AN668 datasheet(6/19 Pages) STMICROELECTRONICS

6 / 19 page

AN668 APPLICATION NOTE

6/19

The most important features of PowerSO-20/36 are:

1) High power structure and process;

2) High current capability;

3) Miniaturization

4) Designed-in surface mount characteristics:

a] Handling;

b] Tape and reel packing;

c] Co-planarity;

d] Solder joint quality and inspection;

e] High reliability after soldering process.

5) Designed-in "hermeticity", for low failure rate (1

ppm).

STRUCTURE AND CHARACTERISTICS

OF POWERSO-20/36

™.

In this section, design criteria listed in previous para-

graph are presented and discussed.

2.1 High power structure and process (>20 W)

The main feature of the power package structure is

the existence of a massive copper slug in the pack-

age onto which the silicon chip is soldered. Its func-

tion is twofold:

a) To provide a low thermal resistance path from

the chip to the external heatsink;

To provide a large thermal capacitance, able to

absorb power peaks in switching conditions.

The slug of PowerSO-20/36, is optimized in order to

get a good compromise between miniaturization and

thermal performance. As a result of this work, Power-

SO-20 has the same junction to case thermal resis-

tance Rth(j-c) of Multiwatt package and the same

thermal impedance up to 0.5 sec. Therefore the

range of application exceeds 20 W, the same as for

traditional power packages.

Section 3 will present thermal design in detail. Here

we will recall the importance of the die attach pro-

cess, typical of power IC packages, which uses a

high melting temperature (300°C) tin based alloy.

The control of this process, in terms of solder thick-

ness and void reduction, is based on SPC methods

and has a CPK > 1.33.

2.2 High current capability (10-20 A)

At first glance, leads of the PowerSO-20/36 seem

very fragile, thin and not compatible with the high cur-

rents (10-20 A) associated with a large number of

power ICs, which require the utilization of large alu-

minum wires having a diameter ranging between

0.010 and 0.020 inch (0.25 to 0.5 mm).

Table 1 reports the comparison of PowerSO-20 with

insertion and surface mount Multiwatt and the follow-

ing considerations are possible:

Electrical resistance of PowerSO-20 leads is

about 760

µΩ, i.e. it is equivalent to insertion

Multiwatt or better;

Electrical resistance of PowerSO-20 leads is

worse than Surface Mount Multiwatt by a factor

of 1.3 to 2;

In any case, lead resistance of PowerSO-20 is

much lower than the wire resistance. Wire re-

sistance is 450-1800

µΩ/mm and it must be

considered that the minimum wire length is 2.5

mm corresponding to 1150-4500

µΩ;

If needed, two or more leads are short circuited

and different frame designs can be developed

providing flexible options (Figure 6). With mul-

tiple wire bonding, current capability can be

very large; for 45A current, if 3 leads are used

with 10 mil diameter, 3 mm long wires, electri-

cal resistance of the interconnection is about

Ω.

Table 1. Electrical resistance data of PowerSO-20

and Multiwatt 15 leads

Copper resistivity : 1.9mW x cm

Aluminum wire resistance:

∅ 10 mils: 1800 mW / mm

∅ 20 mils: 450 mW / mm

Dimensions

(mm)

Multiwatt15

Insertion

Multiwatt15

Surf. Mount

PowerSO-20

Long lead

MW15

23x0.9x0.5

18x0.9x0.5

970

µΩ

–

760

µΩ

–

Short lead

MW15

14x0.9x0.5

9x0.9x0.5

590

µΩ

–

380

µΩ

–

Lead

PowerSO-20

5x0.5x0.25

–

760

µΩ

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