STPCD01 STMicroelectronics, STPCD01 Datasheet - Page 54

• Manufacturer Part Number: STPCD01
• Description: STPC CLIENT DATASHEET - PC COMPATIBLE EMBEDED MICROPROCESSOR
• Manufacturer: STMicroelectronics

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 inFigure
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.
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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 2.2 - October 13, 2000
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 board 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