U.S. patent application number 09/753158 was filed with the patent office on 2002-07-04 for apparatus and method for securing a printed circuit board to a base plate.
This patent application is currently assigned to MOTOROLA, INC.. Invention is credited to Beise, Thomas, Masterton, Patrick J..
Application Number | 20020085359 09/753158 |
Document ID | / |
Family ID | 25029411 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020085359 |
Kind Code |
A1 |
Masterton, Patrick J. ; et
al. |
July 4, 2002 |
Apparatus and method for securing a printed circuit board to a base
plate
Abstract
A PC board assembly employs an apparatus and method for securing
a PC board to a base plate. Multiple compression force distributors
are each attached at one end to the PC board. Each compression
force distributor is preferably implemented as a compressible
standoff that includes two end portions and a compressible body
portion. The compressible body portion transfers a compression
force applied to one end portion to the other end portion for
application to the PC board. The PC board is positioned upon the
base plate and the compression force is applied to the standoffs.
During application of the compression force, the compressible body
portions of the standoffs compress in only one direction toward the
base plate, thereby distributing the compression force to the PC
board to secure the board to the base plate. The method and
apparatus may also be used to secure electrical components to the
base plate.
Inventors: |
Masterton, Patrick J.;
(Carol Stream, IL) ; Beise, Thomas; (Hoffman
Estates, IL) |
Correspondence
Address: |
MOTOROLA, INC.
1303 EAST ALGONQUIN ROAD
IL01/3RD
SCHAUMBURG
IL
60196
|
Assignee: |
MOTOROLA, INC.
|
Family ID: |
25029411 |
Appl. No.: |
09/753158 |
Filed: |
January 2, 2001 |
Current U.S.
Class: |
361/759 |
Current CPC
Class: |
H05K 7/142 20130101;
H05K 7/1405 20130101 |
Class at
Publication: |
361/759 |
International
Class: |
H05K 005/00 |
Claims
What is claimed is:
1. An apparatus for use in securing at least one of a printed
circuit board and an electrical component to a base plate, the
apparatus comprising: a first end portion that includes a
substantially planar outer surface, the substantially planar outer
surface of the first end portion being contactable by a source of a
compression force; a second end portion that includes a
substantially planar outer surface, the substantially planar outer
surface of the second end portion being contactable by the at least
one of the printed circuit board and the electrical component; and
a compressible body portion connecting the first end portion to the
second end portion, the compressible body portion transferring the
compression force applied to the first end portion to the second
end portion for application to the at least one of the printed
circuit board and the electrical component, wherein the
compressible body portion compresses in one direction without
increasing in size in any other direction.
2. The apparatus of claim 1, wherein the second end portion
comprises a solderable material and is soldered to the at least one
of the printed circuit board and the electrical component
substantially when one of the electrical component and at least one
other electrical component is soldered to the printed circuit
board.
3. The apparatus of claim 2, wherein the second end portion
includes two subportions separated by a gap and wherein solder
fills the gap when the second end portion is soldered to the at
least one of the printed circuit board and the electrical
component.
4. The apparatus of claim 2, wherein the solderable material is a
copper alloy.
5. The apparatus of claim 4, wherein the copper alloy is beryllium
copper.
6. The apparatus of claim 1, wherein the compressible body portion
comprises: a first angled member having a first end and a second
end, the first end of the first angled member being connected to a
first end of the first end portion, the first angled member forming
an acute angle with the first end portion; a second angled member
having a first end and a second end, the first end of the second
angled member being connected to a second end of the first end
portion, the second angled member forming an acute angle with the
first end portion; a third angled member having a first end and a
second end, the first end of the third angled member being
connected to the second end of the first angled member, the second
end of the third angled member being connected to a first end of
the second end portion, and the third angled member forming an
acute angle with the second end portion; and a fourth angled member
having a first end and a second end, the first end of the fourth
angled member being connected to the second end of the second
angled member, the second end of the fourth angled member being
connected to a second end of the second end portion, and the fourth
angled member forming an acute angle with the second end
portion.
7. The apparatus of claim 1, wherein the compressible body portion
includes a spring.
8. The apparatus of claim 1, wherein a surface area of the
substantially planar outer surface of the first end portion is
sufficient to facilitate automated pick-and-placement of the
apparatus on the printed circuit board.
9. The apparatus of claim 1, wherein the first end portion, the
second end portion and the body portion form an integrated
device.
10. A printed circuit board assembly comprising: a base plate; a
printed circuit board positioned upon the base plate; a cover that
applies a compression force toward the base plate when the cover is
in a closed position; and a plurality of compressible standoffs
positioned between the cover and the printed circuit board, the
plurality of compressible standoffs transferring the compression
force applied by the cover to the printed circuit board such that
the printed circuit board is secured to the base plate, each of the
plurality of compressible standoffs compressing in one direction
without increasing in size in any other direction.
