AN668 datasheet(9/19 Pages) STMICROELECTRONICS

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AN668 APPLICATION NOTE

package structure has been observed after the

high temperature stress, up to 260°C.

Figure 7. Exposed slug edge for solder joint in-

spection

Figure 8. Loss of thermal performance due to

poor co-planarity and uneven solder

joints

2.5 Hermeticity

Good hermeticity of package interfaces has been ob-

tained by means of slug design and of the selection

of low stress, high adhesion molding compounds.

State-of-the-art molding equipment and process is

used, with CPK in excess of 1.33 for all the key pa-

rameters. This provides repeatability of the results in

large volumes.

Moreover, improved mechanical clamping of the

molding compound is obtained by using a new, pro-

prietary finishing of the slug all along its periphery,

which is provided with undercutting and controlled

surface roughness.

After severe tests consisting of alternate thermal cy-

cling and pressure pot, and after the "pop corn" se-

quence, Scanning Acoustic Microscopy (SAM)

analysis and die penetration test confirms the robust-

ness of the PowerSO-20/36. Figure 9 shows a typical

map of adhesion obtained in SAM evaluation after

stress analysis.

Figure 9. Scanning Acoustic Microscope map

THERMAL DESIGN AND APPLICATIONS

Thermal characteristics of the new PowerSO-20/36

package can cover a wide range of applications, both

in the medium power (1-5W) and in the high power

range (up to 20W). Moreover, as sufficiently large

thermal capacitance is associated with the new pack-

age, performance in switching conditions is equiva-

lent to that of available insertion packages.

In this section, a number of typical applications of the

PowerSO-20/36 will be considered and compared

with existing solutions, taking into account that a ma-

jor role in the thermal design is played by the temper-

ature increase Tj-Ta specified in the different

applications. "Delta" Tj can be as low as 25-30°C in

extreme conditions for automotive or as large as

100°C in more relaxed systems.

Unless differently indicated,

∆Tj of 50°C is assumed

in all the considerations of this note.

3.1 JUNCTION-TO-CASE THERMAL RESIS-

TANCE

The main feature of PowerSO-20/36 packages are

optimization of junction-to-case thermal resistance,

from the junction to the external surface of the slug.

Unlike standard SO packages, which use epoxy die

slug edge accessible

for visual inspection

D94AN003

100

MW SMT

VERSION

Power SO-20/36

D94AN004A

Rth(j-s)

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