Printed circuit board and heat sink arrangement

Making an assembly of a printed circuit board structure and heat sink structure by locating a thermally conductive surface release agent upon a side of one of the structures and causing a flowable thermally conductive material to flow through a hole in the heat sink and lie between the release agent and the other structure. The thermally conductive material lies in heat conductive contact with the release agent and with the other structure and in alignment with an electronic component which is to be cooled upon the board. The release agent ensures easy release of the printed circuit board structure from the heat sink structure for disassembly purposes. An assembly of the two structures is also covered.

This invention relates to printed circuit board and heat sink arrangements. 
In structures of printed circuit boards carrying electronic components, 
heat is generated by the components in use and it is necessary to remove 
this heat so as to prevent overheating which could result in breakdown of 
one or more of the components. To remove the heat, heat sinks are 
conventionally used. For efficient heat exchange to occur, it has 
sometimes been deemed necessary for a heat sink to be intimately attached 
directly to a printed circuit board. However, this raises a problem in 
that it may be required to dismantle a printed circuit board and heat sink 
assembly for inspection, modification, or repair purposes and separation 
of the board from the heat sink may be virtually impossible without 
attendant destruction to one or more parts of the assembly. 
In other suggested structures, heat sinks may be located on the same side 
of a printed circuit board as the electronic components so that the 
components lie between the board and the heat sink. Heat is transferred to 
the heat sink by a heat transfer medium compound from the electronic 
components. Again, there is the problem of disassembly of the parts should 
this be required for any reason. Further, if the compound is applied into 
position before assembly of the parts, this may not result in a 
satisfactory thermal connection between adjacent surfaces for promoting 
heat conduction. This latter method of assembly also is laborious and time 
consuming. Examples of this type of structure are to be found in U.S. Pat. 
Nos. 4,849,856 and 4,914,551. 
In other structures, compressible thermally conductive pads are used to 
provide conductive bridges into heat sinks. Unfortunately, variations in 
gaps exist and which are bridged by these pads. Hence, if pads are used of 
substantially the same thickness then the gap variations lead to 
differences in compression of the pads and result in differences in 
thermal conductivity of the pads. As a result, control in rate of heat 
removal is almost impossible to obtain. Also, there is danger that 
overcompression of pads could result in strains in structures and solder 
joint breakage. Alternatively, to this, because of the variations in the 
gaps, pads of different thicknesses could in theory be used. However, for 
such difference thickness pads to be effective, then in each case the 
width of the gap must be closely ascertained and the corresponding size of 
pad placed in position to obtain the required compression to provide the 
desired thermal conductivity. As may be realized, this alternative method 
of providing pads of different thicknesses is impractical in that it is 
time consuming in determining the size of the gaps and also is impractical 
in the cost and problems associated with having pads of different 
thicknesses. 
In U.S. Pat. No. 5,467,251, granted Nov. 14, 1995 in the name of R. 
Katchmar, there is described a structure in which heat is dissipated 
throughout a printed circuit board from electronic components mounted upon 
the board, the heat then being removed by bridging members extending to a 
heat sink from the board. In this arrangement, electronic components are 
bonded to the printed circuit board by a thermally conductive component 
which has been caused to flow into spaces between the component and the 
heat sink, the thermally conductive component then setting in position. 
As a further development, U.S. patent application Ser. No. 08/601,671, now 
U.S. Pat. No. 5,646,828 describes a method of making a printed circuit 
board structure and heat sink structure assembly in which a heat sink is 
provided with a hole which is directed towards an electronic component 
when a printed circuit board carrying the component (to form the printed 
circuit board structure) is assembled to the heat sink structure. A heat 
conductive path is then created between the heat sink structure and the 
printed circuit board structure by causing a flowable thermally conductive 
material or compound to flow through the hole so as to occupy a spatial 
region between and in heat conductive contact with the heat sink structure 
and the printed circuit board structure. In certain constructions produced 
by the method of Katchmar 3, the heat sinks are removable and replaceable 
if required. 
The present invention seeks to provide a method and a resultant structure 
concerned with a new assembly aspect. 
