Power conversion
New surface mount power
components drive military
power supply modules
By Tracy Autry
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Hermetic surface mount packaging technology is replacing
traditional hermetic through-hole technology in military power
systems. These new hermetic surface mount packages have
brought the advantages of better thermal dissipation, lighter
weight, and smaller volume to the next generation of military
power systems.
While surface mount components have been in use in commercial
power supply applications for many years, designers of the more
demanding military power systems have been forced to continue
to rely on a range of discrete hermetic through-hole components
to implement their power solutions, since
commercial surface mount components fail
to survive in stringent military environments.
Through-hole solutions, however, have poor
thermal transfer characteristics, are heavy,
always use wire bonds (including two terminal devices), and require significantly greater
board space than surface mount devices.
This is changing, however, with the emergence of a range of hermetically sealed surface-mount packages for power applications.
These advanced packages include solutions
for two-terminal devices such as rectifiers,
Schottky diodes, and Transient Voltage Suppressors (TVSs), as well as three-terminal
devices including MOSFETs, SCRs, and
IGBTs. Together, they are rapidly transforming military power supply solutions, providing
better thermal transfer characteristics, lighter
weight, and smaller volume. Targeted for
military applications, these packages are
designed for life cycles that avoid obsolescence in these long-life programs. They can
also be supplied in power module assemblies,
providing a modular circuit solution with the
advantages of surface mount components.
Hermetic SMT packages
Two-terminal packages can take one of two
forms – a disk version often designated
DO-217 for moderate-to-high power ratings,
and a larger square configuration for higherpower applications. Both have an integral
strap on the top to provide a contact lead
to the surface of a printed circuit board.
Figure 1a
Figure1b
Military Embedded Systems ©2007 OpenSystems Publishing. Not for Distribution.
Figures 1a and 1b show the cross-sections of
two-terminal packages having multiple and
single die.
These rugged components sandwich one
or more silicon die between tungsten
slugs that serve as anode and cathode.
In multi-die applications, the silicon dice are
thermally separated from each other using
molybdenum discs. The package seal construction causes compressive forces between these
disks and the silicon dice, improving overall
heat transfer within the package. Their junction
temperature (Tj) can be as high as 200 ˚C. Thermal resistance junction to case (ROjc) can be as
low as 0.2 ˚C/W. The Coefficient of Thermal
Expansion (CTE) is designed to match the
silicon chip, which is 4.5 ppm/˚C.
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Figure 2
Since their double-plug construction has no internal wire bonds
from the die or dice to external contacts in two-terminal devices,
these packages are impervious to the effects of acceleration internally and exhibit lower inductance and higher surge capabilities
than through-hole solutions.
Even though wire bonds are employed here, this package exhibits low inductance due to the lack of ferrous materials. All wire
bonds within the package are mono-metallic Al-to-Al bonds.
This package is suited for integrating gate resistors and protection or zener diodes.
Unlike commercial surface mount packaging that employs soft
solder, these components use hard solder (Au/Sn or Au/Ge)
throughout, eliminating solder creep and recrystallization problems that can be caused by power cycling and high-temperature
operation in military applications. With hard solder, the thermal
resistance junction to case (ROjc) doesn’t increase in operation,
providing longer service life.
Table 1 provides a comparison of three hermetic surface mount packages with standard through-hole packages like the DO-4 for two terminal devices or the
TO-254 for three-terminal components. It can be seen that these
surface mount devices provide a 6x improvement in weight, 3.5x
in thermal resistance, and 2.5x in volume.
Microsemi’s ThinKey package is representative of a higher
power square implementation of this double-plug ceramic and
metal construction. In this case, the silicon die is hard soldered to
a molybdenum disk, and this assembly is then soldered between
two square molybdenum pads that serve as the external terminal
connections. A copper-clad strap brings the top terminal down
to make contact on a circuit board. Typical applications are with
Schottky die (Figure 2).
Larger rectangular surface mount packages are designed for
three-terminal devices. Here, the side of the die with the drain is
soldered onto a copper/molybdenum pad to create one terminal,
while the source and gate on the other side of the die are wirebonded across an insulating alumina bridge to pads that serve
as the other two terminals – all within the hermetically sealed
cavity.
Power module assemblies
In addition, these surface mount components can be used as
building blocks for space-saving hermetic surface mount power
modules, utilizing standard materials and processes to create
application-specific power circuits of high reliability. This construction employs a five-step process designed to pass Quality
Conformance Inspection (QCI) tests for Commercial, TX, TXV,
and JANS equivalent screening levels:
1. Hermetic discrete devices are screened to the required level
(for example, TX or S-level equivalent) so that only “known
good” devices are used in the module’s construction.
2. Terminals are brazed to the substrate at 800 ˚C using Cu/Ag
eutectic.
3. To prevent voiding, the substrate is bonded to its heatsink in a
vacuum and is verified with 100 percent X-ray inspection.
4. Module is populated with the screened discrete devices at
215 ˚C.
DO-4
TO-254
3-terminal SMT
Schottky SMT
Weight
(grams)
10
9
1.5
1
Thermal
resistance (C/W)
1.5
1.5
0.3
0.3
Volume
(cubic inches)
0.25
0.125
0.05
0.02
Diode SMT
0.9
0.4
0.012
Package type
Table 1
Military Embedded Systems ©2007 OpenSystems Publishing. Not for Distribution.
Power conversion
5. Completed module is tested and screened to the appropriate
level, including QCI samples.
Since these hermetic power modules can be fabricated on
ceramic (either metalized or direct bond copper) substrates and
use aluminum composite or copper laminate heatsinks, their
coefficients of thermal expansion will closely match that of the
surface mount components. Integral contact terminations can be
crafted easily from copper alloy or steel to provide an easy-toconnect final assembly.
Employing these surface mount hermetic packages in military
power modules greatly reduces complexity and cost, while
simplifying testing and qualification. Up-front costs such as
tooling and nonrecurring engineering charges are much less than
for conventional hybrid, or multichip module designs. In addition, since the power module approach uses individual hermetic
discrete devices, the modules can be reworked with relative ease
and minimal costs.
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Prototype units can be built rapidly for in-circuit testing. If one
or more of the discretes need to be changed to optimize circuit
performance, the modules can be returned to install new discrete
devices for additional testing.
Military-grade reliability with surface mount
performance
In short, today’s hermetic power module technology can provide
military applications with all the advantages of commercial
surface mount assembly, but with military-grade reliability,
faster prototype development, reduced costs, weight, and size
compared with conventional through-hole hybrid solutions – the
best of both worlds.
Tracy Autry is a module development manager
with Microsemi Corporation. He has 15 years of
experience in electronic packaging development
and 13 years of experience in designing and manufacturing high-reliability power modules for the
military and aerospace markets. He received a BS in Ceramic
Engineering from the University of Missouri-Rolla.
Microsemi Corporation
11861 Western Avenue
Garden Grove, CA 92841
714-372-8050
tautry@microsemi.com
www.microsemi.com
Military Embedded Systems ©2007 OpenSystems Publishing. Not for Distribution.