1998 Mar 23 1
Philips Semiconductors
RF transmitting transistor and
power amplifier fundamentals RF and microwave
transistor packages
4 RF AND MICROWAVE TRANSISTOR PACKAGES
The packages of electronic devices are, in general,
designed to:
Protect the electronics from mechanical damage
Ensure adequate heat transfer to the ambient, and
Provide robust, solderable electrical terminations.
For RF and microwave transistors however, the package
itself forms an important part of the total electronic circuit,
and this places additional requirements upon its electrical
characteristics.
These requirements together with the available
technologies have influenced RF transistor package
design over the years. As a result, there are a variety of
packages on the market today. The design and
characteristics of the main package types are outlined in
the following sections. Information on specific packages is
given in data handbooks SC18: Discrete Semiconductor
Packages, and SC19a: RF & Microwave Power
Transistors, RF Power Modules and Circulators/Isolators.
4.1 Basics of RF and microwave transistor
packages
In general, two (the base/gate and emitter/source) of the
three electrical contacts of a transistor die are on the top of
the die, and are connected to the external package
terminations by bonding wires. The underside of the die is
the third contact (the collector/drain) and connection is
usually made to this contact by bonding the die to an
electrical conductor which also serves as a heatsink.
4.2 Metal-can packages
Included here for historical completeness, metal-can
packages used to house bipolar transistors are rapidly
being replaced by newer, superior alternatives.
In a metal-can package (e.g. TO-39 (SOT5)), the transistor
die is attached to a small, thin metal plate (usually round).
All the external electrical terminations are wire leads. The
collector lead is connected to the plate; the emitter and
base leads are fed through the plate and isolated from it by
a glass seal.
The package is sealed with a metal cap welded onto the
plate. This design combined with stringent well-controlled
manufacture provide a hermetic package.
The power that a metal-can package can handle however
is very limited, because heat is mainly removed from the
die by radiation. And, while mounting the package directly
onto a heatsink in a circuit lowers the packages thermal
resistance, it means the collector is connected to the
heatsink, whereas most applications require a
common-emitter or common-base configuration. The
solution to this drawback was found with the introduction
of ceramic packages.
4.3 Ceramic packages with a copper stud or flange
In a ceramic package, the transistor die is soldered on a
metallized ceramic heat-spreader located on top of a stud
or flange used to mount the transistor and to conduct heat
away from the die. The function of the somewhat
inappropriately named heat-spreader is to electrically
isolate the bottom of the die from the stud or flange,
allowing the transistor package to be mounted directly to a
heatsink.
The electrical terminations are formed by brazing several,
usually flat, leads to the heat-spreader, with wire bonding
from the leads to the two contacts on top of the die. Beryllia
(BeO) used to be the most-commonly used heat-spreader
Fig.4-1 Metal-can package construction.
handbook, halfpage
MGM651
glass
seal metal
plate
metal
cap
die
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Philips Semiconductors
RF transmitting transistor and
power amplifier fundamentals RF and microwave
transistor packages
material since it combines good thermal conductivity
(250 W/mK) with good electrical isolation. A disadvantage
of BeO is that it is toxic. So, in line with Philips’ policy to
eliminate toxic and environmentally harmful substances
from its products, packages with aluminiumnitride (AIN)
heat-spreaders have been developed - the slightly lower
thermal conductivity of AIN being compensated for by
using thinner ceramic.
The first ceramic packages incorporated a copper stud or
flange brazed to the bottom of the heat-spreader. Since
there is considerable mismatch between the thermal
coefficients of expansion (TCE) of copper and beryllia, the
contact area must be limited to prevent the heat-spreader
from cracking. Larger (higher power) packages (e.g.
