Highlighting the possibilities of PCB technology in the field of power electronic substrates

e-motec
November 30, 2020

T. Gottwald, Director Next Generation Products , Dr. Manuel Martina, Christian Rößle of Schweizer Electronics’

The transition from mechanical to electrical power brings new challenges to manufacturers and requires new solutions for the electrical system, where the PCB (Printed Circuit Board) is of crucial importance.

High currents, heat dissipation of power electronic components, low inductances and miniaturization needs are only a few of the requirements that lead to innovative solutions on PCB level.
Chip embedding technologies are meanwhile used to embed thin bare dies of Power Semiconductors into the PCB which leads to powerful alternatives to conventional power electronic modules.

Introduction

The global warming and the pressure on CO2 reduction led to an increasing ratio of Renewable Energy from wind power plants and solar energy systems. As these sources must be connected to the power grid DC to AC, AC to DC, DC to DC converters and the like are applications of growing volume. Because the energy of renewable sources tends to be more costly, the total systems´ efficiency is crucial.

Due to the same reason the Automotive Industry is under legislative pressure to achieve their CO2 reduction targets. That’s why hybrid and electrical drive is bringing momentum to the development of new solutions for the electrification of automotive applications. High power demand leads to increasing challenges for high current and for thermal management of dissipated power losses as well.

The power conversion is done with power semiconductors which have to be assembled on a substrate. This can either be a power module made from Ceramic substrates like Direct Bonded Copper (DBC/DCB) or with a Printed Circuit Board (PCB).

The task of the substrate is to manage high currents, high heat dissipation and high switching frequencies to support the electrical conversion of energy in the best way.

Over the last few years PCBs achieved an increasing share in these applications as they typically have a cost advantage over Ceramics. PCBs offer the opportunity to manage the power stage and the control board in one single substrate while Ceramic power stages always need to have an additional control board and the related interconnection architecture like plugs and cables.

This article highlights the possibilities of PCB technology in the field of power electronic substrates.

  1. Heavy copper PCBs
    Heavy copper PCBs have been used in the automotive industry for a long time, e.g. for fuse- and relay boxes. This technology experiences a revival as the electrical power increases in many applications. The technology is also useful to reduce the parasitic inductance of the conductors by using the heavy copper layers as power lines which can be stacked one above the other in a heavy copper multilayer. Up to 4 layers made from 400 µm (12 oz) Copper can be realized in the inner layers, which leads to a potential ampacity of more than 1000 A. The outer layers of such a heavy copper multilayer should be kept below 150 µm. Otherwise additional effort must be made for the solder mask process to achieve a safe electrical insulation.
Combi Schliff 1024x331

Fig. 1: 6-layer multilayer PCB with 4 inner layers, 400 µm copper each.

  1. Power Combi-Board
    The disadvantage of heavy copper PCB technology is the incompatibility with fine pitch structures which cannot be etched with heavy copper. Therefor a power electronic system typically consists of a power stage with heavy copper design and a separate control board with standard copper thickness for SMT assembly. The installation space must be large enough to host both boards and the connectors between the two boards.
    With the Power Combi Board, a combination of both requirements can be achieved. Heavy copper is partially installed in the inner layers besides standard copper construction. The electrical connection of the whole board is carried out with one common outer layer in SMT compatible copper thickness.
Dickkupfer Schliff 1024x258

Fig.2: Power Combi Board: Heavy copper besides standard copper thickness for power and control in one PCB

For heat dissipation the insulation layer between the heavy copper layers are a barrier for optimal heat transportation in z-axis. Heavy copper PCB technology should therefor preferably be used to manage high currents. If heat dissipation is important for the application, other technologies should also be considered like the Inlay technology.

  1. Insulated Metal Substrates (IMS)
    An Insulated Metal Substrate typically consists of a metal heat sink, a thin insulation layer and a single copper layer on top. The construction is useful for simple designs which host a lot of heat generating components. For more complex components it is not possible to do the routing with one layer only.
    Today IMS substrates can also be manufactured with more than one layer to enable the combination of higher complexity layouts with optimized heat dissipation.
P2 Schliff Bestueckt 1024x575

Fig. 3: Insulated Metal substrate with copper backside

The typical Aluminum backside is a light weight but also a high CTE metal. To increase the reliability of the assembled components Copper was introduced as heat sink metal on the back side. This also improves the thermal capacity and other characteristic as shown in Fig. 4

Fig.4: Comparison of characteristics Copper vs. Aluminum

  1. Inlay Technology
    For minimizing the thermal resistance from the power components to the heat sink the shortest way will lead to the lowest thermal resistance. In most cases the heat is dissipated in z-axis from the assembled top side of a PCB through the board to a heat sink, which is installed at the bottom. By laminating a massive copper element into the PCB, the thermal resistance can be reduced dramatically. If the Inlay is not only used for heat dissipation but also for high currents the lowest ohmic resistance can also be achieved.
P2 Schliff 1024x251

Fig 5: Left: Cross section through an inlay board with 2 mm thick copper inlays in the inner structure. Right: Top and bottom side of an inlay board for 1200 A peak currents.

  1. Embedding Technology
    When it comes to highest performance requirements and lowest installation space, conventional solutions encounter limitations regarding installation space and power density. Miniaturization was the first driver for embedding because space savings are possible if some of the components are installed inside the PCB instead on the outer surface.