11. The printed circuit board assembly of claim 10, wherein the
printed circuit board includes at least one cutout area, the
printed circuit board housing assembly further comprising: at least
one electrical component positioned within the at least one cutout
area of the printed circuit board such that the at least one
electrical component rests upon the base plate; wherein at least
one of the plurality of compressible standoffs is positioned
between the cover and the at least one electrical component such
that the at least one electrical component is secured to the base
plate.
12. The printed circuit board assembly of claim 11, wherein the
cover is pivotally attached to the base plate and includes a
latching mechanism to enable the cover to attach to the base plate
when the cover is in the closed position.
13. The printed circuit board assembly of claim 12, wherein the
base plate forms part of a heat sink.
14. The printed circuit board assembly of claim 10, wherein each of
the plurality of compressible standoffs comprises: a first end
portion that includes a substantially planar outer surface, the
substantially planar outer surface of the first end portion being
contactable by the cover; a second end portion that includes a
substantially planar outer surface, the substantially planar outer
surface of the second end portion being contactable by the printed
circuit board; and a compressible body portion connecting the first
end portion to the second end portion, the compressible body
portion transferring the compression force applied to the first end
portion by the cover to the second end portion for application to
the printed circuit board, wherein the compressible body portion
compresses in one direction without increasing in size in any other
direction.
15. The printed circuit board assembly of claim 14, wherein at
least the substantially planar outer surface of the second end
portion is soldered to a receptacle area of the printed circuit
board.
16. The printed circuit board assembly of claim 10, wherein the
cover is prestressed in a direction of the base plate in order to
supply the compression force.
17. A printed circuit board assembly comprising: a base plate; a
printed circuit board positioned upon the base plate, the printed
circuit board including at least one cutout area; at least one
electrical component positioned within the at least one cutout area
of the printed circuit board such that the at least one electrical
component rests upon the base plate; a cover that applies a
compression force toward the base plate when the cover is in a
closed position; and a plurality of compressible standoffs
positioned between the cover and the printed circuit board and
between the cover and the at least one electrical component, the
plurality of compressible standoffs transferring the compression
force applied by the cover to the printed circuit board and the at
least one electrical component such that the printed circuit board
and the at least one electrical component are secured to the base
plate, each of the plurality of compressible standoffs compressing
in one direction without increasing in size in any other
direction.
18. The printed circuit board assembly of claim 17, wherein each of
the plurality of compressible standoffs comprises: a first end
portion that includes a substantially planar outer surface, the
substantially planar outer surface of the first end portion being
contactable by the cover; a second end portion that includes a
substantially planar outer surface, the substantially planar outer
surface of the second end portion being contactable by one of the
printed circuit board and the at least one electrical component;
and a compressible body portion connecting the first end portion to
the second end portion, the compressible body portion transferring
the compression force applied to the first end portion by the cover
to the second end portion for application to one of the printed
circuit board and the at least one electrical component, wherein
the compressible body portion compresses in one direction without
increasing in size in any other direction.
19. The printed circuit board assembly of claim 17, wherein the
cover is prestressed in a direction of the base plate in order to
supply the compression force.
20. A method for securing a printed circuit board to a base plate,
the method comprising the steps of: attaching a plurality of
compression force distributors to the printed circuit board;
providing a source of a compression force, the source being located
a predetermined distance away from the printed circuit board and
applying the compression force toward the printed circuit board;
and using at least some of the plurality of compression force
distributors to distribute the compression force to the printed
circuit board to thereby secure the printed circuit board to the
base plate.
21. The method of claim 20, wherein the step of attaching the
plurality of compression force distributors to the printed circuit
board comprises the step of soldering the plurality of compression
force distributors to corresponding receptacle areas of the printed
circuit board.
22. The method of claim 21, further comprising the step of: using a
pick-and-place machine to automatically position each of the
plurality of compression force distributors on a corresponding
receptacle area of the printed circuit board prior to the step of
soldering.
23. The method of claim 21, wherein each of the plurality of
compression force distributors, upon application of the compression
force, compresses in a direction of the printed circuit board
without increasing in size in any other direction.
24. The method of claim 20, wherein the step of providing a source
of a compression force comprises the step of providing a cover that
is pivotally attached to the base plate and includes a latching
mechanism, wherein the cover supplies the compression force to the
plurality of compression force distributors when the cover is
pivoted into a closed position and latched to the base plate.
25. The method of claim 20, further comprising the step of: using
at least some of the plurality of compression force distributors to
distribute the compression force to at least one electrical
component attached to the printed circuit board to thereby secure
the at least one electrical component to the base plate.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to printed circuit board
assemblies and, in particular to, an apparatus and method for
securing a printed circuit board and/or electrical components
attached to the printed circuit board to a base plate without the
use of screws or clips.
BACKGROUND OF THE INVENTION
[0002] Printed circuit (PC) board assemblies are known. Such
assemblies typically include a housing and a PC board populated
with electrical components. The housing typically includes a base
plate, side walls, and a cover. Depending on the circuitry attached
to the PC board, the PC board may be secured to the base plate by
clips, clamps, snap-fit mechanisms, glue, epoxy, solder, or screws.
