AN668 APPLICATION NOTE

2/19

1)

They have still the traditional structure for in-

sertion and are mounted manually in the PCB,

with loss of time, productivity, floor space and

money;

2)

They force the use of wave soldering tech-

niques and are not compatible with reflow sol-

dering;

3)

They are not compatible with increasing need

of miniaturization of power systems.

The above points are so important that the develop-

ment of totally new power packages fitting the SMT

requirements is a clear demand of the industry and

can pay back the heavy effort needed in terms of

R&D, engineering and production resources.

1.1 INTERMEDIATE SOLUTIONS: INSERTION

PACKAGES CONVERTED TO SMT

The fastest answer to the requirement discussed

above is the adaptation of existing insertion packag-

es to obtain a kind of surface mount configuration.

This can be conveniently done by redesigning the

lead shape, as in the case of the surface mount Hep-

tawatt package (7 lead TO-220) shown in Figure 1

which was introduced in the market by STMicroelec-

tronics in 1989.

The package of Figure 1 is very attractive in terms of

capital expenditure and time to market; as a minor

modification of existing production line can provide

the surface mount version in a short development

time.

However, the experience of STMicroelectronics with

above solutions is not totally satisfactory, for a num-

ber of reasons:

1)

HANDLING, which unlike all other surface

mount packages, is complicated by the struc-

ture, which is asymmetrical in two directions

(height and thickness). Existing pick and place

tools are not readily compatible with this struc-

ture, as well as the tape and reel packing;

2)

SIZE, which is basically the same as existing

insertion packages; therefore, is not the an-

swer to the demand of miniaturization of power

systems coming from almost all the applica-

tions: automotive, audio and industrial;

3)

COPLANARITY, which can become an issue

for 4-6 mm (0.16-0.24 inch) long leads. It must

be recalled that major coplanarity improvement

is obtained by reducing the lead length, down

to 1-2 mm (0.04-0.08 inch); this cannot be

readily obtained with Multiwatt, whose average

leadframe thickness is about 0.4 mm.

4)

RELIABILITY AFTER THE SOLDERING PRO-

CESS, due to the excess stress caused by the

high temperature (>215°C) even with "zero"

absorbed moisture at the large interface be-

tween slug and molded body, with consequent

delamination;

5)

INSPECTION OF THE SOLDERED JOINT be-

tween slug and substrate, as discussed in next

paragraph;

6)

LIMITED PIN COUNT 7, which is no longer

able to cover the requirements of advanced

smart power, needing more I/Os for the logic

circuitry.

Due to the above reasons, STMicroelectronics pre-

ferred to invest in new especially developed power

structures, with designed-in surface mount charac-

teristics. Moreover, due to the increasing demand of

quality, led by the automotive market, process and

materials were selected in order to obtain intrinsic

long term reliability and a very low failure rate, target-

ed at 1 ppm for early life.

1.2 NEW SOLUTIONS:

ORIGINAL STMicroelectronics PACKAGES

WITH DESIGN-IN SMT CHARACTERISTICS

The well understood dual-in-line configuration was

selected for the new housing (Figure 3), which does

not look very much different than the Small Outline

(SO) package; but, from high dissipation capability,

the main difference is the internal massive slug,

which provides the same thermal impedance as tra-

ditional insertion packages.In view of the larger pin

count needed for smart power products, a family of