AN668 STMICROELECTRONICS | Alldatasheet
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- 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: by P. Casati & C. Cognetti A New High Power IC Surface Mount Package Family: PowerSO-20 ™ & PowerSO-36 Power IC Packaging from Insertion to Surface Mounting 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. 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. PowerSO-20 – Jedec Registration MO-166
of miniaturization of power systems. R&D, engineering and production resources.
1.1 INTERMEDIATE SOLUTIONS: INSERTION
es to obtain a kind of surface mount configuration. Figure 1. HeptawattTM (surface mount) package leadframe thickness is about 0.4 mm.
1.2 NEW SOLUTIONS:
Figure 2. PowerSO-20 & PowerSO-36 packages
Figure 3. PowerSO-20 package (Jedec MO-166) mechanical data. (1) "D and F" do not include mold flash or protrusions.
- Mold flash or protrusions shall not exceed 0.15 mm (0.006").
Figure 4. PowerSO-36 package (Jedec MO-166) mechanical data.
- Critical dimensions are "a3", "E" and "G".
e] High reliability after soldering process.
- STRUCTURE AND CHARACTERISTICS
graph are presented and discussed.
2.1 High power structure and process (>20 W)
absorb power peaks in switching conditions. high melting temperature (300°C) tin based alloy.
2.2 High current capability (10-20 A)
Table 1. Electrical resistance data of PowerSO-20
2.3 Miniaturization
terms of size, weight, height and volume are evident. They range between 22 and 64% improvement. Table 2. Miniaturization of PowerSO-20 vs Sur-
2.4 Designed-in surface mount characteris-
is fully compatible with existing equipment. shipment on the lead coplanarity (Figure 6). Figure 5. Frame options for high current
Figure 6. Embossed cavity tape. mounting and soldering process. der fillets after the soldering process.
high temperature stress, up to 260°C. Figure 7. Exposed slug edge for solder joint in- Figure 8. Loss of thermal performance due to
2.5 Hermeticity
of low stress, high adhesion molding compounds. Figure 9. Scanning Acoustic Microscope map
- THERMAL DESIGN AND APPLICATIONS
lent to that of available insertion packages. 100°C in more relaxed systems. in all the considerations of this note.
3.1 JUNCTION-TO-CASE THERMAL RESIS-
from the junction to the external surface of the slug.
sive copper slug and tin based alloy die soldering. source is uniformly distributed on the die. the heat flows from the silicon to the substrate. Figure 10. Thermal resistance junction-case
3.2 APPLICATIONS WITH 1-2 WATT DISSIPA-
3.2.1 On board dissipating elements
based on the medium power SO. connected to the heat transfer leads (Figure 12). Figure 11. Lead frame for medium power SO Figure 12. Foot print for medium power SO tance of the two areas shown in Figure 12.
and thermal conductivity) and on the die size. power SO, with the slug soldered on the board. Figure 13. Rth(j-a) vs. board dissipating area Figure 14. Rth(j-a) is about 50°C/W in this configura- Figure 14. Pad layout with dissipation elements of 6 sq cm. Figure 15. Footprint with dissipating element
3.3 APPLICATIONS WITH 2-5 WATT DISSIPA-
3.3.1 Dissipating elements and ground layer
Figure 16. Mounting on PCB with ground layer
3.3.2 Via holes and ground layer
Figure 17. Mounting on epoxy FR4 using via Figure 18. Footprint of PowerSO20/36 with via
3.3.3 Via holes and external heatsink
metal plate glued on the opposite side of the board. Figure 19. Mounting on epoxy FR4 using via Figure 20. Thermal Resistance junction-sub-
3.4 HIGH POWER APPLICATIONS (UP TO 20
W)WITH STANDARD SUBSTRATE In order to get an Rth(j-a) of few °C/W, high power applications require large and massive external heat- sinks in close contact with the power device. This is not a simple requirement when cost effective systems using the standard surface mount technolo- gy and the standard PCB substrate are considered. Major concerns are related to the critical assembly of a large heatsink onto a small package; vibrations and thermal excursions can generate unwanted mechan- ical stress, thus damaging the package leads or the integrity of the contact between slug and heatsink. However, the fact that we have a well established, PCB based surface mount technology is pushing the industry to test several directions, similar to those sketched in Figure 21. Rather than proven solutions, they should be consid- ered here as early attempts aimed to explore the ca- pability of existing surface mount processes and materials in the direction of increased heat dissipa- tion at reduced cost. 3.4.1 "Slug-up" package and external heat- sink In the example of Figure 21a, PowerSO-20 is shown in "reverse" or "slug-up" configuration, with a clip mounted external heatsink, which was studied for 3.5W dissipation in still air and 5-6W dissipation in forced ventilation. A similar concept can be used for other applications and the sink size adapted to their specific require- ments. A large variety of solutions will exist which will take advantage of the metal box in which the board can eventually be housed. Figure 21a. "Slug up"package with external heatsink To minimize the mechanical stess on leads and sol- der joints derived from any pressure applied on top of package, the slug-up forming is specifically designed (Fig. 22).
