Electricity: conductors and insulators – Conduits – cables or conductors – Combined
Reexamination Certificate
1998-06-30
2001-05-22
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Combined
C174S13800J, C174S202000, C174S13800J, C174S266000, C174S265000
Reexamination Certificate
active
06235991
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to fasteners for circuit boards, and more particularly to fasteners for coupling a circuit board to a back plate.
BACKGROUND OF THE INVENTION
In many applications it is advantageous to attach a back plate to a circuit board. The back plate is fairly rigid, fabricated out of a metal, and has about the same length and width as the circuit board. The circuit board is thin, on the order of 2 to 100 millimeters, and therefore difficult to handle and to work with because it bends and tends to warp. The additional thickness of an assembly of the circuit board attached to the back plate prevents the circuit board from bending and warping. This makes it easier to install components onto the circuit board, and easier to install the assembly into a device. The circuit board is made out of one or more layers. Some of the layers are made out of plastic or out of a plastic and glass compound. These layers have a conductor, such as copper, deposited on both sides to form a particular pattern of tracks that is the printed wiring. Other layers are made of an epoxy material and hold the circuit board together. Components attach to the circuit board and connect to the printed wiring. The back plate also acts as a heat sink to dissipate some of the heat generated by the components.
Typically, the back plate is attached to the circuit board with an epoxy. Some circuit boards can be attached to the back plates with solder. However, this is very expensive and would drastically increase the cost of the assembly. Other circuit boards, for example circuit boards with low glass transition temperatures, cannot be attached to the back plates with solder because this solder connection will melt when this assembly is heated to attach components to the assembly. Once the back plate and circuit board are attached, a solder paste stencil is applied to the circuit board. The solder paste stencil is a thin metal stencil that has areas cut out that correspond to the areas of the circuit board where the components will be attached. Solder is then applied. The solder goes through the areas cut out on the stencil such that solder is deposited on areas of the circuit board where the leads of the components will be attached. The leads of the components are placed on the solder, soldering the components onto the circuit board. Screws are then added to hold the back plate and circuit board together more securely.
A problem with fabricating the assemblies is that it is difficult and expensive to make an epoxy that can bond the metal back plate to the material of the circuit board. The epoxy must be tailored to the circuit board and back plate. The development of the epoxy takes both time and money, therefore increasing the cost and the time to start production. However, even when the epoxy is tailored to the circuit board and the back plate, the epoxy is not very reliable. A significant percentage of the circuit boards detach from back plates. This reduces the yield and therefore raises the cost and time of fabricating the assemblies.
Attaching the components to the circuit board prior to attaching the circuit board to the back plate solves the problem of using the epoxy, however, it creates a more serious problem.
FIG. 1
shows circuit board
4
with components
6
,
8
attached to circuit board
4
before circuit board
4
is attached to back plate
10
. Attaching components
6
,
8
to thin circuit board
4
before circuit board
4
is reinforced with back plate
10
causes circuit board
4
to warp and bend due to the mismatch in the thermal expansion coefficient between the board and the components
6
,
8
. This warped circuit board
4
is then screwed onto back plate
10
. Back plate
10
is more rigid than circuit board
4
causing warped circuit board
4
to bend when it is screwed on back plate
10
. This can cause the printed wiring on circuit board
4
, circuit board
4
itself, components
6
,
8
, or the solder joints on components
6
,
8
, to crack, which disrupts connections and causes open circuits. This is particularly problematic when the crack in the printed wiring is difficult to detect, and the assembly is placed in a product and fails in the field.
Attaching the back plate to the circuit board with screws before soldering eliminates the need for the epoxy but it creates a more serious problem.
FIG. 2
shows assembly
12
with screws
14
,
16
attaching circuit board
18
to back plate
20
. Back plate
20
also has openings
30
and
32
. Openings
30
and
32
can be used for fasteners that fasten assembly
12
to the devise that receives assembly
12
. Circuit board
18
also has openings
26
and
28
. Openings
26
and
28
can be used for fasteners that fasten assembly
12
to the device that receives assembly
12
. Back plate
20
and circuit board
18
include these openings
30
,
32
, and
26
,
28
, respectively, in addition to the opening for the screws
14
,
16
that attach circuit board
18
to back plate
20
. Circuit board
18
is typically too soft to be able to have threads to hold screws
14
,
16
in place if their heads
22
,
24
are positioned at the bottom of back plate
20
. Therefore, heads
22
,
24
need be positioned above circuit board
18
. Heads
22
,
24
have to be above at least a portion of circuit board
18
otherwise they will not hold circuit board
18
to back plate
20
. In the cases of all but the thickest circuit boards, heads
22
,
24
are taller than, or at least as tall as, the thickness of circuit board
18
. Since heads
22
,
24
need to be positioned above at least a portion of circuit board
18
, at least a portion of heads
22
,
24
protrudes above top surface
34
of circuit board
18
.
Heads
22
,
24
protruding over the top surface of circuit board
18
create raised areas in the top surface of assembly
12
. If the solder paste stencil is placed on top surface
34
of circuit board
18
, a gap is created between circuit board
18
and the stencil. The solder is then applied and will leak out onto circuit board
18
. This will cause the solder to be deposited on areas of circuit board
18
where it does not belong, i.e. the areas of circuit board
18
where components do not need to be attached, resulting in short circuits. When components
36
,
38
are attached to circuit board
18
, some components
36
may attach to the solder that is deposited where it does not belong, resulting in incorrect connections and to open and short circuits. Depositing solder manually onto the circuit board instead of using the solder paste stencil, to avoid the solder leaking over the assembly, increases the cost and the time needed to manufacture assembly
12
to the point where it is prohibitively expensive to manufacture assembly
12
.
SUMMARY OF THE INVENTION
The invention solves the above problems by providing a mechanical fastener that does not protrude above the surface of the assembly. The assembly includes a back plate and a circuit board coupled to the back plate with the mechanical fastener. The fastener has an end and a head with a top surface. The circuit board has an opening that receives the head of the fastener so that the top surface of the head is between the top and bottom surfaces of the circuit board, inclusively. A portion of this opening has a diameter at least as large as the diameter of the widest portion of the head. Another portion of this opening, located closer to the back plate has a smaller diameter.
The assembly is fabricated by forming an opening in the circuit board. A portion of the opening has a diameter as large as the largest diameter of the head of the fastener. An opening is formed in the back plate. The circuit board is attached to the back plate with the fastener. The top surface of the head of the fastener is placed entirely within the opening in the circuit board. If the circuit board is a multi-layer circuit board the circuit board is laminates. An opening is drilled through all of the layers in the circuit board. This opening has a diameter smaller than the lar
Cuneo Kamand
Gaffin Jeffrey
Lager Irena
Lucent Technologies - Inc.
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