Clamping Technique Improves IGBT Installation

Oct 1, 2002 12:00 PM
By C. Panzer, T. Passe, T. Stolze, eupec GmbH, Warstein, Germany and Z. Liang, Infineon-Technologies

Use of clamps eliminates the drilling and tapping of holes to mount modules with screws.

In the past, finding the optimum IGBT module solution was difficult in terms of low cost in combination with high performance and high reliability. Some discrete IGBTs employed a type of silicon rubber foil for isolation. Other IGBTs were packaged in modules with DCB substrates. There were also molded IGBT modules in numerous configurations.

But today, various IGBT devices are available for low-cost consumer applications and low power drives. The EasyPIM/PACK product family (Fig. 1) meets the requirements of low overall cost, joining the list of integrated power modules that replace discrete devices for these applications. A major cost factor is mounting the modules, which can be minimized by using the appropriate process and materials (Fig. 2).

In addition to mounting considerations, the IGBT modules' thermal properties were a concern. For example, some IGBT modules without baseplates are mounted on a heatsink using screws. They eventually face a reduction of thermal conduction between the case and heatsink because, after a period of operation, the screws require retightening to maintain their thermal characteristics.

The EasyPIM/Pack mounting system is similar to existing solutions in mass production for automotive applications. It avoids thermal stress by using clamps. Clamps assure a dynamic attachment of the module to the heatsink, along with a constant force during the life cycle of the application — a cost-efficient and flexible approach (Fig. 3).

Mounting Concept

Clamps hold the EasyPIM/PACK package on the heatsink. Two types are available. Those fixed with screw clamps (Fig. 4) provide all the technical advantages of the clip clamp types, shown in Figs. 5 and 6, on page 58, which minimize the mounting effort. For the clip type, no drilling and trapping of holes is needed to mount the module with screws. Instead, a plug-and-play system is used. First, you place the clamps into the slots on the heatsink. Then, you place the module on the heatsink using thermal paste. Last, you snap the clamps onto the package (Fig. 6).

Contrast this technique with the traditional mounting system that uses screws on plastic, which has a mechanical point of weakness: Without retightening the screws after some period of operation, the plastic compresses under the screws. You must adjust the screw and the washer to accommodate for this creep distance value.

The minimum torque for an M4 screw is about 1.5Nm, which equals 1000N of force. With a washer under the screw head, the pressure on the plastic is about 20.4N/mm². The clamp for EasyPIM2 produces F_max=80N, which means about 14N/mm². Using Fig. 7, you can determine the resulting creep distance values for this screw.

With a traditional mounting system, you must adjust the screw and the washer at about 70 µm, compared with about 40 µm for clamps. As shown in Fig. 8, the force of a screw reduces as it settles down. The reason the module doesn't lose its complete force against the heatsink is because of the effects of thermal bending of the DCB (Direct Copper Bond) — a ceramic insulator with copper circuit etch that acts as the baseplate for the module.

The clamp type automatically pushes the module's DCB back to the heatsink while the screw type does not.

Fig. 9 is the force-length graph for a typical screw vs. clamp. Due to material relaxation during operation, the bow between the DCB and heatsink decreases. Therefore, the thermal glue is pressed out of the area between the heatsink and the case. When temperatures decrease, the DCB cannot contract because of a “vacuum” that is formed. This vacuum holds the DCB closer to the heatsink. The vacuum depends on the properties of the thermal paste and isn't controllable. Thus, common modules without a baseplate and fixed with screws have to be retightened after some period of operation. Otherwise, the mechanical connection between the DCB and heatsink will not be controllable. With the new clamp mounting solution, the force might be reduced by only 4N. Therefore, the force of the clamp pressing the package to the heatsink is nearly constant, resulting in a reliable connection between DCB and heatsink that is independent of the thermal movements between the DCB and heatsink.

To understand the influence of the force pushing the DCB to the heatsink on thermal resistance, this dependence is:

R_th,jh=R_th,jc+R_th,ch

Where:

Rth,jh = Thermal resistance between junction and heatsink

R_th,jc = Thermal resistance between junction and case

R_th,ch = Thermal resistance between case and heatsink

The thermal resistance between junction and case (DCB) is nearly constant, but the resistance between case and heatsink is not. This R_th,ch depends on the force that presses the DCB to the heatsink and the temperature of the ceramic.

