Heatsink Mounting Guideline – Rev: 1.4

Application Note

HEATSINK MOUNTING GUIDELINE FOR TDK-LAMBDA

AMERICAS’ POWER MODULES

The thermal performance and reliability of a power module can be improved with the use of a

heatsink. Proper design and installation of the heatsink is critical to the performance and

reliability of the power module. Improper heatsink design and installation can cause excessive

mechanical stress that can result in premature failure of the module.

Heatsink Design

Heatsinks should be designed such that they minimize the thermal resistance and mechanical

strain between the heatsink and power module. It is recommended that power modules be

coupled with heatsinks if operating in harsh thermal environments. If a custom heatsink/cold

plate is used, it is highly recommended that they have the following features:

 Flat contact surface: The heatsink/cold plate mating surface should be flat with a

maximum of 0.001-inch flatness per inch of surface. For example, a full brick heatsink

measures 4.6 inch long by 2.4 inch wide. Thus the maximum allowable flatness along

the length is .0046 inch and .0024 inch along the width. It is recommended that the

mating surface be machined to meet this flatness requirement. Typical extruded or

casted parts do not meet this requirement.

 Thermal interface material: Due to the flatness irregularities of both the heatsink and

power module baseplate, a thermal interface material must be used between the heatsink

and the power module baseplate to minimize the thermal contact resistance. A

conformal thermally conductive interface material or pad is required to fill the gaps. The

pad should be between 0.005 to 0.008-inch thick and have a minimum thermal

conductivity of 2 W/m-K. We suggest 0.005-inch thick Berquist QPAD3 thermal

interface material. The interface material should completely surround the mounting

hole/slot (see Figure 1). Pad clearance hole diameter should be 1.02 mm (0.040 inch)

larger than heatsink/cold plate mounting hole. This will prevent excessive flexing of the

base plate when torqueing in screws. Thermal grease is not recommended for the

thermal interface material.

 Heatsink/cold plate hole size: For non-threaded heatsink/cold plate, holes are preferred

over slots. If slots are required, closed end slots are preferred. Open ended slots should

not be utilized since they can result in additional strain on the power module when the

heatsink is mounted. Heatsink/cold plate mounting holes should be 3.3 mm (0.130 inch)

in diameter. All holes and sharp edges shall be de-burred.

Heatsink Mounting Guideline – Rev: 1.4

Application Note

Figure 1 - Thermal Pad Design

Assembly

Some TDK-Lambda Americas’ power modules come equipped with mounting studs for

fastening module to PCB and/or heatsink/cold plate. Mounting studs come in two varieties:

threaded and non-threaded. There are certain guidelines to follow when mounting a heatsink or

cold plate to the power module that will help prevent reliability issues.

 Assembly sequence: It is recommended that the module be assembled in the following

order:

o Threaded stud version (Figure 2):

1) Fix power module to PCB.

2) Mount heatsink/cold plate to power module.

3) Solder power module to PCB.

o Non-threaded stud version (Figure 3):

1) Fix power module to PCB and heatsink/cold plate.

2) Solder power module to PCB.

 PCB holes: Refer to appropriate datasheet for mounting stud footprint. Mounting

holes on PCB should be 3.40mm (0.134 inch) diameter plated through holes with

5.5mm (0.217 inch) diameter land pad.

 Hardware (threaded studs): M3 x 0.5 screws should be used. Screws should protrude a

minimum of 3.8mm (0.15 inch) into fastening stud. Locking washers should be used as

shown.

 Hardware (non-threaded studs): M3 x 0.5 or smaller screws should be used. Locking

washers should be used as shown.

Thermal interface

material

Hole