**4. State-of-the-art HEV/EV power module**

Baseplate free- and double-side-cooling modules are proposed for automotive application for their good thermal performance as shown in **Figure 8** [20, 36]. The baseplate-free module can benefit to *R*th j-h, weight, and cost, and the double-side-cooling structure can increase further

Although low *R*th j-c reduces Δ*T*<sup>j</sup> at constant power loss level, the high *T*jmin/*T*jmax together with

**Figure 9.** Improvement of lifetime by copper wire with novel soldering (Left) [37], lifetime comparison of modules

The mechanical shock and vibration affect mostly on the conduct bus bar and pins, which happen frequently in the running of an automobile. The strength of contacts should be enhanced in order to meet automotive standard that requires the module to be tested for 2 h per axis at more than 10 g for vibration, and three times at each direction and more than 100 g for shock. The ultrasonic welding with injection-molded housing (**Figure 10(a)**) as well as pressure contact is designed for achieving the mechanical reliability standard. **Figure 10(b)**

conventional Al wire and soldering, soldered and sintered die attachment [37, 38].

 can degrade gradually module's weakest point such as wire bonds, die attach solder layer, conduct lead, and substrate attach solder layer. The planar and next-generation copperbonding wires with novel soldering technology are effective solutions to this instability. The novel die attachment technologies such as silver sintering and transient liquid phase sintering (TPLS) are verified to improve the power cycling capability by orders of magnitude. **Fig‐ ure 9** shows lifetime comparison of copper wire incorporated with novel soldering and

the heat transfer efficiency. Both the modules are successfully applied in HEV/EV.

**Figure 8.** Baseplate-free (Left) [20] and double-side cooling [36] automotive modules.

**3.4. Technology design for automotive module**

32 Modeling and Simulation for Electric Vehicle Applications

with soldered and sintered die attach [38].

Δ*T*<sup>j</sup>

In this section, the typical state-of-the-art commercial and custom HEV/EV power modules are reviewed and evaluated. The design and manufacture of automotive power module were following industrial power module packaging standard at the beginning. The conventional structure and technologies were applied in automotive module, which was the sandwich structure including plain baseplate and direct bond copper (DBC) substrate interconnected by solder reflowing and wire bonding. The structure and technologies are difficult to meet HEV/ EV requirements in thermal and mechanical performance, as well as in the reliability, lifetime, cost, volume, and weight. Therefore, power semiconductor and automotive industry had developed a series of power modules dedicated for HEV/EV application as described in the following.

#### **4.1. Direct liquid-cooled HEV/EV power module**

Direct liquid cooling (DLC) was supposed to be an efficient solution to HEV/EV modules with its advantages of efficiency, integration, weight, and size [2–4]. A typical DLC module integrates liquid-cooling structure such as pin fins into the baseplate, which can flow through coolant without an external heat sink. Therefore, the traditional thermal interface layer between baseplate and heat sink is eliminated, and the un-uniformity and degradation of thermal grease will be avoided as well. It is reported that the *R*th j-h could be reduced by 50% of plain plate in the application, resulting in much lower Δ*T*<sup>j</sup> and reinforcement of the reliability and lifetime. Therefore, the pin-fin DLC IGBT module is a good solution to HEV/EV power systems not only in the aspects of reliability but also performance, cost, and weight [2–4, 18, 22, 23].

DLC module with pin-fin plate is excellent in delivering higher power than plain base or baseplate-free modules, and the converter system with DLC module is compact and reliable. **Figure 11** shows the commercial DLC modules with pin-fin plate. A technology trend for IGBT module cooling in HEV/EV power-train system uses coolant with elevated temperature, so the power device can share cooling system with engine at up to 105°C liquid [3, 8]. This will simplify power electronics system without separate cooling circuit, resulting in the reduction of overall cost, weight, and volume of whole vehicle. However, high-temperature cooling has huge adverse effects on reliability and lifetime of power module, and may result in exceeding of *T*j max. The direct liquid cooling is generally believed as an efficient thermal management with high cooling efficiency at high-temperature applications. The application of DLC module in HEV/EV has been widely accepted [3–5, 40].

**Figure 11.** The commercial DLC modules for HEV/EV power system.

The manufacture complexity and cost of pin-fin baseplate are high compared to plain plate at the moment, and the new technologies are required to integrate DLC structure into external cooling path.

**Figure 12** shows an integrated cooler structure [2] with the direct bonded Al (DBA) substrates are directly bonded (by brazing) onto specially fabricated cold plate to realize direct cooling of the power module. The integrated module and cooling structure eliminates the conventional baseplate and thermal interface layer. It achieves 30% improvement in thermal performance. The assembly includes a buffer plate with punched holes for releasing the stresses between the cooler and DBA caused by a coefficient of thermal expansion (CTE) mismatch. The Al ribbons were used to replace Al wires for improving the reliability and electric parasitic parameters of die interconnections.

