Direct Bonded Copper Research Articles

Heat Pipe Integrated in Direct Bonded Copper (DBC) Technology for Cooling of Power Electronics Packaging

Thermal dissipation in power electronics systems is becoming an extremely important issue with the continuous growth of power density in their components. The primary cause of failure in this equipment is excessive temperatures in the critical components, such as semiconductors and transformers. This problem is particularly important in power electronic systems for space applications. These systems are usually housed in completely sealed enclosures for safety reasons. The effective management of heat removal from a sealed enclosure poses a major thermal-design challenge since the cooling of these systems primarily rely on natural convection. In this context, the presented paper treats the heat pipes as effective heat transfer devices that can be used to raise the thermal conductive path in order to spread a concentrated heat source over a larger surface area. As a result, the high heat flux at the heat source can be reduced to a smaller and manageable level that can be dissipated through conventional cooling methods. The objective of our work is to describe the feasibility of a cooling system with miniature heat pipes embedded in a direct bonded copper (DBC) structure. The advantage of this type of heat pipe is the possibility for implementation of the component layout on the heat pipe itself, which eliminates the existence of a thermal interface between the device and the cooling system

Influence of titanium on the formation of a “barrier” layer during joining an A1N ceramic with copper by the CDB technique

The paper presents the results concerning the formation of a ‘barrier’ layer on AlN ceramic during its joining with copper by the Copper Direct Bonding (CDB) technique. The surface of the ceramic was modified with titanium, using various amounts of this active component. When deposited by sputtering, Ti layers were about 0.35 μm thick. The implantation doses were 1 × 1016, 5 × 1016, 2 × 1017 and 1 × 1018 ions/cm2, and the ions were accelerated by a voltage of 70 or 15 kV. The modified ceramic was joined with oxidized copper in a nitrogen atmosphere with about 1.5 ppm of oxygen using a belt-type furnace. The results of the examinations of the mechanical strength of the joints and of the microstructural and phase changes induced on the surfaces of the joints during the bonding process formed the basis for establishing the optimum conditions of modification of the AlN surface with titanium ions by introducing them just beneath the surface layer of the ceramic. The modification yielded a multiphase ‘barrier’ layer, which ensured a continuous pore-less contact between the materials being joined. The joints thus produced showed a high mechanical strength. Optimum conditions were also established for joining the AlN ceramic directly with copper by the CDB technique in a nitrogen atmosphere.

Increasing Circuit Functionality and Power Density in Power Hybrids

A new method to increase circuit functionality and power density per unit area in power electronic modules has been developed. This new method uses the present power hybrids production technology platform on ceramic substrates and implements direct bonded copper (DBC) partial multilayer structures on application specific substrates. With this novel approach a more than 50% increased power density and a 40% increased circuit complexity has been achieved. Reliability tests (thermal cycling (TC), power cycling (PC), high temperature reverse bias...) have been performed for several new layout solutions, the most relevant results are shown in this paper.

Boundary element analysis of thermal fatigue effects on high power IGBT modules

The technology of high power IGBT modules has been significantly improved these last years against thermal fatigue. The most frequently observed failure modes, due to thermal fatigue, are the solder cracks between the copper base plate and the direct copper bonding (DCB) substrate and bond wire lift-off. Specific simulation tools are needed to carry out reliability researches and to develop device lifetime models. In other respects, accurate temperature and flux distributions are essential when computing thermo-mechanical stresses in order to assess the lifetime of high power modules in real operating conditions. This study presents an analysis method based on the boundary element method (BEM) to investigate thermal behavior of high power semiconductor packages subjected to power cycling loads. The paper describes the boundary integral equation which has been solved using the BEM and applied to the case of a high power IGBT module package (3.3 kV–1.2 kA). A validation of the numerical tool is presented by comparison with experimental measurements. Finally, the paper points out the effect on the thermal stress of the IGBT chips position on the DCB substrate. In particular, a light shifting of the silicon chips may be sufficient to delay significantly the initiation and the propagation of the cracks, allowing a higher device lifetime of the studied module.

