AN806

Vishay Siliconix

Document Number: 70738

17-Dec-03

www.vishay.com

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

MOSFET, (http://www.vishay.com/doc?72286), for the basis

of the pad design for a LITTLE FOOT TSSOP-8 power MOSFET

package footprint. In converting the footprint to the pad set for a

power device, designers must make two connections: an electrical

connection and a thermal connection, to draw heat away from the

package.

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

are very simple. Pins 1, 5, and 8 are the drain of the MOSFET

for a single MOSFET package and are connected together. In

the dual package, pins 1 and 8 are the two drains. 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 also provide 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 TSSOP-8 Pad

Pattern with Copper Spreading

0.032

0.8

0.018

0.45

0.284

7.6

0.073

1.78

0.118

3.54

0.026

0.66

0.122

3.1

The pad patterns with copper spreading for the single-MOSFET

TSSOP-8 (Figure 1) and dual-MOSFET TSSOP-8 (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.

FIGURE 2. Dual MOSFET TSSOP-8 Pad Pattern with

Copper Spreading

0.026

0.66

0.284

7.6

0.032

0.8

0.122

3.1

0.091

1.65

0.073

1.78

0.018

0.45

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.