BOARD LAYOUT

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Release B

This is preliminary information on a new product now in development or undergoing evaluation. Details are subject to change without notice.

6 BOARD LAYOUT

6.1 Thermal dissipation

Thermal dissipation of the STPC depends mainly

on supply voltage. As a result, when the system

does not need to work at 3.3V, it is interresting to

reduce the voltage to 3.15V for example. This may

save few 100’s of mW.

The second area to look at is unused interfaces

and functions. Depending on the application,

some input signals can be grounded, and some

blocks not powered or shutdown. Clock speed dy-

namic adjustment is also a solution that can be

used along with the integrated power manage-

ment unit.

The standard way to route thermal balls to internal

ground layer implements only one via pad for each

ball pad, connected using a 8-mil wire.

With such configuration the Plastic BGA 388 pack-

age does 90% of the thermal dissipation through

the ground balls, and especially the central ther-

mal balls which are directly connected to the die,

the remaining 10% is dissipated through the case.

Adding a heat sink reduces this value to 85%.

As a result, some basic rules has to be applied

when routing the STPC in order to avoid thermal

problems.

First of all, the whole ground layer acts as a heat

sink and ground balls must be directly connected

to it as illustrated in Figure 6-1.

If one ground layer is not enough, a second

ground plane may be added on solder side.

Figure 6-1. Ground routing

Pad for ground ball

Thru hole to ground layer

TopL

ayer : S

ignals

Groundl

ayer

Powerla

yer

Bottom

Layer:

signals

+ local g

roundla

yer (ifn

eeded)

Note: For better visibility, ground balls are not all routed.