AN807

Vishay Siliconix

Document Number: 70739

26-Nov-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

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

of the pad design for a LITTLE FOOT SOT-23 power MOSFET

footprint . In converting this 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.

The electrical connections for the SOT-23 are very simple. Pin 1 is

the gate, pin 2 is the source, and pin 3 is the drain. As in the other

LITTLE FOOT packages, the drain pin serves the additional

function of providing the thermal connection from the package to

the PC board. The total cross section of a copper trace connected

to the drain may be adequate to carry the current required for the

application, but it may be inadequate thermally. Also, heat spreads

in a circular fashion from the heat source. In this case the drain pin

is the heat source when looking at heat spread on the PC board.

Figure 1 shows the footprint with copper spreading for the SOT-23

package. This pattern shows 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 pin and provides

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

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

ambient air. This pattern uses all the available area underneath the

body for this purpose.

FIGURE 1. Footprint With Copper Spreading

0.114

2.9

0.059

1.5

0.0394

1.0

0.037

0.95

0.150

3.8

0.081

2.05

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.