STPC0380BTI3 datasheet(43/51 Pages) STMICROELECTRONICS

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BOARD LAYOUT

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Issue 1.2

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 may be to reduce

the voltage to 3.15V for example. This may save

few 100’s of mW.

The second area that can be concidered is un-

used interfaces and functions. Depending on the

application, some input signals can be grounded,

and some blocks not powered or shutdown. Clock

speed dynamic adjustment is also a solution that

can be used along with the integrated power man-

agement 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 dissipates 90% of the heat through the ground

balls, and especially the central thermal balls

which are directly connected to the die, the re-

maining 10% is dissipated through the case. Add-

ing a heat sink reduces this value to 85%.

As a result, some basic rules have 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 the 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 (ifne

eded)

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

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