High frequency microelectronic circuit enclosure

An enclosure assembly for a high frequency integrated circuit. The enclosure assembly includes a plurality of lead terminals that are part of a lead frame. The lead terminals are connected to electrical ports of the circuit within the enclosure to connect the circuit to other integrated circuits and/or to a printed circuit board. A portion of the lead terminals outside of the enclosure has a specially shaped flared region that establishes the characteristic impedance of the circuit on the lead lines outside of the enclosure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to an enclosure assembly for one or more 
high frequency microelectronic circuits and, more particularly, to an 
enclosure assembly for one or more high frequency microelectronic circuits 
that includes a specially configured lead frame that is electrically 
connected to the circuits, and extends from the enclosure where a portion 
of the lead frame on the outside of the enclosure includes flared regions 
that are impedance matched to portions of the lead frame within the 
enclosure and to the input and output ports of the circuits. 
2. Discussion of the Related Art 
Currently, many applications exist for high frequency microelectronic 
integrated circuits operating in the GHz range, especially in conjunction 
with communication systems. As is well understood, high frequency signals 
are generally used in communication systems because these signals can be 
effectively transmitted at low power levels. For example, monolithic 
microwave integrated circuits (MMICs) have been extensively developed for 
use in applications such as cellular telephones, satellite receivers, 
personal communication systems, and remote sensing and control devices. 
Usually, these types of integrated circuits incorporate gallium arsenide 
(GaAs) devices including field effect transistors (FETs) and/or 
heterojunction bipolar transistors and associated circuitry that operate 
within the GHz frequency range. 
In order to reduce power losses and protect these types of delicate 
integrated circuits against environmental conditions, it is generally a 
requirement that MMICs, as well as other related types of RF circuits, be 
packaged in some form of enclosure within a particular system. In order to 
meet the requirements of the industry, such an enclosure should not only 
protect the integrated circuit within, but also should be low cost and 
provide low losses at the operating frequencies of the circuit. Because 
improvements in the ability to produce low cost MMICs has significantly 
increased, traditional enclosures formed of one or more of metal, ceramic 
and plastic currently being used for this purpose have generally been 
unable to meet the price/performance requirements that are acceptable for 
the commercial microwave electronics market place. Therefore a need exists 
for a new low cost enclosure for this purpose. 
When designing an enclosure for MMICs, it is necessary to consider 
impedance matching of a lead frame that connects the. MMIC to other 
integrated circuits in a particular system, such as power circuits and the 
printed circuit mother board, so as to reduce or eliminate transitional 
power losses of the signals between the various circuits. Impedance 
matching is particularly important for low power circuits so that the 
signals on the leads are not unnecessarily attenuated resulting in a 
reduced signal-to-noise ratio. For the types of MMICs discussed here, the 
MMIC, as well as the other integrated circuits and the mother board within 
the system, will generally have a characteristic impedance of 50 ohms. 
Because the MMIC is within an enclosure, the lead frame travels through 
different materials having different dielectric constants. A 
characteristic impedance exists for a particular lead frame with respect 
to the geometry of the lead frame, the distance between the lead frame and 
a ground plane and the dielectric material that exists between the lead 
frame and the ground plane. For example, the enclosure and the air outside 
the enclosure have different dielectric constants. Lack of satisfactory 
impedance matching at the transition regions between the different 
dielectric regions causes power losses in the signals being transmitted, 
as well as other types of effects such as reflective phenomenon. 
Therefore, different strategies are incorporated to match the 
characteristic impedance between the transition regions. 
U.S. Pat. No. 5,376,909 issued to Nelson et al. discloses a package for an 
integrated circuit of the type described herein. Nelson et al. discuss 
providing impedance matching of a lead frame connected to the ports of the 
circuit at the location where the lead frame exits the package. The 
impedance matching is achieved by maintaining a predetermined relationship 
between a ground plane and the lead frame. Particularly, the width of lead 
lines of the lead frame are altered with respect to their spacing from the 
ground plane and the dielectric material between the lead lines and the 
ground plane. 
The impedance matching of high frequency integrated circuits can be 
improved over the prior art. Further, the cost of a package encapsulating 
an MMIC circuit can be further reduced. It is therefore an object of the 
present invention to provide such improvements over known integrated 
circuit device packaging. 
