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STPC4HEBI Datasheet(PDF) 84 Page - STMicroelectronics

Part No. STPC4HEBI
Description  X86 Core PC Compatible Information Appliance System-on-Chip
Download  93 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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STPC4HEBI Datasheet(HTML) 84 Page - STMicroelectronics

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DESIGN GUIDELINES
84/93
Release 1.5 - January 29, 2002
When considering thermal dissipation, one of the
most
important
parts
of
the
layout
is
the
connection between the ground balls and the
ground layer.
A 1-wire connection is shown in Figure 6-27. 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
is
2.9°C/W.
This
can
be
easily
improved using four 12.5 mil wires to connect to
the four vias around the ground pad link as in
Figure 6-28. This gives a total of 49 vias and a
global resistance for the 36 thermal balls of 0.5°C/
W.
The use of a ground plane like in Figure 6-29 is
even better.
Figure 6-27. 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 6-28. Recommended 4-wire Ground Pad Layout
4 via pads for each ground ball


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