High-thermal conductivity circuit board

A thermally efficient circuit board has a base layer with high thermal conductivity and a thermal expansion coefficient close to that of silicon, such as aluminum silicon carbide. Above the base layer is a layer of anodized metal, either a separate material, such as aluminum, which is formed on the base and then anodized, or an anodized portion of the base itself. To the anodized metal is then applied a sealant material of lower thermal conductivity, but good electrically insulative and adhesive qualities, such as Teflon FEP. The sealant flows into cavities in the porous anodized metal structure, creating a well-anchored bond. A metal foil layer is then bonded to the surface of the sealant, and used to pattern conductive circuit paths using conventional methods.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to thermal control of board-mounted electronic 
devices and, more specifically, to an improved circuit board construction 
with high thermal efficiency. 
2. Description of the Related Art 
Electronic devices are typically mounted and electrically connected to a 
board, commonly referred to as a circuit board. The circuit board is 
usually comprised of electrical interconnects which are, in turn, 
connected to the electronic devices mounted thereto. The circuit board can 
either be an all-in-one electronic device, or can be plugged into an 
electrical connector which connects it to other external electronic 
devices. 
The electronic devices mounted to a circuit board can be any of a number of 
conventional or hybrid devices such as resistors, capacitors, pin-mounted 
integrated circuit (IC) packages, surface-mounted IC packages, and the 
like. When operated, such devices dissipate energy in the form of heat. If 
the device is unable to transfer enough of its heat to the surrounding 
air, the elevated operating temperature of the device can result in its 
premature failure. 
Generally speaking, heat can be transferred from the electronic device to a 
coolant, (e.g. air) either directly or indirectly. The direct method is 
accomplished by circulating the air over the electronic device. Heat is 
absorbed from the device by the air, which is then exhausted to the 
ambient environment. For low-power devices, the surface area of the 
electronic device itself is sufficient to allow removal of the desired 
amount of heat with a sufficiently-low rise in device temperature. Higher 
power devices, however, often require the use of a heat sink attached to, 
and in thermal contact with, the electronic device. The heat sink is a 
high thermal conductivity material with a large surface area which spreads 
heat from the device over a greater surface area, improving the efficiency 
of the transfer of heat to the circulating air. If designed correctly, a 
heat sink can keep the temperature of the electronic device lower than 
that of a similar device without a heat sink. However, the expense of 
manufacturing a heat sink and attaching it to a device can significantly 
increase the overall cost of the device. 
The indirect method of removing heat from an electronic device involves 
transferring heat from the device to the circuit board. Since the circuit 
board is already attached to the electronic device to be cooled, it can 
act much like a heat sink itself. Heat is conducted (typically downward) 
from the device to the circuit board. Solder bumps and thermal shims or 
posts can be also be used to create high thermal conductivity paths deeper 
into the board, since the circuit board itself is typically not a good 
thermal conductor. 
To improve the conduction of heat from the device to the air, the circuit 
board must conduct heat laterally away from the electronic device. This 
spreads the heat over a larger area of the circuit board and increases the 
surface area from which air circulating over the circuit board may absorb 
the heat. Thus, the efficiency of the thermal spreading within the circuit 
board is an important factor in how well heat can be transferred from an 
electronic device to the ambient air. 
As mentioned above, circuit boards are typically very poor conductors of 
heat. This is mainly due to the fact that they consist of interspersed 
metal and non-metal layers. The non-metal layers are usually poor thermal 
conductors. The metal layers do not significantly improve the thermal 
conductivity of the circuit board because they tend to be thin and 
discontinuous. 
Prior art attempts at reducing the thermal resistance of a circuit board 
include the use of thick metal layers which have no electrical purposes 
(except when such layers are used as a ground plane). The thick metal 
layers are made continuous, and therefore provide significantly improved 
thermal spreading. This, in turn, helps to limit the temperature rise of 
the electronic devices. Typically, the circuit board also includes a layer 
of electrically insulative material between the metal layer and the 
electronic device to provide the necessary electrical isolation. 
One problem with the use of thick metal layers in circuit boards is that 
the metal layer, the insulator layer, and the electronic device itself all 
have different thermal expansion properties. That is, as the temperature 
of the circuit board increases, the device, the insulator and the metal 
all expand by different amounts. This results in thermally-induced 
stresses which can cause the electrical connections to the electronic 
devices to fatigue and fail. 
SUMMARY OF THE INVENTION 
The present invention provides a circuit board which is optimized for two 
thermal factors: the ultimate electronic device operating temperature; and 
the level of thermal stress induced by the heating. High levels of stress 
are developed when there is a mismatch in the degree of thermal expansion 
of materials which are bonded together. Therefore, to minimize the thermal 
expansion stress among the layers of the circuit board and the ICs mounted 
to the circuit board, it is desirable for the materials involved to have 
identical or closely-matched thermal expansion coefficients. However, in 
order to minimize the operating temperature of the IC, it is also 
desirable to use materials with high thermal conductivity. Generally, 
materials that are good thermal conductors, have high thermal expansion 
coefficients, whereas materials with low thermal conduction tend to have 
low thermal expansion coefficients. 