According to the present invention there is provided a method of making an 
assembly of a printed circuit board structure and a heat sink structure 
comprising:--providing a printed circuit board structure comprising a 
printed circuit board and an electronic component mounted upon a first 
side of the printed circuit board; providing a heat sink structure with a 
hole defined through the heat sink structure; disposing a thermally 
conductive surface release agent upon a first side of a selected one of 
the two structures; relatively disposing the printed circuit board 
structure and the heat sink structure with the first sides of the two 
structures facing towards and spaced from each other and with the hole 
having its axis extending in a direction generally towards the electronic 
component; and creating a heat conductive path from the heat sink 
structure to the printed circuit board structure by causing a flowable 
thermally conductive material or compound to flow through the hole so as 
to occupy and be caused to remain in a spatial region:--a) between and in 
heat conductive contact with the thermally conductive surface release 
agent on one side of the spatial region and with the structure disposed on 
the other side of the spatial region; and b) in alignment with the 
electronic component. 
The thermally conductive surface release agent which allows for release of 
one structure from the other may be any agent suitable for the purpose. 
However, in a preferred method the thermally conductive surface release 
agent comprises a tape having a surface release feature on a first side. A 
second side of the tape is heat conductively secured to the first side of 
the selected structure before the flowable thermally conductive material 
is caused to occupy and remain within the spatial area to cause the 
thermally conductive material to remain within the spatial region, it 
hardens then lies in intimate thermal conducting engagement with the 
surface release feature of the tape which being substantially non-secured 
thereto. In a practical structure, the tape needs to be of laminate 
structure. This laminate structure is provided by one layer to provide the 
surface release feature, i.e. a material which will not tend to adhere to 
the thermally conductive material. Such a material for this layer of the 
laminate structure is a thin film of polytetrafluoroethylene (PTFE) or 
fluorinated ethylene propylene. The tape may also need to have heat 
conductive properties from side-to-side and for this purpose the 
polytetrafluoroethylene or fluorinated ethylene propylene layer must be 
carried upon a heat conductive substrate. Such a substrate is conveniently 
formed from metal and this may be aluminum which is convenient for 
practical and economic reasons. However, other materials such as copper 
may be used for this layer. 
Alternatively, the thermally conductive surface release agent is provided 
by coating the first side of the selected structure with a release agent 
in the form of a flowing and settable release material such as a hot 
coating of PTFE. Alternatively, a spray coating of a release agent may be 
all that is required. With the release agent in a set condition, the 
flowable thermally conductive material is caused to flow through the hole 
provided for injection purposes so as to occupy and remain in the spatial 
region. The thermally conductive material after hardening at least to the 
required degree then intimately contacts the release agent while not being 
secured thereto or while having a weak frangible bond therewith. With the 
latter method, the release agent is preferably coated only on selected or 
desired areas of the first side of the selected structure. This is 
conveniently performed by masking other areas of the first side which do 
not require the use of the release agent. Masking need not be required in 
which case, the release agent is applied all over the first side of the 
selected structure. 
Various thermally conductive materials or compounds which are suitable are 
known in the art of thermal management of electronics device power. As an 
example, and as described in U.S. patent application Ser. No. 08/601,671, 
a suitable material is an admixture of a silicon based resin and thermally 
conductive particles (e.g. boron nitride particles). The percentage of the 
particles in the admixture influences the thermal conductive properties of 
the admixture. Such an admixture may be caused to flow to achieve the 
method of the invention for a specific time period after formulation of 
the resin and may, dependent upon the specific resin mix, be curable into 
a set condition either at room temperature or at an elevated temperature 
not sufficiently high to damage components, e.g. in the region of 
60.degree. C. to 80.degree. C. or even higher. A suitable viscosity 
modifier may be required to curtail slump characteristics of the uncured 
compound. An alternative suitable material employs an epoxy based resin in 
admixture with thermally conductive particles, e.g. boron nitride. Optimum 
thermal performance can be attained with certain metal alloys with 
sufficiently low melting points, such as indium/tin alloys which may have 
melting points at 118.degree. C. or lower. However, when using metal 
alloys which have electrically conductive properties, control of the 
dispensing process is critical to ensure that none of the alloy material 
flows beyond its required position in the spatial region so that the alloy 
material does not cause shorting of electrical or electronic components. 
