Heat dissipating box

The present invention relates to a heat dissipating box for a printed board assembly (PBA) using natural convection for cooling. The PBA 64 is fastened between a first part 62 and a second part 61 and the joint is sealed with an elastic material 71. A back plane, which has an opening for a connector of the PBA, is mounted to the other two parts to prevent undesirable substances to get inside the box. The heat dissipating property of the PBA, mounted inside a box, increases rapidly if the PBA's heat dissipating components are placed near or in good thermal contact with the box. This can be obtained by placing bumps, for example supports 66, 68 or ridges 65, on the inside of the box, which are in close contact with the PBA. The surface area of the box may also be enlarged with cooling fins.

TECHNICAL FIELD 
The present invention relates to a heat dissipating box for a printed board 
assembly (PBA) using natural convection for cooling. 
PRIOR ART 
Electronic equipment generally uses one or more fans to cool it down to a 
reasonable level. When mounting such equipment in an area, where there is 
a desire to cool the equipment without using a fan, the cooling can be 
achieved by utilising natural convection. 
A normal way of mounting printed circuit boards in an electronic equipment, 
using natural convection, is to avoid encapsulation for maximum exposure 
of the heat dissipating components mounted on a heat sink. The printed 
circuit boards are then mounted vertically and parallel to each other to 
increase the flow of cooling air between them. The U.S. Pat. No. 5,243,493 
by Jeng et al describes this for cooling of a personal computer using 
natural convection. 
There are a number of useful areas where cooling preferably is performed by 
natural convection and one area is cooling of outdoor assemblies, which 
are dissipating heat in some way. Such a design is described in the U.S. 
Pat. No. 4,371,757 by Debortoli et al. and comprises an enclosure for 
outdoor cross-connect in a telecommunication system including ventilation 
louvers for maintaining the temperature and humidity conditions inside the 
enclosure substantially the same as outside avoiding condensation and the 
like. 
A similar device is described in the U.S. Pat. No. 4,794,487 by Maschek et 
al., where an electronic device is protected from heat radiation from the 
outside, for example solar rays. The purpose is to prevent the electronic 
device to be heated up inside the device using upper and lower ventilation 
slots and achieving natural convection by a chimney effect. 
An enclosure for cooling electronic components is described in the U.S. 
Pat. No. 4,535,386 by Frey, Jr et al., where the cooling is performed 
utilising natural convection. The enclosure comprises an inner chimney 
where the heat dissipating components are mounted at the lower part of 
said chimney. The heated air rises from the components and is led to a 
heat exchanger and a natural turbulence is thereby created. 
SUMMARY OF THE INVENTION 
Mounting of sensitive electronic equipment in a hostile environment, for 
example outdoors in normal weather conditions, is very difficult. The 
normal way of protecting the function of a PBA is to enclose it in a box 
preventing dust and other undesirable substances to reach it. A problem 
arises when electronic equipment dissipates heat inside the box. Normally 
this is taken care of by mounting a fan or the like to cool it off, but a 
fan needs maintenance and power. The preferred way is to utilise natural 
convection instead of fans, but this is not enough due to the heat 
build-up inside the box. 
An other problem with mounting of electronic equipment in a hostile 
environment is that the boxes that hold the PBA's have to be easy to 
manufacture, easy to use and cheap. 
The object of the present invention is to provide an improved heat 
dissipating box for one or more PBA's mounted inside an enclosure and 
cooled utilising natural convection. 
An other object of the present invention is to provide a heat dissipating 
box that is cheap, easy to manufacture and easy to use. 
The present invention solves the problems by introducing an improved box 
for heat dissipation with a high cooling efficiency, including means for 
conducting heat from the PBA to the inside of the box. 
Most of the heat from a PBA originates from a few numbers of electronic 
components. The heat dissipating property of the PBA, mounted inside a 
box, increases rapidly if the heat dissipating components are placed near 
or in contact with the box. A good thermally contact can be obtained by 
means of filling the distance between the heat dissipating components on 
the PBA and the box with for example gap-fillers, which are thermally good 
conductors. An alternative way of creating means for a good thermal 
contact is to place bumps, for example supports or ridges, on the inside 
of the box, which are in close contact with the PBA. 
