Active Metal Brazing Research Articles

Recent Advances in Active Metal Brazing of Ceramics and Process

Ceramic to metal joining has its potential applications in microelectronics packaging, metal–ceramic seals, vacuum tubes, sapphire metal windows, etc. But there are many limitations in joining this duo of materials that range from their structures, nature of bonding, physical properties to a complex phenomenon like wetting, spreading and adhesion. The current review discusses these critical issues from the aspects of thermodynamics, the role, and type of active elements, Ag–Cu–Ti brazing filler system and the reliability factors like residual stress, coefficient of thermal expansion, material reliability, pores and unbonded regions on the surface which affect the mechanical reliability of the joint.

“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.

Low-Pressure Silver Sintering of Automobile Power Modules with a Silicon-Carbide Device and an Active-Metal-Brazed Substrate

To improve the efficiency of power modules in environmentally friendly vehicles, silicon-carbide (SiC) chips and silicon-nitride (Si3N4) active metal-brazed (AMB) substrates were bonded by low-pressure silver (Ag) sintering at 220°C and 1 MPa using Ag paste. The initial bond strength of the sintered joint was 35.7 MPa, and the void content and bonding-layer thickness of the sintered joint were 4–7% and 94–99 μm, respectively. To verify the reliability of the silver-sintered joints, we conducted thermal cycling tests (TCTs) and high-temperature storage tests (HTSTs). The bond strength of the SiC chip/Si3N4 AMB substrate after the TCT decreased to 16.9 MPa, but increased to 43.6 MPa after the HTST. Thermomechanical fatigue cracks from the difference in the thermal-expansion coefficient occurred at the sintered joint interface during the TCT, which decreased the sintered joint strength. As the sintering process progressed continuously during the long test time at a high temperature, the densification of the sintered joint increased to 98.4%, which lead to an increase in bonding strength. In this study, a SiC device was sinterbonded to a Si3N4 AMB substrate using silver paste at a low pressure, and optimization at a commercialized level was achieved.

Performance comparison of Al-Si-Ti and Co-Si-V-Ti braze alloys in the vacuum brazing of reaction-bonded silicon carbide

AlSiTi and CoSiVTi braze alloys were used in the active metal brazing of reaction-bonded silicon carbide (RB-SiC). The brazing tests were conducted in vacuum furnace under different isothermal holding temperatures (1200 to 1300 °C). The wetting behavior, interfacial microstructure, elemental diffusion, and morphology of fractured joint surface after mechanical testing were investigated using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Both braze alloys demonstrated good wettability and spreadability on RB-SiC under tested conditions. During brazing, elements from the braze alloys diffused through the base substrate (BS) and reacted with RB-SiC to form silicides, leading to the formation of a diffusion-affected zone (DAZ). Shear strength of joints produced by the above two braze alloys under different brazing conditions were also measured, and the results showed a maximum shear strength of 95 MPa and 43 MPa for braze joints produced by AlSiTi braze alloy and CoSiVTi braze alloy, respectively.

Parametric study of electronics components joining using reactive films for high temperature applications