With the use of the method of the invention, the printed circuit board and 
heat sink structures are assembled together into their relative positions 
before the thermally conductive material or compound flows into position. 
Because the thermally conductive material or compound is caused to flow 
between the two structures subsequent to their assembly together, then the 
flowing material intimately contacts both the thermally conductive surface 
release agent and the structure disposed on the other side of the spatial 
region thereby maximizing the thermal conductive efficiency of the 
assembly. In addition, it is convenient to manufacture the assembly by the 
above method because the thermally conductive material is not positioned 
upon one of the structures before the other structure is located in 
position and thus any slow and messy assembly steps are avoided. In 
contrast, because the flowable material is caused to flow through a hole 
in the heat sink structure then the method is particularly suitable for 
use with injection apparatus, i.e. by location of an injection nozzle into 
the hole for injection of the material between the two structures. Hence, 
the process step of locating the thermally conductive material in its 
correct position is easily, quickly, efficiently and cleanly accomplished. 
The resultant assembly of the printed circuit board and heat sink 
structures optimizes heat removal from components mounted upon a printed 
circuit board and is thus particularly relevant to removal of heat from 
printed circuit board structures in which the heat generated could result 
in electronic component failure from malfunction if not conducted away 
with suitable efficiency. All of these advantages in manufacture and in 
use are obtained with the thermally conductive surface release agent 
located in position and in thermal conductive engagement on either one or 
both of its two sides with the selected structure and with the thermally 
conductive material or compound. Although thermal conductive pathways are 
produced between the electronic components and the heat sink, nevertheless 
the surface release agent enables the two structures to be dismantled one 
from the other for disassembly purposes. 
In the method according to the invention defined above the first side of 
the printed board structure may be the side at which is located the 
electronic component. In this case it may be that the electronic component 
surface itself provides the first side of the printed circuit board 
structure against which the release agent is located. Hence, in this 
structure, the electronic component is connected to the heat sink for 
thermal conduction purposes firstly through the thermally conductive 
release agent then through the thermally conductive material or compound 
and into the heat sink itself. Alternatively, with the electronic 
component disposed on a second side of the printed circuit board structure 
then the thermally conductive surface release agent may be provided upon 
the printed circuit board itself which provides the first side of that 
structure. Of course the release agent may be disposed instead upon the 
first side of the heat sink structure.

In a first embodiment as shown by FIG. 1, a printed circuit board structure 
10 comprises a printed circuit board 12 having a plurality of electronic 
components 14 mounted upon one side of the board (only one component 14 
being shown). For removal of heat from the printed circuit board structure 
during use a heat sink structure 16 is provided. This heat sink structure 
comprises a planar body 18 from one side of which a plurality of parallel 
and spaced apart heat cooling fins 20 extend. The two structures 10 and 16 
are detachably secured together, by mechanical attachment means not shown, 
with the fins 20 directed outwardly from the side of the planar body 18 
remote from the printed circuit board structure 10. Also, in the assembly 
the components 14 are located upon the board between the two structures 
and with the distances between the components and the planar body 18 of 
the heat sink being dependent upon the sizes of the components and varying 
from one component to another. The structures 10 and 16 are maintained in 
fixed spaced-apart relationship by spacer columns 22 (one being shown) 
which are held in position by bolts or securing screws 24 extending either 
through the planar body 18 of the heat sink as shown or through the 
printed circuit board 12. 
