Leadless surface mountable assembly

A leadless surface mountable assembly (100) includes a heat dissipating cover (110), a substrate (120), a heat generating semiconductor device (130), and a spacer (140). The substrate (120) is mounted on the interior surface (115) of the cover (110). The heat generating semiconductor device (130) is thermally coupled to the cover (110) and electrically coupled to the substrate (120). The spacer (140) has an interface portion (147) which provides an electrical interface to the substrate (120). Electrical contacts (144) are disposed on the interface portion (147) of the spacer (140) and are electrically coupled to the substrate (120) through electrical conductors (145) within the spacer (140).

TECHNICAL FIELD 
This invention relates to electrical assemblies, and in particular, to 
surface mountable assemblies. 
BACKGROUND 
Products utilizing electrical assemblies which generate an appreciable 
amount of heat, usually require that heat dissipation paths be designed in 
to prevent overheating of these assemblies. One popular solution is to 
locate a heat sink, such as a metal plate, at or near the site of the heat 
generating assembly, and providing a thermal path for heat to travel from 
this heat sink to other heat dissipating locations. For example, a 
relatively large power amplifier can be implemented on a substrate, and 
this substrate is mounted on a heat sink, and the heat sink is mounted on 
another substrate such as a printed circuit board. A heat dissipation path 
could be created by thermally connecting the heat sink to the frame or 
chassis of the host device. Leads provide the electrical connection 
between the substrate containing the power amplifier and the printed 
circuit board. Similar configurations are quite common in the art. 
Despite some of the obvious manufacturing advantages of using leadless 
surface mountable assemblies, the use of leads has persisted to provide 
electrical connection between the assembly containing the heat generating 
electrical components and the general circuitry for the application. This 
results from the traditional design of placing the heat sink, which can be 
rather bulky, directly under the assembly, so that the electrical 
connections must be routed around the heat sink. Thus, leads are often 
used to accomplish this routing. 
However, there are several problems which are inherent in the use of leaded 
electrical assemblies in the assembly of electronic circuitry. One such 
problem is that of lack of planarity, i.e., where the ends of the leads 
are not all on the same level or plane. Lack of planarity may prevent 
proper electrical connection between the component and the supporting 
substrate, thus affecting the ease of manufacture and the reliability of 
the assembly. Automatic placement of leaded components can also be 
difficult, depending on the size and orientation of the assembly, and hand 
placement may be necessary. Some applications, such as those involving 
wireless communications, require shielding of certain electrical 
assemblies to minimize electrical interference and spurious radiation 
emissions from the assembly. With leaded assemblies, the shields need 
openings to accommodate the leads, or alternatively, the shields need to 
be shaped to encompass the leads. Assuming the application is implemented 
on a printed circuit board (PCB), the PCB layout must accommodate these 
shields and this can reduce the space available for the other circuit 
components. 
A further problem exists in current designs addressing the heat dissipation 
path between the electrical assembly and the chassis of the host device. 
When the assembly is mounted on the heat sink and the heat sink mounted on 
the printed circuit board, an excision is typically made through the 
printed circuit board to access the heat sink. This processing of the 
printed circuit board increases the number manufacturing operations and 
add to manufacturing costs. Moreover, an excision in the printed circuit 
board reduces the surface area available for electrical components, 
thereby increasing the product size. 
The problems described thus far are particularly prevalent with the use of 
power amplifiers in the wireless communications industry. The packaging of 
high output power amplifiers or other high energy assemblies with large 
heat dissipation requirements have traditionally relied on leads to 
electrically connect the assembly to the printed circuit board. However, 
the demand for increase product quality, reduced manufacturing costs, and 
product miniaturization requires new solutions to the packaging design for 
these assemblies. As such, it is desirable to eliminate the leads for 
these electrical assemblies and to provide improved electrical performance 
in a smaller package, while maintaining the heat dissipating 
characteristics of prior art electrical assemblies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Generally, the present invention provides for a leadless surface mountable 
assembly with heat dissipation characteristics. The assembly includes a 
circuit carrying substrate, mounted within a heat dissipating cover, which 
also functions as an electrical shield. An electrical interface to the 
assembly is provided through a spacer or mounting frame, which is 
electrically coupled to the substrate. The mounting frame may be soldered 
directly to a printed circuit board. 