SUMMARY OF THE INVENTION 
In accordance with the teaching of the present invention, an enclosure 
assembly for a high frequency integrated circuit is disclosed that 
provides impedance matching of a lead frame connected to the circuit as it 
exits the enclosure. A portion of lead terminals of the lead frame within 
the enclosure that connect the electrical ports of the circuit to other 
integrated circuits and/or a printed circuit board have a conventional 
rectangular shape and an impedance that is substantially matched to the 
characteristic impedance of the circuit. A portion of the lead terminals 
adjacent an outside wall of the enclosure has a specially shaped flared 
region that establishes the characteristic impedance of the circuit on the 
lead lines outside of the enclosure relative to a ground plane of a 
printed circuit board. 
Additional objects, advantages and features of the present invention will 
become apparent from the following description and appended claims, taken 
in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following discussion of the preferred embodiments directed to an 
enclosure assembly for a microelectronic circuit is merely exemplary in 
nature and is in no way intended to limit the invention or its 
applications or uses. 
FIG. 1 shows a side plan view of an enclosure 10 for enclosing and 
packaging a microelectronic circuit 12 (see FIG. 2). In one embodiment, 
the microelectronic circuit 12 is an MMIC operating in the GHz frequency 
range at relatively low power levels. The microelectronic circuit 12 would 
generally be one circuit component of a larger circuit device (not shown) 
such as a communication system. The enclosure 10 is mounted on a printed 
circuit board 14 on which various other integrated circuits (not shown) of 
the device would also be mounted. It will be appreciated by those skilled 
in the art, that the enclosure 10 is useful for packaging a plurality of 
integrated circuits of the type of circuit 12, as well as other types of 
RF circuits that do not operate at high frequencies. In one embodiment, 
the printed circuit board 14 includes a protective top layer 16, made of a 
printed circuit board material such as Teflon, on which the enclosure 10 
is mounted, and a metal ground plane 18 below the top layer 16. Printed 
circuit boards of this type are well known in the art. 
The enclosure 10 includes a cover 20. FIG. 2 shows a top plan view of the 
enclosure 10 with the cover 20 removed to expose the microelectronic 
circuit 12. In this view, the enclosure 10 has been separated from the 
printed circuit board 14. The enclosure 10 further includes a metal base 
layer 22 and a bottom layer 24 formed on the base layer 22. A single piece 
side wall sealing ring 26 is positioned between the cover 20 and the 
bottom layer 24 around the microelectronic circuit 12. The cover 20, the 
bottom layer 24 and the sealing ring 26 are secured together by heat and 
pressure, and are preferably made of a material that provides a low cost, 
high quality protective packaging material that has low losses at the 
microwave operating frequencies of the circuit 12. Teflon is one material 
that has been shown to have these suitable characteristics. Other 
materials, including different types of plastics, also have been shown to 
be adequate for the purposes described herein. Preferable materials for 
these layers would be known to those skilled in the art. The manner in 
which the various layers are secured together may vary depending on the 
particular material. 
FIG. 3 shows a cut-away side sectional view of the enclosure 10. An opening 
28 is formed by a suitable process, such as etching, through the bottom 
layer 24 to expose the metal base layer 22 beneath. The microelectronic 
circuit 12 is mounted to the metal base layer 22 within the opening 28 by 
an appropriate adhesive layer 30. The base layer 22 acts as a ground plane 
and a heat sink for the circuit 12. Therefore, the adhesive layer 30 must 
be made of an appropriate heat conducting material. A ground trace 32 is 
deposited on the top layer 16 and defined by an etching step to provide a 
ground connection to the metal base layer 22. A plurality of via holes 
(not shown) are formed through the top layer 16 by an appropriate 
fabrication step such as etching so as to make an electrical ground 
contact between the ground plane 18 and the ground trace 32. 
A plurality of microstrip lead terminals 34 that are part of a lead frame 
are deposited on a top surface of the bottom layer 24 and defined to have 
a rectangular shape within the enclosure 10 as shown. The lead terminals 
34 are microstrips that act in electrical association with the metal base 
layer 22 within the enclosure 10, and the ground plane 18 outside of the 
enclosure 10. In one embodiment, the lead terminals are copper strips, but 
can be any suitable conductive material. The lead terminals 34 extend 
through appropriately configured openings in the side wall sealing ring 26 
to allow the lead terminals 34 to exit the enclosure 10 so as to connect 
the microelectronic circuit 12 to the various other circuits that are also 
mounted on the printed circuit board 14. 
