Amplifiers – Involving structure other than that of transformers per se – With printed circuits
Reexamination Certificate
2000-12-14
2003-06-17
Nguyen, Patricia (Department: 2817)
Amplifiers
Involving structure other than that of transformers per se
With printed circuits
C330S250000, C361S704000, C257S712000
Reexamination Certificate
active
06580316
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to 9904595-7 filed in Sweden on Dec. 15, 1999; the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD
The present invention relates to a power transistor module intended for radio frequency applications, particularly for use in an amplifier stage in a radio base station or in a ground transmitter for television or radio, to a power amplifier comprising said power transistor module and a method in the fabrication of the power amplifier.
DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION
Power transistor modules for amplification at high frequencies have to comply with a number for detailed requirements as regards power amplification, raggedness, break-down voltages, noise, distortion, capacitances, input and output impedences, etc. at a specified feed voltage and operation frequency. The demand on output power vary from a few watts to several hundred watts, in the latter case several components connected in parallel in a module being utilized. Power transistors operate at high signal levels and thus high current densities. The operation frequency is within the radio and microwave frequencies.
The power transistor modules are the critical components in power amplifiers in radio transmitters. The performance of the modules is the limiting factor as regards output power, efficiency and reliability.
In
FIG. 1
is shown, schematically in cross-section, a power transistor module
1
mounted at a heat sink
3
and electrically connected to conductor pattern (not shown) on a circuit board or a printed circuit board
5
according to known practice.
Module
1
comprises a rectangular electrically conducting and heat conducting bottom plate or flange
7
, preferably of a metallic material, onto the upper surface of which are mounted a transistor chip
9
and typically a first
11
and a second
13
capacitor chip. Further, module
1
comprises an insulating, preferably ceramic, frame arrangement or isolator
15
mounted on the upper surface of flange
7
so that the arrangement surrounds said chips, and a cover
17
attached to frame
15
.
The transistor chip
9
is typically a chip of the LDMOS (Lateral Double Diffused Metal Oxide) type and comprises a row of blocks connected in parallel, which each comprises a large number of transistor cells connected in parallel. The higher output power that is desired, the more transistor cells are needed. In
FIG. 2
is shown a schematic layout of the transistor chip
9
comprising seven blocks. In each block all gates are connected in parallel to a gate connection or pad and all drains are connected to a drain connection or pad. In the uppermost block the gate pad is indicated by
19
and the drain pad is indicated by
21
. The sources have connections at the back side/bottom side of the chip, which are connected to ground, e.g. via flange
7
.
High power transistors have at high frequencies very low input and output impedances. To match these to surrounding circuits on the circuit board
5
impedance matching networks are needed close to the active chip with the transistors. Usually, these matching networks are implemented by using bonding wires
23
,
25
,
27
,
29
and capacitor chips
11
,
13
inside the power transistor module
1
. The bonding wires are further connected to straight butt connection contacts
31
,
33
, preferably of alloy
42
, which are projecting from module
1
for connection to said surrounding circuits. In
FIG. 2
bonding wires
25
,
27
are indicated at the uppermost block of the chip. Each other block is certainly connected via its own bonding wires (not shown). Difficulties to attain tight tolerances during manufacture of the modules give rise to undesired variations in different critical electrical parameters.
The module
1
may be fastened to heat sink
3
by screws or bolts, whereafter the butt connection contacts
31
,
33
are manually soldered to the surrounding circuits of circuit board
5
through soldered seams
35
,
37
. Flange
7
and heat sink
3
typically comprise apertures or recesses for receiving screws or bolts (not shown) for the mounting of module
1
.
The matching network gives an extra contribution to the impedances before an outer impedance matching is performed on the circuit board. The placement of the transistor chip in the module is very critical, since it affects the length and form of the bonding wires and thereby the impedance matching. Length and form of the bonding wires may be difficult to define.
The low output impedance of the transistor chip is caused by the capacitance of the transistor chip together with a relatively low feeding voltage. The possibilities to achieve a good matching to the surrounding circuits are limited by these facts and by inductance in the conductors (particularly the bonding wires) between the power transistor module and the circuit board.
The impedance matching, which is required, therefore has to be performed both inside the power transistor module and outside on the circuit board. Variations in the properties of the transistor module, in the dimensions of the circuit boards and in the placement of the components will imply that a power amplifier comprising said components ought to be functionally trimmed to possess the best performance. Tolerance equalization through trimming, however, is not desirable by amplifier manufacturers. Manufacture without trimming will, however, rarely provide for optimal performance but results is discarding of transistor modules.
Further, high power transistor modules of the above kind cannot be surface soldered (i.e. machine soldered) together with other surface soldered circuits on the circuit board and thereafter be mounted, i.e. fastened by screws, to a heat sink, since the modules are mechanically stressed during the screwing. The force arisen hereby is directly transferred to the soldered seams through the butt connector contacts, such that the soldered seams will be damaged, e.g. they will crack. This involves that the transistors have to be soldered manually as indicated, which is time-consuming, costly and results in large quality variations.
Due to the straight butt connector contacts, which are very rigid, the movements, which arise due to the temperature variations, will direct affect the soldered seams between the module and the circuit board. During these stresses the solder will plasticize and relax, which results in congruence displacements. When this is repeated sufficiently many times the soldered seams start to crack, which constitutes the main source of failure during operation. The lifetime of an amplifier will in this case be 5-10 years instead of the desired 30 years.
Existing capsules for transistors have preferably a metallic flange and a ceramic isolator. Both the flange and the isolator must have a certain thickness due to purely mechanical reasons and the transistor module will therefore be thicker than the circuit board (which normally is 0.8 mm). A recess must therefore be milled in the heat sink in order to be able to lower the flange of the transistor module, see FIG.
1
. This involves increased manufacturing expenditures and a risk for irregularities in the bottom of the recesses. Such an irregular mounting surface results in a risk that the capsules are bent during the fastening by screws, so that the chips crack. Further, the cooling is deteriorated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a power transistor module intended for radio frequency applications, particularly for use in an amplifier stage in a radio base station, which is lacking at least some of the problems which are associated with the prior art.
It is in this respect a particular object of the present invention to provide a power transistor module, which reduces the problems of endurance stress of the soldered seams.
It is a further object of the invention to provide a power transistor module, which may be machine soldered to a circuit board.
Further objects of the present invention wil
Ahl Bengt
Olofsson Lars-Anders
Burns Doane Swecker & Mathis L.L.P.
Nguyen Patricia
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