For example, PC boards that include circuitry with low power
dissipation (e.g., PC boards that do not require attachment to a
heat sink) are typically secured to the base plate or bottom of the
housing using clips, snap-fit mechanisms, or other techniques that
do not involve the use of screws. On the other hand, PC boards that
include circuitry with high power dissipation (i.e., PC boards that
require attachment to a base plate of a heat sink) are typically
secured to the base plate with screws. In certain applications,
some electrical components that are attached to the PC board must
also be secured to the base plate for power dissipation purposes.
For example, in power amplifier applications, the power transistors
are typically secured to the heat sink with screws in order to
insure sufficient heat transfer away from the transistor during
operation. Failure to provide a high thermal conductivity path
between the power transistor and the heat sink could result in
transistor failure during operation.
[0003] Although screws are commonly used to attach PC boards and
other electrical components to heat sink base plates in high power
applications, the use of screws has many drawbacks. For example,
screws require manual insertion and, therefore, are subject to
human errors, such as omitted screws, stripped screw heads, and
damaged electrical components due to slippage of the hand screw
driver or electric torque driver off of the screw head. In
addition, the use of screws requires tooling of each heat sink base
plate to accommodate the screws. Thus, in most cases, each high
power PC board design requires a unique heat sink base plate
because PC board designs do not typically utilize the same base
plate screw hole placements. Unique heat sink base plates add
undesired costs to PC board assemblies.
[0004] Therefore, a need exists for an apparatus and method for
securing a printed circuit board and/or electrical components
attached to the printed circuit board to a base plate that may be
used in high power dissipation applications and do not require the
use of screws.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 illustrates perspective and side views of a
compressible standoff in accordance with a preferred embodiment of
the present invention.
[0006] FIG. 2 illustrates perspective and side views of a
compressible standoff in accordance with an alternative embodiment
of the present invention.
[0007] FIG. 3 illustrates side views of compressible standoffs in
accordance with further alternative embodiments of the present
invention.
[0008] FIG. 4 is a cross-sectional view of a printed circuit board
assembly in accordance with a preferred embodiment of the present
invention.
[0009] FIG. 5 is a perspective view of a printed circuit board
assembly with the cover removed in accordance with an alternative
embodiment of the present invention.
[0010] FIG. 6 is a side view of the printed circuit board assembly
of FIG. 5 illustrating pivotal attachment of the cover to the base
plate in accordance with the present invention.
[0011] FIG. 7 is a side view of the printed circuit board assembly
of FIG. 5 illustrating the application of a compression force by
the cover when the cover is in a closed position in accordance with
the present invention.
[0012] FIG. 8 is a logic flow diagram of steps executed to secure a
printed circuit board and/or one or more electrical components to a
base plate in accordance with the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0013] Generally, the present invention encompasses an apparatus
and method for securing a printed circuit (PC) board and/or
electrical components to a base plate. Multiple compression force
distributors are attached to the PC board and/or the electrical
components through an attachment technique, such as soldering. Each
compression force distributor is preferably implemented as a
compressible standoff that includes two end portions and a
compressible body portion. A first end portion of the standoff
includes a substantially planar outer surface for contacting a
source of a compression force, such as a PC board assembly cover. A
second end portion of the standoff includes a substantially planar
outer surface for contacting the PC board and/or an electrical
component. The compressible body portion transfers the compression
force applied to the first end portion to the second end portion
for application to the PC board and/or the electrical component.
The PC board and/or the electrical components are positioned upon
the base plate and the compression force is applied to the
standoffs. During application of the compression force, the
compressible body portions of the standoffs compress in only one
direction toward the base plate, thereby distributing the
compression force to the PC board and/or the electrical components
to secure the PC board and/or the electrical components to the base
plate.
[0014] By using compressible standoffs or similar devices to secure
a PC board and/or an electrical component to a base plate in this
manner, the present invention eliminates the use of screws to
provide such attachment as in the prior art, without jeopardizing
thermal reliability of a PC board assembly that includes the PC
board, high power dissipation electrical components and the
compressible standoffs. In addition, the present invention
accommodates the use of an automated pick-and-place machine to
automatically position the standoffs in their proper locations on
the PC board and/or the electrical components, thereby limiting
manual operations to merely arranging the base plate, the populated
and reflowed PC board, and the PC board assembly cover. Further,
since the standoffs preferably compress toward the base plate only,
without increasing in size in any other direction, the standoffs
are designed to minimize any negative impact on the electrical
components or other circuitry that may be adjacent to the
standoffs. Still further, by preferably using a PC board assembly
cover as the source of the compression force, a PC board assembly
that includes the cover, the compressible standoffs, and the base
plate is a self-securing, reliable system.
[0015] The present invention can be more fully understood with
reference to FIGS. 18, in which like reference numerals designate
like items. FIG. 1 illustrates perspective and side views of a
compressible standoff 100 in accordance with a preferred embodiment
of the present invention. The compressible standoff 100 includes a
substantially planar top end portion 101, a substantially planar
bottom end portion formed by two bottom end subportions 103, 104,
and four angled body members 106-109. The compressible standoff 100
is preferably fabricated as an integrated device from a length of
solderable material having high elasticity when formed
substantially as illustrated in the FIG. The solderable material is
preferably a copper alloy, such as beryllium copper, although other
materials may be used. The length of solderable material is formed
in accordance with known techniques to substantially coincide with
the shape of the standoff 100 depicted in FIG. 1.
