Preliminary
Macroblock
___________________Application Note – How to Use Thermal Pad V2.00 _______________
Application Note – How to Use Thermal Pad
Foreword
When an IC starts to work, the heat dissipation becomes the heat effect and affects the life time of the IC.
Therefore, removing the heat dissipation from IC is an important challenge when it comes to PCB design. A
package with thermal pad is designed to enhance the heat dissipation to provide superior thermal
performance in a standard size package. The thermal pad can be mounted on Print Circuit Board (PCB) to
enhance the heat sink area of package. A good design on PCB with a thermal pad helps the users with more
flexible applications. This article provides a guideline on how to use the thermal pad on PCB design.
PCB Design Considerations
To remove heat dissipation from IC, only a package with thermal pad is insufficient; a good PCB design also
needs to be considered. At least, an area of solderable copper underneath the thermal pad package is
needed, and this area is called thermal land. If possible, the larger area of solderable copper for thermal pad
helps to remove the more heat dissipation from package, and the size of the area is determined by the size of
the thermal pad, PCB construction, and how much of heat that needs to be removed. Generally, if the area of
thermal land is insufficient for heat dissipation, adding thermal vias on thermal land is helpful for heat removal,
or using an external heat sink is another choice.
Usually, thermal pad is designed as GND pad of IC; that helps designers get a more stable system due to
large ground plane.
Thermal Land Design
A thermal land is required on the surface of PCB, where underneath the body of IC’s package. During surface
mount reflow solder operation, the thermal pad on the underside of the package is soldered to its thermal land,
which can create an efficient thermal path. Generally, the larger exposed solder area gets the better effect on
the heat sink, but it might cause short circuit problem during surface mount reflow solder operation. The
recommendation is to set the exposed solder area equal to the thermal pad, and the other area uses a solder
mask to cover the copper. Figures 1 to 4 show the configurations of thermal pad for different packages.
The purpose of thermal land is to transfer heat dissipation from package, but if thermal land doesn’t connect
with thermal pad, the heat dissipation can’t be removed effectively. Apply solder paste on the exposed solder
area before surface mount reflow operation, and after that operation, the thermal pad can be mounted on
exposed solder area.
Thermal Vias Design
Thermal via is the primary method to transfer the heat from the surface of PCB thermal land to the internal
copper planes. Thermal vias increase the contact area between the thermal land and the air. Therefore, the
number, the size, and the construction of thermal vias will affect thermal performance of PCB’s thermal land
and heat dissipation on the package with thermal pad. For small packages, 5 to 9 vias are adequate for most
applications, and for larger ones, users have more space to design with more vias for efficient heat dissipated.
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Preliminary
Macroblock
___________________Application Note – How to Use Thermal Pad V2.00 _______________
Generally, a thermal vias with 0.33mm (13mil) diameter and plated with 1oz are recommended.
In addition, users can place a small number of vias with 0.64mm diameter on the solder mark area to get
another heat path between the thermal land and other planes.
10.3mm
1.8mm
Thermal Vi as
9xψ1mm
3mm
Exposed Sol der
Area
9.65mm
2.23mm x 6
2.38mm x 6
Solder Mask
Over Copper
2.5mm
0.51mm
1.27mm
1.7mm
0.94mm
Solder Mask Opening
2.79mm
Non Sol dermask Defined Pad
Pad Geometry
0.07mm
All Around
Figure 1 Package and thermal pad configuration of TO-265
2.286mm
1.11mm
Thermal Vi as
12xψ0.3mm
Solder Mask
Over Copper
6.198mm
3.988mm
0.57mm x 9
2.286mm
0.76mm x 8
Exposed Solder
Area
0.41mm
1.27mm
4.98mm
Non Sol dermask Defined Pad
0.41mm
Solder Mask Opening
1.27mm
Pad Geometry
0.07mm
All Around
Figure 2 Package and thermal pad configuration of SOP8
Preliminary
Macroblock
___________________Application Note – How to Use Thermal Pad V2.00 _______________
2.2mm
Thermal Vi as
12xψ0.3mm
0.75mm
Thermal Vi as
6xψ0.64mm
Solder Mask
Over Copper
0.55mmx9
1.5mm
4.4mm 6.4mm
0.5mmx8
Exposed Solder
Area
0.3mm
0.65mm
6.6mm
Non Sol dermask Defined Pad
0.35mm
Solder Mask Opening
0.8mm
0.07mm
All Around
Pad Geometry
Figure 3 Package and thermal pad configuration of TSSOP20
4.1mm
2.88mm
13
0.38mm
18
12
19
0.23mm
2.88mm
4.1mm
0.3mm
0.52mm
0.3mm
10
9
0.5mm
0.5mm
7
24
6
Via Layout Design
May be Varied
Depending on Heat Sink Size
0.96mmx6
1
0.5mm
Non Soldermask Defined Pad
0.06mm
0.23mm Typ.
(0.18mm~0.30mm)
Solder Mask Opening
Thermal Vias
9xψ0.3mm
0.43mm
0.07mm 0.30mm
All Around
Pad Geometry
Figure 4 Package and thermal pad configuration of QFN24
0.96mmx6