STPC STMicroelectronics, STPC Datasheet - Page 44

• Manufacturer Part Number: STPC
• Description: PC Compatible Embeded Microprocessor
• Manufacturer: STMicroelectronics

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BOARD LAYOUT
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/
m. K). This high value is due to the thickness (34
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 resis-
tance for the 36 thermal balls of 0.6 C/W.
The use of a ground plane like in Figure 6-4 is
even better.
44/51
Figure 6-2. Recommended 1-wire ground pad layout
Figure 6-3. Recommended 4-wire ground pad layout
m) of the copper on the external side of the PCB.
Issue 1.2
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 m for external layers and 17 m 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.
Pad for ground ball (diameter = 25 mil)
Solder Mask (4 mil)
Connection Wire (width = 10 mil)
Via (diameter = 24 mil)
Hole to ground layer (diameter = 12 mil)
4 via pads for each ground ball
1 mil = 0.0254 mm