AN4444 datasheet(51/68 Pages) STMICROELECTRONICS

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AN4444

Design guidelines

10.1

PCB layout guidelines

10.1.1

Thermal performance

The ST7590 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 to keep it operating in safe

conditions.

Even if the ST7590 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 an exposed pad has been chosen for the ST7590 device to have

a very good thermal performance. However, in order to take full advantage from 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 5 x 5 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 has also the advantage to reduce the amount of solder paste used, thus avoiding

bridges with perimeter pads.

A suitable example for the QFN48 package is shown in Figure 42.

Figure 42. 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.

1.35 x 1.35 mm squares

Coverage: 68%

at 1.65 mm pitch

AN4444 Datasheet(PDF) 51 Page - STMicroelectronics