AN668 datasheet(8/19 Pages) STMICROELECTRONICS

8 / 19 page

AN668 APPLICATION NOTE

8/19

Figure 6. Embossed cavity tape.

C] Coplanarity

Lead coplanarity is considered a major issue for

surface mount devices. In the case of Power sur-

face mount packages, whose slug is eventually

soldered on a substrate, coplanarity is also refer-

enced to the relative position of the slug and the

leads.

In the PowerSO-20/36, lead extremities and

package slug are positioned inside a 4 mil (0.1

mm) layer; this condition provides a high yield

mounting and soldering process.

D] Solder joint quality and inspection

When the package slug is soldered onto the sub-

strate, as in the many applications developed so

far, the need exists for producing a high quality

solder joint between the slug and its solder pad on

the board.

The PowerSO-20/36 structure is ideal from this

viewpoint: symmetry existing in the x-y plane pro-

vides a very good balance of the wetting forces,

once the solder paste is melted. A kind of self

centering effect has been observed, which is

mostly due to the dual-in-line outline and to the

excellent wettability of both leads and slug. This

results in a solder joint well controlled in thickness

and consistency.

An additional advantage of the PowerSO-20/36

outline is the presence of two exposed inspection

points on the slug, which has been intentionally

obtained in the package body (Figure 7). Propri-

etary processes keeps these two points free of

molding compound flash and allows evident sol-

der fillets after the soldering process.

Due to the symmetrical structure of PowerSO-

20,/36 the presence of solder fillet in both inspec-

tion points is a good indicator of joint quality and

planarity. It should be noticed that the same kind

of control is not possible in the surface mount

Multiwatt structure, which can show a good solder

fillet in the exposed section of the slug even with

a non-parallel joint and possible voiding. In this

case, the degradation of the junction to substrate

thermal resistance Rth(j-s) is more evident, as

shown in Figure 8.

E] Reliability after soldering process

Extensive evaluation of frame design and materi-

als provides the PowerSO-20/36 excellent reli-

ability performance after the soldering process,

as discussed in section 5. No degradation of the

8.60

13.00

4.50

11.50

24.00

1.50 min

R0.50 Typical

4.00

2.00

1.5 +0.1/-0.0

1.75

11.50

24.00 ±0.3

D94AN002

30 ±0.05

STRETCH

FORM

1.00

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