful towards the development of high entropy brazing fillers in the future.

Active Brazing of SiC-Base Ceramics to High-Temperature Alloys

Active metal brazing of SiC-base ceramics and composites to Ti, Cu-clad-Mo, and nickel was carried out to screen promising commercial braze compositions with liquidus temperatures of 810-1040 °C. Microstructure, elemental composition, and microhardness were characterized using FESEM, EDS, and Knoop test in joints brazed using Ag-Cu-Ti alloys (Cusil-ABA and Ticusil) and a Ni-base alloy (MBF-20). Ti- and Si-rich interfacial layers developed in joints brazed using Ti-containing brazes, whereas Ni- and Si-rich layers formed in joints brazed using Ni-base brazes. Monolithic SiC/Mo and SiC/Kovar joints with Cusil-ABA had sound interfaces and compressive shear strengths of 25-30 MPa. In the case of composites, surface preparation influenced bond quality: ground SiC-SiC samples led to sound joints and unground samples led to interfacial decohesion at low thermal strains whereas joints cracked regardless of the surface preparation at large thermal strains. Among SiC-SiC/metal joints, SiC-SiC/Cu-clad-Mo had the best microstructure and bond quality. Microstructures of joints made using MBF-20 mixed with low-expansion Si-X (X: Y, Ta, Hf, Ti, B) eutectic powders to control the thermal expansion are also presented.

Interfacial and Cross-sectional Studies of Thermally Cycled Alumina-Monel Brazed Joint

Alumina and monel superalloy was successfully joined by active metal brazing technique using Ticusil (68.8Ag-26.7Cu-4.5Ti in wt%) filler alloy in a vacuum furnace at 910°C for 10 min under a vacuum of 5×10–6 Torr. Phase analysis of the monel-filler alloy and alumina-filler alloy interfaces was conducted by X-ray diffraction analysis. Interfacial and cross-sectional microstructural investigation and determination of elemental composition were performed by scanning electron microscopy and energy dispersive X-ray analysis. No crack was found at the joint interfaces after thermal cycling test for 100 cycles between 50° and 600°C. Microhardness was estimated by Vickers hardness tester at the cross-section of the joint after thermal cycling test. Typical brazing strength of the thermally cycled joint was above 23 MPa. Helium leak tests indicated good hermiticity of the thermally cycled brazed joints.

Active metal brazing of graphite foam-to-titanium joints made with SiC-Coated foam

Medium-density, high-conductivity carbon foams were joined to titanium using a two-step process that first exposed foam to SiO vapor at 1450 °C for 30 min. under vacuum followed by vacuum brazing Ti using Cusil-ABA to the sides of prismatic foam pieces along the ‘with-rise’ (WR) or foaming direction and the ‘against rise’ (AR) or transvers direction. Well-bonded joints with braze-infiltrated foam and Ti-rich interfaces formed along WR and AR. The un-bonded foam was stronger along WR (785 kPa) than along AR (277 kPa) as were the joints made using coated and uncoated foams. Foam thickness minimally affected joint strength along WR but along AR, joints with thick foam were 58 % stronger. The coating marginally (9 %) lowered joint strength along WR but led to a nearly 50 % strength drop along AR. The experimental foam is more robust and amenable to coating and joining along foaming direction than transverse to it.