Abstract In this paper, in order to assemble electronic components onto substrates, a local rapid soldering process using an exothermic reactive foil sandwiched between solder preforms was evaluated. Among others, the main interest of this technique is that it can allow the use of high temperature melting solders, without the need to heat the whole assembly above this melting temperature. The reactive foil is commercially available and is formed from alternatively stacked nanolayers of Ni and Al until it reaches the total film thickness. Once the film is activated by using an external power source, a reaction takes place and releases such an amount of energy that is transferred to the solder preforms. If this amount of energy is high enough, solder preforms melt and insure the adhesion between the materials of the assembly. The process was evaluated using a standard SAC305 and a high temperature Au80Sn20 preforms. The influences of the applied pressure, the reactive film thickness as well as the solder and the attached materials nature and thicknesses were investigated. The initial joint quality was evaluated using scanning acoustic microscopy, scanning electron microscopy, and shear strength measurements. It was shown that the applied pressure during the process has a strong effect on the joint initial quality. The voids ratio between metallized diode dice and an Active Metal Braze (AMB) substrate decreases from 64% to 26% for pressure values between 0.5kPa and 100kPa respectively. Otherwise, under a constant low pressure of 13kPa, reducing the substrate metal thickness on a low thermal conductivity insulator allows the improvement of the initial joint quality and a voids ratio of about 15% was reached when using 35μm of copper on FR4 substrate. The use of aluminum instead of copper as a metal for the ceramic metallized substrate (with the same gold finishing layer) led to a reduction in the void ratio in the joint. The microstructure of the AuSn joint achieved using the reactive films shows very fine phase distribution compared to the one obtained using conventional solder reflow process in the oven. The mechanical properties of the joint were evaluated using shear tests performed on 350μm thick silicon diodes assembled on AMB substrates under a pressure of 100kPa. The reactive films were 60μm thick and were sandwiched between two 25μm thick SAC preforms. The void ratio was about 37% for the tested samples and shear strength values above 9.5MPa were achieved which remains largely higher than MIL-STD-883H requirements. Finally, the process impact on the electrical properties of the assembled diodes was compared with a commonly used solder reflow assembly and results show a negligible variation.

Finite element analysis of the effect of TiC or graphite modified composite fillers on the thermal residual stress of AMB ceramic substrates

Abstract Ag–Cu–Ti–In + X (X = TiC, graphite) composite fillers are developed to improve the comprehensive mechanical performance of active metal brazing (AMB) ceramic substrates (AlN/filler/Cu). This paper reports the effect of TiC or graphite modified composite fillers on the thermal residual stress of AMB ceramic substrates. Models based on force and moment balances and thermomechanical finite element analysis are applied to estimate the distributions of residual stresses. Results indicate that the graphite modified composite filler provides better thermal residual stress reduction on AMB ceramic substrates compared to the TiC modified composite filler. Furthermore, submodels are established to investigate the mechanism of the influence of TiC or graphite particles on the residual stress of AMB ceramic substrates. The stress distribution trends obtained from the submodels are consistent with those obtained from the corresponding global simulations, that is, stress is concentrated on the ceramic side. This further proves the rationality of adopting the submodels. It is indicated that softer graphite particles can be appropriately deformed like a pore structure to decrease stress. For TiC particles, higher hardness and stronger load transfer ability can alleviate the deformation of a metal layer, and the compressive stress concentration transfers to the particles.

Microstructures and properties of Ag–Cu–Ti–In composite fillers for electronic packaging applications

With excellent thermal cycling performance, active metal brazing (AMB) ceramic substrates have been highly competitive materials for high power packaging. In related researches, composite filler is an effective way to improve the comprehensive performance of ceramic substrates. The effects of particle hardness on microstructure and properties (i.e., coefficient of thermal expansion (CTE), elastic modulus and compression properties) of Ag–Cu–Ti–In composite fillers were studied in this paper. The results showed that the microstructures of composite fillers were refined with the increase of graphite and TiC content. Meanwhile, the reduction of CTE of composite fillers added with TiC particles is better than that of graphite particles. The CTE of G10 sample is 7.4% lower than that of the sample without particle, and T10 sample is 8.9% lower. Furthermore, TiC particles increased both elastic modulus and yield strength of the composite fillers while graphite particles mainly had an opposite effect. The bulk modulus of the composite fillers drops sharply from 95.96 to 12.64 GPa when the content of graphite particles was 10 wt% compared with that of the filler metal without particles. The yield strength of composite filler with graphite content of 1 wt% reaches 380 MPa, which is caused by the in situ reaction between graphite and Ti. However, the elastic modulus and the yield strength of the composite fillers increases with the increase of TiC content.