**Figure 12.** Integrated automotive power system with baseplate-free module and cold plate [2].

### **4.2. Baseplate and solder-free automotive power module**

**Figure 11** shows the commercial DLC modules with pin-fin plate. A technology trend for IGBT module cooling in HEV/EV power-train system uses coolant with elevated temperature, so the power device can share cooling system with engine at up to 105°C liquid [3, 8]. This will simplify power electronics system without separate cooling circuit, resulting in the reduction of overall cost, weight, and volume of whole vehicle. However, high-temperature cooling has huge adverse effects on reliability and lifetime of power module, and may result in exceeding of *T*j max. The direct liquid cooling is generally believed as an efficient thermal management with high cooling efficiency at high-temperature applications. The application of DLC module

The manufacture complexity and cost of pin-fin baseplate are high compared to plain plate at the moment, and the new technologies are required to integrate DLC structure into external

**Figure 12** shows an integrated cooler structure [2] with the direct bonded Al (DBA) substrates are directly bonded (by brazing) onto specially fabricated cold plate to realize direct cooling of the power module. The integrated module and cooling structure eliminates the conventional baseplate and thermal interface layer. It achieves 30% improvement in thermal performance. The assembly includes a buffer plate with punched holes for releasing the stresses between the cooler and DBA caused by a coefficient of thermal expansion (CTE) mismatch. The Al ribbons were used to replace Al wires for improving the reliability and electric parasitic

**Figure 12.** Integrated automotive power system with baseplate-free module and cold plate [2].

in HEV/EV has been widely accepted [3–5, 40].

34 Modeling and Simulation for Electric Vehicle Applications

**Figure 11.** The commercial DLC modules for HEV/EV power system.

cooling path.

parameters of die interconnections.

The state-of-the-art IGBT modules are based on a solder construction for chips attaching to substrate and substrate attaching to baseplate. Investigations have shown that these solder layers constitute the weakness of power semiconductor module as they demonstrate fatigue when exposed to active and passive temperature cycling. **Figure 13** shows an automotive power module named SKiM by Semikron, which is designed with high reliability to meet the demands of automotive applications in terms of shock and vibration stability, as well as hightemperature capability and service life [31].

**Figure 13.** Baseplate and solder-free automotive power module [31].

The module features a pressure-contact low-profile housing that boasts the advantages of 100% solder-free module, Pb-free, and spring contacts for auxiliary contacts. The chips are sintered by silver on substrate, achieving a very high-power cycling capability. The sinter joint is a thin silver layer whose thermal resistance is superior to that of a soldered joint. Due to the high melting point of silver (960°C), no joining fatigue occurs, resulting in an increased service life [31].

The pressure contact of bus bar and auxiliary pins results in very low thermal and ohmic resistance and high thermal reliability. The laminated sandwich main terminals as shown in **Figure 14** benefits to a very low stray inductance and therefore improves the reliability, efficiency, and electrical performance. The single chip is connected symmetrically in **Fig‐ ure 15**, leading to similar stray inductances for the individual chips and a homogeneous current distribution [31]. The baseplate-free structure has advantages of low volume and lightweight, but a thermal interface layer must be applied to improve the contact between substrate and heat sink, which deteriorates the thermal performance and reliability.

**Figure 14.** Main terminals with sandwich structure and low inductance [31].

**Figure 15.** The substrate with symmetrically chip layout and terminal pressure contact areas [31].

#### **4.3. Direct lead bond automotive module**

A Transfer-mold power (TPM) packaged by direct lead bond (DLB) technology was released to automotive power electronics market by Mitsubishi and Bosch [20, 21, 41], which makes HEV/EV applications more reliable and compact. **Figure 16** shows the power module samples, the low profile, and compact package achieved by the concept.

**Figure 16.** TPM automotive module prototypes from Mitsubishi and Bosch (Right) [20, 41].

The internal cross section of the packaging structure is shown in **Figure 17**. The transfer-mold case chips are bonded on heat spreader and on lead frame directly (DLB) by lead-free solder, the TCIL is attached on the heat spreader for electrical isolation and contact with external heat sink.

**Figure 17.** The internal cross section of TPM module with DLB [20].

DLB is the key feature of the module, by which the internal lead resistance is decreased to 50% and the self-inductance is decreased to 60% compared with the classically wire-bonded TPM module. The large solder contact area of DLB results in a uniform chip surface temperature distribution and a small thermal resistance. The integrated heat spreader reduces the contact thermal resistance and transient thermal impedance. The construction provides a larger area of heat flow between junction and case. The features of DLB TPM automotive has enhanced almost 30 times of power and thermal cycling capability compared with the conventional module case assembled with wire bond technology. In addition, the on-chip temperature and current sensors are integrated into the IGBT die, enabling a precise, safe, and fast over temperature protection, and detects and turns off a short-circuit situation without the IGBT entering a de-saturation phase [20, 21, 41].

