Direct Bonded Copper Substrates Research Articles

Flexible PCB-Based 3-D Integrated SiC Half-Bridge Power Module With Three-Sided Cooling Using Ultralow Inductive Hybrid Packaging Structure

Silicon carbide (SiC) devices are capable of high switching speeds and also enable high switching frequency in power electronic converters. However, this feature poses substantial challenges to packaging, especially limiting the loop inductance. The traditional wire-bonding packaged power module has large parasitic inductance, which will cause voltage overshoot, oscillation, parasitic turn-on, and EMI issues. In order to reduce the parasitic inductance, this paper proposes a flexible printed circuit board (FPCB) based full SiC half-bridge power module with a novel low inductive hybrid packaging structure and three-dimensional (3-D) integration method. This hybrid packaging structure has an ultrathin FPCB substrate stacked on a direct bonding copper (DBC) substrate, which forms a multilayer 3-D power loop. The SiC chips are soldered on the DBC substrate for good thermal dissipation through a cavity in the FPCB substrate. After power loop optimization, the power loop inductance of a 1200-V/120-A SiC power module is only 0.79 nH. The power module consists of three submodules, which are connected by the bendable FPCB substrate. The bendable power module enables maximum utilization of 3-D space. The gate drive, decoupling capacitors, and dc-link capacitors are also integrated and 3D-structured using rigid-flexible PCBs. Moreover, the cooling system is a high-efficiency three-sided cooling structure for the bendable power module. The simulation results show that the three-sided cooling structure reduces the heatsink volume by 50%. Applying this method, the converter can be designed as a system-in-package and a 3D-structured compact system. The power density of a 20-kW three-phase inverter will reach 19.3 kW/L based on this power module. In this paper, the 1200-V/120-A power module fabrication and assembly processes are given. Finally, the static and dynamic experimental comparisons are done for a commercial power module and the proposed power module. The experiment results show that the voltage overshoot of the proposed module reduces about 5.8 times and are consistent with the simulation results. Meanwhile, the proposed power module switching speed is 1.8 times faster than the commercial module under zero external gate resistors and the switching loss can reduce by about 60%.

Thermal Management of High-Power Switching Transistors Using Thick CVD-Grown Graphene Nanomaterial

Graphene is a promising material for thermal management. Heating in power electronics is a severe issue since heat generation can lead to rise in device temperature which can cause performance degradation and device failure. However, current approaches are not so efficient because of the extremely small thermal mass of monolayer or few layers graphene. Here we show a new type of chemical vapor deposition (CVD)–grown thick graphene network with in-plane thermal conductivity of 1100 W/mK on ceramic substrates for thermal management of power switching devices. The demonstration of graphene network's thermal management capabilities was carried out on a 10-Ω resistor module and a 1.2-kV SiC mosfet package module. The results show that the thermal management performance using graphene coated on a low cost Al2O3 direct-bonded copper (DBC) is significantly superior to the industry standard, costlier AlN DBC substrates. Specifically, the temperature of hot spots is lower by at least 25 °C when using CVD-grown graphene networks for thermal management. Numerical simulations suggest that the high in-plane thermal conductivity of thick graphene networks helps in quick transfer of heat away from the device. These demonstrations represent a transformative change on thermal management of WBG high-power switching devices.

Pressureless sintering multi-scale Ag paste by a commercial vacuum reflowing furnace for massive production of power modules

Air is critical to Ag sintering. The Cu surface of the non-metallized direct-bonding-copper (DBC) substrate can be oxidized seriously for bonding power devices using Ag paste in air. In this paper, we presented a pressureless sintering approach for a multi-scale Ag paste in the formic acid vapor by a commercial vacuum reflowing furnace. The study found that the activation temperature of the formic acid vapor had an obvious effect on the sintering behavior of the multi-scale Ag paste. The multi-scale Ag paste could obtain much denser sintered Ag when the formic acid vapor was rapidly injected at 180 °C by the vacuum reflowing furnace. The sintered Ag necks larger than 0.4 μm and the porosity lower than 11.3% were achieved in formic acid vapor, which had better sintering behavior than Ag nanoparticles (NPs) paste. The thermal resistance of 1200 V/50 A half-bridge IGBT modules by sintering the multi-scale Ag paste in formic acid vapor was 0.41 °C/W, which was ~ 12% lower than those of the commercial IGBT modules using Pb92.5Sn5Ag2.5. This work could overcome the contradiction between the Ag sintering and the Cu oxidation of the non-metallized DBC substrate by the formic acid vapor in a commercial vacuum reflowing furnace. The method is extremely useful for the traditional manufacturer, who do not have to invest any new facilities for the sintering.