Thermal Optimization and Characterization of SiC-Based High Power Electronics Packages With Advanced Thermal Design

A single-phase high power electronics package is designed and developed in this paper. The developed power package achieves a significant thermal performance improvement compared with the conventional wire-bonded power package. The improvement is attributed to two features of the package design. First, the SiC chips are embedded into the active metal brazed (AMB) substrates with specially designed cavities, as such the heat transfer path from the embedded SiC chips to the liquid-cooled heat sink attached to the bottom side of the AMB substrate is shortened. Hence, the thermal performance of the package is improved. Moreover, customized copper clips are introduced as the electrical interconnections between the SiC chips and the top metal layer of the substrate at the same level, and the top surface of the power package remain flat. As such another heat sink can be added to the top side of the package to further improve the thermal performance of the power package through the double-side cooling (DSC) scheme. The simulation results show that the junction-to-case thermal resistance (Theta JC) of the optimized power package is about 50% less than the Theta JC of the conventional wire-bonded power package with the same package size and the same power rate. Further applying the DSC scheme to the proposed power package, which is not suitable to the conventional wire-bonded power package, the Theta JC of the proposed power package reduces another 20%. In addition, the effects of the core layer (i.e., material and thickness) and the metal layer (i.e., materials and thicknesses) of the AMB substrate, as well as the die attach (i.e., material and thickness) on the Theta JC of the proposed power package are investigated systematically. As such the thermal performance of the power inverter package is further elaborated. Finally, the thermally enhanced power package is fabricated and assembled. Thermal characterization has been conducted, and the thermal performance of the developed power package has been evaluated. The simulation results and characterization results match well with each other.

Pressureless Silver Sintering Property of SiC Device and ZTA AMB Substrate for Power Module

We have developed pressureless silver sintering module using zirconia toughened alumina (ZTA) substrate made of active metal brazing (AMB) and SiC device for power module. The silver paste was printed with a metal mask of 80, 120, and 150 μm thickness, respectively. The void content was measured by x-ray images after pressureless silver sintering under a vacuum and nitrogen atmosphere. In order to evaluate the durability against thermal stress of the sintered module, the thermal cycle test (TC) and the high temperature storage test (HTS) were conducted. The void content and bond strength before and after the environment test were measured. As a result, the void content did not change, but the bonding strength was decreased after TC and increased after HTS. Crack was generated at between silver sintered joint and ZTA substrate after TC test. Also, we observed that Cu-Ag alloy layer was formed between the copper AMB and silver sintered layer after HTS, which finally leaded to improve the bonding strength. Key words: Power module, Silver paste, Pressureless, Silver sintering, Bonding strength, Interface

On crack propagation in homogeneous and composite materials under mixed mode loading conditions

The direction of crack propagation under mixed mode loading conditions is investigated by FEM simulation in comparison with delamination experiments performed in the 4 point bending mode. Composites consisting of aluminum nitride attached to copper by active metal brazing were delaminated with use of a central notch for crack initiation. The main crack line propagated along the interface direction. However, SEM micrographs revealed that the crack line propagating along the interface is subjected to bifurcation. Small cracks digress from the main delamination line under kinking angles of 90° until they are stopped at the copper substrate. The observed behavior of crack propagation is studied by Linear Elastic Fracture Mechanics and by nonlinear Finite Element Analysis. In fact, the bifurcation angle of 90° can be considered as an indication for dominant influence of geometrical nonlinearities at the crack tip. Finite Element Analysis is performed in two stages starting from macroscopic level and then the analysis of crack tip behavior is continued on a microscopic level. On the macroscopic level, the whole experimental setup is simulated in order to derive an approximation of the stress distribution around the crack tip. Thereafter, the analysis is further extended on microscopic level, whereby local mesh refinement is continued until the element size approaches the dimension of the lattice constant. Thus, high stress values at the crack tip lead to considerable amounts of material rotation. Consequently, the observed direction of crack extension may be derived from the maximum hoop stress criterion.