Figure 6: Cross section of a Smart p² Pack power PCB (left) and a half bridge (right).

To improve heat dissipation from the inside of the PCB to the heat sink Schweizer Electronic [3] and Infineon Technologies [4] developed the so-called p² Pack® Technology which uses a power semiconductor assembled in a lead frame which acts as a heat spreader and reduces the thermal resistance significantly. The top side contacts are connected with a heavy copper layer using copper filled micro vias which replace the bond wires, which typically are used in conventional power modules. With this technology not only the heat dissipation but also electrical parameters could be improved as follows.

System PCB With Integrated Power MOSFETs 2 1024x683

Fig. 7: Inverter PCB in Smart p² Pack Technology.

Electrical performance [1]

On State Resistance: The part of the package resistance associated with the bond wires is virtually eliminated with chip embedding. The exact value depends on the respective semiconductor technology generation, the voltage class, and the semiconductor package.

Thermal Resistance: Due to the excellent heat spreading which is achieved with a lead frame in p² Pack technology the systems thermal resistance significantly improves. Also, the thermal impedance and therefor the robustness of the devices is improved due to the thermal capacity of the lead frame.

Switching performance: Low parasitic inductance is achieved as a result of the almost flat connection between the top of the chip and the vias, and short distances between the DC-link capacitors and power semiconductors. This enables faster switching, with lower losses especially with fast switching devices like GaN and SiC semiconductors.

Miniaturization: Many systems for current and future applications need to be shrinked while simultaneously providing additional functionality. Chip embedding can save valuable space on PCB level.

Higher Reliability: Replacing bond wires or DCB ceramics substantially increases reliability. In power cycling tests with a temperature difference dT of 120 K, designs were able to withstand more than 700,000 active cycles.

System Cost Reduction: With savings on plug connectors and cables, optimized cooling, reductions in required chip surface areas for power components, smaller passive components, fewer EMC issues, the insulation already built in and overall space savings, system cost savings are considerable.

Roadmap to high voltage and Wide Bandgap Semiconductors
PCB embedding technologies like the p² Pack technology will enhance the performance of power electronic applications furthermore. Due to its´ very low parasitic inductance this new technology supports low loss switching at high frequencies which is of utmost importance when so called wide band gap semiconductors come into play. The next generation of Automotive drives, which will be equipped with SiC and with GaN devices, is already in development and showed outstanding results. [2]

With a built-in insulation it will be possible to assemble the Smart p² Pack directly on the Heat sink. The TIM (Thermal Interface Material) can be chosen either as electrically non-conductive or as electrically conductive material.

Additional features

Standardization and modularization are important factors for the success of a new technology. Therefore, half bridge designs were built as demonstrators with current sensing by using shunt elements for the measurement of the phase current of an electrical motor. As shunts are relatively large components, miniaturization efforts are supported while reliability improves: Solder joints are replaced by micro – vias for the connection to the board.

Shunt Roentgen 1024x602

Figure 9: X-Ray image of a Half-Bridge design with embedded shunt (left). X-sectional view of Half-Bridge design with embedded shunt in the middle (right)

By embedding the shunt into the p² Pack the heat dissipation from the shunt is improved dramatically which increases the possibility to use shunts for current measurements even for very high currents e.g. 300 A per phase.

  1. Conclusions
    New PCB technologies have the potential to support e-mobility systems by minimizing form factors, increasing the systems´ performance and by reducing the system cost when determined on system level.
    The embedding of power electronic devices is able to replace conventional power modules, also improves system performance and reliability significantly and is useful for low voltage applications with highest currents as well as for wide band gap devices in high voltage applications. [2], [5]
  2. Acknowledgement
    The authors acknowledge the contribution of Infineon Technologies AG and the Schweizer Electronics’ Innovations Team to this work.
  3. References
    [1] Adrian Röhrich and Christian Rössle, Chip Embedding of Power Semiconductors in Power Circuit Boards, ATZ elektronics worldwide, 06/2018
    [2] C. Marczok, M. Martina, M. Laumen, S. Richter, A. Birkhold, B. Flieger, O. Wendt, T. Päsler: SiC modul – Modular high-temperature SiC power electronics for fail-safe power control in electrical drive engineering. Proceedings CIPS 2020, 11th International Conference on Integrated Power Electronics Systems
    [3] https://www.schweizer.ag/de/produkteundloesungen/ embedding/p2_Pack.html
    [4] https://www.infineon.com/cms/en/about-infineon/press/market-news/2019/INFATV201905-068.html
    [5] Thomas Gottwald, Christian Roessle : Minimizing form factor and parasitic inductances of Power Electronic Modules: The p² Pack Technology. 7th Electronic System-Integration Technology Conference (ESTC 2018)
  4. Glossary
    MOSFET: Metal-Oxide-Semiconductor Field-Effect Transistor
    PCB: Printed Circuit Board
    DBC/DCB: Direct Bonded Copper / Direct Copper Bonding
    SMT: Surface Mount Technology
    GaN: Gallium nitride
    SiC: Silicon carbide
    Al2O3: Aluminum oxide
    AlN: Aluminum nitride
    Si3N4: Silicon nitride
    CTE: Coefficient of thermal expansion

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