Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-07-21
2001-03-20
Thompson, Gregory (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C174S252000
Reexamination Certificate
active
06205028
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to circuit substrates having a printed circuit board on which electronic devices generating much heat and requiring cooling (hereinafter referred to as heat-generating electronic devices) are mounted, and more particularly, to a circuit substrate which permits the influence of heat from the heat-generating devices upon the entire printed circuit board and other peripheral electronic devices by localizing the conduction of the heat to the mounted area of the electronic devices or to a particular direction, thereby restricting the conduction of heat in the direction along the surface of the printed circuit board.
2. Description of the Background Art
Devices such as CPUs installed in electronics such as work station and personal computer generate significant heat. Thus, the process of radiating heat from such a heat-generating electronic device is indispensable in order to provide satisfactory operation specs.
In conventional devices, in order to radiate heat generated from a heat-generating electronic device installed on a top surface layer of a printed circuit board, there are provided a number of through holes at the printed circuit board, and the through holes are plated. Then, the heat-generating electronic device and the through holes are connected. Further by connecting the through holes at the back surface layer of the printed circuit board to a heat radiation plate, heat generated from the electronic device is allowed to escape onto the heat radiation plate through the printed circuit board via these through holes.
In the conventional structure, however, a significant amount of heat is still transmitted from the heat-generating electronic device to the printed circuit board, the heat is transmitted in the vertical direction from the top surface layer to the back surface layer of the circuit board, in other words in the thickwise direction of the substrate, but part of the heat is also diffused in the crosswise direction of the printed circuit board, in other words in the direction along the surface, and the heat generated from the mounted portion of the printed circuit board adversely affects other peripheral electronic devices. Furthermore, the temperature of solder generally used as means for connecting electronic devices is raised at soldered parts of the printed circuit board by the conduction of heat, which could adversely affect the entire circuit substrate and is likely to cause operation faults or shorten the useful life of the circuit substrate.
FIGS. 1A and 1B
are a plan view and a cross sectional view, respectively of a circuit substrate in a first conventional example disclosed by Japanese Patent Publication No. 5-52079. The circuit substrate shown in
FIGS. 1A and 1B
is incorporated into a personal computer or the like, and an electronic device generating much heat (heat-generating electronic device)
22
is provided on a printed circuit board
21
.
Printed circuit board
21
includes a top surface layer
21
a
, a bottom surface layer
21
b
, a pair of inner layers
21
c
provided between top and bottom surface layers
21
a
and
21
b
and serving as a signal layer having signal lines for a particular circuit, and a ground plane layer
21
d
provided between the pair of inner layers
21
c
. There is provided at the top surface layer
21
a
or bottom surface layer
21
b
of printed circuit board
21
, a die attaching pad
23
serving as a mounting portion electrically and physically insulating within circuit board
21
.
Note that a denotes the die area.
Heat-generating electronic device
22
takes the form of TCP (Tape Carry Package), and the central portion of heat generation (die)
24
is secured to the die area a of die attaching pad
23
on the top surface layer
21
a
of the printed circuit board by a thermally conductive adhesive, a die attaching adhesive
25
. Printed circuit board
21
having heat-generating electronic device
22
mounted thereon has its back surface side connected to a heat radiation plate
27
through solder or a thermal compound
26
.
Thus, in the first conventional example, one or more plated through holes
28
are provided at printed circuit board
21
, heat from heat-generating electronic device
22
is transmitted onto the back surface layer
21
b
of printed circuit board
21
through through holes
28
, and the heat is further transmitted to heat-radiation plate
27
.
FIGS. 2A and 2B
are a plan view and a cross sectional view, respectively of a circuit substrate in a second conventional example disclosed by Japanese Patent Publication No. 5-52079. In the second conventional example, one or more relatively large, un-plated through holes
29
are provided at printed circuit board
21
.
Through hole
29
is formed larger than die area a, and a fitting portion
27
a
extended as a projection from heat-radiation plate
27
is fitted therein. Heat-generating electronic device
22
is directly connected to a surface of radiation plate fitting portion
27
a
exposed from through hole
29
using die attaching adhesive
25
. As a result, heat from heat-generating electronic device
22
may be directly transmitted to radiation plate
27
without being passed through printed circuit board
21
.
FIG. 3
shows a third conventional example, in which only heat-generating electronic device
22
is separately provided at a dedicated printed circuit board
21
, so that heat generated from heat-generating electronic device
22
will not be transmitted to other peripheral electronic devices
30
,
31
and
32
. These other peripheral electronic devices
30
,
31
and
32
are mounted on a printed circuit board
21
a
which is separately prepared, and a stacking connector
33
or the like is provided to connect these printed circuit boards
21
and
21
a.
The first to third conventional examples are encountered with the following problems. In the third conventional example, printed circuit board
21
dedicated to heat-generating electronic device
22
should be provided separately from printed circuit board
21
a
for other peripheral electronic devices, which is disadvantageous in terms of cost for materials as well as in terms of mounting efficiency and electrical characteristics as compared to the case of mounting heat-generating electronic device
22
and other devices together at a single printed circuit board
21
.
In contrast, the first conventional example is advantageous in terms of the efficiency, because heat generated from electronic device
22
is transmitted from the top surface layer
21
a
of printed circuit board
21
to bottom surface layer
21
b
, and then onto heat-radiation plate
27
. However, since the heat conduction is through a number of through holes
28
provided with plated layers at printed circuit board
21
, and therefore the part of printed circuit board
21
having through holes
28
is exposed to high heat from heat-generating electronic device
22
.
As a result, as heat diffuses onto the entire surface of printed circuit board
21
, the temperature of printed circuit board
21
is raised, and thermal stress imposed on the other peripheral electronic devices or the soldered portion cannot be avoided.
In the second conventional example, heat radiation plate fitting portion
27
a
is engaged into one or more relatively large un-plated through holes
29
provided at printed circuit board
21
, and therefore fitting portion
27
a
and heat-generating electronic device
22
are directly connected. Thus, heat conduction is allowed without transmitting heat from heat-generating electronic device
22
to printed circuit board
21
. However, die attaching adhesive
25
, a generally heat conductive adhesive, is used as means for connecting the surfaces of heat-generating electronic device
22
and heat radiation plate
27
, and therefore die attaching adhesive
25
should be removed and re-adhesion is necessary during disassembling and replacement working for repairing or the like.
Since the adhesion process by die attaching
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Thompson Gregory
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