**3. Overview of design for HEV/EV module**

In HEV/EV applications, power modules must have superior performance and reliability than industrial products as the working environment is harsh in temperature, humidity, and vibration [9, 10–13]. The power modules are stressed heavily and frequently by electrical, thermal, and mechanical actions, so the device itself and the packaging parts are required to be robust enough during their operational life. Moreover, the system and device are restricted by space, weight, and cost of the whole vehicle [2–4]. For these reasons, extensive efforts have been taken to improve the performance and reliability of automotive modules, and a series of optimized design and packaging solutions have been proposed [18–26].

The design of automotive power module should address the performance and reliability issues related to electrical, thermal, and mechanical. They are the main functional aspects that a power module has to serve. The power devices, module structure, materials, and packaging technol‐ ogies are responsible for these performances, reliability, cost, volume, and weight. The components and technologies affecting power module's overall performance are listed in **Table 3**, and the module's reliability and lifetime are limited by the most unstable parts in the packaging.


**Table 3.** Performance and reliability design on automotive power module.

The electrical performance is essential to power module application, with the main parameters affecting system performance are power density, operation temperature, blocking voltage, switching frequency, power dissipation/efficiency, and reverse/short-circuit safety-operating areas (RB/SCSOA). These performances are affected primarily by IGBT and FWD chips. However, the thermal and mechanical performances are mainly dependent on module packaging aspects. Thermal design is a critical step for the enhancement and optimization of thermal resistance, high-/low-temperature storage, thermal cycling, and power cycling, and the mechanical design is beneficial to the module resistant to shock and vibration [14].

#### **3.1. Power chips for HEV/EV module**

It is generally believed that the electrical performance and reliability are mainly controlled by power switches. **Figure 4** shows the vertical structure of an advanced IGBT used in HEV/EV modules. The thin-wafer technology, trench gate, and field stop layer are introduced to tradeoff conduction and switching losses by which the frequency and efficiency are improved. The power dissipation results from leakage currents are reduced by the optimization of chip design. High power density, high RBSOA, and SCSOA capabilities are essential to HEV/EV power train, which are objectives of automotive chip design [2–4, 27–31].

**Figure 4.** Vertical structure of a trench gate, field stop IGBT in HEV/EV modules [31].

At the moment, the standard fourth-generation IGBT technology is widely adopted by various industries. The thickness of 650 and 1200 V chips is reduced to about 70 and 120 μm, along with trench gate, the saturation voltage and conduction loss are reduced substantially compared to thick and planar gate devices. The frequency and switching loss are also opti‐ mized by these structures and field stop layer. Low power dissipation power chips are crucial to HEV/EV industry, which will result in high efficiency and energy saving, low rise of junction temperature (*T*<sup>j</sup> ) and therefore the high thermal reliability.

#### **3.2. Design for low stray inductance**

by space, weight, and cost of the whole vehicle [2–4]. For these reasons, extensive efforts have been taken to improve the performance and reliability of automotive modules, and a series of

The design of automotive power module should address the performance and reliability issues related to electrical, thermal, and mechanical. They are the main functional aspects that a power module has to serve. The power devices, module structure, materials, and packaging technol‐ ogies are responsible for these performances, reliability, cost, volume, and weight. The components and technologies affecting power module's overall performance are listed in **Table 3**, and the module's reliability and lifetime are limited by the most unstable parts in the

> Gate leakage Gate oxide, packaging cleanness Power loss Gate, field stop, thickness, parasitics

Storage Integrity of plastic, passivation, glue, gel Temperature cycling/shock Joining, interconnection, materials

Frequency/SOA Power chip, parasitics

Power cycling Joining, interconnection

The electrical performance is essential to power module application, with the main parameters affecting system performance are power density, operation temperature, blocking voltage, switching frequency, power dissipation/efficiency, and reverse/short-circuit safety-operating areas (RB/SCSOA). These performances are affected primarily by IGBT and FWD chips. However, the thermal and mechanical performances are mainly dependent on module packaging aspects. Thermal design is a critical step for the enhancement and optimization of thermal resistance, high-/low-temperature storage, thermal cycling, and power cycling, and the mechanical design is beneficial to the module resistant to shock and vibration [14].

