1. ABSTRACT

The trend from through-hole packages to low-cost SMD-applications is marked by the improvement of

chip technologies. "Silicon instead of heatsink" is therefore possible in many cases. Many applications

today use PCBs assembled with SMD-technologies, the emphasis being on Power ICs in SMD packages

mounted on single-sided PCBs laminated on one side.

The printed circuit board (PCB) itself becomes the heatsink. In early fabrications a solid heatsink was

either screwed or clamped to the power package. It was easy to calculate the thermal resistance from the

geometry of the heatsink.

In SMD-technology, this calculation is much more difficult because the heat path must be evaluated: chip

(junction) - lead frame - case or pin - footprint - PCB materials (basic material, thickness of the laminate)

- PCB volume - surroundings.

As the layout of the PCB is a main contributor to the result, a new technique must be applied. Surface

mount board layout is a critical portion of the total design. The footprint for the semiconductor packages

must be the correct size to ensure proper solder connection interface between the board and the

package.

The power dissipation for a SMD device is a function of the drain pad size, which can vary from the

minimum pad size for soldering to a pad size given for maximum power dissipation. The measurements

achieved on SMD packages for different drain pad size show that by increasing the area of the drain pad

the power dissipation can be increased.

Although one can achieve improvements in power dissipation with this method, the tradeoff is to use

valuable board area.

8™, PowerFLAT™ (5x5) and (6x5), TSSOP8 and PowerSO-10™) with their recommended footprints.

For each of these packages we will show the power dissipation for the minimum footprint and for a large

as well.

Finally we will make a thermal performance comparison for all SMD packages analysed.

Information regarding the mechanical dimensions for each SMD package can be found on the related

datasheets.

2. THERMAL MEASUREMENTS

The most practical method of optimizing thermal performance is to characterize the MOSFET on the

PCB where it will be used. The basis of this method is to dissipate a known amount of power in the

MOSFET, and to measure the amount of temperature rise this causes in the junction, given the data

June 2003

1/22

AN1703

APPLICATION NOTE

R.Gulino

GUIDELINES FOR USING ST’S MOSFET SMD PACKAGES