Thermal Cycling of Sintered Silver (Ag) Joint as Die-Attach Material

Sintered silver is a promising die-attach material capable of operating at temperatures of more than 200°C. However, while sintering reliably bonds Ag paste on Ag-plated substrate, reliability studies are needed to understand the behavior of this sintered bond on copper and direct bond copper (DBC) substrates. Here, we thermally cycled micron-Ag and nano-Ag joints created via sintering on Cu, DBC, and Ag-plated substrates between − 65°C and 150°C in order to understand their evolving microstructures and reliability. Short periods at high temperature did not oxidize the substrate, but the absence of copper oxides did not prevent adhesive failure of the nano-Ag joint at the Cu interface. We found that Ag filler size influences pore shapes, pore sizes, and shear strength; the micron-Ag joint produced a mixture of irregular and regular spherical pore shapes that reduced bond strength more than the predominantly spherical pores present in the nano-Ag joint.

Reliability Evaluation of SiC Power Module With Sintered Ag Die Attach and Stress-Relaxation Structure

Silicon carbide (SiC) power modules with Ag sinter-bonding die attach were designed on the basis of thermal stress analysis for reliable high-temperature operations. Both the finite-element analysis (FEA) simulations and preliminary experiments confirmed that inserting the direct-bonded-copper (DBC) substrates can effectively reduce the maximum thermal stress in the module. A prototype SiC power module using sintered Ag die attach with a DBC substrate was designed and fabricated. The modules exhibited excellent durability in power cycling between 65 °C and 250 °C up to 20 000 cycles. FEA calculations of cumulative thermal strain and stress distributions adequately predicted the initial cracking position in the specimens after prolonged power cycles, observed by scanning electron microscopy.

SiC chip attachment sintered by nanosilver paste and their shear strength evaluation

In this study, a nanosilver paste was prepared by silver nanoparticles and organics, by which an SiC ship could be sintered on the direct bonding copper (DBC) substrate at 250–300 °C. The nanosilver paste firmly sintered the SiC chip and DBC substrate, and die attachment had an excellent bonding interface. Die-shear results showed that the shear strength increased with the increase of sintering temperature and was more than the value required by the MIL-STD-883J standard. When high assisted pressure was applied, the sintering temperature could be lowered and sintering time also could be shortened to realize attaching high bonding strength. The assisted pressure also influenced the porosity of the bondline. X-ray non-destructive method was used to measure the porosity of die attachment. The results showed that the shear strength of die attachment decreased with the increase of bondline porosity.

Microstructural evolution of laser-brazed joint of Mg2Si and HMS on DBC substrate for thermoelectric generator

Abstract Joints properties of high temperature thermoelectric (TE) modules, which are composed of n-type Mg2Si and p-type higher manganese silicide (HMS), were investigated using the laser brazing process. In this process, the direct-bonded copper (DBC) and the TE materials were bonded using the Ag Cu based brazing filler metal. Heating and cooling rates were controlled by changing laser power and irradiation time gradually for preventing from thermal shock damage. The properties of joints were investigated by die shear test and microstructural analysis. As a result, improved joints of TE modules were fabricated through the laser brazing. Suitable heat input condition was deducted to optimize inter-diffusion between the filler metal and TE materials for obtaining reliable bond strength. In order to evaluate the high temperature stability, heat treatments were performed with TE modules using the furnace. As the heat treatment time increased, voids was formed at the joint but the diffusion distance of Cu and Ag did not increase.

