Integrated Power Electronics Module

Seminar Report ‘03 Module

Integrated Power Electronics

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-bygeneration 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.

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Integrated Power Electronics

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 threedimensional 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.

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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.

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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].

Fig 1: DPS System

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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.

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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.

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

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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 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. Plated Cu Sputtered Cu Sputtered Ni Sputtered Cr/Ni Al pad on chip

Dielectric and passivation

Si .

IGBT or MOSFET chip Drain contact

FIG 3: contact scheme in embedded power connection

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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.

Ceramic cutting

Si

Device mount

Dielectric pattern

Metalization pattern

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Figure 4: Process flow chart of embedded power

Steps Ceramic

Description Openings in flat Al2O3 or AlN plate by laser cutting

frame Die mount Dielectric

Dispense dielectric Void free precision dielectric pattern, good adhesion by

pattern Metalization

screen - printing or/and photolithography. Adhesion, barrier, low stress, low resistance by sputtering of Ti/Cr-Cu thin film. Thicker (>5mil), low stress, low resistance, solderable, precision pattern by electroplating of Cu, etching

Table1: processing step for embedded power stage. 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 × 27.3mm. The exploded view of active IPEM is shown in figure 5.

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Fig.5: Three Dimensional View

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

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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 (c) prototype

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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 Icore. 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.

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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.

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Table:2 Comparison of wire bond and IPEM

Parameters Substrate area (mm2) Inductance (nH) Capacitance (pF) No. of passive

Wire bond 40 x 30 10 4 6

IPEM 28.5 x 27.3 3 20 1

components Volume of passives No. of terminals Volume of terminals Total passive volume System profile System power density

173 15 170 343 20 1

82 5 5 87 10 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.

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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

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• 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.

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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]

[6]

http:// www.scholar.lib.vt.edu/thesis/avalable

B. G. Streetman, S. Banerjee, ”Solid state electronic devices, PHI, pp 150-160

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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.

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CONTENTS

•

Introduction

1

•

Features of IPEM

3

•

DPS system

4

•

Embedded power technology

6

•

Spiral integration technology

10

•

Performance of IPEM

12

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Advantages and disadvantages

•

Applications

14

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Conclusion

15

•

14

References

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ACKNOWLEDGEMENT

I extend my sincere gratitude towards Prof . P.Sukumaran Head of Department for giving us his invaluable knowledge and wonderful technical guidance

I express my thanks to Mr. Muhammed kutty our group tutor and also to our staff advisor Ms. Biji Paul for their kind co-operation and guidance for preparing and presenting this seminar.

I also thank all the other faculty members of AEI department and my friends for their help and support.

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