The lead terminals 34 are electrically connected to terminal pads 36 of the 
microelectronic circuit 12 by wire bonds 38. The lead terminals 34 are 
separated into signal terminals 40 that transfer high frequency signals to 
and from the circuit 12, and power terminals 42 that operate at low 
frequency and provide power to the circuit 12. A widened portion 44 of the 
signal terminals 40 adjacent to the opening 28 is provided where the bond 
wire 38 contacts the signal terminal 40. The signal terminals 40 are 
electrically connected to signal traces 46 deposited on the top layer 16 
outside of the enclosure 10. 
Because the signal terminals 40 connect the circuit 12 to other device 
circuits at high frequency and low power as discussed above, it is 
generally necessary that the signal terminals 40 be impedance matched to 
the characteristic impedance of the circuit 12 and to the other circuit 
components of the device. Generally, the characteristic impedance of 
circuits of the type of the circuit 12 is about 50 ohms. Of course, other 
impedances can be matched within the scope of the invention. As is 
apparent from a review of FIG. 3, as the signal terminals 40 exit the 
enclosure 10, the metal base layer 22 is no longer below the signal 
terminals 40. Therefore, the microstrip ground plane for this region is 
the ground plane 18. Further, because the signal terminals 40 are no 
longer surrounded by the material of the enclosure 10, the dielectric 
constant of the material surrounding the signal terminals 40 changes. 
Therefore, the characteristic impedance on the signal terminals 40 changes 
accordingly. 
In accordance with an embodiment of the present invention, the signal 
terminals 40 include a flared region 48 adjacent to an outside surface 50 
of the sealing ring 26. The flared region 48 extends away from the 
enclosure 10 and towards the circuit board 14 as depicted in the figures. 
A rectangular end portion 52 of the signal terminals 40 adjacent to the 
flared region 48 is electrically connected to the signal trace 46 on the 
printed circuit board 14. The width of the end portion 52 is greater than 
the width of the signal terminal 40 within the enclosure. 
FIG. 4 shows a cut-away sectional top view of the enclosure 10 around the 
area where the signal terminal 40 exits the enclosure 10 and contacts the 
signal trace 46. The signal terminal 40 begins to flare at symmetrical 
curved sections 54 as shown within the sealing ring 26. In one embodiment, 
the curved sections 54 have a radius of curvature of about 0.01". The 
curved sections 54 of the flared region 48 continue into outwardly flaring 
edges 56 that flare out in a symmetrical nature to rounded edge areas 58, 
as shown. The rounded edge areas 58 continue into inwardly flared edges 60 
in a symmetrical nature, and end at the rectangular end portion 52 as 
shown. Not only does the flared region 48 form this flared configuration 
when viewed in a plane perpendicular to the plane of the bottom layer 24, 
the flared region 48 also descends towards the printed circuit board 14 
such that the inwardly flared edges 60 contact the signal trace 46 as 
shown. 
The flared region 48 of the signal terminal 40 adjacent to the outside 
surface 50 of the ring 26 increases the conductive area of the signal 
terminals 40 as they exit the enclosure 10. Because air has a lower 
dielectric constant than the dielectric constant of the material of the 
sealing ring 26, the increased area of the flared region 48 causes the 
part of the signal terminals 40 outside of the enclosure 10 to be 
impedance matched to the part of the signal terminals 40 inside of the 
enclosure 10. The size and angle of flare of the flared region 48 varies 
depending on the value of the impedance being matched, the thickness of 
the signal terminal 40, the distance between the signal terminal 40 and 
the ground plane 18 and the material of the sealing ring 26. 
Experimentation gives the specific dimensions of the flared region 48 for 
these parameters. However, the shape of the flared region 48 will 
generally have the shape as shown, and will not have sharp transition 
regions. 
The foregoing discussion discloses and describes merely exemplary 
embodiments of the present invention. One skilled in the art will readily 
recognize from such discussion, and from the accompanying drawings and 
claims, that various changes, modifications and variations can be made 
therein without departing from the spirit and scope of the invention as 
defined in the following claims.