The present invention can be more fully understood with references to FIGS. 
1 and 2. FIG. 1 is a perspective view of a surface mountable assembly 100 
in accordance with the present invention. The assembly 100 includes a heat 
dissipating cover or housing 110, a circuit carrying substrate 120 having 
electrical components 127, 130 thereon, and a spacer 140 or mounting 
frame. In a typical application, the assembly 100 mounted onto a support 
substrate, such as printed circuit board 190. A thermally conductive path 
is provided between the assembly 100 and a heat dissipating structure, 
such as the chassis 185 or frame of a host device. 
The cover 110 is formed from a thermally conductive material, such as 
nickel, silver, aluminum, copper, or other suitable heat dissipating 
material. The cover 110 functions as a heat sink for the assembly 100. The 
cover 110 has a base portion 112 with depending sidewalls 113 to form a 
structure having an interior surface 115 and an exterior surface 116. The 
sidewalls 113 are of sufficient height to allow clearance of the substrate 
120, the electrical components 127, 130, and the spacer 140, when the 
assembly 100 is fully assembled. The sidewalls terminate to form a 
mounting surface 111 for the assembly 100. Mounting extensions 117 on the 
interior surface 115 of the cover 110 help to properly locate the 
substrate 120 within the cover 110. Additionally, a device mount 118 
projects from the interior surface 115 of the cover 110, the function of 
which will be explained below. Preferably, the cover 110 is also 
electrically conductive, such that the cover 110 can function as an 
electrical shield to help reduce or prevent electrical interference 
between the electrical components 127, 130 of the assembly 100 and other 
electrical components of a host device. 
The substrate 120 is formed from materials typically used to construct 
printed circuit boards, such as ceramic and the like. Mounting slots 121 
located on the substrate 120 accommodate the mounting extensions 117 on 
the interior surface 115 of the cover 110, to locate and align the 
substrate 120. Similarly, an aperture 123 extending through the substrate 
120 accommodates the device mount 118 of the cover 110. The substrate 120 
has a first planar surface 124 having circuitry thereon, and a second 
planar surface 125 having conductive material disposed thereon to form an 
electrical ground plane. The second surface 125 is mounted to the interior 
surface 115 of the cover 110 and thermally engages the cover 110. In the 
preferred embodiment, the substrate 120 carries the electrical circuitry 
and components of a power amplifier. As such, the electrical components 
127, 130 include a power transistor 130, which is a heat generating 
semiconductor device. The power transistor 130 is electrically coupled to 
the substrate 120 through wire bonding, or other coupling methods such as 
by using flexible circuits, flip chip technology, and the like. The power 
transistor 130 generates heat which must be dissipated to prevent damage 
to the power amplifier, and other surrounding components. Consequently, 
the power transistor 130 is mounted on the device mount 118, thereby 
thermally coupling the transistor 130 to the cover 110, and providing a 
thermally conductive path for rapid transfer of heat from the transistor 
130. Thus, the device mount 118 is a convenient thermal conduit for 
transferring heat from the power transistor 130, or other device, to the 
cover 110. 
The spacer 140 is formed from circuit board material, similar to the 
substrate 120. The spacer 140 can be integrally formed on the substrate 
120 and may comprise one or more structural members in combination. 