Effect of Sintering Conditions on the Mechanical Strength of Cu-Sintered Joints for High-Power Applications.

In this study, the feasibility of low-cost Cu-sintering technology for power electronics packaging and the effect of sintering conditions on the bonding strength of the Cu-sintered joint have been evaluated. A Cu paste with nano-sized Cu powders and a metal content of ~78% as a high-temperature bonding material was fabricated. The sinter-bonding reactions and mechanical strengths of Cu-sintered joints were evaluated at different sinter bonding pressures, temperatures, and durations during the sintering process. The shear strength of the Cu-sintered joints increased with increasing sintering pressure. Good interfacial uniformity and stable metallurgical microstructures were observed in the Cu joints sintered at a high sintering pressure of 10 MPa, irrespective of the sintering time. It was confirmed that a high-pressure-assisted sintering process could create relatively dense sintered layers and good interfacial uniformity in the Cu-sintered joints, regardless of the sintering temperatures being in the range of 225–300 °C. The influence of the sinter bonding pressure on the shear strengths of the Cu-sintered joints was more significant compared to that of the sintering temperature. Durations of 10 min (at 300 °C) and 60 min (at 225 and 250 °C) are sufficient for complete sintering reactions between the Si chip and the direct bond copper (DBC) substrate. Relatively good metallic bonding and dense sintered microstructures created by a high sintering pressure of 10 MPa resulted in high shear strength in excess of 40 MPa of the Cu-sintered joints.

Design and Performance Evaluation of Al2O3-SiC Composite for Direct-Bonded Copper Substrate

A computational material design approach is applied to propose a novel ceramic material for direct-bonded copper (DBC) substrate with enhanced thermal and structural performance. The material design inherently consists of many competing requirements that require careful decisions regarding key trade-offs in terms of material composition, inclusion size, shape, and distribution to achieve the target properties. The alumina-silicon (Al2O3-SiC) composite, as compared to commercial alumina, used in DBC is found to be the most suitable design among other candidates with improved thermal and structural properties. In order to study the performance characteristics and the effects of the new ceramic composite with improved properties in terms of structural behavior and fatigue life of the DBC substrate, the normal working and extreme thermal cycling conditions were simulated and analyzed using finite element method. The temperature, strain, and localized stress distribution within the substrate at a steady-state condition were analyzed, and the improved Coffin–Manson law was used to calculate the fatigue life of the substrate under extreme thermal cycling conditions. The proposed Al2O3-SiC composite is found to be more robust than the commercial alumina as DBC substrates considering the thermal–mechanical performance. The fatigue life cycle of the DBC substrate with the proposed material is predicted to be about two times longer than the commercial alumina DBC ceramic under transient thermal cycling test.

Microstructural and mechanical evolution of silver sintering die attach for SiC power devices during high temperature applications

Silver sintering is a promising die attach technology to ensure high thermal reliability. The long-term reliability of SiC device sintered by nano-Ag paste has been evaluated by the high temperature storage (HTS) process at 350 °C in air and vacuum, respectively. Although the SiC chip and direct bonding copper (DBC) substrate could be bonded firmly by the nano-Ag paste after sintering at low temperature, the microstructure and shear strength of sintered die attachment experienced the huge evolution during HTS process. The bondline of die attachment became compact, and some pores grow up and pore distribution became nonuniform after HTS in air. While the densification of bondline was significantly delayed because residual organics inhibited the growth and migration of pores during HTS in vacuum. The shear strength of die attachment first increased then decreased slowly with the increasing of storage time. The fracture surface showed that the Ni(P) layer was oxidized, and the formed NiO layer provided the failure location. The results indicated that the electroless nickel/immersion gold (ENIG) surface of DBC substrate was not the ideal metallization when the sintered die attachment applied at the long-term high temperature.

