The full name of IGBT is insulated gate bipolar transistor. It is characterized by voltage control, high voltage withstanding, small saturation voltage drop, fast switching speed, energy saving, etc. The power module is the core component of the electric vehicle inverter, such as 2000w pure sine wave inverter or 3000w inverter, and its packaging technology has a crucial impact on system performance and reliability.
Traditional single-sided cooling power modules have been one of the most common packaging structures in automotive applications. And IGBT double-sided cooling model has been developed lately, which is a big upgrade for the application of inverter IGBT.
Main content:
1. The importance of IGBT heat dissipation
With the development of power electronic devices towards high density, high power and miniaturization, the large-scale use of electronic devices brings convenience to our lives. At the same time, higher and higher power makes the heat dissipation problem of electronic devices more and more serious. Therefore, heat dissipation is a very critical technology, and the quality of heat dissipation directly affects the performance and life of the home solar power system.
In device packaging, due to different packaging materials, the differences in thermal expansion coefficients of the packaging materials lead to varying degrees of deformation and thermal stress accumulation in the device, eventually leading to failure problems such as detachment of bonding wires, delamination of solder, cracking and delamination of plastic packages, etc.
The increase in temperature will also reduce the performance of the device, causing problems such as current load capacity and gate voltage impact. The heat generated by the power chip loss of traditional single-sided heat dissipation is conducted to the radiator in one direction through the insulating substrate and bottom plate.
Although this method can solve certain heat dissipation needs, it cannot solve the heat dissipation needs of some large amounts of heat. Using a single-sided heat dissipation solution, the heat transfer channel is limited and the thermal resistance is large, resulting in a large temperature difference between the chip and the heat dissipation surface. During long-term use, the chip is easily burned due to excessive temperature.
In traditional power module packages, the top side of the power semiconductor device is used only for electrical connections, while the bottom side is usually connected to a DBC (Direct Bonded Copper) substrate for electrical connections and heat transfer.
Wire bonding has been the interconnect method used in power module packaging due to its ease of use and low production cost. However, this asymmetric packaging structure has a series of defects such as large parasitic electrical parameters and mold bending under thermal stress.
Although there have been some improvements in wire bonding technology, including the use of Cu or Al stripline bonding to replace Al, due to higher thermal stress at the connection point and relatively low connection strength, wire bonding is still the weakest part to power module reliability. Lead bonding methods are also a major source of parasitic losses. More importantly, the presence of wire bonds prevents the possibility of heat dissipation from the top of the power semiconductor device.
2. Advantages of double-sided cooling
To move away from wire bonding as an interconnection method and introduce alternative interconnection technologies, power semiconductor devices are directly connected to copper conductors through solder or sintering so that heat can be dissipated and transferred through both sides of the power semiconductor device.
Due to the elimination of wire bonds, the additional path on the top of the power semiconductor device makes two parallel cooling paths possible, thus forming a double-sided cooling power module.
In recent years, there have been more and more studies on double-sided cooling of power modules. Compared with the single-sided heat dissipation structure, the double-sided cooling structure has insulating and heat-conducting substrates welded on both sides of the power chip.
All power terminals are connected to the insulating and heat-conducting substrates, and a radiator is installed on the outside of the insulating and heat-conducting substrates. This design provides better heat transfer and significantly lowers the effective temperature. In theory, double-sided cooling can reduce the Rth between the device and the coolant by 50%.
Double-sided cooling IGBT packaging structure:
Compared with single-sided cooling power modules, the advantages of double-sided cooling power modules include:
- Improved thermal performance will reduce temperature fluctuations and thermal stress within the power module.
- Eliminating wire bonds also eliminates one of the major failure modes in traditional power module packaging. As a result, the power cycling capabilities and reliability of double-sided cooling modules have been proven to be an order of magnitude higher than single-sided cooling modules, resulting in longer service life.
- Double-sided cooling of the package requires a planar power package, minimizing the current loop area. It reduces electrical parasitic inductance, optimizing the resistance reduction resulting from larger bonding areas. Wireless bonding configurations are key for silicon carbide devices due to their lower parasitic inductance and higher packaging density.
3. Current status of IGBT double-sided cooling technology
At present, there are mainly 4 IGBT double-sided heat dissipation packaging structures:
① Denso double-sided cooling power module
- Chip:
- Spacer:
In order to balance the chip height difference between the IGBT and the diode, a copper spacer is placed on top of each of the IGBT and the diode. This copper spacer also provides a buffer for thermal diffusion when passing current. Spectral analysis shows that Denso used a pure copper spacer, rather than a copper-molybdenum alloy, which usually has a coefficient of thermal expansion closer to silicon.
- Electrical connections:
For the planar package, both sides of the chip are connected using conventional solder connections, but the IGBT gate uses aluminum bonding wires and a bonding process.
- Copper substrate:
Instead of using the common ceramic substrate, Denso uses thicker copper substrates on the top and bottom sides to connect the positive and negative collectors and emitters of the IGBTs respectively, and to lead out the terminals. Compared to ceramic substrates, copper substrates undoubtedly have a greater advantage in thermal conduction.
