STPC12 datasheet(101/111 Pages) STMICROELECTRONICS

101 / 111 page

DESIGN GUIDELINES

Issue 1.0 - July 24, 2002

101/111

When considering thermal dissipation, one of the

most

important

parts

the

layout

the

connection between the ground balls and the

ground layer.

A 1-wire connection is shown in Figure 6-32. The

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

of 105°C/W assuming copper is used (418 W/

m.°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

2.9°C/W.

This

can

easily

improved using four 12.5 mil wires to connect to

the four vias around the ground pad link as in

Figure 6-33. This gives a total of 49 vias and a

global resistance for the 36 thermal balls of 0.5°C/

The use of a ground plane like in Figure 6-34 is

even better.

Figure 6-32. Recommended 1-wire Power/Ground Pad Layout

Solder Mask (4 mil)

Pad for ground ball (diameter = 25 mil)

Hole to ground layer (diameter = 12 mil)

Connection Wire (width = 12.5 mil)

Via (diameter = 24 mil)

1 mil = 0.0254 mm

Figure 6-33. Recommended 4-wire Ground Pad Layout

4 via pads for each ground ball

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