A Multichip Phase-Leg IGBT Module Using Nanosilver Paste by Pressureless Sintering in Formic Acid Atmosphere

This paper proposed a new approach to attach power devices on nonmetallized direct bonding copper (DBC) substrates by pressureless sintering of nanosilver paste in poor-oxygen sintering atmosphere and formic acid reduction atmosphere. The average shear strength of die-attach joints could reach 25 MPa, and the copper oxidation issue of the nonmetallized DBC substrates was avoided. Based on the pressureless sintering approach of nanosilver paste, insulated-gate bipolar transistor (IGBT) modules were prototyped to verify the feasibility of this approach for mass production of power modules. The electrical and thermal characteristics of the IGBT modules bonded with sintered nanosilver were then compared with those of the commercialized ones using Pb92.5Sn5Ag2.5 solder alloy. This approach could extend the applications to bonding power devices on nonmetallized DBC substrates by nanosilver paste, and guide fabricating power modules in a mass productive, low facility costs, and high-yield way.

Comparative studies on microstructures, strengths and reliabilities of two types of AlN direct bonding copper substrates

Oxidation of AlN ceramic plates was performed via two different techniques of Cu2+ activation followed by oxidation and direct oxidation. Two types of AlN direct bonding copper (DBC) substrates were then fabricated by combining the pre-oxidized Cu foils with the AlN plates. The interface structures, the peel strengths, and the temperature cycling reliabilities of the samples are investigated. The results showed that after Cu2+ activation followed by oxidation, rod-like CuAl2O4 particles were distributed on the dense α-Al2O3 layer oxide layer of the AlN plates. This microstructure is beneficial in improving the interface bonding strength and the wettability of the AlN-DBC substrates. Interface layers of both the AlN-DBC substrates were composed of two sublayers of Cu2O+CuAlO2 and residual Al2O3. During the peeling process, fractures occurred in the AlN plates for the Cu2+ activated and oxidized AlN-DBC substrates, but at the interface layer for the directly oxidized AlN-DBC substrates. The peel strength and the temperature cycling performance of the Cu2+ activated and oxidized AlN-DBC substrates were higher than the directly oxidized AlN-DBC substrates.

How to determine surface roughness of copper substrate for robust pressureless sintered silver in air

It is significant to attach power devices on low-cost copper (Cu) substrates by pressureless silver-sintering in air. The Cu surface in direct-bond-copper (DBC) substrates could have a wide range of surface roughness due to the various processing techniques. It is crucial to find out how the surface roughness influences the bonding strength of the sintered-silver (Ag) die-attachment. In this study, we did die-attach on Cu substrates with surface roughness Ra = 0.15–8.41 µm and Rz = 1.22–50.97 µm by pressureless sintering of nanosilver paste in air. When the surface roughness (Ra of 0.65 μm) is slightly larger than the silver nanoparticles (diameter of 0.6 μm), the high shear strength in excess of 50 MPa was achieved, likely due to both strong metallic bonds and mechanical interlock. This work could help to prepare DBC substrates with optimized surface roughness to accommodate the viscosity, the particle size, and the sintering shrinkage of the silver paste for well using the sintered-Ag die-attach technology.

Prediction and Mitigation of Vertical Cracking in High-Temperature Transient Liquid Phase Sintered Joints by Thermomechanical Simulation

Transient liquid phase sintering (TLPS) is a novel high-temperature attach technology. It is of particular interest for application as die attach in power electronic systems because of its high-melting temperature and high thermal conductivity. TLPS joints formed from sinter pastes consist of metallic particles embedded in matrices of intermetallic compounds (IMCs). Compared to conventional solder attach, TLPS joints consist to a considerably higher percentage of brittle IMCs. This raises the concern that TLPS joints are susceptible to brittle failure. In this paper, we describe and analyze the cooling-induced formation of vertical cracks as a newly detected failure mechanism unique to TLPS joints. In a power module structure with a TLPS joint as interconnect between a power device and a direct bond copper (DBC) substrate, cracks can form between the interface of the DBC and the TLPS joint when large voids are located in the proximity of the DBC. These cracks do not appear in regions with smaller voids. A method has been developed for the three-dimensional (3D) modeling of paste-based TLPS sinter joints, which possess complex microstructures with heterogeneous distributions of metal particles and voids in IMC matrices. Thermomechanical simulations of the postsintering cooling process have been performed and the influence of microstructure on the stress-response within the joint and at the joint interfaces have been characterized for three different material systems (Cu + Cu6Sn5, Cu + Cu3Sn, Ni + Ni3Sn4). The maximum principal stress within the assembly was found to be a poor indicator for prediction of vertical crack formation. In contrast, stress levels at the interface between the TLPS joint and the power substrate metallization are good indicators for this failure mechanism. Small voids lead to higher joint maximum principal stresses, but large voids induce higher interfacial stresses, which explain why the vertical cracking failure was only observed in joints with large voids.