SOT121 and SOT171) were therefore designed using a
copper pedestal to which the heat-spreader was attached
(brazed) with the die on top. The pedestal allows a larger
heat-spreader (and hence die) to be used whilst
maintaining the metal-to-ceramic contact area well below
the practical limit. Even with this design, however, the size
of the heat-spreader is limited as only the region directly
above the pedestal has a low thermal resistance, and thus
conducts heat effectively. Those areas of a transistor die
and heat-spreader extending beyond the top of the
pedestal have a higher thermal resistance. Nevertheless,
since such packages can be mounted directly onto a
heatsink in the application, the power handling, though still
restricted, is much better than that of the standard design.
This type of package (with or without pedestal) is sealed by
epoxy-glueing a ceramic cap to the top of the package.
Though forming a high-quality reliable seal, epoxy resin
does not provide a hermetic barrier. To ensure that the
package is completely sealed and that there are no
pinholes in the epoxy, the packages are tested for gross
leaks. Note that all epoxy glues start to degrade at
temperatures close to 300 °C and, for long-term stability,
standard ceramic packages should not be exposed to
temperatures above about 150 °C. Short exposure to
higher temperatures is allowed (e.g. during reflow
soldering). In addition, during fluxing and cleaning,
minimize exposure to liquids, for example by dipping.
Though not strictly hermetic, all of Philips’ standard
ceramic packages contain glass-passivated transistor
dies. Effectively isolating the die from its surroundings,
glass passivation contributes to extremely high levels of
transistor reliability.
4.4 Ceramic packages with special flange
materials
As indicated above, the size of ceramic packages with
copper flanges is limited by the different TCEs of copper
and ceramic. This limitation was overcome by replacing
the copper by a material with a much lower TCE.
Nowadays, two materials are commonly used which
combine a much lower TCE with a still acceptable thermal
conductance:
A tungsten-copper alloy (e.g. SOT262), and
A copper-molybdenum-copper sandwich (e.g.
SOT468).
These materials allow the contact area between flange
and ceramic to be much larger while the ceramic can be
even thinner without increased risk of cracking. Since
these materials are at present very expensive, packages
with a copper flange remain in widespread use. For
improved RF grounding at high frequencies, flanged
packages with through-plated holes in the ceramic have
been developed.
Fig.4-2 Construction of a typical ceramic
package with flange.
handbook, halfpage
copper flange
ceramic heatspreader
die
metal bridge
connecting both
outer emitter leads
ceramic cap
epoxy glue
base
lead collector
lead
pedestal
leadframe connection
between both
outer emitter leads
soldered or
brazed region
MGM652
1998 Mar 23 3
Philips Semiconductors
RF transmitting transistor and
power amplifier fundamentals RF and microwave
transistor packages
4.5 Hermetic ceramic packages
A hermetic transistor package provides the highest levels
of reliability in extremely harsh environments as required
in aerospace and military applications for example.
Ceramic packages can be made hermetic, however a
special design is required. The leads are no longer brazed
directly onto the heat-spreader as in a standard ceramic
package but are brazed onto a two-layer alumina frame
(i.e. consisting of two frames sintered together). The
bottom frame is larger than the top one, thus forming inner
and outer ledges (electrically connected by metallization
on the bottom frame) The external leads are brazed to the
outer ledge, while the inner ledge is used to make a
connection to the transistor die by wire-bonding. The top
side of the second frame is flat and completely metallized
to enable a metal or ceramic cap to be soldered to it.
4.6 LDMOST packages
Whereas the abovementioned packages are suitable for
bipolar and VDMOS transistors, packages for the LDMOS
transistors are somewhat different. This is because the
bottom of an LDMOST die is the source, not the drain
(collector) as in a standard MOS (bipolar) transistor. This
is a major advantage as it is no longer necessary to
electrically isolate the die from the heatsink in the
application - the die is mounted directly on the flange. The
flange material therefore must have a good thermal
conductivity and a TCE close to that of silicon. Two
materials in current use are a tungsten-copper alloy
flange, and the copper-molybdenum-copper sandwich
mentioned earlier. In order to electrically isolate the drain
and gate leads from the flange, an alumina frame is brazed
onto the flange, with the leads brazed on top of this frame.
The package is completed by a ceramic cap sealed with
epoxy.
