STPCI01 STMicroelectronics, STPCI01 Datasheet - Page 64

• Manufacturer Part Number: STPCI01
• Description: STPC INDUSTRIAL - PC COMPATIBLE EMBEDED MICROPROC
• Manufacturer: STMicroelectronics

BOARD LAYOUT
When considering thermal dissipation, the most
important - if not the most obvious - part of the
layout is the connection between the ground balls
and the ground layer.
A 1-wire connection is shown in
use of 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 improved by
using four 10 mil wires to connect to the four vias
around the ground pad link, as in
gives a total of 49 vias and a global resistance for
the 36 thermal balls of 0.6°C/W.
The use of a ground plane, as shown in
4, is even better.
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This is preliminary information on a new product now in development or undergoing evaluation. Details are subject to change without notice.
Figure 6-3. Recommended 4-wire Ground Pad Layout
Figure 6-2. Recommended 1-wire Ground Pad Layout
Figure
Figure
Issue 2.4 - February 11, 2002
Figure 6-
6-3. This
6-2. The
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 can be tolerated.
The thickness of the copper on PCB layers is
typically 34 µm for external layers and 17 µm for
internal layers. The resulting thermal dissipation is
not good, with areas of high temperature being
concentrated around the devices, falling off
quickly with increased distance.
Where possible, place a metal layer inside the
PCB. This will improve dramatically the spread of
heat and hence improve the 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