Microstructure and Mechanical Properties of DBC on Sputter Deposited Copper on Alumina Substrate

The process of Direct Bonding Copper (DBC) is performed by a spinel reaction between CuO and Al2O3. In order to develop DBC on alumina substrate with high bonding strength, alumina substrate was preformed as follows: Cu was sputter-deposited on alumina substrate. Sputter-Deposited Cu (SDC) on alumina substrate was oxidized at 673K for 30min in air atmosphere and then stabilized at 1273K for 30min in N2 gas atmosphere to improve bonding strtrength between preformed alumina substrate and SDC layer. Subsequently, the Cu-foil (300µm) was bonded on preformed-alumina substrate in N2 gas atmosphere at 1342~1345K. It was found that optimum condition of DBC on preformed-alumina substrate could be successfully obtained at 1345K for 30min. Consequently, bonding strength of DBC on alumina substrate was the high value of 80N/cm. Observation and analysis of microstructure for Cu sputtered DBC showed that reaction compounds such as CuAlO2 and CuAl2O4 approved to be formed in the vicinity of interface between Cu and alumina substrate.

Preoxidation of the Cu layer in direct bonding technology

Between the metal melting point and the eutectic temperature of the metal–oxygen, direct bonded copper (DBC) depends on the eutectic composition to join the copper and ceramic. The key of this technology is to import the oxygen to the bonding surface. In this paper, we do some research on the preoxidation law of the copper layer in direct bonding technology, so we can optimize the rang of preoxidation temperature and time. After we investigate the phase diagram of copper–oxygen, by the “conservation of mass” and “level principle”, we find there are some relationship between the preoxidation condition and the bonding process, and we offer a experiential formula to help control the bonding process. Finally, we also analyze the interface reaction production between the Cu layer and alumina ceramic, and discuss some possible reasons.

Advantages and new development of direct bonded copper substrates

Direct bonded copper (DBC) substrates have become the most important electronic circuit boards for multichip power semiconductor modules. They are replacing complicated assemblies based on leadframes and refractory metallized substrates due to ease of assembly and the low temperature coefficient of expansion of DBC which matches silicon in spite of thick copper metallization. The DBC technology allows bonding of copper to alumina and aluminum nitride, fusing of copper to copper has been developed to establish efficient water cooling devices with sophisticated internal micro channel structures for cooling power laser diodes and other high power density electronics. Low weight, hermetically sealed DBC packages are replacing more and more heavy weight metal can packages. Increasing requirements for temperature cycling reliability and mechanical stability in automotive, avionics and space applications is the driving force for the development of a new type of DBC material with a 60% increase in flexural strength and more than 100% increase of temperature cycles: This new type of DBC allows 35% thicker copper layers on 20% thinner ceramic, for example 0.4 mm copper on 0.32 mm ceramic. By this the thermal resistance of multichip power modules can be reduced substantially without decreasing reliability. In many applications this will be an economic solution where the price of AlN and Si 3N 4 substrates would be too high.

