AN668 datasheet(15/19 Pages) STMICROELECTRONICS

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AN668 APPLICATION NOTE

3.5 HIGH POWER APPLICATIONS (UP TO

20W) WITH INSULATED METAL SUB-

STRATES - IMS

In the last few years, several Companies developed

the idea of a cost effective Insulated Metal Substrate

(IMS) having:

A copper printed layer supported by an alumi-

num base, with epoxy or polyimide isolation in

between; or

A flexible printed circuit (polyimide) glued onto

an aluminum base.

In both cases, the following conditions are offered:

The substrate is compatible with standard SMT

processes, including infrared reflow soldering;

It is compatible with passive and active devic-

es, including PowerSO-20/36;

It has an acceptable thermal resistance loss

between the PowerSO-20/36 slug and the alu-

minum base, due to optimized thermal conduc-

tivity of the isolating layer;

The aluminum baseplate has variable thick-

ness, up to 5 mm, in order to sink different

amounts of heat.

Utilization of IMS is considered for several applica-

tions both for automotive and industrial systems and

is particularly fruitful when associated with PowerSO-

20/36 packages (Figure 23)

Figure 23.

Thermal performance has been measured for both

substrates described above. The first result is related

to the contribution of isolation to the total thermal re-

sistance. When the footprint of Figure 14 is used,

with the 96 sq mm slug soldered onto a 155 sq mm

pattern, this contribution is about 0.5°C/W .

Therefore, the thermal resistance from the junction to

the aluminum baseplate ranges from 1.0 to 2.5°C/W

depending on the chip size.

For large dissipation, an external heat sink is applied

to the IMS by means of screws, bolts or rivets. The

total thermal resistance is obtained by adding the

heat sink resistance to the junction to baseplate re-

sistance (1.0-2.5°C/W).

This solution is capable of 20-25 W per device and if

a large sink is used, as massive parts existing in the

car, more than one device can be assembled on the

same ISM, thus obtaining a functional power sub-

system with several tens of Watt dissipation.

It is interesting to notice that the aluminum plate itself

has good dissipation properties to the ambient, de-

pending on its size.

The PowerSO-20/36 soldered onto a 40 mm square,

1.5 mm thick IMS shows a thermal resistance of 7°C/

W, for about 7 W dissipation.

3.6 HIGH POWER PULSES AND THERMAL

IMPEDANCE

In several applications, large power pulses are deliv-

ered by the device for a short time.

In this case, discussed in the STMicroelectronics pa-

per "Designing with Thermal Impedance" (Semith-

erm Conf. 1988), the quantity which rules the junction

temperature up to the time t0 is the thermal imped-

ance of the three different elements: silicon chip,

package slug and heatsink.

For pulse durations in the range of 0.1-1.0 sec, the

package slug has the strongest influence on the sys-

tem performance, depending on the associated ther-

mal

capacitance,

i.e.

the

capability

heat

accumulation.

In the presence of properly sized slugs with suitable

capacitance, it is possible to maintain a low junction

temperature for the switching time of most applica-

tions, which seldom exceeds 1 sec.

For popular power packages like TO-220 and Multi-

watt, the value of the thermal impedance in the first

second is lower than the value of Rth(j-c) in steady

state.

In order to compare the PowerSO-20/36 packages

with the insertion equivalent, Fig. 24 is very helpful. It

shows the thermal impedance of different packages

in the first second of the power pulse, in the same

conditions of power intensity and die size.

When similar comparisons are performed with medi-

um power "bat wing" SO, the enhancement provided

Copper

foil

Insulation

D94AN016

Aluminium

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