The PBA is placed in the box, which is divided into three parts, a first 
part, a second part and a back plane. The PBA is fastened between the 
first part and the second part and the joint is sealed with an elastic 
material. The back plane has an opening for a connector of the PBA and the 
back plane is mounted and sealed to the other two parts to prevent 
undesirable substances to get inside the box. The box can have apertures 
for electrical connectors and indicators on the front side and the surface 
area of the box may be enlarged with cooling fins or equal. The 
gap-fillers can be omitted if the PBA is in good thermal contact, through 
bumps of any form, with the first and second part of the box. 
The present invention has a major advantage in a high cooling efficiency 
when exposed to cooling by natural convection even though the box is 
sealed. 
Another advantage is that the same type of box may be used for many 
different types of PBA's with only minor adjustments, which means lower 
costs and increased flexibility. 
A better understanding of the present invention can be obtained when the 
following detailed description of the preferred embodiments is considered 
in conjunction with the following drawings.

PREFERRED EMBODIMENTS 
FIG. 1 shows a complete assembly 1 comprising an enclosure 2 with an 
enclosure door 3. The enclosure has several ventilation louvers 4 in the 
lower part of the enclosure and at least one ventilation slit 5 on the top 
of the enclosure. The angled top 6 of the enclosure is designed to prevent 
snow or falling leaves to block the ventilation slit and interrupt the 
natural convection cooling. The enclosure is divided into two parts by 
means of a partition wall 9 on which main frame connectors 10 are mounted. 
Apertures for mounting of cables 7 are provided in the base 8 on the back 
side of the partition wall. The back of the partition wall is a sealed 
compartment and contains interconnections between the main frame 
connectors and incoming cables. Boxes 11 containing electronic devices, 
such as PBA's, are fastened to the main frame connectors 10 on the front 
side of the partition wall 9 and sealed to the partition wall by means of 
an elastic material, such as an O-ring. 
The boxes 11 are cooled, when they dissipate heat, by a flow of air in 
through the ventilation louvers 4, upwards between the boxes 11 and out 
through the top slit 5. This assembly is designed to be placed outdoors 
which means exposure to solar rays, rain and other types of weather 
conditions. This requires that the connections and the electronic devices 
in the boxes must be protected from the outside environment. 
FIGS. 2a-2c show views of a first preferred embodiment. This box 11 is 
designed for high cooling performance and the surface area of the box is 
enlarged with cooling fins 12. The box comprises at least two main parts 
13 and 14 and a separate back plane 19. A PBA is squeezed between the two 
main parts that are fastened together with some type of fastening devices, 
preferable screws 15 but other fastening means can be used, for example 
glue or rivets. The front plane 16 of the box can be equipped with 
apertures for connectors 17, for example test ports and indicators 18 if 
so is desired. The back plane 19 of the box is equipped with at least one 
opening for a connector 20 from the PBA, called PBA-connector. The 
PBA-connector is plugged in to the main frame connector 10 during normal 
operation of the assembly and the back plane is sealed to the partition 
wall using an elastic material, for example an O-ring. 
The design of the heat dissipating box 11 can be adjusted for low heat 
dissipating PBA's, where the need to enlarge the surface area is less. 
This type of box comprises at least two plain main parts, without the 
cooling fins, that are fastened together in a similar way as described in 
FIGS. 2a-2c and the same type of back plane for the PBA-connectors. The 
box can also be equipped with the same apertures for test ports and 
indicators on the front plane. 
FIG. 3 shows a cross-sectional view of a heat dissipating box 11, section 
A--A FIG. 2b. A first part 14 and a second part 13 forms a cavity 21, 
where a PBA 22 is mounted. The edge of the PBA is placed in a niche 33, 
see FIG. 4, created by the main parts 13 and 14 and the height h1 of the 
niche must be greater than the thickness h2 of the PBA's printed circuit 
board. The joint between the main parts are sealed with an elastic 
material 23, which protects the cavity 21 and the PBA 22 from the outside 
environment. 
The first and second parts are fastened together by screws 15, where each 
screw is placed through a drilled hole 24 in the second part 13, through 
an aperture in the PBA and fastened in a threaded hole 25 in the first 
part 14. The holes 24 and 25 are drilled through second supports 30 and 
into first supports 31, where the hole is threaded. The first and second 
supports prevents the PBA from deforming when the screws are tightened. A 
close contact between the supports 30 and 31 and the PBA enhances the 
possibility to conduct heat from the PBA's circuit board. This can be 
difficult to obtain due to variations in manufacture of the main parts 13 
and 14. 
