AN823

Vishay Siliconix

Document Number: 71743

27-Feb-04

www.vishay.com

1

Surface mounted power MOSFET packaging has been based on

integrated circuit and small signal packages. Those packages

have been modified to provide the improvements in heat transfer

required by power MOSFETs. Leadframe materials and design,

molding compounds, and die attach materials have been

changed. What has remained the same is the footprint of the

packages.

The basis of the pad design for surface mounted power MOSFET

is the basic footprint for the package. For the TSOP-6 package

In converting the footprint to the pad set for a power MOSFET, you

must remember that not only do you want to make electrical

connection to the package, but you must made thermal connection

and provide a means to draw heat from the package, and move it

away from the package.

In the case of the TSOP-6 package, the electrical connections are

very simple. Pins 1, 2, 5, and 6 are the drain of the MOSFET and

are connected together. 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 shows the copper spreading recommended footprint for

the TSOP-6 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 overlays the basic pattern on

pins 1,2,5, and 6. 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. Notice that the

planar copper is shaped like a “T” to move heat away from the

drain leads in all directions. This pattern uses all the available area

underneath the body for this purpose.

FIGURE 1.

Recommended Copper Spreading Footprint

0.049

1.25

0.010

0.25

0.014

0.35

0.074

1.875

0.122

3.1

0.026

0.65

0.167

4.25

0.049

1.25

Since surface mounted packages are small, and reflow soldering

is the most common form of soldering for surface mount

components, “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.

REFLOW SOLDERING

Vishay Siliconix surface-mount packages meet solder reflow

reliability requirements. Devices are subjected to solder reflow as a

test preconditioning and are then reliability-tested using

temperature cycle, bias humidity, HAST, or pressure pot. The

solder reflow temperature profile used, and the temperatures and

time duration, are shown in Figures 2 and 3.

Ramp-Up Rate

+6_C/Second Maximum

Temperature @ 155 " 15_C

120 Seconds Maximum

Temperature Above 180_C

70 − 180 Seconds

Maximum Temperature

240 +5/−0_C

Time at Maximum Temperature

20 − 40 Seconds

Ramp-Down Rate

+6_C/Second Maximum

FIGURE 2.

Solder Reflow Temperature Profile