- Filler:
The gaps between the copper substrates are sealed and molded by injection molding through a transfer mold.
- Insulation:
Since no ceramic substrate is used, this power module is not insulated. Therefore, before assembly with the heat sink, two layers of silicon nitride insulation on both sides are also required for electrical insulation.
- Thermally conductive silicone grease:
In order to minimize the interfacial thermal resistance caused by the insulating layer, thermally conductive silicone grease is applied to both sides of each piece of insulation. In early designs, Denso used 24 pieces, 12 layers of power modules, in order to ensure an even distribution of pressure.
It can be imagined that the thickness of the thermal grease design is not thin. For each power module, the 4 layers of thermal grease on the outside actually offset the low thermal resistance of double-sided cooling to some extent.
② Hitachi double-sided water cooling heat dissipation
- Chip:
Consists of two sets of power components in series, using two-in-one half-bridge package. Considering that the area of the pin fins is only about 6cm×4cm, the position left for the IGBT and the continuity diode in each component is only 2cm×4cm.
So it is suspected that its application in hybrid models contains only one piece of IGBT and one piece of continuity diode for each switch, whereas each switch contains two pieces of IGBT and continuity diode connected in parallel for high-power pure electric vehicle models. 2 groups of IGBTs in the unit are placed in the same direction. The IGBTs are placed in the same direction in the two groups of units.
- Electrical connection:
In order to realize the flat package, both sides of the chip are connected by conventional soldering, and the latest nano-silver paste sintering process is not used, so it may be challenging to deal with high-temperature application scenarios of silicon carbide later on. Only the IGBT gate is bonded.
- Copper lead frames:
Thicker copper lead frames on both sides are used to connect the positive and negative collectors and emitters of the IGBTs respectively, and lead to the terminals.
- Insulation layer:
Two insulating layers are used to realize electrical insulation, which are affixed to the outside of the two layers of copper lead frames. The patent describes this insulating layer as an epoxy resin doped with thermally conductive fillers, with a thickness of about 120 microns.
In addition, Hitachi emphasizes that no thermally conductive silicone grease or thermal interface material (TIM) to reduce the interface thermal resistance, to ensure high thermal performance.
- Aluminum housing:
This housing has a pin-finned area on each side, which allows the IGBT module to be placed directly into the water cooling channel for direct double-sided water cooling without the need for thermally conductive silicone grease or an additional heat dissipation substrate.
The top of the aluminum housing also provides the sealing structure for the cooling channels and the connections for the high and low car battery voltage terminals. On the inside of the aluminum, there is an anodized layer of thickness of about 50 microns.
This anodized layer helps to connect with the insulating layer and also serves as an insulating layer, which enhances the electrical insulation properties of the package module.
- Fillers:
The gaps between the copper leadframes are sealed by injection molding in the first transfer mold; the gaps inside the aluminum metal housing are sealed by injection molding in the second transfer mold.
③ Viper double-sided water cooling heat dissipation
- Chip:
Each module contains only one set of power components, including one piece of IGBT and one piece of continuity diode, which is equivalent to one-in-one package.
- Electrical connection:
In order to realize the flat package, both sides of the chip are connected by conventional tin soldering, including gate connection, without using any binding wires, so the reliability of the module as a whole has been greatly improved.
- Ceramic substrate:
Delphi uses a relatively rare aluminum-based aluminum nitride ceramic substrate. The thickness of the three layers of the upper substrate is 200 microns for the outer layer of aluminum, 700 microns for the middle layer of aluminum nitride, and 350 microns for the inner layer of aluminum.
The thickness of the three layers of the lower substrate is 200 microns for the outer layer of aluminum, 700 microns for the middle layer of aluminum nitride, and 270 microns for the inner layer of aluminum.
- Filler:
Aluminum-based aluminum nitride ceramic substrates between the gap through the transfer mold injection sealing molding.
- Thermally conductive silicone grease:
Because this module is indirectly cooled by water, it requires the use of thermally conductive silicone grease between both sides of the module and the heat sink to reduce the interface thermal resistance.
④ Double-sided water cooling heat dissipation
- Chip:
Contains two groups of power elements, each group includes one IGBT and one continuity diode, equivalent to a two-in-one half bridge package. The two groups of power components are placed in the same direction.
- Spacer:
In order to balance the chip height difference between IGBT and diode, there is a spacer on top of each IGBT and diode. This spacer also provides a buffer for heat diffusion when conducting current.
Instead of the common copper-molybdenum alloy, the spacer is made of aluminum silicon carbide, which moderately sacrifices thermal conductivity but takes into account the coefficient of thermal expansion.
- Electrical connection:
In order to realize flat package, both sides of the chip and the top side of the spacer are connected by conventional tin soldering, and the IGBT gate adopts aluminum binding line and bonding process, not using the latest nano-silver sintering process.
- Copper substrate:
The use of common copper-based alumina ceramic substrate, cost-effective, and compared to Denso's double-sided cooling package, copper-based alumina ceramic substrate comes with insulation, making the module itself insulated, but also eliminates the need for insulation layer.
- Filler:
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