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AN668 APPLICATION NOTE

3.4 HIGH POWER APPLICATIONS (UP TO 20

W)WITH STANDARD SUBSTRATE

In order to get an Rth(j-a) of few °C/W, high power

applications require large and massive external heat-

sinks in close contact with the power device.

This is not a simple requirement when cost effective

systems using the standard surface mount technolo-

gy and the standard PCB substrate are considered.

Major concerns are related to the critical assembly of

a large heatsink onto a small package; vibrations and

thermal excursions can generate unwanted mechan-

ical stress, thus damaging the package leads or the

integrity of the contact between slug and heatsink.

However, the fact that we have a well established,

PCB based surface mount technology is pushing the

industry to test several directions, similar to those

sketched in Figure 21.

Rather than proven solutions, they should be consid-

ered here as early attempts aimed to explore the ca-

pability of existing surface mount processes and

materials in the direction of increased heat dissipa-

tion at reduced cost.

3.4.1 "Slug-up" package and external heat-

sink

In the example of Figure 21a, PowerSO-20 is shown

in "reverse" or "slug-up" configuration, with a clip

mounted external heatsink, which was studied for

3.5W dissipation in still air and 5-6W dissipation in

forced ventilation.

A similar concept can be used for other applications

and the sink size adapted to their specific require-

ments. A large variety of solutions will exist which will

take advantage of the metal box in which the board

can eventually be housed.

Figure 21a. "Slug up"package with external

heatsink

To minimize the mechanical stess on leads and sol-

der joints derived from any pressure applied on top of

package, the slug-up forming is specifically designed

(Fig. 22).

3.4.2 Cavity board and external heatsink

In the example of Figure 21b, the PowerSO-20/36 is

mounted onto an epoxy board, with a through cavity

fabricated to correspond to the package slug. The ex-

ternal heatsink is directly applied in contact with the

slug, secured by means of a spring system or glued.

Also in this case, a high level of dissipation can be

achieved with properly designed heatsinks.

Figure 21b. Mounting on cavity board and ex-

ternal heatsink applied

Heatsink

D94AN014

Epoxy board

Epoxy board

with through cavity

D94AN015

Heatsink