**Figure 15.** The substrate with symmetrically chip layout and terminal pressure contact areas [31].

the low profile, and compact package achieved by the concept.

**Figure 16.** TPM automotive module prototypes from Mitsubishi and Bosch (Right) [20, 41].

**Figure 17.** The internal cross section of TPM module with DLB [20].

A Transfer-mold power (TPM) packaged by direct lead bond (DLB) technology was released to automotive power electronics market by Mitsubishi and Bosch [20, 21, 41], which makes HEV/EV applications more reliable and compact. **Figure 16** shows the power module samples,

The internal cross section of the packaging structure is shown in **Figure 17**. The transfer-mold case chips are bonded on heat spreader and on lead frame directly (DLB) by lead-free solder, the TCIL is attached on the heat spreader for electrical isolation and contact with external heat

**4.3. Direct lead bond automotive module**

36 Modeling and Simulation for Electric Vehicle Applications

sink.

As the evolution of the first generation of DLB module, a six-in-one HEV/EV module bonded by DLB and integrated with direct water-cooled Al fin was developed [21]. The adoption of these innovative technologies has led to improved thermal performance of 30%, and has reduced the footprint by 40% and the module weight by 76%. **Figure 18** shows the module prototypes and internal structure. The Al cooling fin was integrated into module for direct liquid cooling. DLB is employed that has extensive advantages to power density, thermal and electrical performance, reliability, etc. The Al cooling fins have lower thermal conductivity compared to Cu pin-fin structure, but they have high durability when exposing directly to coolant and are much lighter. Compared to the first-generation DLB modules of **Figure 16**, as much as 76% weight reduction and 30% thermal performance improvement were achieved based on the same current and voltage for three-phase HEV/EV motor drives [21].

**Figure 18.** The prototypes and package structure of a high performance, compact size, and light-weight HEV/EV pow‐ er module [21].

The custom power module in Nissan LEAF pure EV shown in **Figure 19** has the same concept of DLB [2]. The power semiconductor dies are attached onto Cu plate, which is an electrical terminal and is wire bonded to other terminals to form a half-bridge configuration. The largearea Cu bus bars act as heat spreader and are mounted onto external cold plate through a separated electrical insulator sheet. The sheet has a special composition and offers high thermal conductivity.

**Figure 19.** The custom module for Nissan LEAF and its schematic of cross-sectional view [2].

#### **4.4. Planar interconnection and double-sided-cooled automotive module**

In a conventional module packaging, the top electrodes of die are electrically interconnected by bonding Al wires, while the whole bottom metal surfaces are soldered onto insulating ceramic with direct bond copper or aluminum surfaces. This asymmetric package structure has a series of drawbacks such as large parasitic electric parameters, deformation of die subjected to thermomechanical stress, small thermal conduction path through the top of die, etc.

Therefore, changing the top interconnection configuration to a planar or symmetric package will bring comprehensive benefits to thermal, electrical, and reliability. With a planar inter‐ connection, the die can be connected to cold plates at both sides to achieve double-sided cooling, and the thermal performance can be enhanced accordingly. This will eliminate the traditional bonding wires but require that front metal of chips must be solderable [19].

The concept of planar IGBT packaging without bonding wires is shown in **Figure 6**, and the planar modules were developed for HEV/EV and aerospace industries. By soldering or sintering semiconductor chips to copper leads directly or to DBC system, the module can be cooled by liquid or forced air at both sides, which provides 70% higher cooling efficiency than a conventional single-side cooling module. A joining layer on a chip active area will spread heat easily and result in low-junction temperature and high reliability [15, 19]. The removal of bonding wires has advantages on reliability as wire bonds are prone to failure during operation because of the high intermittent temperature cycling from the junction. On the other hand, the parasitic resistance and inductance are reduced accordingly by large area contact, which improves efficiency and dynamic performance such as the safe operating areas of RBSOA and SCSOA [2, 6].

IR has presented a new power management platform approach for HEV/EV to help address the need to reduce the size, weight, and system cost of electric power-train components while increasing system reliability for long lifetime, low maintenance, and low warranty cost. The packaging platform named CooliR2TM characterizes wire bond frees and transfer-mold technologies that addresses all the HEV/EV module packaging challenges. The IGBT and diode called CooliR2 DIE were designed for the platform. The IGBT has reduction of conduction and switching losses, increases of blocking voltage, and compatibility with wire bond-free interconnection techniques, and the switching frequency and maximum *T*<sup>j</sup> were increased to

20 kHz and 175°C, respectively. The diode was optimized for automotive traction by fast-speed soft recovery with oscillation-free behavior [15, 19, 42].