The PBA 22 comprises heat dissipating electronic components 26 mounted on 
at least one side of the board, the primary side, and to get a good 
thermal contact between the components 26 and the inside of the cavity 27 
a gap-filler 28 can be used. Large gap-fillers 29 may also be used to 
dissipate heat from the secondary side of the PBA. One type of gap-filler 
is a silicon gap-filler and an other type is a copper gap-filler. 
FIG. 4 shows a top view of the first part 14 together with a 
cross-sectional view of a mounted back plane 19 of the heat dissipating 
box in FIG. 3. The first supports 31 are placed inside the cavity 21 to 
uniformly squeeze the first and second parts together. 
An elastic material 23 is placed around three sides of the cavity in a 
groove and is adjusted to seal apertures for eventual test ports 17 and 
indicators 18 using for example elastic o-rings 36. The fourth side is 
sealed with an elastic material 35 when the back plane 19 is mounted. 
Small gaps 34 will appear between the elastic material around the cavity 
23 and the elastic material in the back plane 35. These gaps are used to 
lead humidity created inside the cavity out to the ambient area. The PBA 
22 is outlined in the figure, with the dashed line, including apertures 
32, which are smaller than the size of the top of the first supports 31. 
FIG. 5 shows a cross-sectional view of a second preferred embodiment of a 
heat dissipating box 40. A first part 42 and a second part 41 forms a 
cavity 43, where a PBA is mounted, not shown in the figure. The edge of 
the PBA is placed in a niche 44 created by the main parts 41 and 42 and 
the height h1 of the niche must be greater than the thickness of the PBA's 
printed circuit board. The joint between the main parts is sealed with an 
elastic material 45, which protects the cavity 43 and the PBA from the 
outside environment. 
The parts are fastened together by using at least four screws, not shown in 
the figure. The PBA is squeezed between rows of bumps, where each of said 
rows consists of at least one ridge 46 and 47. The rows are parallel and 
are placed on the first 43a and second 43b surface of the cavity and are 
extending in a first direction perpendicular to the back plane 19. The 
height h3 and h4 of each ridge extends from the first and second surface 
of the cavity towards the centre of the cavity in a second direction which 
is perpendicular to the first direction and the direction of the back 
plane. 
There are at least three rows of first ridges 46 placed on the first 
surface 43a of the cavity 43 with an intermediate distance d1 in a third 
direction parallel to the direction of the back plane. The second surface 
43b of the cavity has at least two rows of second ridges 47 with an 
intermediate distance d2 in the third direction. The first and second 
ridges must be dislocated a distance d3 in the third direction when the 
first and second parts are positioned on top of each other. The distance 
d2 is preferable equal to: 
d2=d1, 
and the distance d3 is preferable equal to: 
d3=d2/2. 
The height of the ridges is adjusted so that the intermediate distance h5 
in the second direction, between the tops of the first and second ridge is 
less than the minimum thickness of the PBA's printed circuit board and 
greater than: 
h5&gt;minimum thickness-d3/200. 
A lesser distance can cause the PBA to be damaged from the pressure of the 
ridges. 
For example: One type of standard PBA has a length of 265 mm and for that 
kind of PBA the intermediate distance d2 between the ridges is selected to 
be 75 mm and the intermediate distance d3 between the first and second 
rows of ridges is selected to be 75 mm/2=37.5 mm. If the minimum thickness 
of the PBA's printed circuit board is 1.8 mm the distance h5 between the 
tops of the ridges is equal to: h5=1.6125. 
No apertures are needed in the PBA for fastening the PBA inside the cavity, 
but there is a need to design a component layout on the circuit board in a 
way that permits the ridges in the cavity to be in contact with the PBA. A 
close contact between the ridges and the PBA enhances the possibility to 
conduct heat from the PBA's circuit board without using any kind of 
gap-fillers and an even better conductivity can be obtained if the ridges 
are in contact with the conductive pattern and connected to for example 
earth potential. 