3.4.2 Cavity board and external heatsink
In the example of Figure 21b, the PowerSO-20/36 is mounted onto an epoxy board, with a through cavity fabricated to correspond to the package slug. The ex- ternal heatsink is directly applied in contact with the slug, secured by means of a spring system or glued. Also in this case, a high level of dissipation can be achieved with properly designed heatsinks. Figure 21b. Mounting on cavity board and ex- ternal heatsink applied Heatsink D94AN014 Epoxy board Epoxy board with through cavity D94AN015 Heatsink
Figure 22. PowerSO-36 Slug-up
3.5 HIGH POWER APPLICATIONS (UP TO
20W) WITH INSULATED METAL SUB- STRATES - IMS In the last few years, several Companies developed the idea of a cost effective Insulated Metal Substrate (IMS) having: a) A copper printed layer supported by an alumi- num base, with epoxy or polyimide isolation in between; or b) A flexible printed circuit (polyimide) glued onto an aluminum base. In both cases, the following conditions are offered: a) The substrate is compatible with standard SMT processes, including infrared reflow soldering; b) It is compatible with passive and active devic- es, including PowerSO-20/36; c) It has an acceptable thermal resistance loss between the PowerSO-20/36 slug and the alu- minum base, due to optimized thermal conduc- tivity of the isolating layer; d) The aluminum baseplate has variable thick- ness, up to 5 mm, in order to sink different amounts of heat. Utilization of IMS is considered for several applica- tions both for automotive and industrial systems and is particularly fruitful when associated with PowerSO- 20/36 packages (Figure 23) Figure 23. Thermal performance has been measured for both substrates described above. The first result is related to the contribution of isolation to the total thermal re- sistance. When the footprint of Figure 14 is used, with the 96 sq mm slug soldered onto a 155 sq mm pattern, this contribution is about 0.5°C/W . Therefore, the thermal resistance from the junction to the aluminum baseplate ranges from 1.0 to 2.5°C/W depending on the chip size. For large dissipation, an external heat sink is applied to the IMS by means of screws, bolts or rivets. The total thermal resistance is obtained by adding the heat sink resistance to the junction to baseplate re- sistance (1.0-2.5°C/W). This solution is capable of 20-25 W per device and if a large sink is used, as massive parts existing in the car, more than one device can be assembled on the same ISM, thus obtaining a functional power sub- system with several tens of Watt dissipation. It is interesting to notice that the aluminum plate itself has good dissipation properties to the ambient, de- pending on its size. The PowerSO-20/36 soldered onto a 40 mm square, 1.5 mm thick IMS shows a thermal resistance of 7°C/ W, for about 7 W dissipation.
3.6 HIGH POWER PULSES AND THERMAL
In several applications, large power pulses are deliv- ered by the device for a short time. In this case, discussed in the STMicroelectronics pa- per "Designing with Thermal Impedance" (Semith- erm Conf. 1988), the quantity which rules the junction temperature up to the time t0 is the thermal imped- ance of the three different elements: silicon chip, package slug and heatsink. For pulse durations in the range of 0.1-1.0 sec, the package slug has the strongest influence on the sys- tem performance, depending on the associated ther- mal capacitance, i.e. the capability of heat accumulation. In the presence of properly sized slugs with suitable capacitance, it is possible to maintain a low junction temperature for the switching time of most applica- tions, which seldom exceeds 1 sec. For popular power packages like TO-220 and Multi- watt, the value of the thermal impedance in the first second is lower than the value of Rth(j-c) in steady state. In order to compare the PowerSO-20/36 packages with the insertion equivalent, Fig. 24 is very helpful. It shows the thermal impedance of different packages in the first second of the power pulse, in the same conditions of power intensity and die size. When similar comparisons are performed with medi- um power "bat wing" SO, the enhancement provided Copper foil Insulation D94AN016 Aluminium
reliability target is 1 ppm failure rate. associated to these tests is given in Table 4. are the same as those for standard power packages. Table 4. Reliability tests description. Table 5. ABS Voltage Regulator (die size 5.3 x 5.28mm)
Table 6. TDA7350A (Bridge Audio Amplifier) 1) Good thermal characteristics. They are able to handle die with the same size as Multiwatt. 2) Versatility. A wide variety of intelligent power products with a wide range of options can be managed. to the customers for power dissipation. 4) High power density through reduced package volume and height. sembly. Reliable and inspectable solder joints can be achieved. allow the PowerSO-20/36 to pass successfully severe tests like pressure pot and die penetrant. where power ICs are becoming more and and more common.
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics 2001 STMicroelectronics - All Rights Reserved ®MULTIWATT, PENTAWATT are registered trademarks of STMicroelectronics PowerSO-20™ is a trademark of STMicroelectronics STMicroelectronics GROUP OF COMPANIES Australia - Brazil - China - Finland - France - Germany - Hong Kong - India - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A. http://www.st.com AN668 APPLICATION NOTE