**Figure 20** shows the CooliR2 DIE as building blocks and the construction of a half-bridge package by using the die. The building-block approach of CooliR2TM platform has advantages of cost reduction and mechatronics enabler. The electrical performance of package is improved with lower resistance and parasitic inductance. The cooling method is flexible for no baseplate cooling, or attaching a baseplate or direct liquid-cooled heat sink to substrate. The transient thermal impedance and die temperature distribution are improved in the packaging. In addition, the reliability and power density are increased by the wirebond less, dual-sided cooling and higher *T*jmax solutions.

**Figure 19.** The custom module for Nissan LEAF and its schematic of cross-sectional view [2].

38 Modeling and Simulation for Electric Vehicle Applications

**4.4. Planar interconnection and double-sided-cooled automotive module**

etc.

SCSOA [2, 6].

called CooliR2

In a conventional module packaging, the top electrodes of die are electrically interconnected by bonding Al wires, while the whole bottom metal surfaces are soldered onto insulating ceramic with direct bond copper or aluminum surfaces. This asymmetric package structure has a series of drawbacks such as large parasitic electric parameters, deformation of die subjected to thermomechanical stress, small thermal conduction path through the top of die,

Therefore, changing the top interconnection configuration to a planar or symmetric package will bring comprehensive benefits to thermal, electrical, and reliability. With a planar inter‐ connection, the die can be connected to cold plates at both sides to achieve double-sided cooling, and the thermal performance can be enhanced accordingly. This will eliminate the traditional bonding wires but require that front metal of chips must be solderable [19].

The concept of planar IGBT packaging without bonding wires is shown in **Figure 6**, and the planar modules were developed for HEV/EV and aerospace industries. By soldering or sintering semiconductor chips to copper leads directly or to DBC system, the module can be cooled by liquid or forced air at both sides, which provides 70% higher cooling efficiency than a conventional single-side cooling module. A joining layer on a chip active area will spread heat easily and result in low-junction temperature and high reliability [15, 19]. The removal of bonding wires has advantages on reliability as wire bonds are prone to failure during operation because of the high intermittent temperature cycling from the junction. On the other hand, the parasitic resistance and inductance are reduced accordingly by large area contact, which improves efficiency and dynamic performance such as the safe operating areas of RBSOA and

IR has presented a new power management platform approach for HEV/EV to help address the need to reduce the size, weight, and system cost of electric power-train components while increasing system reliability for long lifetime, low maintenance, and low warranty cost. The packaging platform named CooliR2TM characterizes wire bond frees and transfer-mold technologies that addresses all the HEV/EV module packaging challenges. The IGBT and diode

switching losses, increases of blocking voltage, and compatibility with wire bond-free

interconnection techniques, and the switching frequency and maximum *T*<sup>j</sup>

DIE were designed for the platform. The IGBT has reduction of conduction and

were increased to

**Figure 20.** CooliR2 DIE building block and the construction of a half-bridge package by using CooliR2 DIE [42].

**Figure 21** shows a custom automotive power module for Toyota LS600, in which two planar Cu plates are directly soldered onto power electrodes on the dies from both surfaces. The module is encapsulated with transfer-molded compound while keeping the Cu plates exposed to the outside for acting as heat sinks to transfer device heat to a cold plate (cooling tube) from two surfaces [2]. Therefore, the module's thermal resistance is reduced dramatically. Insulator layers are required at both sides between power module and cold plate as the module is nonelectrically isolated.

**Figure 21.** Custom power modules of Toyota LS600 and its schematic of cross-sectional view [2].

In **Figure 22**, Delphi planar [36] power module for dual-side cooling is shown. The DBC isolates module to external heat sink. It is a co-packaged IGBT and diode unit that needs next-level interconnection to form power inverters, so the pressure must be controlled to ensure the press contact between all package units and cold plates for double-sided cooling. However, the assembly complexity of electrical interconnections is difficult and costly at inverter-level packaging.

**Figure 22.** Delphi planar bond power module with dual-side cooling [36].

The Semikron double-sided planar power module using SkiN technology is shown in **Figure 23**. The die top connection is a flex circuit board, and all the joining interfaces between two sides of die and substrate, and DBC and heat sink, are bonded by Ag-sintering process. This provides very high thermal and power cycling reliability, as well as good thermal and electrical performance [43].

**Figure 23.** Schematic of cross-sectional view of Semikron SKiN power module [43].