Power electronics thermal solutions using thermally conductive polyimide films

Increased adoption of hybrid and electrical vehicles as well as renewable energy systems are driving the innovation in power module packaging. Thermal substrate, one of the major components of power modules, is not an exception, and technological advancements are necessary to meet increased reliability requirements. DuPont has developed a thermally conductive polymer film that provides very low thermal resistance and very high insulation. The film can be bonded to conductive and thick metallic layers and this polymer equivalent of DBC shows very high reliability in addition to high performance characteristics. Electrically insulating layers within a power electronics module are critical for separating circuitry from thermal management layers. Electrical insulating substrates typically used in power electronics modules utilize a ceramic layer, comprised most commonly of either Al2O3, AlN, or Si3N4. Thin Cu layers are bonded to either side of the substrate using a direct bond Cu (DBC) or active metal brazing (AMB) process. These processes involve bonding metallization layers to both sides of the ceramic at a high temperature as bonding to only one side would cause deformation during the cooling phase. Typical metal thickness bonded to either side of the ceramic is about 0.3–0.6 mm as the high temperature manufacturing process does not allow very thick metals to be bonded and this limits the heat spreading capability of the thermal substrate. DuPont's new Temprion™ Organic Direct Bond Copper (ODBC) address aforementioned problems, increasing thermal durability and reliability as well as enabling system layer suppression. Temprion™ ODBC's dielectric layer will absorb thermo-mechanical stress from the metals due to CTE mismatch, dramatically improving durability of the system. In addition, various kinds of metals including Cu and Al can be easily bonded to Temprion™ DB films through simple process. There are no thickness limitations on bonding metal sheets and metal attached at the bottom can be used as an integrated heat sink/baseplate. Al2O3 and Si3N4-based substrates were utilized as a baseline for reliability comparison with the DuPont substrates. The industry-standard substrates in used in this study have a thickness of 0.3 and 0.8 mm for the Cu metallization layers and 0.38 and 0.32 mm for the insulating layer respectively for Al2O3 an Si3N4 insulators. DuPont ODBC substrates were fabricated by attaching a polyimide layer to a layer of 0.8-mm-thick Cu. The polyimide and bottom Cu layer cross-sectional footprints are both 50.8 mm × 50.8 mm. The corners of both layers were filleted with various radii (0.5, 1.0, 2.0, and reversed 2.0 mm) to explore the impact of different stress concentrations between the metallization and insulating layers. The top Cu metallization was inset 2.0 mm from the perimeter of the electrically-insulating substrate and bottom Cu metallization.10 samples each of the DuPont ODBC and industry Al2O3 substrates were placed in a thermal shock chamber and cycled between temperature extremes of −40°C and 200°C. Substrates were inspected every 1000 cycles. After 5000 cycles, the ODBC substrates experienced no hipot failures, but preliminary edge delamination was visually observed. Al2O3 substrates all failed after 50 thermal cycles.Five DuPont ODBC samples were placed in a thermal chamber and subjected to an elevated temperature of 175°C. After 2000 hours, no hipot failures were observed, but edge delamination was again observed.Five DuPont ODBC samples were attached to a cold plate with Kapton tape. Heater cartridges were attached to the top of the substrates with Kapton tape and thermocouples were placed in several locations through the package. The heater cartridges were alternated between on and off states to allow for the substrates to cycle between −40°C and +200°C. While the change between the maximum and minimum temperatures is smaller for the power cycling test compared to the thermal cycling test, the heater cartridge and cold plate create a thermal gradient within the samples that is not possible with passive thermal cycling. After 2000 hrs cycles of testing, no hipot failures or edge delamination have been observed. Herein we show that the DuPont ODBC substrate design is a promising alternative to traditional industry substrates based on ceramic insulators. The reliability of the substrate design has been demonstrated under several thermomechanical accelerated tests and the electrical and thermal performance has been measured. Future work will include reliability comparisons to other industry substrates, including thermal shock testing of substrates with HPS, AlN, and Si3N4 ceramic layers. Thermal models will correlate thermal resistance values measured by the transient thermal tester and compare the ODBC substrate performance to industry substrates within a commercialized power electronics module. The modeling will also optimize the thickness of the metallization layers within the ODBC substrates to minimize the junction temperature of the switching devices.