It is generally believed that the electrical performance and reliability are mainly controlled by power switches. **Figure 4** shows the vertical structure of an advanced IGBT used in HEV/EV modules. The thin-wafer technology, trench gate, and field stop layer are introduced to tradeoff conduction and switching losses by which the frequency and efficiency are improved. The power dissipation results from leakage currents are reduced by the optimization of chip

optimized design and packaging solutions have been proposed [18–26].

28 Modeling and Simulation for Electric Vehicle Applications

**Reliability issues Design optimization**

Mechanical Shock/vibration Bonding, housing

**Table 3.** Performance and reliability design on automotive power module.

**3.1. Power chips for HEV/EV module**

Electrical performance Blocking Chip field depletion, passivation

Thermal performance Resistance, *R*th Module structure, material, technology

packaging.

The parasitic parameters such as resistance (*R*), stray inductance (*L*S), and capacitance have adverse effects on power dissipation, switching speed, and RB/SCSOA. One of the main objectives of automotive module design is to achieve low parasitic parameters. *L*<sup>S</sup> is considered as the chief factor affecting IGBT module's performance and reliability. During the switching, an overshoot voltage (*V*OS), equal to the product of *L*S and current-varying rate, will be applied on the device terminals. If the sum of *V*OS and DC-link voltage is higher than that of deviceblocking voltage (*V*CES), IGBT will be broken down. RB/SCSOA is then reduced because of the *V*OS accordingly. The speed of automotive modules is much higher than industrial applications, resulting in high *V*OS and reliability problems.

*L*S of an IGBT module results from the substrate metal parts, bonding wires, conduct bus bars, control, and auxiliary pins. Design rules for minimizing the parasitic effects are proposed including reductions of current loop geometrical length and area [32], laminated bus bar, planar chip interconnection by using metal lead or PCB [2, 19, 32]. **Figure 5** shows an optimized substrate layout with minimum stray inductance. In this half-bridge substrate, the commutate path and area through DC+ and DC− are reduced to relatively small levels, leading to a small *V*OS during IGBT switching-off verified by both simulation and module test [32]. It is supposed that the commutation loop length and area are valuable indicators of low *L*S substrate design.

**Figure 5.** Optimized current loop dimensions with minimum stray inductance (Right) [32].

Thick and short bus bar and wires are effective to minimize *L*<sup>S</sup> of a module. Due to substrate design, this bus bar may not be applicable. Furthermore, the laminated sandwich layout bus bars are verified as effective low *L*S design solution [31]. The bonding wire-free concept, such as direct lead bond (DLB) [20, 21], double-side soldering/sintering on PCB or top-layer substrate [31] are good solutions to lower *L*S.

**Figure 6** shows a novel concept of double-side bonding in which bonding wire is eliminated. Therefore, *L*<sup>S</sup> can be reduced, wire bonds failure is avoided, and the heat transfer efficiency is enhanced significantly by spreading through both sides of the chips. The planar IGBT module has been developed and applied in HEV/EV with great interest by the industry [20, 21].

**Figure 6.** A planar interconnection concept for automotive module assembly [14].

#### **3.3. Thermal design for automotive module**

Thermal performance and reliability are of most importance for automotive IGBT modules as the ambient temperature is very high under the hood. On the other hand, the active power cycling and surging are more frequent than other applications that happen in the acceleration and deceleration stages. Therefore, large passive and active temperature excursions always occur in an automotive module operation. For the sake of cost and system complexity, customers prefer the traction inverter to share cooling system with the engine, meaning that the temperature of coolant could be up to 105°C in the near future. The abovementioned problems result in serious reliability problems on power module joining and interconnection parts. The solder layers of chip attach, substrate attach, and bus bar attach are prone to delamination and failure because of fatigue finally due to high absolute temperature and high temperature swing (Δ*T*<sup>j</sup> ), and the bonding wires will be cracked or lifted off [33–35].

Reliability and lifetime of a power module is limited by the weakest point of the above parts. It is reported that power module lifetime reduces exponentially with the minimum/maximum junction temperature (*T*jmin/*T*jmax) and temperature swing. An outstanding thermal design gives smaller Δ*T*<sup>j</sup> from the low thermal resistance of junction to case (*R*th j-c) and junction to heat sink (*R*th j-h) and enhances reliability [33–35].