Miniature Piezoelectric Sensor for In-Situ Temperature Monitoring of Silicon and Silicon Carbide Power Modules Operating at High Temperature

Owing to the introduction of wide-bandgap power devices, recent power modules operate at higher temperatures. This increased module temperature leads to significant reliability problems, which render thermal monitoring very important. Several thermal sensors have been used in power modules; however, each of these sensors has drawbacks such as limited maximum temperature, nonlinearity, power source requirement, or limited miniaturization. This paper proposes a new temperature sensor that utilizes the piezoelectric effect. A new piezoelectric material (PZT-PZNM) with a high Curie temperature (260 °C) is soldered onto a direct bonded copper substrate. When the power module is operating at a high temperature, the substrate deflects and induces thermal stress, which is converted into an electric voltage via a piezoelectric mechanism. The voltage can be maximized by optimizing the location of the PZT-PZNM; the best location was determined to be away from the power devices. This observation confirms that the piezoelectric temperature sensor has a negligible influence on the performance of the power devices. Moreover, the sensor volume was miniaturized (approximately 74% reduction compared with preliminary work) to ensure that it can be readily integrated into power modules. The developed piezoelectric sensor demonstrates excellent linearity ( R 2 = 0.99, approximately) up to 250 °C, which is sufficient for monitoring temperature in both silicon and silicon carbide power modules.

Formation of Cu Nanodots on Diamond Surface to Improve Heat Transfer in Cu/D Composites

Diamond‐dispersed copper matrix (Cu/D) composite materials with different interfacial configurations are fabricated through powder metallurgy and their thermal performances are evaluated. An innovative solution to chemically bond copper (Cu) to diamond (D) has been investigated and compared to the traditional Cu/D bonding process involving carbide‐forming additives such as boron (B) or chromium (Cr). The proposed solution consists of coating diamond reinforcements with Cu particles through a gas–solid nucleation and growth process. The Cu particle‐coating acts as a chemical bonding agent at the Cu–D interface during hot pressing, leading to cohesive and thermally conductive Cu/D composites with no carbide‐forming additives. Investigation of the microstructure of the Cu/D materials through scanning electron microscopy, transmission electron microscopy, and atomic force microscopy analyses is coupled with thermal performance evaluations through thermal diffusivity, dilatometry, and thermal cycling. Cu/D composites fabricated with 40 vol% of Cu‐coated diamonds exhibit a thermal conductivity of 475 W m−1 K−1 and a thermal expansion coefficient of 12 × 10−6 °C−1. These promising thermal performances are superior to that of B‐carbide‐bonded Cu/D composites and similar to that of Cr‐carbide‐bonded Cu/D composites fabricated in this study. Moreover, the Cu/D composites fabricated with Cu‐coated diamonds exhibit higher thermal cycling resistance than carbide‐bonded materials, which are affected by the brittleness of the carbide interphase upon repeated heating and cooling cycles. The as‐developed materials can be applicable as heat spreaders for thermal management of power electronic packages. The copper‐carbon chemical bonding solution proposed in this article may also be found interesting to other areas of electronic packaging, such as brazing solders, direct bonded copper substrates, and polymer coatings.