[0016] When the standoff 100 is an integrated device formed from a
length of material, the bottom end portion of the standoff 100
preferably includes two subportions 103, 104 separated by a gap
111. The gap 111 is preferably less than one (1) millimeter and, in
the preferred embodiment, is filled with solder, epoxy or some
other adhesive used to attach the standoff 100 to a PC board or an
electrical component as discussed in more detail below. The surface
area of the outer surface of the top portion 101 of the standoff
100 is preferably sufficient to enable a vacuum head of an
automated pick-and-place machine to retrieve the standoff 100 from
a tape and reel attached to the pick-and-place machine and position
the standoff 100 onto a solder pad or other receptacle area of a PC
board or an electrical component without the standoff 100 having a
tendency to fall from the vacuum head. In addition, the outer
surface of the top portion 101 of the standoff 100 is preferably
substantially planar to facilitate optimum retention by the vacuum
head of the automated pick-and-place machine and to facilitate
maximum contact with a source of a compression force to be applied
to the top portion 101 during operation of the standoff 100.
Operation of the standoff 100 is described in detail below with
respect to FIGS. 4-7. In one tested embodiment of the standoff 100,
the length of material forming the standoff 100 had a width 112 of
approximately three and one-quarter (3.25) millimeters, although a
width 112 of at least four (4) millimeters is preferred for the top
portion 101 of the standoff 100 for the vacuum retention and
compression force contact reasons indicated above. The height 113
and width 114 of the standoff 100 will vary with the particular
application, but one tested embodiment of the standoff 100 had a
height 113 of approximately twelve and three-quarter (12.75)
millimeters, and a width 114 of approximately six and one-half
(6.5) millimeters. Consequently, the outer surface of the top
portion 101 of the tested embodiment of the standoff 100 had a
surface area of approximately twenty-one (21) square
millimeters.
[0017] The angled body members 106-109 are preferably configured in
a shape similar to an hourglass as depicted in FIG. 1. That is, one
end of angled body member 106 is connected to one end of the top
end portion 101, such that angled body member 106 forms an acute
angle 115 with the top end portion 101. The other end of angled
body member 106 is connected to one end of angled body member 109.
The other end of angled body member 109 is connected to one end of
the bottom end portion (i.e., the non-gap end of bottom end
subportion 103), such that angled body member 109 forms an acute
angle 117 with the bottom end portion. Angled body member 107 is
connected at one end to the other end of the top end portion 101,
such that angled body member 107 forms an acute angle 116 with the
top end portion 101. The other end of angled body member 107 is
connected to one end of angled body member 108. The other end of
angled body member 108 is connected to the other end of the bottom
end portion (i.e., the non-gap end of bottom end subportion 104),
such that angled body member 108 forms an acute angle 118 with the
bottom end portion. Although the connections of the angled body
members 106-109 with each other and with the end portions 101, 103,
104 are shown to be without a smooth radius, such connections
preferably include a smooth radius or curve to reduce the
likelihood that the connections will crack as the standoff 100 is
compressed and decompressed over time.
[0018] FIG. 2 illustrates perspective and side views of a
compression standoff 200 in accordance with an alternative
embodiment of the present invention. The standoff 200 depicted in
FIG. 2 is substantially identical to the standoff 100 depicted in
FIG. 1, except that the bottom end portion 201 is a single element
instead of including two subportions 103, 104 separated by a gap
111 as in FIG. 1. This embodiment of the standoff 200 may be
fabricated using a known extrusion process to form a profile from
which several standoffs 200 may be produced using conventional
cutting or sawing techniques.
[0019] FIG. 3 illustrates side views of two compression standoffs
301, 303 in accordance with further alternative embodiments of the
present invention. The first alternative standoff 301 depicted in
FIG. 3 includes two end portions 305, 306 connected together by a
compressible, spring body portion 308. The second alternative
standoff 303 depicted in FIG. 3 also includes two end portions 311,
312 connected together by a compressible body portion, except that
the compressible body portion in this embodiment includes two
tubular members 314, 315 and a spring 317. The elasticity and
thickness of the spring 308, 317 in each embodiment would be
selected based on the amount of spring compression desired upon
application of a compression force to one of the end portions 305,
311. The end portions 305, 306, 311, 312 of each embodiment are
preferably substantially planar and are fabricated from a copper
alloy or other solderable material to facilitate soldering of the
standoffs 301, 303 to receptacle areas of a PC board and/or
electrical components attached to a PC board.
[0020] FIG. 4 is a cross-sectional view of a PC board assembly 400
in accordance with a preferred embodiment of the present invention.
