ST AN668 APPLICATION NOTE
AN668
APPLICATION NOTE
A New High Power IC Surface Mount Package Family: PowerSO-20
& PowerSO-36 Power IC Packaging from Insertion to Surface Mount
by P. Casati & C. Cognetti
A new, high power IC surface mount package family is
introduced in this note. It is called Power SO famil y and
has the Jedec registration MO-166.
STMicroelectronics developed PowerSO in order to
answer the increasing demand of miniaturization and
quality in power applications. Automotive, industrial,
audio and telecom markets will take advantage of the
new package, by introducing the use of S urface Mount
Technology in the production of power systems.
PowerSO-20 and PowerSO-36 are the elements of the
MO-166 family having 20 leads at 0.050 inch pitch
(1.27 mm) and 36 leads at 0.026 inch pitch (0.65 mm)
respectively. These packages are in mass production
since 1995.
PowerSO-20 – Jedec Registration MO-166
™
ing
This note is intended to compare the PowerSO-20/36 with alternative surface mount solutions and to
the existing Multiwatt package, the well known "double TO-220" developed by STMicroelectronics in
late 70s.
Data presented here demonstrates that PowerSO-20 is the real successor of Multiwatt for surface mount
applications and is becoming a milestone in power package technology with PowerSO-36 as Multiwatt
did 20 years ago.
1. POWER DEVICES AND SURFACE MOUNTING
Use of Surface Mount Technology (SMT) has dramatical ly increased in the last 20 years, moving from consumer
to professional applications and serving highly demanding markets like telecom, industrial and automotive.
Major advantages expected from SMT are size reduction, automated board mounting, high reliability and cost
effectiveness; larger density of functions is achieved in smaller systems.
Evolution of SMT drove the development of several new packages for discrete and IC devices: SOT23,
SOT194, TO263, SO, PLCC, PQFP with many options in pin pitch, size and thickness.
All of them are compatible with the surface mount technique, based on fast picking and placing from tapes or
trays, followed by mass soldering. Mounting lines are almost totally automated, with high throughput and high
yield.
Only a few devices are not y et compati ble w ith SMT princi ples: a few "exotic" components like l arge capac itors,
resistors, inductors, varistors, etc. and almost all the power semiconductor packages. Several drawbacks are
associated with existing power packages:
July 2001
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AN668 APPLICATION NO TE
1) They have still the traditional structure for insertion and are mounted manually in the PCB,
with loss of time, productivity, floor space and
money;
2) They force the use of wave soldering techniques and are not compatible with reflow soldering;
3) They are not compatible with increasing need
of miniaturization of power systems.
The above points are so important that the development 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 packages 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 Heptawatt package (7 lead TO-220) shown in Figure 1
which was introduced in the market by STMicroelectronics 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 number of reasons:
Figure 1.
HeptawattTM (surface mount) package
ture, which is asymmetrical in two directions
(height and thickness). Existing pick and place
tools are not readily compatible with this structure, as well as the tape and reel packing;
2) SIZE, which is basically the same as existing
insertion packages; therefore, is not the answer to the demand of miniaturization of power
systems coming from almost all the applications: 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 tha t maj or c oplanar ity im provement
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 PROCESS, due to the excess stress caused by the
high temperature (>215°C) even with "zero"
absorbed moisture at the large interface between slug and molded body, with consequent
delamination;
5) INSPECTION OF THE SOLDERED JOINT between 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 preferred to invest in new especially developed power
structures, with designed-in surface mount characteristics. 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, targeted at 1 ppm for early life.
1) HANDLING, which unlike all other surface
mount packages, is complicated by the struc-
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1.2 NEW SOLUTI ONS:
ORIGINAL STMicroelectronics P ACKAGES
WITH D ESIGN-IN SMT CHAR ACTERISTIC S
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 traditional insertion packages.In view of the larger pin
count needed for smart power products, a family of
AN668 APPLICATION NOTE
packages has been designed, cov eri ng from 2 0 to 36
leads. The well established concept of "variable pitch
in a fixed body" has been used, with 1.27 mm (0.05
inch), 1.0 mm, 0.8 mm and 0.65 mm.
Figure 2.
PowerSO-20 & PowerSO-36 packages
PowerSO-20
The new family, named PowerSO, is Jedec registered as MO-166.
PowerSO-20 and PowerSO-36 are presented in this
technical note (Figure 3 & 4)
PowerSO-36
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AN668 APPLICATION NO TE
Figure 3. PowerSO-20 package (Jedec MO-166) mechanical data.