Low cost flex substrates for miniaturized electronic assemblies

Electronic power converters have been designed, produced, and disseminated to the market in mass quantities utilizing a number of fabrication techniques; ranging from standard printed circuit board (PCB) technologies for low cost applications, to conventional thick film on ceramic, to direct bond copper (DBC) approaches for high power, higher cost applications. Each of these approaches holds a share of the power packaging market, but they all demonstrate a limitation to conventional two dimensional “flat board” strategy. PCBs, thick films, and DBCs are all technologies which restrict, for the most part, circuit and package designs to two dimensional boards. The one potential pathway into the third dimension is through the use of multilayers; an approach, which becomes increasingly difficult with each additional layer added beyond the first, and with the exception of high performance solutions is typically cost prohibitive for the majority of applications. This paper will demonstrate the feasibility and viability of flexible polymer substrates. Flex technology employs industry standard PCB and/or thick film processes, offers the lower cost, higher performance solutions inherent with the majority of polymer plastics, and as a final bonus, essentially frees the designer to more efficiently utilize all three dimensions of space. The researchers have demonstrated the feasibility of this low cost alternative solution through the fabrication and testing of integrated power modules, which utilize flexible polymer substrates in conjunction with both surface mount and bare dice. These DC/DC power converters transform 120 V/240 V inputs to 9 V, 7-W outputs, and illustrate through their unique geometrical design the miniaturization advantages of fully utilizing the three dimensional space offered by flex circuitry.

A fritless copper conductor system for power electronic applications

Copper coated aluminium nitride (AlN) ceramic is a preferred substrate material utilized for high power, high frequency electronic applications. The use of metallized ceramic requires bonding of the metallization onto the substrate. AlN, however, is chemically inert to Cu. At present time the direct copper bonding (DCB) technology is used to manufacture such structures. In this project, for reasons of simplicity and economy and as alternative to the standard DCB-technology, the metallization on the AlN ceramic is built up by active metal brazing of a screen printable AgCuTi thick film ink. The mean advantage of this technology is, that the metal/ceramic braze compound can be processed in a conventional belt furnace at inert firing atmosphere usually available at hybrid microelectronics industry. Aspects of paste preparation ranging from the derivation of the metallic powder to the assortment of an appropriate organic screen-printing medium were investigated. The physical properties of the phases occurring at the interface between the TiCuAg metallization and AlN ceramic and their contribution to the thermal conductivity and the thermomechanical performance of the metallized substrate have been characterized theoretically as well as by experiment and compared with different commercial glass frit containing copper thick film metallization systems.

Double-sided cooling for high power IGBT modules using flip chip technology

A new technique for the packaging of IGBT modules has been developed. The components are sandwiched between two direct bond copper (DBC) substrates with aluminum nitride. Wire bonds are replaced with flip chip solder bumps, which allows cooling of components on both sides. Microchannel heat sinks are directly integrated in the package to decrease the thermal resistance of the module. Thus, a very compact module with high thermal performance is obtained. A prototype with two insulated gate bipolar transistors (IGBTs) and four diodes associated in parallel was realized and tested. In this paper, the innovative packaging technique is described, and results of thermal tests are presented.

A modelling approach to assess the creep behaviour of large-area solder joints

Packages of power modules are designed to show a slightly convex shape of the base-plate in order to achieve a reasonable thermal contact between the module and the heat sink when the power module is mounted. During manufacturing, soldering of materials with differences in thermal expansion coefficients will lead to bending of the “bi-material” composite, e.g. the silicon chip to substrate (usually DCB = direct copper bond Al2O3 or AlN) or the DCB-substrate to base-plate joint (Cu or MMC = metal matrix composite). Especially the large area solder joint between DCB-substrate and Cu base-plate is known to induce a certain curvature into the base-plate immediately after cooling down from soldering. The curvature will relax to a certain extent during module finishing, storage, shipping, and use which makes the prediction of the shape of the base-plate difficult. The present paper proposes a model to assess the creep behaviour of such large area solder joints in order to predict the state of bending after soldering and the amount of relaxation of bending with time.

Direct silver bonding – an alternative for substrates in power semiconductor packaging

Bonding between silver and ceramics like Al2O3, ZrO2, MgO, AlN, sapphire or quartz glass is obtained by a “liquid phase bonding process” based on the pseudo-binary eutectic between Ag and CuO (at 1 mol %). It melts 15 K below the melting point of pure Ag in air. Excellent wetting between the eutectic liquid and the ceramic surfaces gives mechanically strong, reliable bonds. The bonding mechanism is similar to the well known direct copper bonding (DCB)-process. Our new process is simple and works at 1219±2 K in plain air. It therefore has the potential of massive cost reductions compared to the more complicated DCB-process.

