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Advancements in High-Power IGBT Modules: The Benefits of Full Copper Technology

Posted Date:2023-09-07 11:22:49

The Insulated Gate Bipolar Transistor (IGBT) is a core component responsible for electrical energy conversion in electrified rail transportation equipment. The long-term reliability of high-voltage, high-capacity IGBT modules is crucial for the safe operation of traction inverters. As high-speed and heavy-load rail transportation technology advances and the demand for greener and smarter power electronic devices increases, there are ever-increasing requirements for the performance and reliability of high-power IGBT modules. These requirements include higher power density, higher operating temperatures, and increased operational reliability to meet the demands of the next generation of traction power applications.

Advancements in High-Power IGBT Modules: The Benefits of Full Copper Technology

One of the main mechanisms behind the fatigue and aging failure of IGBT modules is the thermal expansion coefficient mismatch between heterogeneous materials inside the modules. Traditional semiconductor processes are based on aluminum metallization and interconnection techniques, and most high-power IGBT modules still use aluminum-based technologies, including aluminum metallization on the chip’s surface, aluminum wire bonding for chip interconnection, and soldering of power terminals to ceramic substrates. Due to the inevitable thermal expansion coefficient mismatch between heterogeneous materials inside high-power modules and the relatively high thermal expansion coefficient of aluminum, long-term exposure to temperature and power cycling can lead to issues such as solder joint cracking and degradation, affecting the long-term reliability of the modules.

Advantages of Full Copper Technology

Advancements in High-Power IGBT Modules: The Benefits of Full Copper Technology

The advancement of power semiconductor packaging technology is greatly influenced by material and manufacturing process developments. In order to reduce the difference in thermal expansion coefficients between materials within the module, improve power cycling capabilities, and enhance long-term operational reliability, the industry has introduced full copper technology, which mainly includes copper metallization of chips, copper wire bonding for interconnection, and ultrasonic welding of copper power terminals.

Compared to aluminum technology modules with similar packaging structures, full copper technology modules have two main improvements:

First, a thick copper layer is grown on the chip’s surface. This not only helps reduce chip conduction losses and improve heat dissipation but also mitigates the impact force during copper wire bonding, thus enhancing the reliability of the bonding points. Second, copper wire bonding interconnects between chips can reduce module parasitic resistance losses and minimize the impact of wire self-heating effects.

Infineon has introduced XT technology, which includes new techniques such as copper metallization of IGBT chips and copper wire bonding. By applying this technology to medium and low-voltage modules, it has been demonstrated that this approach significantly improves module efficiency and lifespan.

Research shows that compared to traditional aluminum technology, full copper technology modules reduce conduction losses by 10%, increase surge current capability by 20%, and improve power cycling capability by 16 times, enhancing the operational resilience and application reliability of power modules.

Introduction to Full Copper Technology

  1. Copper Wire BondingAdvancements in High-Power IGBT Modules: The Benefits of Full Copper Technology

The reliability of high-power IGBT module packaging largely depends on the reliability level of the wire bonding process between chips, as the material and reliability of the wire bonds directly affect the module’s power cycling capability during application.

Comparison of physical properties between copper and aluminum materials:

  • Copper has 60% lower electrical resistivity than aluminum, making it a better conductor.
  • It has a 2-3 times higher elastic modulus than aluminum, making it more ductile and suitable for bonding operations.
  • Copper exhibits better resistance to electromigration compared to aluminum.

Replacing aluminum with copper as the material for chip metallization and interconnection wires is advantageous in reducing IGBT power losses, improving chip heat dissipation, increasing module power density, and enhancing reliability.

Currently, high-power module packaging primarily uses aluminum wires or aluminum strips for interconnection, achieved through ultrasonic bonding. During long-term power cycling, cracking near the root of aluminum wires and chip bonding points may occur. Aluminum has a 50% higher thermal expansion coefficient than copper, and the thermal expansion coefficient mismatch between aluminum and silicon is even more severe. The use of silicon-aluminum alloys (1.2% Si-2% Al) can partially mitigate material performance degradation caused by the thermal expansion coefficient mismatch between the two materials and improve resistance to electromigration and bonding quality. However, silicon-aluminum alloys can impact electrical and thermal conductivity. Increasing bond strength can improve bond quality but carries the risk of damaging the chip.

Traditional aluminum interconnection is nearing its technological limit. Copper, with its low resistivity, high thermal conductivity, low thermal expansion coefficient, and high current-carrying capacity, offers about two orders of magnitude improvement in resistance to electromigration. Using copper wires of the same diameter increases current-carrying capacity by 70%, making copper wire bonding a promising direction for IGBT technology development, especially in high-end applications like rail transportation and power systems.

  1. Copper Metallization of Chips

To implement copper wire bonding, it is essential to ensure that the chip’s surface metallization electrode is made of copper. Therefore, copper metallization of the chip is the foundation for copper wire bonding interconnection. Since copper has lower resistivity and higher thermal conductivity compared to aluminum, a thick copper metal layer on the chip’s surface helps reduce chip conduction losses, improve chip surface heat capacity, and enhance heat dissipation. Copper technology (FRD) significantly improves the operational resilience compared to aluminum technology (FRD).

Copper ions are considered harmful impurities in chips, and compatibility issues with process platforms hinder the application and development of copper technology in power devices like IGBTs. To prevent copper ions from entering the chip, a barrier layer must be introduced between the copper metal layer and the silicon chip. Refractory metals and their nitrides, such as W, Ti/TiN, Ta/TaN, are ideal barrier layer materials due to their excellent conductivity and thermal stability.

Composite structures of metals and their nitrides, such as Ti/TiN and Ta/TaN, have denser structures and offer better copper ion blocking effects. A thin layer of copper is deposited on the barrier layer as a seed layer for electroplating, followed by thick copper electroplating. To prevent copper oxidation, a thin layer of oxidation-resistant metal, such as Ni/Pd/Au, is deposited on the surface.

Chip copper metallization is based on the traditional aluminum process flow but is more complex. It involves changing the surface metal material and structure and involves a longer process.

Due to the mismatch in thermal expansion coefficients between multiple metal layers, stress differences can lead to wafer warpage, reducing the yield during wafer processing. Therefore, precise control of the copper plating and subsequent annealing process is crucial in minimizing wafer warpage caused by imbalanced internal stresses.

  1. Ultrasonic Welding of Copper Terminals

After parallel connection at the chip and substrate levels, the current is collected through copper terminals and connected electrically to external circuits. Therefore, in high-power modules, the terminal soldering points are where current and heat are concentrated, making them one of the weak links with a higher failure rate.

Traditional power terminal interconnections use soldering processes, which have drawbacks such as high electrical/thermal contact impedance, high and uneven temperatures. Ultrasonic welding technology eliminates the interface degradation issues caused by the mismatch in thermal expansion coefficients between different materials and long-term thermal fatigue seen in traditional soldered connections.

The use of ultrasonic welding for busbar connections eliminates contact resistance effects caused by solder layers, thus reducing parasitic resistance conduction losses. Ultrasonic-welded terminal connections exhibit stable mechanical strength and electrical/thermal contact status. Compared to traditional aluminum technology, copper technology IGBT modules offer lower conduction losses, higher surge current capability, longer power cycling lifetimes, and are better suited for demanding

Source: https://www.slw-ele.com/advancements-in-high-power-igbt-modules-the-benefits-of-full-copper-technology.html

Source from: https://www.shunlongwei.com/advancements-in-high-power-igbt-modules-the-benefits-of-full-copper-technology/

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