Before assembly of the two structures 10 and 16 together, the heat sink 16 
is provided with a thermally conductive surface release agent at selective 
positions to enable subsequent disassembly of the two structures 10 and 16 
after their final assembly and without causing damage to any part of the 
structures. As shown by FIG. 2, the release agent is disposed so as to 
cover and adhere to the unfinished surface of the planar body 18 in 
discrete areas which are directly opposed to the positions of the 
electronic components 14 upon assembly. Hence, in this embodiment, the 
thermally conductive release agent comprises a plurality of short lengths 
26 of thermally conductive tape which are disposed in the required 
positions. As shown by FIG. 2a, which is a cross-section through a typical 
length 26 of tape, each length of tape comprises a basic substrate layer 
28 which is aluminum foil approximately 2 mil thick. On the one side of 
the foil is provided a layer 30 of a thin acrylic based pressure sensitive 
adhesive which may be of any required thickness, but in this case is 0.5 
mil thick. Any other suitable adhesive may be used. On the other side of 
the layer 28 is a layer 32 of fluorinated ethylene propylene or 
polytetrafluoroethylene which provides a surface release feature as will 
be described. The thickness of the layer 32 need only be in the region of 
1 mil. Hence, each length 26 is composed of thermally conductive 
materials. Each tape 26 is positioned exactly in its required location 
with the acrylic layer 30 securing the length 26 to the planar body 18 of 
the heat sink structure. 
After assembly of the structures 10 and 16 as shown in FIG. 1 and as 
described above, it is necessary to thermally conductively connect the 
structures 10 and 16 together for removal of heat from the structure 10 
out through the heat sink structure. This is effected by injecting a 
thermally conductive and at least partially settable flowing material or 
compound through a hole 34 provided in the planar body 18 in respect of 
each of the components 14. Each hole 34 extends through a tape length 26. 
If, each tape length symmetrically or centrally positioned relative to its 
respective electronic component, it is preferable to have its hole 34 
positioned substantially centrally with regard to its tape length. Of 
course, the tape length may be offset relative to its component in which 
case the hole 34 may be central with the tape. To inject the thermally 
conductive material 36 through the hole 34, an injection nozzle 38 (FIG. 
3) is disposed into the opening of the hole 34 and a mass of the material 
36 is injected through the hole so as to flow into and expand 
progressively across a spatial region between the component 14 and the 
corresponding tape length 26. Care needs to be taken to ensure that the 
thermally conductive material is not excessively applied in each case. For 
this purpose, an observation aperture 40 is provided in the planar body 18 
with respect to each component 14. The observation aperture 40 is spaced 
from a respective hole 34 a distance which needs to be determined to 
ensure that only sufficient of the thermally conductive material has been 
injected into position. The observation aperture must also be provided 
within the bounds of the tape length 26 to ensure that the thermally 
conductive material 36 does not extend beyond the edges of the tape length 
and into contact with the planar body 18. The latter may set up permanent 
and undesirable adhesive contact with the planar body 18 which is against 
the teachings of the present invention. Also, because the material 36 is 
likely to flow at substantially the same rate in all directions away from 
the hole 34, it is prudent to have all of the tape lengths 26 of 
substantially square configuration as shown in FIG. 2 and with the hole 34 
positioned as centrally as possible and as shown by that Figure. Where the 
hole 34 is not centrally positioned either with regard to its tape or with 
regard to its electronic component, extreme care must be taken to prevent 
contact between the material 36 and the planar body 18. 
It follows therefore that as injection of the material 36 proceeds through 
the nozzle 38, then as soon as the material is seen by observation through 
the aperture 40, then no more of the material should be injected for that 
particular component 14. The material 36 is then allowed to set or harden 
in position thereby providing a heat conductive bridge between each of the 
components 14 and the heat sink structure 16 through a tape length 26. 
The thermally conductive material 36 may be of any suitable material having 
the required thermal properties and which will intimately contact the 
surfaces against which it has been injected. Such a material is as 
described in the earlier application (Katchmar 3) and is of material 
having viscous properties and possibly also adhesive properties to be 
disposed within the spatial region between each component and a tape 
length 26. The material may have a low modulus of elasticity preferably 
well below 5000 lbs. per square in., although certain exceptions may 
permit moduli much greater than this. The material 36 is an admixture of a 
silicon based resin and boron nitride particles. The quantity of the 
particles, in percentage by weight of the total admixture, controls the 
thermally conductive properties of the admixture which may thus be as 
desired. For the purpose of disposing the thermally conductive medium 
within each space, the material may be injected at room temperature 
through each of the holes 34 from the outside of the heat sink. The 
thermally conductive material is then cured either at room temperature or 
at around 60.degree. C. to 80.degree. C. which should have no 
disadvantageous effects upon the circuitry for the components of the 
printed circuit board structure. 