The PC board assembly 400 includes a base plate 401, a PC board 403
positioned upon the base plate 401, one or more electrical
components 405 (one shown) positioned upon the base plate 401 and
attached (e.g., soldered) to the PC board 403, a cover 407, and
multiple compressible standoffs 409-412. The base plate 401 is
preferably fabricated from a metal, such as aluminum or copper, and
forms part of a heat sink. The base plate 401 may optionally be
coupled to a set of heat sink fins 416 as is known in the art to
improve heat removal in high power dissipation applications. The PC
board 403 may be fabricated from any presently known or
later-developed PC board material. Exemplary PC board materials
include cyanate ester, polyimide, alumina ceramic,
polytetraflouroethylene (PTFE) or any form of bizmalemide triazine
(BT) resin, such as the readily available FR4 resin. The PC board
403 may be a single layer PC board or a multi-layer PC board.
Electrical component 405 may be any component that dissipates
substantial heat during operation and, therefore, requires
substantially direct connection to the base plate 401. For example,
the electrical component 405 may be a radio frequency (RF) power
transistor. Attachment of an electrical component, such as
electrical component 405, to a PC board is described below with
respect to FIG. 5. Various other electrical components (not shown)
may be attached (e.g., soldered) to various locations of the PC
board 403 in accordance with known techniques before the PC board
403 is positioned upon the top surface of the base plate 401.
[0021] The cover 407 is preferably constructed of a metal, such as
aluminum or steel; however, any other substantially rigid material
may be used. The cover 407 applies a compression force to the top
portions of the standoffs 409-412 when the cover 407 is in a closed
position. To insure application of an appropriate amount of
compression force, the cover 407 may be prestressed in a direction
of the base plate 401, such that, in an open position, the cover
407 includes a slightly convex curvature with respect to the base
plate 401 and/or PC board 403, but in the closed position, the
cover 407 is substantially flat. The cover 407 is also preferably
electrically coupled to the base plate 401 to form a shielded
enclosure. For example, the PC board assembly 400 may also include
metallic walls (not shown) extending from the base plate 401 toward
the cover 407 and the cover 407 may include extrusions about the
cover's periphery extending toward the base plate 401. The
electrical connection of the cover 407 to the base plate 401 may be
accomplished by attaching the cover, or its extrusions, to the base
plate 401 or the base plate walls using screws or some other means,
such as a snap fitting mechanism. Alternatively, the cover 407 may
pivotally engage the base plate 401 at one end and attach to the
base plate 401 at an opposite end using a latching mechanism. Such
an alternative embodiment is described in more detail below with
respect to FIGS. 5-7.
[0022] The standoffs 409-412, which may be any one of the standoffs
100, 200, 301, 303 described above with respect to FIGS. 1-3, are
positioned between the PC board 403 (and/or particular electrical
components 405) and the cover 407 as shown. The standoffs 409-412
are positioned on receptacle areas 414 (e.g., 0.004-0.1 millimeter
thick copper pads) of the PC board 403 and/or on one or more
electrical components 405 to be attached to the PC board 403. With
respect to positioning of the standoffs 411, 412 on the electrical
components 405, the standoffs 411, 412 may be positioned on a
flange of the component 405 (as depicted in FIG. 4) or may be
positioned on a flange to which the component 405 is attached, in
which case the external flange and the component 405 together form
a single, composite component for purposes of the present
invention. The aforementioned preferred positioning of the
standoffs on predefined receptacle areas 414 of the PC board 403
(e.g., solder pads) applies only when the standoffs 409, 410 are
going to be soldered to the receptacle areas 414. In cases in which
the standoffs 409, 410 are going to be attached to the PC board 403
by other means, such as through the use of epoxy, predefined
receptacle areas 414 may not be necessary.
[0023] The standoffs 409-412 are preferably positioned on the
receptacle areas 414 using an automated pick-and-place machine
during population of the PC board 403 with surface mountable
electrical components. The positioned standoffs 409-412 are then
preferably soldered or otherwise attached to the PC board 403
and/or any particular electrical components 405 substantially when
the automatically-placed components (which may include component
405) are likewise soldered or otherwise attached to the PC board
403. For example, the standoffs 409-412 may be soldered to the PC
board 403 during reflow or wave soldering process. With respect to
positioning and attaching standoffs 411, 412 to electrical
components 405, each standoff 411, 412 should be attached to a
flange or other portion of the component 405 in such a manner that
the standoff 411, 412 does not interfere with the component's
operation. Standoffs 411 and 412 are depicted as being attached to
the flange of electrical component 405. In addition, the heights of
the standoffs 409-412 are preferably such that the standoffs
409-412 are at least two (2) to three (3) millimeters taller than
the tallest components (e.g., walls of electrical shields) on the
PC board 403 in order to allow the standoffs 409-412 to receive
adequate compression force from the cover 407 and to prevent the
cover from contacting such components. Further, as depicted in FIG.
4, the standoffs 409, 410 used to secure the PC board 403 to the
base plate 401 may be of a different height or heights than the
standoffs 411, 412 used to secure any electrical components
405.
