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ABSTRACT
IPEM is an improved power processing technology through advanced integration of power electronics components. It provides high frequency synthesis, resulting in important improvements in performance, size, and cost.
Currently, assemblies of power semiconductor switches and their associated drive circuitry are available in modules. Though the module contains a small size power switching part, the associated control, sensing, electro magnetic power passives and inter connect structures are very bulky. In IPEM, the reduction in size and weight is provided by planar metalization that allows 3-D integration of power devices and power passives to increase the power density.
This paper addresses the improvements of power processing technology through advanced integration of power electronics. The fundamental functions in electronic power processing, the materials, processes, and integration approaches and future concepts are explained.
INTRODUCTION
In power electronics, solid-state electronics is used for the control and conversion of electric power .The goal of power electronics is to realize power conversion from electrical source to an electrical load in a highly efficient, highly reliable and cost effective way. Power electronics modules are key units in a power electronics system. These modules contain integration of power switches and associated electronic circuitry for drive control and protection and other passive components.
During the past decades, power devices underwent generation-by-generation improvements and can now handle significant power density. On the other hand power electronics packaging has not kept pace with the development of semiconductor devices. This is due to the limitations of power electronics circuits. The integration of power electronics circuit is quite different from that of other electronics circuits. The objective of power electronics circuits is electronics energy processing and hence require high power handling capability and proper thermal management.
Most of the currently used power electronic modules are made by using wire-bonding technology [1,2]. In these packages power semi conductor dies are mounted on a common substrate and interconnected with wire bonds. Other associated electronic circuitries are mounted on a multi layer PCB and connected to the power devices by vertical pins. These wire bonds are prone to resistance, parasitic and fatigue failure. Due to its two dimensional structure the package has large size. Another disadvantage is the ringing produced by parasitic associated with the wire bonds.
To improve the performance and reliability of power electronics packages, wire bonds must be replaced. The researches in power electronic packaging have resulted in the development of an advanced packaging technique that can replace wire bonds. This new generation package is termed as 'Integrated Power Electronics Module' (IPEM) [1]. In this, planar metalization is used instead of conventional wire bonds. It uses a three-dimensional integration technique that can provide low profile high-density systems. It offers high frequency operation and improved performance. It also reduces the size, weight and cost of the power modules.
FEATURES OF IPEMS
The basic structure of an IPEM contains power semi conductor devices, control/drive/protection electronics and passive components. Power devices and their drive and protection circuit is called the active IPEM and the remaining part is called passive IPEM. The drive and protection circuits are realized in the form of hybrid integrated circuit and packaged together with power devices. Passive components include inductors, capacitors, transformers etc.
The commonly used power switching devices are MOSFETs and IGBTs [3]. This is mainly due to their high frequency operation and low on time losses. Another advantage is their inherent vertical structure in which the metalization electrode pads are on two sides. Usually the gate source pads are on the top surface with non-solderable thin film metal Al contact. The drain metalization using Ag or Au is deposited on the bottom of chip and is solderable. This vertical structure of power chips offers advantage to build sand witch type 3-D integration constructions.
In IPEM integration of active part is done by using 'Embedded Power Technology' [4] and that of passive part is done by using 'spiral Integration Technology' [5]. Embedded power technology provides a high-density integration of active components with negligible parasitic effects, and spiral integration of passive components with a large amount of volume reduction.
To describe these advanced integration techniques we can use a typical power electronics system. A Distributed Power Supply (DPS) system can be used for this purpose
DPS SYSTEM
The complete circuit diagram of DPS system is shown in figure 1. In this DC-DC converter, the primary dc supply is split in with the help of capacitors and then inverted to high frequency ac by MOSFET by half bridge inverter. The resonant capacitor voltage is transformer coupled, diode rectified and then filtered to get the output dc voltage [6].
ACTIVE PART
1 1 U^J
Fig 1: DPS System
The active part of the DPS system contains a power switching stage with two MOSFETs and their drive circuit. The drive circuit contains high and low drivers with over current protection and the drive control logic. The passive part contains capacitors, transformer and current doubling inductors.
