STPCINDUSTRIAL datasheet(48/55 Pages) STMICROELECTRONICS

48 / 55 page

BOARD LAYOUT

48/55

Issue 1.1

When considering thermal dissipation, the most

important - and not the more obvious - part of the

layout is the connection between the ground balls

and the ground layer.

A 1-wire connection is shown in Figure 6-2. The

use of a 8-mil wire results in a thermal resistance

of 105

°C/W assuming copper is used (418 W/

°K). This high value is due to the thickness (34

µm) of the copper on the external side of the PCB.

Considering only the central matrix of 36 thermal

balls and one via for each ball, the global thermal

resistance is 2.9

°C/W. This can be easily im-

proved by using four 10 mil wires to connect to the

four vias around the ground pad link as in Figure

6-3. This gives a total of 49 vias and a global re-

sistance for the 36 thermal balls of 0.6

°C/W.

The use of a ground plane like in Figure 6-4 is

even better.

To avoid solder wicking over to the via pads during

soldering, it is important to have a solder mask of

4 mil around the pad (NSMD pad), this gives a di-

ameter of 33 mil for a 25 mil ground pad.

To obtain the optimum ground layout, place the

vias directly under the ball pads. In this case no lo-

cal boar d distortion is tolerated.

The thickness of the copper on PCB layers is typ-

ically 34

µm for external layers and 17 µm for inter-

nal layers. This means thermal dissipation is not

good and temperature of the board is concentrat-

ed around the devices and falls quickly with in-

creased distance.

When it is possible to place a metal layer inside

the PCB, this improves dramatically the heat

spreading and hence thermal dissipation of the

board.

Figure 6-2. Recommended 1-wire ground pad layout

Figure 6-3. Recommended 4-wire ground pad layout

Solder Mask (4 mil)

Pad for ground ball (diameter = 25 mil)

Hole to ground layer (diameter = 12 mil)

Connection Wire (width = 10 mil)

Via (diameter = 24 mil)

1 mil = 0.0254 mm

4 via pads for each ground ball

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