Offering superb electrical and thermal performance, and
ease of heatsinking, Philips’ LDMOS packages are an
attractive solution in an increasing number of applications.
Fig.4-3 Typical hermetic package construction
(Package: SOT422A).
handbook, halfpage
soldered or
brazed region
beryllia
flange
inner ledge 2-layer
alumina
frame
cap
collector
lead die
MGM653
epoxy glue
Fig.4-4 Typical LDMOST package construction
(Package: SOT467).
handbook, halfpage
metal flange
die
alumina
frame
ceramic cap
lead
soldered or
brazed region
MGM654
epoxy glue
1998 Mar 23 4
Philips Semiconductors
RF transmitting transistor and
power amplifier fundamentals RF and microwave
transistor packages
4.7 Flangeless and SMD packages
In order to reduce the board space required by transistors,
packages without a flange have been introduced (e.g.
SOT333). Eliminating the flange however also eliminates
the mounting holes, so other mounting methods have to be
used. These include clamping or even reflow soldering.
Eliminating the flange is in general only possible with
packages for bipolar devices in which the dies are
mounted on a ceramic heat-spreader. However, for
Philips’ SOT391B package, eliminating the flange alone
was not an option as this package has through-plated
holes in the heat-spreader. Without a flange, the package
could not be sealed adequately. The solution is to braze a
thin copper plate to the back of the package. This has two
advantages. First, the holes are sealed, and second, the
lead height of the package can be optimized to suit
standard printed board materials.
With LDMOST packages, the entire flange cannot be
eliminated as the dies are mounted on top of the flange.
The board space required can be reduced however by
reducing the length of the flange, most effectively by
shortening it to the size of the alumina frame. Packages
with such modified flanges are often referred to as
‘earless’. Clamping or reflow soldering are again the
recommended mounting methods.
Besides flangeless and earless packages, Philips has
introduced a leaded surface-mount package (SOT409).
This package, which is based upon the plastic SO8
package, has a copper backpad, a ceramic (alumina or
AIN) heat-spreader, a copper leadframe extending beyond
the heat-spreader and a ceramic cap. The backpad
enables the package to be soldered onto a PCB. To
ensure reliable solder joints and low thermal resistance,
the specification stipulates that the leads are J-shaped
such that backpads and leads are coplanar to within
0.1 mm, and that the leads never extend beyond the
backpad plane.
An even more effective way of reducing the required board
area is to use leadless packages. Besides saving board
space, these are highly cost-effective as they can be
mounted in a standard automated SMD reflow soldering
process. This kind of package consists of a ceramic (AIN)
heat-spreader and a ceramic cap. Leads are replaced by
plated contacts on the package sides. To increase the
soldering area for the electrical connections (and to obtain
more reliable solder joints), the plating is extended onto
the back of the package.
Reflow soldering footprints are available for all SMD
packages for optimum soldering results. For optimum heat
flow between package and heatsink (through a printed
circuit board), it is recommended to incorporate vias in the
board. The optimum size and location of these vias for
each package are available.
4.8 Coefficients of linear thermal expansion of
packages
The data of Tables 4-1 and 4-2 (together with
manufacturers’ board and heatsink data) can be used to
obtain good thermal matching in practical amplifiers.
Fig.4-5 SOT391B - a typical flangeless ceramic
package.
handbook, halfpage
MGM813
Fig.4-6 SOT409B - a ceramic surface-mount
transistor package derived from the SO8
package.
handbook, halfpage
MGM814
Fig.4-7 SOT511 - a typical leadless ceramic
surface-mount package.
