STPCATLAS datasheet(102/111 Pages) STMICROELECTRONICS

102 / 111 page

DESIGN GUIDELINES

102/111

Issue 1.0 - July 24, 2002

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

6-35 illustrates such an implementation.

Figure 6-34. 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 6-35. Use of Metal Plate for Thermal Dissipation

Metal planes

Thermal conductor

Board

Die

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