DIM.
mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A 3.6 0.142
a1 0.1 0.3 0.004 0.012
a2 3.3 0.130
a3 0 0.1 0.000 0.004
b 0.4 0.53 0.016 0.021
c 0.23 0.32 0 .009 0.013
D (1) 15.8 16 0.622 0.630
D1 9.4 9.8 0.370 0.386
E 13.9 14.5 0.547 0.570
e 1.27 0.050
e3 11.43 0.450
E1 (1) 10.9 11.1 0.429 0.437
E2 2.9 0.114
E3 5.8 6.2 0.228 0.244
G 0 0.1 0.000 0.004
H 15.5 15.9 0.610 0.626
h 1.1 0.043
L 0.8 1.1 0.031 0.043
N 10˚ (max.)
S
8˚ (max.)
T10 0.394
(1) "D and F" do not include mold flash or protrusions.
- Mold flash or protrusions shall not exceed 0.15 mm (0.006").
- Critical dimensions: "E", "G" and "a3"
OUTLINE AND
MECHANICAL DATA
JEDEC MO-166
PowerSO20
E2
h x 45
DETAIL B
PSO20MEC
R
lead
a3
Gage Plane
BOTTOM VIEW
E
DETAIL B
0.35
S
D1
L
c
a1
DETAIL A
slug
- C -
SEATING PLANE
GC
(COPLANARITY)
E3
NN
a2
A
b
DETAIL A
e3
H
D
T
1
e
1120
E1
10
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Figure 4. PowerSO-36 package (Jedec MO-166) mechanical data.
AN668 APPLICATION NOTE
DIM.
mm inch
MIN. TYP. MAX. MIN. TYP. MAX.
A 3.60 0.141
a1 0.10 0.30 0.004 0.012
a2 3.30 0.130
a3 0 0.10 0 0 .004
b 0.22 0.38 0.008 0.015
c 0.23 0.32 0.009 0.012
D (1) 15.80 16.00 0.622 0.630
D1 9.40 9.80 0.370 0.385
E 13.90 14.50 0.547 0.570
e 0.65 0.0256
e3 11.05 0.435
E1 (1) 10.90 11.10 0.429 0.437
E2 2.90 0.114
E3 5.80 6.20 0.228 0.244
E4 2.90 3.20 0.114 0.126
G 0 0.10 0 0.004
H 15.50 15.90 0.610 0.626
h 1.10 0.043
L 0.80 1.10 0.031 0.043
N10°(max.)
S8°(max.)
(1): "D" and "E1" do not include mold flash or protrusions
- Mold flash or protrusions shall not exceed 0.15mm (0.006 inch)
- Critical dimensions are "a3", "E" and "G".
OUTLINE AND
MECHANICAL DATA
PowerSO36
NN
a2
A
1936
0.12 AB
⊕
e
M
E1
DETAIL B
lead
a3
B
Gage Plane
PSO36MEC
BOTTOM VIEW
DETAIL B
0.35
S
L
E
DETAIL A
(COPLANARITY)
E2
h x 45˚
DETAIL A
118
A
e3
H
D
b
c
a1
slug
E3
D1
- C -
SEATING PLANE
GC
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AN668 APPLICATION NO TE
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).
2. STRUCTURE AND CHARACTERISTICS
OF POWERSO-20/36
.
In this section, design criteria listed in previous paragraph 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 package onto which the silicon chip is soldered. Its function is twofold:
a) To provide a low thermal resistance path from
the chip to the external heatsink;
b) 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, PowerSO-20 has the same junction to case thermal resistance 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 process, 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 thickness 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 compatibl e with the high c ur-
rents (10-20 A) associated with a large number of
power ICs, which require the utilization of large aluminum 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 surfac e mount Mul tiwatt and the following considerations are possible:
1) Electrical resistance of PowerSO-20 leads is
about 760
µΩ
, i.e. it is equivalent to insertion
Multiwatt or better;
2) Electrical resistance of PowerSO-20 leads is
worse than Surface Mount Multiwatt by a factor
of 1.3 to 2;
3) In any case, lead resistance of PowerSO-20 is
much lower than the wire resista nce. Wire resistance is 450-1800
µΩ
/mm and it must be
considered that the minimum wire length is 2.5
mm corresponding to 1150-4500
µΩ
;
4) If needed, two or more leads are short circuited
and different frame designs can be developed
providing flexible options (Figure 6). With multiple wire bonding, current capability can be
very large; for 45A current, if 3 leads are used
with 10 mil diameter, 3 mm long wires, electrical resistance of the interconnection is about
Ω
2m
.
Table 1.
Electrical resistance data of PowerSO-20
and Multiwatt 15 leads
Dimensions
(mm)
Long lead
MW15
23x0.9x0.5
18x0.9x0.5
Short lead
MW15
14x0.9x0.5
9x0.9x0.5
Lead
PowerSO-20
5x0.5x0.25 – – 760µΩ
Multiwatt15
Insertion
970µΩ
–
590µΩ
–
Multiwatt15
Surf. Mount
–
760µΩ
–
380µΩ
PowerSO-20
–
–
–
–
Copper resistivity : 1.9mW x cm
Aluminum wire resistance:
∅
10 mils: 1800 mW / mm
∅
20 mils: 450 mW / mm
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