Thermal management of power electronics modules packaged by a stacked-plate technique

The research presented in this paper is part of a multidisciplinary research program of the Center for Power Electronics Systems at Virginia Tech. The program supported by the Office of Naval Research focuses on the development of innovative technologies for packaging power electronics building blocks. The primary objective of this research is to improve package performance and reliability through thermal management, i.e., reducing device temperatures for a given power level. The task of thermal management involves considering trade-offs in the electrical design, package layout and geometry, materials selection and processing, manufacturing feasibility, and production cost. Based on the electrical design of a simple building block, samples of packaged modules, rated at 600 V and 3.3 kW, were fabricated using a stacked-plate technique, termed metal posts interconnected parallel plate structure (MPIPPS). The MPIPPS technique allows the power devices to be interconnected between two direct-bond copper substrates via the use of metal posts. Thermal modeling results on the MPIPPS packaged modules indicate that the new packaging technique offers a superior thermal management means for packaging power electronics modules.

Reliability of radio frequency/microwave power packages: the effects of component materials and assembly processes

Abstract The mechanical performance and reliability of various Al2O3 ceramics were evaluated with intended applications as insulator frames for radio frequency/microwave power packages. Considering variations in base-flange material properties (CuW, CuMoCu), base-flange thickness (0.040 in., 0.020 in.), assembly process material (AgCu, Cu) assembly process temperature (860°C, 1080°C), and Al2O3 insulator material (96%, 99%, 20% ZrO2/Al2O3, ZTA), finite element analysis (FEA) spatially resolved the fabrication-induced stresses during the assembly of CuW/Al2O3 and CuMoCu/Al2O3 structures and showed the critical regions to be the frame corners at the metal base-flange/Al2O3 interface. Bend strengths (four-point) and Weibull distributions were determined for each Al2O3 material and coupled with the FEA-predicted stresses for the various package configurations and assembly processes, the failure probabilities (Pf ) for the various CuW/Al2O3 and CuMoCu/Al2O3 structures were calculated. The CuMoCu-based structures exhibited the greatest warp deformation (concave upward, +), and highest stress and failure probability for all Al2O3 insulator varieties. The CuW-based structures exhibited an order-of-magnitude lower warp deformation (concave downward, −), an order-of-magnitude lower stress in the Al2O3, and in excess of seven orders-of-magnitude lower failure probability than CuMoCu-based structures, for all Al2O3 insulator varieties. Confirmational experiments were conducted using AgCu and direct-bond copper (DBC) assembly processes for selected varieties of CuW/Al2O3 and CuMoCu/Al2O3 structures. Al2O3 failure sites were identified using radiographic, ultrasonic and optical techniques and were in good agreement with model predictions of suspect structures and failure location. The strength and reliability data were considered in conjunction with relative cost for the Al2O3 ceramics to select an optimum frame insulator for the application.

Thermal analysis of high-power modules

A highly descriptive method for displaying heat flow in power modules is presented. Heat flow is studied for three different transistor-stack types: direct bond copper (DEC), thick-film printed substrate, and insulated metal substrate (IMS). DEC and thick film are thermally superior to IMS, but IMS shows potential. In addition, the effect of case-to-sink interface conductivity on heat flow is studied and shown to be of extreme importance in a proper thermal simulation.

Hybrid Substrates for Power Electronic Applications: An Investigation of Thermal Resistance and Mechanical Stress

A new hybrid substrate technology for power electronic applications has been characterised by thermal resistance and mechanical stress measurements. The new substrate utilises thermal spray technology for deposition of dielectric layer and electrical conductors. The results are compared with the more established technology of alumina substrates with direct copper bonding (DCB) metallisation. Silicon test chips for thermal resistance and mechanical stress measurement were used for the characterisation. The experimental results were compared with finite element analysis and a reasonable agreement was found.