Reliability of Die Attach on DBC Substrates With Different Ni Surface Finishes Using BiAgX Solder

Ni plating is a commonly used barrier layer for direct bond copper substrates used in high-temperature and high-power applications. The Ni plating method can be electrolytic or electroless and the composition options include pure Ni, Ni:P, Ni–Co, or Ni:B. BiAgX solder paste is a promising lead-free solder for applications below 200 °C. The reactivity of these different Ni layers with the BiAgX solder paste has been compared. Specially the reaction to form NiBi3 during 200 °C storage and −55 °C to + 195 °C thermal cycling test. The initial shear strength of all these Ni plating is ~4.8 kg/mm2. A degradation in shear strength was observed in further aging. Among them, the electrolytic Ni:P plated samples had the lowest degradation. The reason of degradation is determined to be the formation of NiBi3 intermetallic, since it had been found in all the other Ni plating samples beside the electrolytic Ni:P. Similar results were observed in thermal cycling test.

Cantilever testing of sintered-silver interconnects

Cantilever testing is an underutilized test method from which results and interpretations promote greater understanding of the tensile and shear failure responses of interconnects, metallizations, or bonded joints. The use and analysis of this method were pursued through the mechanical testing of sintered-silver interconnects that joined Ni/Au-plated copper pillars or Ti/Ni/Ag-plated silicon pillars to Ag-plated direct bonded copper substrates. Sintered-silver was chosen as the interconnect test medium because of its high electrical and thermal conductivities and high-temperature capability—attractive characteristics for a candidate interconnect in power electronic components and other devices. Deep beam theory was used to improve upon the estimations of the tensile and shear stresses calculated from classical beam theory. The failure stresses of the sintered-silver interconnects were observed to be dependent on test-condition and test-material-system. The experimental simplicity of cantilever testing, and the ability to analytically calculate tensile and shear stresses at failure, result in it being an attractive mechanical test method to evaluate the failure response of interconnects.

Reliability of AuGe Die Attach on DBC Substrates With Different Ni Surface Finishes

Plated Ni is a commonly used barrier layer for direct bond copper (DBC) substrates used in high-temperature and high-power applications. The Ni plating method can be electrolytic or electroless, and the composition may be pure Ni, Ni:P, or Ni:B. With a melting point of 356 °C, eutectic Au88Ge12 (wt.%) solder preforms can be used for die attach in high-temperature applications up to 325 °C. The reactivity of these different Ni layers with the Ge in the AuGe solder preform has been compared. With Ni and Ni:B, the reaction was rapid during the reflow process, and initial shear strengths were low. With electroless and electrolytic Ni:P, the initial shear strengths were high and decreased ~30% after 3000 h aging at 300 °C and 325 °C. The formation of Ni–Ge intermetallic compounds (IMCs) during high-temperature storage was studied, and the growth kinetics of the Ni–Ge IMCs were calculated. A −55 °C to +300 °C thermal cycling test was conducted, in which the Cu-to-alumina DBC substrate interface failed before the die attach.

Reliability of Ag Sintering for Power Semiconductor Die Attach in High-Temperature Applications

Low-temperature Ag sintering provides a lead-free die attachment method that is compatible with high-temperature (300 °C) power electronics applications. The reliability of sintered Ag die attach for Si and SiC die has been studied on both thick film substrates for lower current power applications and direct bond copper (DBC) substrates for higher current power applications. Pressureless and low-pressure sintering were evaluated. Sintering with low pressure yielded lower porosity (15–17%) versus pressureless sintering (∼30%). Reliability was evaluated with thermal aging (300 °C) and thermal cycling (–55 °C to + 300 °C) tests. Reliable Ag sintered die attach was achieved with assemblies having Ag-bearing surface finishes on both the die and the substrate. In contrast, the shear strength after 300 °C aging was greatly reduced when Au metallization was used either on the die or on substrate surface. In some cases, low-pressure sintering delayed the failure of the sintered Ag die attach to Au surfaces when aged at 300 °C compared to the pressureless sintering. The reliability with Pd-containing substrate metallizations was intermediate between Ag and Au metallizations. The thermal cycle reliability on DBC substrates was limited by failure at the Cu-to-alumina interface over the wide temperature range, while on the thick film substrates high adhesion was maintained after 1000 thermal cycles.