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 Mounting

by P. Casati & C. Cognetti

A new, high power IC surface mount package family is introduced in this note. It is called PowerSO family 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 Surface 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-20ä – Jedec Registration MO-166

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 dramatically 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 yet compatible with SMT principles: a few "exotic" components like large capacitors, resistors, inductors, varistors, etc. and almost all the power semiconductor packages. Several drawbacks are associated with existing power packages:

July 2001

1/19

AN668 APPLICATION NOTE

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.

1.1INTERMEDIATE 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

1)HANDLING, which unlike all other surface mount packages, is complicated by the struc-

2/19

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 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 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.

1.2NEW 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 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, covering from 20 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

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-20	PowerSO-36

3/19

AN668 APPLICATION NOTE

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.)
	T		10			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

N	N			R
			a2 A			c
	b	e		DETAIL B		a1
	DETAIL A				E
	e3
	H			lead		DETAIL A
	D					slug
				a3
				DETAIL B
	20	11			0.35
				Gage Plane		- C -
				S	L	SEATING PLANE
						G C
E2			E1	BOTTOM VIEW		(COPLANARITY)

T

E3

1	10
	D1
h x 45	PSO20MEC

4/19

AN668 APPLICATION NOTE

Figure 4. PowerSO-36 package (Jedec MO-166) mechanical data.

	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
	N			10°(max.)
	S			8 °(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

N	N
				a2	A			c
			A	e	DETAIL B		a1
		DETAIL A					E
			e3
			H		lead		DETAIL A
			D
					a3		slug
	36			19	BOTTOM VIEW
					B			E3
E2					E1
					DETAIL B		D1
					0.35
	1		1	8	Gage Plane		- C -
					S	L	SEATING PLANE
			b	0.12 M A B			G	C
h x 45˚					PSO36MEC		(COPLANARITY)

5/19

AN668 APPLICATION NOTE

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-20/36ä.

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, Power- SO-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 compatible with the high cur-

6/19

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 surface mount Multiwatt and the following considerations are possible:

1)Electrical resistance of PowerSO-20 leads is about 760μΩ, 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 resistance. Wire resistance is 450-1800μΩ/mm and it must be

considered that the minimum wire length is 2.5 mm corresponding to 1150-4500μΩ;

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, electri-

cal resistance of the interconnection is about 2mΩ.

Table 1. Electrical resistance data of PowerSO-20 and Multiwatt 15 leads

	Dimensions	Multiwatt15	Multiwatt15	PowerSO-20
	(mm)	Insertion	Surf. Mount
	Long lead
	MW15	970mW
	23x0.9x0.5		–	–
	18x0.9x0.5	–	760mW	–
	Short lead
	MW15	590mW	–	–
	14x0.9x0.5
	9x0.9x0.5	–	380mW	–
	Lead
	PowerSO-20			760mW
	5x0.5x0.25	–	–

Copper resistivity : 1.9mW x cm

Aluminum wire resistance:

10 mils: 1800 mW / mm

20 mils: 450 mW / mm