Thermal design of IGBT module lies in the chip and packaging structure and materials. By elevating *T*jmax of chips, the reliability will be enhanced as the improvement of electrical performance, and the requirement of module design will be mitigated. Currently, the fourth IGBT chips have a *T*jmax of 150°C, and it is proposed that *T*jmax of next-generation automotive module should reach 175°C, which requires redesigns in chip-doping profile, passivation, and metal materials.

**Figure 5.** Optimized current loop dimensions with minimum stray inductance (Right) [32].

**Figure 6.** A planar interconnection concept for automotive module assembly [14].

**3.3. Thermal design for automotive module**

substrate [31] are good solutions to lower *L*S.

30 Modeling and Simulation for Electric Vehicle Applications

Thick and short bus bar and wires are effective to minimize *L*<sup>S</sup> of a module. Due to substrate design, this bus bar may not be applicable. Furthermore, the laminated sandwich layout bus bars are verified as effective low *L*S design solution [31]. The bonding wire-free concept, such as direct lead bond (DLB) [20, 21], double-side soldering/sintering on PCB or top-layer

**Figure 6** shows a novel concept of double-side bonding in which bonding wire is eliminated. Therefore, *L*<sup>S</sup> can be reduced, wire bonds failure is avoided, and the heat transfer efficiency is enhanced significantly by spreading through both sides of the chips. The planar IGBT module has been developed and applied in HEV/EV with great interest by the industry [20, 21].

Thermal performance and reliability are of most importance for automotive IGBT modules as the ambient temperature is very high under the hood. On the other hand, the active power cycling and surging are more frequent than other applications that happen in the acceleration and deceleration stages. Therefore, large passive and active temperature excursions always occur in an automotive module operation. For the sake of cost and system complexity, customers prefer the traction inverter to share cooling system with the engine, meaning that the temperature of coolant could be up to 105°C in the near future. The abovementioned problems result in serious reliability problems on power module joining and interconnection parts. The solder layers of chip attach, substrate attach, and bus bar attach are prone to

To enhance reliability and prolong lifetime, power dissipated in chips and parasitic compo‐ nents must be spread with high efficiency, which can be achieved by low *R*th i-c. Design for low *R*th j-c is dependent on the optimization of module structure and material. The high thermal conductivity ceramic such as AlN and Si3N4, and Cu or AlSiC baseplate with optimized thickness, direct cooling structure without using thermal grease are proved effective solutions to reduce the overall *R*th j-h. However, the thermal performance should be traded off with reliability, weight, and cost. **Figure 7(a)** shows that a direct cooling pin-fin baseplate can reduce the *R*th j-h of conventional module by about 50% because of eliminating the grease layer [4, 18, 22, 23]. The direct liquid cooling (DLC) pin fins can be optimized in terms of efficiency, shape, layout, material, and cost. **Figure 7(b)** shows an automotive IGBT module with optimized Al in-line pin fins, in which the weight and cost are saved by maintaining merits of low thermal resistance and high reliability. Thermal simulation shows that the power switches work at the safe temperature envelop during the highest transient and continuous power output stages of a passenger car sharing 105°C cooling of the engine. Lifetime of the module is predicted under a real mission, which shows that it is capable of meeting the requirements with high coolant temperature [3].

**Figure 7.** Comparison of *R*th j-h between conventional and direct cooling modules (a) and automotive IGBT module with optimized Al in-line pin fins (b).

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 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**

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 Δ*T*<sup>j</sup> 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 conventional Al wire and soldering, soldered and sintered die attachment [37, 38].

**Figure 9.** Improvement of lifetime by copper wire with novel soldering (Left) [37], lifetime comparison of modules with soldered and sintered die attach [38].

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)** shows that the reliability of bonding can be enhanced by ultrasonic welding, as negligible degradation of bonding tensile strength was found [39].

**Figure 10.** Ultrasonic welded terminals and pins in a HEV/EV module (a), and the reliability comparison with soldered terminals (b) [39].
