SC19 RF Trans Packages 1 - Philips Semiconductors / NXP Semiconductors

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

1998 Mar 23 2

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 −−− −−−−−−√

1998 Mar 23 6

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.