[0024] The thickness of the base plate 401 varies depending on the
particular application in which the PC board assembly 400 is being
used. For example, the thickness of the base plate 401 will likely
be greater in high power dissipation applications, such as when the
PC board assembly 400 implements a power amplifier. In low power
applications, the base plate 401 may be very thin (on the order of
approximately two (2) millimeters); whereas, in high power
applications the base plate may be very thick (on the order of
approximately ten (10) millimeters). The base plate 410 may also
include one or more recesses 418 to accommodate attachment of
electrical components 405 to the base plate 401 such that leads or
tabs of the components 405 align properly with receptacle areas
(shown in FIG. 5) of the PC board 403.
[0025] The thickness of the cover 407 will vary based on the
attachment mechanism employed and the amount of compression force
necessary to adequately secure the PC board 403 and/or any
electrical components 405 to the base plate 401 for a particular
application. For example, when a prestressed aluminum cover 407 is
used to apply approximately sixty-eight (68) kilograms of force,
and the cover 407 is attached to the base plate 401 using screws,
the thickness of the cover 407 may be in the range of approximately
one (1) to two (2) millimeters. On the other hand, when a latching
mechanism as depicted in FIG. 7 is used to attach an unstressed
aluminum cover 407 to the base plate 401 in order to apply a
similar amount of compression force, the thickness of the cover 407
may be in the range of approximately two (2) to three (3)
millimeters.
[0026] The thicknesses or heights of one or more of the base plate
401, the PC board 403, the cover 407, the flange of the electrical
component 405, the standoffs 409-412, and the receptacle areas 414
have been exaggerated in FIG. 4 to illustrate the features of the
present invention. Exemplary thicknesses of some of these elements
401-414 have been provided above to illustrate particular
embodiments of the present invention.
[0027] An alumina-filled paraffin (not shown), such as "POWERSTATE"
compound that is commercially available from Power Devices, Inc. of
Newburyport, Mass., or another thermally conductive, compressible
material may be used between the PC board 403 and/or the electrical
component 405 and the base plate 401 to fill any air gaps that may
exist between the PC board 403 and/or the electrical component 405
and the base plate 401 to further improve the transfer of heat away
from the board 403 or component 405. Such air gaps are typically a
result of tolerance variations in the flatness of the contact
surface of the base plate 402 and the flatness of the contact
surface of the flange of the electrical component 405 and/or the
bottom side of the PC board 403.
[0028] When the cover 407 is closed, the cover 407 applies a
compression force toward the base plate 401 and to the top portions
of the standoffs 409-412. The standoffs 409-412 compress slightly
(e.g., approximately one (1) to three (3) millimeters) toward the
base plate 401 upon application of the compression force, and act
as compression force distributors by distributing the compression
force applied by the cover 407 to the PC board 403 and any
electrical components 405 to which the standoffs 409-412 are
attached. In a preferred embodiment, each compressible standoff
409-412 applies approximately twenty-five (25) pounds
(approximately 11.4 kilograms) of compression force to the PC board
403 or an electrical component 405. The amount of compression force
applied by each standoff 409-412 may be varied by varying the
thickness of the standoff material (e.g., beryllium copper) or the
width 112 of the standoff 409-412. In addition, multiple standoffs
may be grouped side-by-side to increase the amount of compression
force applied to particular areas of the PC board 403.
[0029] Since compressible standoffs 409-412 are used instead of
screws to secure the PC board 403 and/or certain electrical
components 405 to the base plate 401, the base plate 401 does not
need to be machined with tapped screw holes to accommodate
fastening of the board 403 and/or the components 405. Therefore,
the present invention facilitates use of a common base plate/heat
sink platform for various PC board-implemented electrical circuit
designs. Further, the present invention renders the design of the
electrical circuitry on the PC board 403 independent of the base
plate tooling, particularly when no electrical components 405 are
directly connected to the base plate 401 or when electrical
components 405 that are directly connected to the base plate 401 do
not require the use of recesses 418 in the base plate 401.
[0030] To minimize any negative impact on electrical components 405
to which the standoffs 411, 412 may be attached or on electrical
components positioned on the PC board 403 near the standoffs 409,
410, the standoffs 409-412 preferably compress in only one
direction (toward the base plate 401), without increasing in size
in any other direction. Using the exemplary x, y, and z-coordinate
system depicted in FIG. 4, the standoffs 409-412 preferably
compress or decrease in length in the z-direction only when a
compression force is applied, without increasing in size in the x
or y-directions.
[0031] In one tested embodiment of the present invention in which
(1) standoffs similar to those depicted in FIG. 1 were used to
secure an RF power transistor to a heat sink base plate, (2) an
alumina-filled paraffin was used to fill any air gaps between the
flange of the transistor and the base plate, and (3) approximately
six kilograms of compression force were applied to each standoff,
thermal conductivity test results showed substantially equal
thermal conductivity with the present invention as compared with
the thermal conductivity resulting from securing the transistor to
the base plate using two screws, each fastened with an electric
torque driver set at twenty inch-pounds (approximately 0.230
meter-kilograms) of torque, and with no alumina-filled paraffin.