FIG. 6 shows a top view of the first part 42 together with a 
cross-sectional view of a mounted back plane 19 of the heat dissipating 
box in FIG. 5. The ridges 46 are placed in parallel rows on the first 
surface 43a of the cavity in the first direction perpendicular to the back 
plane 19 of the box. The total length (L1+L2) of the ridges in a row must 
be equal to or exceed 50% of the inside distance L3 of the cavity in the 
same direction. The first part also include at least four threaded holes 
50 to fasten the first and second part of the box with screws or a similar 
fastening device. 
An elastic material 45 is placed around three sides of the cavity in a 
groove and is adjusted to seal apertures for eventual test ports 17 and 
indicators 18 using for example elastic o-rings 37. The fourth side is 
sealed with an elastic material 35 when the back plane 19 is mounted. 
Small gaps 38 will appear between the elastic material 45 around the 
cavity and the elastic material in the back plane 35. These gaps are used 
to lead humidity created inside the cavity out to the ambient area. The 
PBA 39 with the PBA-connector 20 is outlined in the figure, with the 
dashed lines. 
FIG. 7 shows a cross-sectional view of a third preferred embodiment of a 
heat dissipating box 60. A first part 62 and a second part 61 forms a 
cavity 63, where a PBA 64 is mounted. This embodiment is a combination of 
the two previous described embodiments. 
The first part 62 has at least three rows of a first type of bumps 65 and 
at least two rows of a second type of bumps 66. The first type of bumps, 
described in the second embodiment, comprises at least one ridge 65. The 
second type of bumps, described in the first embodiment, comprises at 
least three first supports 66, which contains a threaded hole 67. The rows 
are all parallel and the rows with the first supports 66 are placed 
between the rows of the ridges 65 extending in a first direction 
perpendicular to the direction of the back plane and on the cavity's first 
surface 63a. 
The second part 61 has at least two rows comprising at least three second 
supports 68 containing a drilled hole 69, described in the first 
embodiment. A difference from the previously described first embodiment is 
that the second supports have a larger top area compared to the first 
supports top area, and the tops of the supports coincide when the second 
part 61 and first part 62 are fastened together. Screws 70 are mounted 
through the hole 69 in the second part and fastened in the threaded part 
67 in the first part. 
The PBA 64 is placed between the rows of bumps 65 and 66 and in a niche 49. 
The PBA's printed circuit board is squeezed in the same way as described 
in the second preferred embodiment, where the ridges on the second surface 
are replaced with the first and second supports with an intermediate 
distance h6 calculated the same way as for the intermediate distance h5 in 
the second preferred embodiment. 
FIG. 8 shows a top view of the first part 62 together with a 
cross-sectional view of a mounted back plane 19 of the heat dissipating 
box in FIG. 7. The ridges 65 are placed in parallel rows inside the cavity 
63 in the first direction perpendicular to the back plane 19 of the box. 
The total length of the ridges in a row must be equal to or exceed 50% of 
the inside distance of the cavity in the same direction as previously 
described in FIG. 6. 
The first part also comprises two rows of first supports 66 with the 
threaded hole 67 to fasten the first and second part of the box with 
screws 70 or a similar fastening device. 
An elastic material 71 is placed around three sides of the cavity in a 
groove and is adjusted to seal apertures for eventual test ports 17 and 
indicators 18 using for example elastic o-rings 72. The fourth side is 
sealed with an elastic material 35 when the back plane 19 is mounted. 
Small gaps 73 will appear between the elastic material 71 around the 
cavity and the elastic material in the back plane 35. These gaps are used 
to lead humidity created inside the cavity out to the ambient area. The 
PBA 64 is outlined in the figure, with the dashed line, including 
apertures 74, which are larger than the size of the top of the first 
supports 66. It is essential that the second supports does not pass 
through the PBA to establish the squeezing effect that will create the 
thermal contact between the PBA and the main part via the bumps. 
This design has a big advantage compared to the other described 
embodiments. The demanded space for the supports on the primary side of 
the PBA is less, which means that there is a larger area where the 
components of the PBA can be mounted. 
A PBA that is encapsulated in a heat dissipating box according to the 
invention can obtain a heat dissipation up to 5 times greater compared 
with a PBA without any enclosure or cooling fins. If the box is plain, 
which means no cooling fins, this effect is reduced 2-3 times.