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.

High reliable and high bonding strength of silver sintered joints on copper surfaces by pressure sintering under air atmosphere

AbstractSilver sintering is a promising die attach technology for high temperature power electronics packaging. Our previous studies have revealed that highly reliable sintered joints was obtained on silver and gold surfaces by either non-pressure or pressure sintering. In this paper, we extended our study to die attachment on copper surfaces by pressure sintering under air atmosphere. We attached Ag metallized die on silicon nitride active metal braze copper substrates with Ag metallization and without metallization by silver sintering at 230°C with a pressure of 10 MPa for 3 min. We observed that the average initial die shear strength for bare Cu substrate is lower than for Ag metallized substrate. This observation is attributed to the self-diffusion of Ag is faster than the interdiffusion between Ag and Cu. However, the average die shear strength for all samples increased considerably after temperature cycling test (−40°C/+150°C) and high temperature storage at 250°C. It is highly likely that sintering process is not yet completed under the sintering conditions used in this study and consequently Ag and Cu continued to diffuse during thermal cycling and high temperature storage and as a result strengthen the sintered joints. It is believed that after a certain time of storage at 250°C the sintering process is completed as we observed the average die shear strength remained relatively constant after 250 h storage. Voids, drying channels and delamination in the sintered joints were not detected by scanning acoustic microscopy for the samples before and after 2000 thermal cycles.

Joining Using Reactive Films for Electronic Applications: Impact of Applied Pressure and Assembled Materials Properties on the Joint Initial Quality

The use of local and rapid heating of electronic assemblies can significantly reduce the degradation of temperature-sensitive materials and substrate bowing commonly encountered in electronic applications during the high temperature reflow process. It can also allow assembling electronic packages on a non-planar surface and/or on massive structures that are very complex using a conventional oven for soldering. In order to attach electronic components to substrates, a rapid soldering process using an exothermic reactive foil sandwiched between solder preforms was evaluated. Once the film was activated and reacted, the solder preforms were melted to ensure the adhesion between the assembled materials. The effect of applied pressure on the joint quality, the reactive film thickness, as well as the attached material thickness and physical properties were assessed. Using a 60 μm thick reactive foil with two 25 μm thick SnAgCu305 preforms, results show that the fraction of void-free interfacial area between a metallized diode and an active metal braze substrate increased from 34% to 74% with pressure values between 0.5 kPa and 100 kPa, respectively. At a constant pressure of 13 kPa, increasing the reactive foil thickness from 40 μm to 60 μm leads to an increase in the void-free interfacial attach area ratio from 20% to 40%, and a value of 54% was achieved by using two 60 μm foils under the same conditions. The substrate metallization and solder thickness also affect the joint quality. The experimental results are analyzed and correlated with the duration of liquid solder using thermal models.

Effect of Surface Roughening of Temperature-Cycled Ceramic-Metal-Bonded Substrates on Thermomechanical Reliability of Sintered-Silver Joints

We studied the reliability of power chip die-attach by sintered-silver on two types of ceramic-metal-bonded substrates: 1) aluminum-nitride direct-bond-aluminum (DBA) and 2) active-metal-brazed (AMB) silicon nitride-copper. The samples underwent a temperature-cycling test from −55 °C to 250 °C. Consistent with what we reported earlier, the surface roughness on both substrates grew with number of temperature cycles. The roughening rate on the DBA substrate was two times faster than that on the AMB substrate. To evaluate the effect of surface roughening on the sintered-silver joint reliability, we measured the transient thermal impedance of the bonded power chips. Despite the higher roughening rate, the sintered-silver bond on the DBA substrate had a longer lifetime—defined by a 20% increase in the transient thermal impedance of the bonded device—than that on the AMB substrate. Cross-sectional scanning electron microscopy showed the formation of vertical cracks in the sintered bond-line on the DBA substrate, as opposed to horizontal cracks on the AMB substrate. Formation of the vertical cracks in the bond-line relieved stresses without significantly impacting the thermal performance.

