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STPC4EDBI Datasheet(PDF) 85 Page - STMicroelectronics

Part No. STPC4EDBI
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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STPC4EDBI Datasheet(HTML) 85 Page - STMicroelectronics

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DESIGN GUIDELINES
Release 1.5 - January 29, 2002
85/93
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
diameter 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
local board distortion is tolerated.
6.4.5.3. Heat dissipation
The thickness of the copper on PCB layers is
typically 34 µm for external layers and 17 µm for
internal
layers.
This
means
that
thermal
dissipation is not good; high board temperatures
are concentrated around the devices and these
fall quickly with increased distance.
Where possible, place a metal layer inside the
PCB; this improves dramatically the spread of
heat and hence the thermal dissipation of the
board.
The possibility of using the whole system box for
thermal dissipation is very useful in cases of high
internal
temperatures
and
low
outside
temperatures. Bottom side of the PBGA should be
thermally connected to the metal chassis in order
to propagate the heat flow through the metal.
Thermally connecting also the top side will
improve furthermore the heat dissipation. Figure
6-30 illustrates such an implementation.
Figure 6-29. Optimum Layout for Central Ground Ball - top layer
Via to Ground layer
Pad for ground ball
Clearance = 6mil
diameter = 25 mil
hole diameter = 14 mil
Solder mask
diameter = 33 mil
External diameter = 37 mil
connections = 10 mil
Figure 6-30. Use of Metal Plate for Thermal Dissipation
Metal planes
Thermal conductor
Board
Die


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