However, the orientation of the spacer 140 and the substrate 120 must be 
such as to allow accommodation of the electrical components 127, 130 on 
the substrate 120. In the preferred embodiment, the spacer 140 is a 
rectangular ring-shaped structure having a cavity 142 therethrough, and is 
formed to fit along the border of the first surface 124 of the substrate 
120. The spacer 140 is of sufficient height and the cavity 142 
sufficiently large to accommodate the electrical components 127, 130. One 
function of the spacer 140 is to provide a leadless electrical interface 
to the substrate 120. Thus, the spacer 140 has a first planar surface 148 
which contacts the second planar surface 125 of the substrate 120, and a 
second planar surface 147 which forms the electrical interface portion of 
the assembly 100. Conductive vias 145 form electrical conductors extending 
through the spacer 140 to electrically couple the first surface 148 to the 
interface portion 147. The conductive vias 145 can be formed to have 
desirable impedance matching characteristics by varying the dimensions, 
and other attributes of the vias 145. This is a significant advantage of 
using the spacer 140 as an electrical interface versus leads commonly 
found in the prior art. The spacer 140 also provides electrical isolation 
between input and output signals to the assembly 100. The spacer 140 is 
selectively plated with conductive material along the sides 141 to 
facilitate an electrical ground connection which provides electrical 
isolation for the electrical conductors represented by the conductive vias 
145. As indicated earlier, the spacer 140 provides the electrical 
interface or electrical ports for the assembly 100. Solder deposits in the 
form of collapsible solder balls 144 are located on the interface portion 
147 of the spacer 140 to provide or facilitate the electrical contacts. 
The assembly 100 is typically soldered to a printed circuit board 190, or 
other support substrate such as flexible circuit board, and the like. 
Referring to FIG. 2, a cross-sectional view of the assembly 100 is shown. 
The assembly 100 is mounted to the printed circuit board 190, and is in 
thermal and electrical contact with the chassis 185 of the host device. 
The substrate 120 is attached to the cover 110 with solder 187, such that 
the substrate 120 is thermally and electrically coupled to the cover 110. 
Moreover, an electrical coupling is established between the cover 110 and 
the plated sides 141 of the spacer 140. Thus, an effective electrical 
ground connection is established between the cover 110, the ground plane 
of the substrate 120, and the plated sides 141 of the spacer 140. The 
power transistor 130 is mounted onto the device mount 118 and is 
electrically coupled to the power amplification circuitry of the substrate 
120. Electrical conductors 133 (see FIG. 1) convey electrical signals from 
electrical ports 134 (see FIG. 1) on the substrate 120, through the 
conductive vias 145 of the spacer 140, to the solder balls 144 on the 
interface portion of the spacer 140. Preferably, the interface portion 147 
is substantially coplanar with the mounting surface 111 of the cover 110 
to present a planar mounting surface for the assembly 100. This 
facilitates handling by automated parts placement equipment during 
manufacturing operations utilizing the assembly 100. 
Referring to FIG. 3, a front view of a communication device that 
incorporates the assembly 100 is shown. The communication device 300 is a 
conventional two-way radio, as known in the art. The radio 300 has a 
housing 303 in which the printed circuit board 190 is situated. The 
printed circuit board carries well-known communications circuitry for 
communicating over a radio frequency channel. The circuitry incorporates 
the assembly 100, which is a power amplifier, mounted on the printed 
circuit board 190. 
As evident from the description above, the present invention addresses many 
problems present in the prior art. The essential components of the 
assembly 100 are substantially enclosed within a cover 110, which 
functions as a heat sink and also as an electrical shield. This packaging 
presents a readily accessible path for the establishment of thermal 
contact between the chassis 185 and the heat sink without having to make 
structural accommodating provisions to the printed circuit board 190, or 
other support substrate. No excisions are needed in the printed circuit 
board 190 to access the heat sink 110, thus leaving more room for 
electrical components, which facilitates product miniaturization. 
Additionally, the package avoids the problems associated with leads, such 
as excess space consumption, lack of lead coplanarity, electrical 
interference among leads, among others. Electrically isolated input and 
output electrical ports are provided, which can be tuned for impedance 
matching. Furthermore, the assembly 100 provides a surface mountable 
package which facilitates automated assembly 100, and which is smaller 
than traditional packages. The concepts embodied in this invention are 
particularly useful for power amplifiers and other applications requiring 
significant heat dissipation and electrical shielding. Manufacturers 
utilizing the present invention can realize increase manufacturing 
efficiency and lower manufacturing costs.