VISHAY SILICONIX

Application Note 808

Document Number: 70740

www.vishay.com

Revision: 18-Jun-07

1

Wharton McDaniel

Surface-mounted LITTLE FOOT power MOSFETs use

integrated circuit and small-signal packages which have

been been modified to provide the heat transfer capabilities

required by power devices. Leadframe materials and

design, molding compounds, and die attach materials have

been changed, while the footprint of the packages remains

the same.

See Application Note 826, Recommended Minimum Pad

Patterns With Outline Drawing Access for Vishay Siliconix

basis of the pad design for a LITTLE FOOT SO-8 power

MOSFET. In converting this recommended minimum pad

to the pad set for a power MOSFET, designers must make

two connections: an electrical connection and a thermal

connection, to draw heat away from the package.

In the case of the SO-8 package, the thermal connections

are very simple. Pins 5, 6, 7, and 8 are the drain of the

MOSFET for a single MOSFET package and are connected

together. In a dual package, pins 5 and 6 are one drain, and

pins 7 and 8 are the other drain. For a small-signal device or

integrated circuit, typical connections would be made with

traces that are 0.020 inches wide. Since the drain pins serve

the additional function of providing the thermal connection

to the package, this level of connection is inadequate. The

total cross section of the copper may be adequate to carry

the current required for the application, but it presents a

large thermal impedance. Also, heat spreads in a circular

fashion from the heat source. In this case the drain pins are

the heat sources when looking at heat spread on the PC

board.

Figure 1. Single MOSFET SO-8 Pad

Pattern With Copper Spreading

Figure 2. Dual MOSFET SO-8 Pad Pattern

With Copper Spreading

The

minimum

recommended

pad

patterns

for

the

single-MOSFET SO-8 with copper spreading (Figure 1) and

dual-MOSFET SO-8 with copper spreading (Figure 2) show

the starting point for utilizing the board area available for the

heat-spreading copper. To create this pattern, a plane of

copper overlies the drain pins. The copper plane connects

the drain pins electrically, but more importantly provides

planar copper to draw heat from the drain leads and start the

process of spreading the heat so it can be dissipated into the

ambient air. These patterns use all the available area

underneath the body for this purpose.

Since surface-mounted packages are small, and reflow

soldering is the most common way in which these are

affixed to the PC board, “thermal” connections from the

planar copper to the pads have not been used. Even if

additional planar copper area is used, there should be no

problems in the soldering process. The actual solder

connections are defined by the solder mask openings. By

combining the basic footprint with the copper plane on the

drain pins, the solder mask generation occurs automatically.

A final item to keep in mind is the width of the power traces.

The absolute minimum power trace width must be

determined by the amount of current it has to carry. For

thermal reasons, this minimum width should be at least

0.020 inches. The use of wide traces connected to the drain

plane provides a low impedance path for heat to move away

from the device.

0.027

0.69

0.078

1.98

0.2

5.07

0.196

5.0

0.288

7.3

0.050

1.27

0.027

0.69

0.078

1.98

0.2

5.07

0.088

2.25

0.288

7.3

0.050

1.27

0.088

2.25