Highly Reliable Pressure-Less Silver Sintering Joints

Abstract Pressure-less silver sintering pastes with high reliability tested by temperature cycling test (TCT) under −40°C to 175°C were developed. The silver joints formed between Si die and Si3N4 active metal brazing (AMB) substrate can survive at least 1500 cycles without decrease in shear strength. In contrast, the strength of reference solders decreases to 62% to 86% of their initial values. The optimized silver joints can achieve average total void% of <1.3% and maximum single void% of <0.3%. Die tilt angle can be controlled within < 0.3°. Failure mode analysis of the joints reveals that during TCT in the bulk phase, silver sintering continues resulting in the formation of larger grain and pore size; while at the interface, the adhesion strength at the silver-substrate interface was gradually enhanced as compared to that at the silver-Si interface. Temperature cycling makes the adhesion strength at both interfaces more balanced accounting for the constant shear strength. SEM cross-section shows that for the as-sintered sample the porosity within the joint is not evenly distributed; the center area normally has a low porosity of < 4%, while at the edge there is a higher porosity of 18% to 24%. The average total porosity of the joint is ~ 13%. This value does not change after 800 cycles of TCT, but increases to 19% at 1500 cycles. The effect of TCT on the joints under conditions such as different surface finished substrate, varied bond line thickness and Si die size was also studied.

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.

Thermal-cycling-induced surface roughening and structural change of a metal layer bonded to silicon nitride by active metal brazing

We applied thermal cycles with a range of −40 to 250 °C to metalized silicon nitride substrates, to both sides of which copper layers had been bonded using the active metal brazing method, and systematically evaluated the effects of the thermal cycles on the roughening of the surface of the metal layer, and on the changes in the structure of the copper layer. The roughening of the metal surface increased with the number of thermal cycles. The period of the long-periodic component of the roughened surface (surface waviness) was 100–300 μm. This value is almost identical to the diameter of the copper grains. The results of electron back-scatter diffraction pattern analysis revealed strain spanning the entire copper crystal, with severe crystal strain appearing near the high-angle grain boundaries in the copper layers subjected to the above thermal cycles, due to repeated cyclic thermal stress arising from a difference in the coefficients of thermal expansion between the metallic and ceramic layers. In a sample subjected to 1000 thermal cycles, the strain within a grain or in the vicinity of a grain boundary becomes more severe as the distance from the ceramic join decreases, while the strain tends to be smaller at the surface of the metal layer. This appears to be due to the copper grains near the surface being subjected to stress, giving rise to out-of-plane displacement, such that the strains tend to be released.

Active metal brazing of SiO2–BN ceramic and Ti plate with Ag–Cu–Ti + BN composite filler

SiO2–BN ceramic and Ti plate were joined by active brazing in vacuum with Ag–Cu–Ti+BN composite filler. The effect of BN content, brazing temperature and time on the microstructure and mechanical properties of the brazed joints was investigated. The results showed that a continuous TiN–TiB2 reaction layer formed adjacent to the SiO2–BN ceramic, whose thickness played a key role in the bonding properties. Four Ti–Cu compound layers, Ti2Cu, Ti3Cu4, TiCu2 and TiCu4, were observed to border Ti substrate due to the strong affinity of Ti and Cu compared with Ag. The central part of the joint was composed of Ag matrix, over which some fine-grains distributed. The added BN particles reacted with Ti in the liquid filler to form fine TiB whiskers and TiN particles with low coefficients of thermal expansion (CTE), leading to the reduction of detrimental residual stress in the joint, and thus improving the joint strength. The maximum shear strength of 31MPa was obtained when 3wt% BN was added in the composite filler, which was 158% higher than that brazed with single Ag–Cu–Ti filler metal. The morphology and thickness of the reaction layer adjacent to the parent materials changed correspondingly with the increase of BN content, brazing temperature and holding time. Based on the correlation between the microstructural evolution and brazing parameters, the bonding mechanism of SiO2–BN and Ti was discussed.

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.