Although the tested embodiment of the PC board assembly utilized an
alumina-filled paraffin between the transistor flange and the base
plate, use or non-use of such a paraffin material to fill air gaps
between the base plate and electrical components or the PC board
will be based on the particular application.
[0032] FIG. 5 is a perspective view of a PC board assembly 500 with
the cover 505 removed in accordance with an alternative embodiment
of the present invention. Similar to the PC board assembly 400 of
FIG. 4, the PC board assembly 500 of FIG. 5 includes a PC board 501
positioned upon a base plate 503, a cover 505, a plurality of
compressible standoffs 507 or other compression force distributors,
and one or more optional electrical components 509 (one shown)
positioned upon the base plate 503. The PC board 501 preferably
includes metallic receptacle areas 506 upon which the standoffs 507
are placed during component population of the PC board 501. The
standoffs 507 are preferably soldered to the receptacle areas 506
during the reflow or wave soldering process used to solder other
components (not shown) to the PC board 501. The standoffs 507 may
alternatively be attached to the PC board 501 with an epoxy or
other adhesive.
[0033] The PC board 501 also includes one or more cutout areas 511
(one shown) to accommodate positioning and placement of certain
electrical components 509 (e.g., RF power transistors) within the
cutout areas 511, such that the components 509 rest upon the base
plate 503. The components 509 positioned upon the base plate 503
include tabs or leads 513 (one shown) that are soldered or
otherwise attached (e.g., using conductive epoxy) to corresponding
PC board receptacle areas 515 (one shown) either contemporaneously
with the soldering or attachment of other PC board components or
some time thereafter. When electrical components, such as component
509, are included in the PC board assembly 500, standoffs 507 are
preferably positioned upon one or more portions of each component
509 (e.g., on each end of the flange of an RF power transistor) and
are used to secure the component 509 to the base plate 503 upon
receipt of a compression force from a compression source, such as
the cover 505. That is, the standoffs 507 are preferably placed at
locations on the PC board 501 and on certain electrical components
509 where screws would ordinarily be used to secure the PC board
501 and the components 509 to the base plate 503.
[0034] Since special machining of the base plate 503 is not
necessary when using compressible standoffs 507 or other
compression force distributors in accordance with the present
invention, many more standoffs 507 may be positioned on the PC
board 501 than would be screws in the prior art to distribute the
applied compression force as desired for a particular application
of the PC board assembly 500. For example, in RF applications in
which improper grounding can critically affect performance of the
assembly 500, substantially more standoffs 507 may be used than
screws as in the prior art in order to achieve a substantially
continuous contact between a ground plane on the bottom side of the
PC board 501 and the contact or top surface of the base plate 503.
Except as described below, the materials and other characteristics
of the PC board assembly components 501, 503, 505-507, and 509 are
similar to corresponding components described above with respect to
the PC board assembly 400 of FIG. 4.
[0035] The base plate 503 in this embodiment includes two slots
517, 518 to accommodate insertion of a pivoting mechanism forming
part of the cover 505. The pivoting mechanism is depicted more
clearly in FIGS. 6 and 7 and is described below. The cover 505 also
includes a latching mechanism 519 that enables the cover 505 to
attach to the bottom surface of the base plate 503 when the cover
505 is in a closed position. The compression force is applied by
the cover 505 as the cover 505 engages the top portions of the
standoffs 507 and compresses the body portions of the standoffs 507
toward the base plate 503 until the latching mechanism attaches the
cover 505 to the base plate 503. The compression force remains
substantially constant while the cover 505 is closed due to the
latching mechanism.
[0036] FIG. 6 is a side view of the PC board assembly 500 of FIG. 5
illustrating pivotal attachment of one end of the cover 505 to the
base plate 503 in accordance with the present invention. As
depicted, a pivoting mechanism (hooks 601) of the cover 505 is
positioned through the slots 517, 518 of the base plate 503, such
that the pivoting mechanism allows the cover 505 to rotate with
respect to a pivot axis. In the embodiment depicted in FIG. 6,
counterclockwise rotation of the cover 505 about the pivot axis
opens the cover 505; whereas, clockwise rotation closes the cover
505. Of course, the base plate slots 517, 518 and cover pivoting
mechanism may be arranged such that clockwise rotation of the cover
505 about the pivot axis opens the cover 505; whereas,
counterclockwise rotation closes the cover 505.
[0037] When the cover 505 is closed, as depicted in FIG. 7, the
cover 505 applies a compression force to the standoffs 507, which
in turn transfer the applied force to the PC board 503 and/or
particular electrical components 509 as described above. The
latching mechanism 519 engages the bottom surface of the base plate
503 to keep the cover 505 in the closed position. Release of the
latching mechanism 519 and rotation of the cover 505 opens the
cover 505 and removes the compression force. The cover 505 may be
prestressed in the direction of the base plate 503 as discussed
above with respect to FIG. 4 depending on the amount of compression
force necessary to secure the PC board 501 and/or any electrical
components 509 to the base plate 503 when the cover 505 is closed.