The active and passive parts of DPS system can be implemented using the above mentioned techniques and are discussed in the following part.
EMBEDDED POWER TECHNOLOGY
Embedded power technology is a three-dimensional, multilayer integrated packaging technology. Figure 2 shows the conceptual structure of an embedded power packaged module.
Components
(a) Substrate
FIG 2: Conceptual structure of embedded power module
It consists of three parts: embedded power stage, electronics circuitry and base substrate, which are soldered together to build a module. The electronics circuitry (components in figure) includes gate drive, control and protection components. A hybrid (thick film) drive circuitry with high density interconnect is employed to shrink the module size.
Base substrate
The base substrate provides electrical interconnection and thermal path of power chips. An Al2O3 or AlN direct bonded copper (DBC) substrate with 25mil ceramic and 10mil thick copper is used as the base substrate. It is bonded to a 3mm thick heat spreader to improve thermal management and to provide suitable mechanical stability.
Embedded Power stage
The core element in this structure is the embedded power stage that comprises of ceramic frame, power chips (Si in figure), isolation dielectric and metalization circuit. Inside the power stage, multiple bare power semi conductor dies, featuring vertical semi conductor structures with topside and backside electrode pads, are buried in a ceramic frame.
Planar metallization
Plated Cu Sputtered Cu Sputtered Ni Sputtered Cr/Ni Al pad on chip
IGBT or MOSFET chip
Drain contact
In this module, wire bonds are replaced b metalization using copper layers. It is shown in figure 3. Here the deposited Cu pattern layers connect the Al pads of the power chips with external circuitry.
FIG 3: contact scheme in embedded power connection
To accomplish this metallurgical interconnect the under bump metalization (UBM) schemes are employed in our approach. These UBM schemes such as Ti-Ni-Cu or Cr-Ni-Cu deposited layers provide low film stress with good adhesive and electrical/ thermal conduction. For carrying high current an electro plated Cu layer is added to this thin Cu layer of UBM. The process flow chart of embedded power module is given in figure 4. Table 1 summarizes the fabrication steps. They are ceramic cutting, device mounting, dielectric printing and metalization [6]. One of the features of this technology is its mask based processing. The metalized base substrate is patterned using photolithography, the dielectric polymer is applied with a screen-printing method, and the chip-carrier ceramic frame is fabricated by computer controlled laser machining.
II
it
Si
ii
Ceramic cutting
Device mount
Dielec patten tric n
Metalization
pattern
Figure 4: Process flow chart of embedded power
By assembling the three parts, i.e. the substrate, the power stage and the gate driver, we get the active module. It has a substrate area of 28.5 x 27.3mm. The exploded view of active IPEM is shown in figure 5.
SPIRAL INTEGRATION TECHNOLOGY
In order to integrate the electromagnetic power passive components used in power electronic converters in to modules, we use spiral integration technology. This integration technology for power passives can best be described by first considering a simple bifilar spiral winding as shown in figure 6.
Fig:6 Spiral Integrated LC structure
This structure consists of two windings (A-C and B-D), separated by a dielectric material. This resultant structure has distributed inductance and capacitance and is best described as an electro magnetically integrated LC resonant structure for which equivalent circuit characteristics depends on the external connections. Even more complex integrated structures can be realized by adding more winding layers.
Design of these structures requires deliberate increase and modification of naturally existing structural impedances, like intra winding capacitance, to realize a particular equivalent circuit function. These models will provide power densities of 29W/cm3 at frequencies up to 1MHz. In our example of DPS system the passive part contains decoupling capacitor, current doubler inductors and isolation transformer. Because of the current doubler configuration, passive IPEM can be realized by stacking two transformers and using only one DC blocking capacitor as illustrated in figure 7.
