Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-05-29
2001-09-18
Gaffin, Jeffrey (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S718000, C361S719000, C361S720000, C361S761000, C361S764000, C361S809000, C174S252000, C174S260000, C257S707000, C257S711000, C257S728000
Reexamination Certificate
active
06292374
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to assemblies, and more particularly to assemblies that have a back plate that can be attached to a circuit board.
2. Description of the Related Art
In many applications it is advantageous to attach a back plate to a circuit board.
FIG. 1
is an exploded view of circuit board assembly
8
. Assembly
8
has back plate
10
that attaches to circuit board
12
. Back plate
10
has a plurality of openings. Some of the openings, such as opening
18
, go through a portion of back plate
10
to accommodate components. Other openings, such as openings
20
,
22
,
24
,
26
,
28
, run through the entire back plate
10
, and are typically for fasteners. The fasteners can be used to fasten back plate
10
to circuit board
12
and to the equipment or device that receives assembly
8
. Back plate
10
is typically fabricated out of metal and is preferably thick enough to be rigid.
Typically, back plate
10
is fabricated out of inexpensive lightweight metal, such as aluminum. The length, width, and thickness of back plate
10
are large enough to receive circuit board
12
and keep the circuit board fairly rigid when it is attached to the back plate. The length and width of back plate
10
are typically about the same as the length and width of circuit board
12
. Top side
16
of back
10
plate fits on one side
28
of circuit board
12
. Circuit board
12
is thin, and therefore difficult to work with because it bends and warps. The additional thickness provided when circuit board
12
is attached to back plate
10
makes circuit board
12
fairly rigid, preventing circuit board
12
from bending or warping. It is much more accurate and economical to install components with automatic surface mount equipment. It easier to install components onto circuit board
12
when it is rigid because if the circuit board surface is warped the circuit board may not be able to go through the automatic surface mount equipment due to its shape, and when it can go through the equipment, it is difficult to deposit components in the right locations due the circuit board continuing to bend as component are deposited. It is also easier to install circuit board
12
into equipment when it does not bend or warp since it is easier to machine flat surfaces to receive circuit board
12
in the equipment than to machine surfaces that can receive a warped or bent circuit board.
Circuit board
12
has one or more layers
30
,
32
,
34
,
36
,
38
. Some of layers, such as layers
30
,
34
,
38
have a conductor, such as copper, deposited on one or on both sides to form a particular pattern of tracks that is the printed wiring. These layers
30
,
34
,
38
are formed out of plastic or out of a plastic and glass compound. Other layers, such as layers
32
,
36
are formed of an epoxy material and hold the layers of circuit board
12
together. Tracks are used to provide a power grid, a ground grid, and other electrical connection for components
56
,
58
. For example, in circuit board
12
designed for both digital components
58
and RF components
56
: layer
34
has tracks that are the connections for digital component
58
on the top side of layer
34
, and tracks that are the digital ground on the bottom side of layer
34
. Circuit board
12
also has layer
30
with tracks that are the connections for RF components
56
on the top side of layer
30
, and tracks that serve as RF ground on the bottom side of layer
30
. Circuit board
12
has openings
44
,
46
48
,
50
,
52
that run through all of the layers. These openings are for the fasteners that connect circuit board
12
and back plate
10
.
Circuit board
12
designed to work with RF component
56
has connect openings
54
,
55
located near the location of the RF components. Connect openings
54
,
55
are through layer
30
that has the tracks are the RF ground. There is a connection through connect openings
54
,
55
between RF component
56
are the RF ground, this connection provides an electrical conduction path between RF component
56
and the RF ground.
Circuit board
12
has component openings
40
,
42
through the entire depth of circuit board
12
. Components
56
,
58
fit through component openings
40
,
42
, respectively. A thin metal layer, hereinafter flange
60
,
62
, at the bottom of components
56
,
58
, contacts back plate
10
. Components
56
,
58
are either integrated circuits or discrete devices connected to circuit board
12
by leads
64
,
66
, and
68
,
70
, respectively. Leads
64
,
66
,
68
,
70
are attached to the printed wiring on top surface
72
of circuit board
12
.
As described above, typically back plate
10
is fabricated out of inexpensive lightweight metal, such as aluminum. One problem with fabricating back plate
10
out of aluminum is that the thermal expansion coefficient of aluminum is very different from the thermal expansion coefficient of the metal that forms flanges
60
,
62
. This makes it difficult to solder components
56
,
58
to back plate
10
. The temperature of flanges
60
,
62
and back plate
10
are raised during the soldering process. As flanges
60
,
62
and back plate
10
cool after they are soldered together, flanges
60
,
62
contract at a different rate than back plate
10
, thus pulling apart the solder and breaking the connection. This can cause components
56
,
58
to break away from the rest of assembly
8
.
Fabricating back plate
10
out of a material such as a copper, that has a thermal expansion coefficient that is close to the thermal expansion coefficient of flanges
60
,
62
creates a different problem. Since back plate
10
should be large enough to support circuit board
12
and to provide a good thermal conduction path away from circuit board
12
. Fabricating a large back plate from a metal that is a good conductor would tremendously increase the cost of the assembly, going against the long term industry goals and trends of reducing cost. Furthermore, because copper is a heavy material, a large copper back plate would make the assembly heavy and therefore more difficult to support. Additionally, the current trend is to combine more and more components on a single circuit board, requiring larger circuit boards and therefore larger back plates, which further increases the cost and weight of the back plate.
Another problem with assembly
8
is that the electrical connection between RF component
56
and the RF ground is through connect openings
54
,
55
. The size of connect openings
54
,
55
is restricted by the area that can be apportioned on leads
64
,
66
. Typically, connect openings
54
,
55
are small. Because connect openings
54
,
55
are small it is difficult to ensure that a good connection is made between component
56
and ground.
SUMMARY OF THE INVENTION
The invention solves the above problems by providing an assembly that has an insert that fits onto the back plate. The insert either fits into a recessed area in the back plate or attaches to the top of the back plate. When the insert fits into the recessed area the insert is either about the same height as the depth of the recessed area, or its height is larger than the depth of the recessed area, to create a raised area on the back plate. A component can be attached to the assembly. The bottom surface of the component is attached to the top surface of the insert. The insert is made out of a material having a thermal expansion coefficient similar to the thermal expansion coefficient of the bottom surface of the component. This allows the component to be securely soldered to the insert and therefore to the assembly. Preferably the insert is also made out of a good conductor to provide a good electrical conduction path between the component and the ground plane of the circuit board that contacts the insert.
In an alternate embodiment the assembly has a circuit board having a contact opening and a back plate with a raised area that fits into the contact opening. The contact opening exposes a portion of
Johnson Michael Gunnar
Sosnowski Janusz B.
Gaffin Jeffrey
Lager Irene
Lucent Technologies - Inc.
Vigushin John B.
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