New Trends in Power Microelectronics DCB Packages

Direct copper bonded (DCB) alumina substrates are among sophisticated technical materials suitable for application in electronics, automotive and engineering industries. DCB substrates resist very severe operational conditions and assure high reliability. This technology provides alumina substrates which are very strongly bonded to copper foil, due to the solidified eutectic alloy Cu2O‐Al2O3 or a suitable vitreous solder. Such substrates are ideal for the construction of elements in microelectronics hybrid circuits, power circuits and devices. As the bonding layer of a solidified eutectic or vitreous solder is very thin, the thermal resistivity stays sufficiently low; in addition, the dilatation properties of DCB substrates are close to those of silicon. This enables soldering of large silicon chips without the risk of creating mechanical stress and allows for the production of devices with very high construction effectivity. Examples of such applications are potential‐free modules, bridge circuits, automotive ignition modules and Peltier elements. The vitreous bonding layers are advantageous for certain applications. State‐of‐the‐art DCB bonding technology was developed for DCB aluminium nitride substrates, which have even better thermal qualities.

High thermal conductivity aluminum nitride ceramic substrates and packages

Breakthrough technologies for achieving a whole line of AlN products for wider use in the electronics market are described. These products are: (1) plain substrates; (2) metallized substrates; (3) direct bond copper (DBC) substrates; (4) substrates for thin-film circuits; (5) substrates for thick-film circuits; and (6) cofired multilayer packages. The basic properties of AlN and problems with AlN from the manufacturing process point of view are summarized. The discussion of the breakthrough technologies needed for AlN applications focuses on four problem areas: the realization of the highest thermal conductivity by sintering, pin/lead brazing, the metallization problem for the cofired case and the postfired case, and polishing for a thin-film circuit. AlN applications and market trends are discussed. >

Thick Film and Direct Bond Copper Forming Technologies for Aluminum Nitride Substrate

Thick film and direct bond copper (DBC) were successfully formed on aluminum nitride (AIN) ceramics. Commercially available gold (Au), silver-palladium (Ag-Pd), and copper (Cu) thick film pastes for alumina (AI 2 O 3 ) were formed on AIN under exactly the same firing conditions as on AI 2 O 3 . From X-ray micro analysis (XMA) and X-ray diffraction (XRD) analysis, copper (Cu), lead (Pb), silicon (Si), and oxygen (O) were observed at the adhesive boundary of the thick films and ceramics. On the contrary, poor bondings were observed when bismuth (Bi) segregated at the adhesive boundaries. Pre-oxidation of AIN ceramics at high temperature around 1250°C was required for the bonding of DBC to AIN. Cu 2 O was formed at the boundary between the Cu metal and oxidized AIN, thus copper was directly bonded to AIN firmly. Bonding strength, peel strength, and wire bondability were enough for practical use in applications to high power modules and large-scale integrated (LSI) packages.

Modular SCR Assemblies in VSCF Systems

The success of the cycloconverter based VSCF electrical system has clearly been dependent upon the development of the silicon-controlled rectifier (SCR) as a high power electrical switch. The purpose of this paper is to describe the innovative work in process to improve the thermal characteristics of the SCR; and, thereby, improve the cycloconverter power quality and efficiency. The key parameter to improvements in the SCR performance is to reduce the thermal impedance from the semi-conductor junction to case. By improving this characteristic, lower junction temperature or smaller devices may be used in a given application. Significant improvements in this area, for 18 mm SCR pellets and larger, have been severely limited by the necessity for dry interfaces. Up to now, the thermal stresses developed at the silicon-copper interface during low temperature excursion preclude solder or other direct bonding techniques. The unique approach described in this paper eliminates the dry interface, incorporates built-in electrical isolation, and features structured copper, solder bonding, direct bond copper on beryllia, and a hermetic-sealed aluminum enclosure.