To disassemble the two structures 10 and 16 after loosening or removal of 
the mechanical attachment means, it is possible to use each of the 
observation apertures 40 or perhaps only selective ones of these 
apertures. For this purpose, a disassembly tool may be inserted through 
the required apertures 40 to be pushed with care against either a 
component 14 or against the printed circuit board 12. Any slight adherence 
between each mass of material 36 and a tape length 26 is overcome thereby 
forming a line of severance at the interface between these two elements 
and forcing the heat sink away from the printed circuit board structure. 
As indicated, this may need to be performed at various locations across 
the facial area of the heat sink for the full effect of the disassembly 
operation to be realized. For total control in applying load at each 
selected location, it is desirable for each observation aperture 40 to be 
screw-threaded as shown by FIG. 4 and the disassembly tool 42 is 
complementarily screw-threaded for reception within the hole. As may be 
realized, screwing of the tool into position gradually moves it axially so 
as to gradually and progressively increase the load applied against a 
component 14 or again the printed circuit board 12 until separation occurs 
between a mass of material 36 and its respective tape length 26. FIG. 4 
shows the relative positions of the structures 10 and 16 as they begin to 
move apart during disassembly. 
As may be seen therefore in the first embodiment of the invention provides 
a process and a structure for bridging gaps between electronic components 
mounted on a board and a heat sink in which these gaps may vary from 
component-to-component and without applying any undue strain or stresses 
upon any of the elements of the two structures. Furthermore, the invention 
enables complete and clean separation between the two structures without 
harming or breaking any of the elements of the structures. Apart from 
providing an extremely efficient heat exchange arrangement therefore for 
use in printed circuit boards, an extremely clean and efficient method is 
provided for disassembly for maintenance purposes. 
In the invention, it is not essential for the release agent to be applied 
to the heat sink structure 16 nor is it necessary for the agent to 
comprise a tape length as discussed above. For instance, as shown by FIG. 
5, in a total assembly which is similar to that described in the first 
embodiment and in which parts bearing the same reference numerals are 
similar or identical, a tape length 26 in each case is attached to an 
electronic component 14 instead of to the heat sink 16. Thus upon 
separation of this structure, as shown in FIG. 5 the heat sink 16 is 
removed together with the masses of material 36 and the tape lengths 26 
remain carried upon the components 14. 
In a third embodiment as shown by FIG. 6, with the structure having similar 
parts to that shown in the first embodiment, a printed circuit board 
structure 10 is positioned upon the heat sink 16 with the components 14 on 
the other side of the printed circuit board. With this arrangement the 
tape length 26 in each case may be positioned upon the planar body 18, as 
in the first embodiment or as shown in FIG. 6. Each tape length is 
actually applied to a surface of the printed circuit board 12 which faces 
the heat sink 16. In the latter case, care should be taken to ensure that 
each of the tape lengths does not extend into contact with an electric 
conductor upon the surface of the board as electrical conductivity 
properties of the tape length could disastrously affect the desired 
operation of the circuitry. 
In a fourth embodiment, a release tape length 26 or any other release tape 
is not used. As shown by FIG. 7, the unfinned surface of the planar body 
18 of the heat sink is provided with a mask 44 except in areas provided by 
windows 46 in the mask. In these areas, a release agent is provided 
selectively by filling the windows with a coating material having suitable 
thermally conductive properties. Such a suitable release agent is a 
flowable settable release material which sets either after cooling or with 
heat treatment accompanied by a curing action. A suitable material for 
this function is provided by Teflon which is heated to render it flowable. 
The Teflon is then caused to fill each of the windows 46 to form release 
agent pads 48 as shown in FIG. 7 and which are thermally conductively in 
intimate contact with the heat sink 16. 
As an alternative to the fourth embodiment, no mask 44 is used and the 
entire unfinned surface of the planar body 18 is coated with release 
agents.