AN668 datasheet(12/19 Pages) STMICROELECTRONICS

12 / 19 page

AN668 APPLICATION NOTE

12/19

3.3.2 Via holes and ground layer

If via holes are used as in Figure 17, a more direct

thermal path is obtained from the slug to the ground

layer. The number of vias is chosen according to the

desired performance.

Figure 18 shows a typical high performance foot print

for PowerSO20/36 with via holes

Rth(j-a) can be reduced down to 15-20°C/W and dis-

sipation increased up to 2.5-3.3W with Delta Tj =

50°C.

Figure 17. Mounting on epoxy FR4 using via

holes for heat transfer of ground layer

Figure 18. Footprint of PowerSO20/36 with via

holes

3.3.3 Via holes and external heatsink

The solution considered in Figure 19 combines a Pow-

erSO-20/36 soldered onto a via hole structure, with a

metal plate glued on the opposite side of the board.

The final thermal resistance value is given by the re-

sistance of via holes added to the resistance of the

plate to the ambient.

Figure 20 shows the thermal resistance associated

with 16 holes and reports its behavior with the dissi-

pated power. Hole depth of 1.5 mm is an extreme

case, existing in rather thick substrates only; the ex-

perimental value is about 9°/W, including the contri-

bution of Rth(j-c). In practical cases, structures with

Rth(j-a) = 12-15°C/W are obtained for 3.3-4.2W dis-

sipation.

Figure 19. Mounting on epoxy FR4 using via

holes for heat transfer and external

metal plate

Figure 20. Thermal Resistance junction-sub-

strate

Copper

foil

Ground

layer

D94AN011

via holes

Copper

foil

FR4 board

D94AN012

Via holes

Metal plate

0123456789

D94AN013

Dissipated Power (W)

8.4

8.6

8.8

9.0

9.2

9.4

Rthj-s(˚C/W)

Mounted on FR4

substrate with

vias holes

Thickness: 1.5mm

Infinite heatsink

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