When the cover 505 is closed and the compression force is applied,
the standoffs 507 act as compression force distributors by
distributing the compression force applied by the cover 505 to the
PC board 501 and any electrical components 509 to which the
standoffs 507 are attached. Thus, in the completed assembly 500,
the standoffs 507 are positioned between the cover 505 (compression
source) and the PC board 501 and/or any electrical components 509
that are to be secured to the base plate 501.
[0038] FIG. 8 is a logic flow diagram 800 of steps executed to
secure a PC board and/or one or more electrical components to a
base plate in accordance with a preferred embodiment of the present
invention. The logic flow begins (801) when compressible standoffs,
such as those described above with respect to FIGS. 1-3, or any
other comparable compression force distributors are positioned
(803) in receptacle areas of the PC board and/or on receptacle
areas of electrical components. In the preferred embodiment, the
standoffs are automatically positioned in their respective
receptacle areas by an automated pick-and-place machine during the
time period that the pick-and-place machine is positioning other
electrical components on the PC board. Each standoff is preferably
fabricated from a length of a copper alloy and is formed as
described above with respect to FIG. 1. The standoffs are
preferably packaged in a tape and reel to be used by the
pick-and-place machine.
[0039] After the compressible standoffs or other comparable
compression force distributors are positioned on the PC board
and/or the electrical components, the standoffs are soldered or
otherwise attached (805), such as through use of an epoxy (e.g., a
conductive epoxy), to their respective receptacle areas (e.g.,
copper solder pads) on the PC board and/or the components. The
standoffs are preferably soldered to the PC board contemporaneously
with the soldering or attachment of other surface mount PC board
components. After the standoffs have been soldered or otherwise
attached to the PC board and/or the electrical components, the PC
board and the electrical components (if any) are positioned (807)
upon the base plate (e.g., of a heat sink). A source of a
compression force is then provided (809), such that the source is a
predetermined distance (e.g., slightly less than the height of the
standoffs) away from the PC board and applies the compression force
to the standoffs or comparable compression force distributors. The
compression source is preferably a cover of the PC board assembly
that includes the PC board and the base plate. In one embodiment,
the cover may be pivotally attached to the base plate and include a
latching mechanism substantially as described above with respect to
FIGS. 5-7. The cover supplies the compression force to the
standoffs or other compression force distributors when the cover is
in a closed position.
[0040] Once the compression force has been applied to the standoffs
or other compression force distributors, the standoffs are used
(811) to distribute the applied compression force to the PC board
and/or the electrical components to which the standoffs are
attached and, thereby, secure the PC board and/or the electrical
components to the base plate, ending (813) the logic flow. That is,
the standoffs or other comparable compression force distributors
preferably transfer the compression force applied to their top
portions, through their compressible bodies, to their bottom
portions and to the PC board or electrical components attached
thereto. In the preferred embodiment, the standoffs compress, under
application of the compression force, only in the direction of the
PC board and the base plate, but not in any other direction to
minimize any negative impact on the operation of the PC board
assembly.
[0041] The present invention encompasses an apparatus and method
for securing a PC board and/or electrical components to a base
plate. With this invention, a PC board and/or electrical components
may be properly secured to a base plate without using screws or
other conventional fastening mechanisms that require specialized
base plate or heat sink tooling, thereby supporting common base
plate or heat sink platforms for various PC board electrical
designs. In addition, part of the securing mechanism (i.e., the
compressible standoffs or other compression force distributors) may
be surface-mountable and, therefore, capable of being automatically
positioned on the PC board by an automated pick-and-place machine.
Prior art securing mechanisms do not include any auto-placeable
components. Further, since the standoffs can be automatically
placed, they are not subject to the human errors typically
encountered when screws are used as the securing means.
[0042] In the foregoing specification, the present invention has
been described with reference to specific embodiments. However, one
of ordinary skill in the art will appreciate that various
modifications and changes may be made without departing from the
spirit and scope of the present invention as set forth in the
appended claims. For example, compression force distributors other
than the compressible standoffs described above with respect to
FIGS. 1-3 may be developed and used by one of ordinary skill in the
art. In addition, although the benefits of the present invention
have been presented above primarily with respect to a PC board
assembly used in a high power dissipation application, the present
invention is also applicable in low and medium power applications
to help automate the PC board securing process. Accordingly, the
specification and drawings are to be regarded in an illustrative
rather than a restrictive sense, and all such modifications are
intended to be included within the scope of the present
invention.
[0043] Benefits, other advantages, and solutions to problems have
been described above with regard to specific embodiments of the
present invention. However, the benefits, advantages, solutions to
problems, and any element(s) that may cause or result in such
benefits, advantages, or solutions, or cause such benefits,
advantages, or solutions to become more pronounced are not to be
construed as a critical, required, or essential feature or element
of any or all the claims. As used herein and in the appended
claims, the term "comprises," "comprising," or any other variation
thereof is intended to refer to a non-exclusive inclusion, such
that a process, method, article of manufacture, or apparatus that
comprises a list of elements does not include only those elements
in the list, but may include other elements not expressly listed or
inherent to such process, method, article of manufacture, or
apparatus.
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