handbook, halfpage
MGM815
Bottom view
1998 Mar 23 5
Philips Semiconductors
RF transmitting transistor and
power amplifier fundamentals RF and microwave
transistor packages
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Table 4-1 Overview of materials used in packages
Source: Suppliers’ data sheets
PACKAGE
FLANGE LEADFRAME BACKPAD CERAMIC
INSULATOR
COPPER TUNGSTEN
-COPPER Cu-Mo-Cu ALLOY 42
(Fe58/Ni42) NICKEL KOVAR
(Fe54/Ni29) COPPER COPPER BeO AIN
SOT119 √−− −−−
SOT121 √−− −−−
SOT123 √−− −−−
SOT161 √−− −−−
SOT171 √−− −−−
SOT262 −√− −−−
SOT268 −√− −−−
SOT273 √−− −−−
SOT279 √−− −−−
SOT289 −√− √−−
SOT324 −√− −−−
SOT333 √−− −−−
SOT390 −√− −−−
SOT391 −√− −−−
SOT391B −−− −−√
SOT409 −−− −√√
SOT422 √−− −−−
SOT423 √−− −−−
SOT437 −√− −−−
SOT439 √−− −−−
SOT440 √−− √−−
SOT443 −√− √−−
SOT445 √−− √−−
SOT448 −√− −−−
SOT460 −√− −−−
SOT467 −√− −−−
SOT468 −−√ −−−
SOT502 −√− −−−
SOT511 −−− −−−
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Philips Semiconductors
RF transmitting transistor and
power amplifier fundamentals RF and microwave
transistor packages
Table 4-2 Coefficients of linear thermal expansion, α, (in ppm/K) of package materials between 25 and 150 °C;
COPPER TUNGSTEN
COPPER Cu-Mo-Cu ALLOY 42
(Fe58/Ni42) NICKEL KOVAR
(Fe54/Ni29/Co17) BERYLLIA ALUMINIUM
NITRIDE
17.9 6.6 9.5-6.0 4.5 11.6 4.4 6.7 4.0
Source: Suppliers’ data sheets
4.9 Mounting recommendations
When mounting transistors, observing the following
recommendations will ensure good thermal and good
electrical contact between transistor package and heatsink
- a requisite for trouble-free, reliable operation.
4.9.1 Heatsink preparation
For transistors dissipating up to 80 W, heatsink
thickness should be:
- At least 3 mm for copper heatsinks (>99.9% ETP-Cu)
- At least 5 mm for aluminium heatsinks (99% Al)
These thicknesses should be increased proportionally
for transistors dissipating more power.
Minimum depth of tapped holes in heatsinks: 6 mm
Ensure holes in heatsinks are free of burrs
Ensure that the mounting area is at a level such that
there is a small positive clearance between the
transistor leads and the printed circuit board. This
prevents any upward bending of the leads which can
damage the ceramic heat-spreader and/or the
encapsulation.
Flatness of the mounting area: better than 0.02 mm
Mounting area roughness: <0.5 µm
Mounting area should be free of oxidation.
4.9.2 Printed circuit board preparation
Tin and wash the printed circuit board.
4.9.3 Transistor preparation
Transistor leads are gold plated. To avoid brittle solder
joints (due to too much gold in the joint), pre-tin the
leads, for example, by dipping their full length into a
solder bath at a temperature of about 230 °C. Minimize
the use of flux.
Apply a thin, evenly-distributed layer of heatsink
compound to the flange
Recommended heatsink compounds are:
- ‘WPS II’ (silicone free) Austerlitz-Electronics
- ‘340’ from Dow Corning.
When using a thermal pad, take special care with
respect to the size as well as the positioning of the pad.
If the pad does not cover the entire flange, the package
can be stressed so much that the ceramic heat-spreader
cracks. Ensure that mounting screws do not contact the
thermal pad (prevents the pad wrinkling if the screws are
turned).
4.9.4 Mounting sequence
Position the device with the washers in place
Use 4-40 UNC-2A cheese-head screws with a flat
washer to spread the joint pressure
Tighten the screws until finger tight (0.05 Nm)
Further tighten the screws until the specified torque is
reached (do not lubricate); for torques, refer to the
package outlines section of each data handbook
To lock mounting screws, allow about 30 minutes for
them to bed-down after the specified torque has been
applied, re-tighten to the specified torque and apply
locking paint
Solder the transistor leads onto the printed circuit board.