Fig:7 Components of passive IPEM (a) equivalent circuit ( b) Exploded view of passive IPEM © prototype
In this circuit, the two magnetic structures i.e. inductors and transformers, can be integrated in to one physical structure through integrated magnetics technology. The equivalent magnetizing inductance is used to realize the current doubler inductors.
The transformers are built with two planar E-cores that share a common I-core. The ac flux is partially cancelled in the shared I-core. The D C blocking capacitor is now implemented in only transformer T1 using the hybrid winding technology. This technology is implemented using Cu traces on both sides of the winding and a dielectric layer placed in the middle to enhance the capacitive component of the winding. For this we use a high permittivity ceramic [Er >12000]. The Transformer T2 is a conventional planar low-profile transformer.
To get the complete IPEM, we mount the active and passive IPEMs on a single ceramic chip carrier with metalization and then bonded to the heat spreader. Figure 8 shows an IPEM based DPS system and its wire bonded version.
Fig:8 Comparison of wire bond and IPEM
PERFORMANCE OF IPEM
The performance of IPEM can be evaluated using various parameters. A comparison of IPEM and wire bonding technology is given in table 2. As given in the table, IPEM has achieved 35% reduction of foot print area as compared to the wire-bonding version. The planar interconnects in IPEM reduces the structural inductance by a factor of three when compared to the wire bonding. But the structural capacitance is increased by a factor of five.
Table:2 Comparison of wire bond and IPEM
Parameters Wire bond IPEM
Substrate area (mm2) 40 x 30 28.5 x 27.3
Inductance (nH) 10 3
Capacitance (pF) 4 20
No. of passive components 6 1
Volume of passives 173 82
No. of terminals 15 5
Volume of terminals 170 5
Total passive volume 343 87
System profile 20 10
System power density 1 X3.6
In IPEM, the volume of passive components is reduced to half of that in wire bonding and total passive volume is reduced to 1/3rd of that in wire bonding. Also the system power density is increased by a factor of 3.6.
ADVANTAGES AND DISADVANTAGES
There are several advantages for integrating power electronics system using the IPEM concept. The main advantages are
1. Modular approach:
This modular approach reduces the design and implementation time cycles as well as simplifies the assembly process.
2. Improved usage of space
The reduction in volume increases the power density and reduces the profile of the system.
3. Reduction of components and inter connects.
This improves the system reliability and also increases the speed.
4. Reduction in structural packaging inductance.
It leads to improved electrical performance, which in turn leads to reduced voltage ringing across the power switches. It increases the switching frequency.
The main disadvantage of this system is that it is very complex compared to other 2-D modules. Also it requires efficient combination of a large number of different technologies.
APPLICATIONS
IPEM can be used for most of the power electronic circuits. Hence it has a wide range of applications. It includes
¢ Motor drives
¢ UPS systems
¢ Power supplies
¢ Inverters
¢ Converters etc.
CONCLUSION
In power electronics modules further improvements in performance, reliability and cost can be achieved by using IPEMs. Various experiments have proved its manufacturability and other features of this technology. The impacts of system integration via IPEM will enable a rapid growth of power electronics that can be compared to the impacts in computer applications brought about by VLSI technology.
REFERENCES
[1] F.C.Lee, J.D.Van Wyk, D. Boroyevich, G.Q. Lu, Z. Liang and P. Barbosa "Technology trends towards system in a module in power electronics ", IEEE circuits ^systems, Vol. 2, No 4, Vth quarter, pp 6-21, 2002.
[2] X. Liu, and G. Q. Lu "Power chip inter connection: from wire bonding to area bonding," Advancing microelectronics, Vol. 28.No 4, July/August
2001.
[3] B.K.Bose, " Modern power electronics ", Jaico Books, pp 8-32
[4] Z. Liang, F.C. Lee, "Embedded power technology for IPEM packaging applications", IEEE proceedings on APEC 2001 pp 1057-1061
[5] http:// scholar.lib.vt.edu/thesis/avalable
[6] B. G. Streetman, S. Banerjee, "Solid state electronic devices, PHI, pp 150-160