For T_DCB = 25°C, the bow between case and heatsink is, for example, about 40µm (Fig.10). The cavity between the DCB and heatsink depends on the DCB temperature. Tests show a reduced bow with increasing temperature and time. Table 1 lists the present test results for the clamped technique, referring to the clamped type; however, the bowing process with temperature is true for both.

The mechanical tension reduces after a thermal treatment. A relaxed state of the DCB will snuggle itself to the heatsink. As a result, the reliability of thermal conduction (between the DCB and heatsink) improves and the R_th,ch decreases for the application.

In the mounted state, the bow between the DCB and heatsink is filled with thermal paste (l=1W/m/K). The power dissipation in the module creates a thermal distribution above this gap that depends on its thickness. The relationship between force and temperature rise is shown in Fig. 11. It quantifies the influence between the force on the DCB and the temperature difference T_dcb,top to T_heatsink.

The R_th value becomes saturated with growing force. Due to the reduction of screw force, the thermal resistance between DCB and heatsink increases, and retightening is necessary after a certain period of operation. Without tightening, the lifetime of the module decreases because of the increasing junction temperature (Fig. 12).

T_j (R_th,jh)=T_h+P×R_th,jh

Using the clamping approach described, this lifetime reduction will be prevented. During the lifetime of the application, the force will only be slightly reduced. Therefore, the junction temperature and the reliability are nearly independent of mechanical influences.

IPM-equivalent vs. IPM

To support applications with IPM-type functions but with lower system cost, more flexibility, and high reliability, an evaluation board for EasyPIM modules has been developed.

It combines an EasyPIM1 with additional electronic components, such as IGBT gate drivers on a PCB assembly. Fig. 13 shows the major function of a 3-phase inverter with single-phase rectifier, additional level-shift functions, gate drivers, and fault protection.

This subsystem solution is a 600V/10A IPM equivalent suitable for ac drives up to 1kW using single-phase mains. The circuit is on a small p. c. board (Fig. 14.) and can be connected to or implemented in the drives electronic circuit board. All components used in this IPM equivalent are standard devices. This is done to maximize availability and reduce cost. Features of the IPM solution include the following:

• Single-phase input and 3-phase output.
• Control input and output: six gate signals and one fault signal (active low).
• Gate drive with built-in level shift.
• Bootstrap diodes and capacitors.
• Undervoltage lock out (UVLO).
• Protection (INHIBIT) for overcurrent or overtemperature condition.
• Timing of INHIBIT with external capacitor.
• Using +15V single power supply.

Pin headers are mounted on the p.c. board to mate with input/output pins of the module. The module accepts six PWM input signals for independent switching of the six IGBTs in the three half bridges of the inverter. As a result of this approach, it's suitable for use by both ac (induction motor and synchronous motor) and brushless dc drives.

This board has built-in level-shift and bootstrap circuits and is powered by a single power supply. Based on an external sense resistor, the module can detect the dc-link current and trip (switch off all six IGBTs) for overcurrent protection. The value of the sense resistor sets the overcurrent trip level. With the EasyPIM1's built-in NTC thermistor, the module can also detect the temperature of the upper DCB and trip for overtemperature protection. Once the trip occurs, timing starts based on an external timing capacitor. When the timing is over, the module is automatically reset to its normal state.

The EasyPIM product scope (Table 2) is optimized to cover a wide range of low power applications. The range is for drive applications of 500W to 2.2kW. Looking at Figs. 15 and 16, you can see the various configurations. The EasyPACK has a six-pack inverter configuration, as you can see in Fig. 17, and offers currents up to 50A for 5.5kW motors.

EasyPIM1 includes a single-phase input rectifier and a 3-phase inverter section. EasyPIM2 is designed with a single-phase or 3-phase input rectifier section and includes a chopper IGBT with chopper diode, as shown in Fig. 17. All EasyPIM/PACK modules have an internal NTC thermistor for thermal management.

For more information on this article, CIRCLE 335 on Reader Service Card