AN4068 datasheet(43/63 Pages) STMICROELECTRONICS

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AN4068

Design guidelines

Doc ID 022923 Rev 2

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Design guidelines

9.1

PCB layout guidelines

9.1.1

Thermal performance

The ST7580 device can operate within the standard industrial temperature range, from -40

to 85 °C ambient temperature. Especially in high ambient temperature conditions, the effect

of the power dissipation of the device must be considered in order to keep it operating in

safe conditions.

Even though the ST7580 features a built-in thermal shutdown circuitry which turns off the

power amplifier (PA) when the die temperature (TJ) exceeds 150 °C, it is recommended not

to exceed 125 °C during normal operation to ensure the functionality of the IC.

A QFN48 package with exposed pad has been chosen for the ST7580 device in order to

obtain very good thermal performance. However, in order to take full advantage of this, the

PCB must be designed to effectively conduct heat away from the package.

To get a low impedance thermal path to the PCB, a 5x5 mm thermal pad has been realized

on the top layer under the device. In order to effectively remove the heat, the exposed pad

must be well soldered to the PCB thermal pad. Therefore, the out-gassing phenomenon due

to the soldering process must be controlled to reduce solder voids between the QFN48

exposed pad and the PCB thermal pad. To achieve this, smaller multiple openings in the

solder paste stencil should be used instead of one big opening on the thermal pad region.

This also has the advantage of reducing the amount of solder paste used, therefore avoiding

bridges with perimeter pads.

A suitable example for the QFN48 package is shown in Figure 34.

Figure 34.

Example of stencil openings for the QFN48 package

Another technique to improve heat conduction on the top layer is to fill all unused areas with

copper tied to the dissipating ground plane.

In order to have an effective heat transfer from the top layer of the PCB to the bottom layer,

thermal vias need to be included within the thermal pad area. If properly designed, thermal

vias are the most efficient paths for removing heat from the device.

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