The present invention comprises a multi-layered circuit board with a 
relatively thick base layer of material having relatively high thermal 
conductivity and a thermal expansion coefficient close to that of the IC 
material (typically silicon). In the preferred embodiment, this material 
is aluminum silicon carbide. The high thermal conductivity provides good 
thermal spreading within the board, while the matched coefficients of 
thermal expansion minimize thermal stress between the circuit board and 
the IC. 
In the present invention, the thermal conductivity of the 
electrically-insulating layer between the base and the electrical circuit 
paths of the circuit board is maximized. This is accomplished by combining 
a relatively thin layer of anodized metal with an insulating sealant. The 
anodized metal may different than the base material, or may be an anodized 
portion of the base material itself. In the preferred embodiment, a layer 
of aluminum is formed on the surface of the base layer. The aluminum is 
then anodized by exposure to sulfuric acid and electric current to create 
an aluminum oxide layer on the outer surface. This aluminum oxide layer is 
typically 0.002-in-thick and consumes a 0.001-in-thick portion of the 
original aluminum coating. 
The anodized metal, whether part of the base or a separate material, has a 
porous structure. An electrically insulative sealant material is then 
applied to the anodized metal. In the preferred embodiment, the sealant is 
Teflon FEP. Both aluminum oxide and Teflon FEP are relatively good 
electrical insulators, and provide the necessary isolation of the base 
layer from the printed circuit paths of the circuit board. The Teflon also 
acts as an adhesive to which an upper layer may be attached. In the 
present invention, a conductive foil (preferably copper) is laminated to 
the top of the sealant-filled aluminum oxide while the sealant is still in 
its liquid state. When the sealant hardens, the foil is firmly attached to 
it and, consequently, to the base. The foil is then post-processed in a 
conventional manner (such as photoetching) to form electrical traces that 
are used to provide electrical connection to electronic devices on the 
circuit board. 
While the sealant material has a thermal conductivity which is low compared 
to the copper and the aluminum silicon carbide, the thermal effect of the 
sealant is minimized by its integration with the aluminum oxide. Aluminum 
oxide has relatively good thermal conductivity, and its porous structure 
allows it to serve as a matrix to the sealant material which flows into 
its pores. While the microscopic cavities of the anodized metal allow 
anchoring of the sealant material, the portions surrounding the cavities 
(which are closer to the metal foil) provide natural thermal vias between 
the copper foil and the base layer. Thus, the thermal conductivity of the 
anodized metal/sealant is better than other composite layers, and 
significantly better than that of a sealant alone. Furthermore, the added 
bonding strength provided by the anchoring action of the aluminum oxide 
pores allows the overall thickness of this layer to be kept to a minimum, 
thus further minimizing the thermal resistance between the board surface 
and the base layer. 
Single or double-layer tapes, such as those made of Kapton.RTM. (a 
registered trademark of E.I. du Pont de Nemours & Co., Inc.), can also be 
used with present invention to provide multi-layered circuit boards. This 
structure is commonly referred to as a flexible ("flex") circuit. Flex 
circuits can have very fine electrical traces with pitches on the order of 
0.004 inch. This allows components to be densely located on the circuit 
board as compared to conventional circuit board layouts. Electronic 
devices are typically mounted directly to the Kapton tape but, 
alternatively, holes can be etched into the Kapton to allow the electronic 
devices to be directly mounted to the Teflon for better thermal conduction 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The fabrication steps for the circuit board of the present invention are 
described herein with reference to FIGS. 1-3. Shown in FIG. 1 is a base 
layer 101 formed from a material having good thermal conductivity, and a 
thermal expansion coefficient close to that of silicon (i.e. is relatively 
low). The thermal expansion coefficient is matched to silicon since most 
ICs tend to be silicon-based and, thus, thermal stress due to mismatched 
thermal expansion coefficients is minimized. 
The thermal conductivity of the base layer 101 provides good lateral heat 
spreading within the circuit board. In the preferred embodiment, the base 
material is aluminum silicon carbide. However, those skilled in the art 
will recognize that other materials having similar properties may also be 
used. The base 101 is significantly thicker than other layers of the 
circuit board. However, the base thickness may be customized to a 
particular board design, recognizing that the thermal spreading is better 
with a thicker base layer. 
Formed on the surface of the base 101 in a conventional manner is an 
anodizeable metal, which in the preferred embodiment is aluminum layer 
102. The layer 102 may be unnecessary if the base material is an 
anodizeable metal, such as aluminum. That is, it is possible to simply 
anodize the surface of the base material, rather than adding layer 102. 
However, in the preferred embodiment, the base material is aluminum 
silicon carbide, and layer 102 is therefore added to provide the 
anodizeable layer. Thus, the following description will refer to layer 102 
as being a different material, although those skilled in the art will 
understand that layer 102 may simply be a region of base material 101 
which is anodized. 
Aluminum layer 102 may be formed onto the surface of base 102 as part of 
the fabrication process, or layers 101 and 102 may be purchased together, 
as aluminum silicon carbide is commercially available in sheet form with 
aluminum metal skins. In the preferred embodiment, the aluminum layer 102 
is approximately 0.005 inch thick. It is noted that other anodizeable 
metals may be substituted for aluminum. These include niobium, molybdenum, 
tantalum, titanium and vanadium. 
After its formation, the aluminum layer 102 is anodized using sulfuric acid 
and an appropriate electric current. This anodizing causes the formation 
of a layer 103 of aluminum oxide on the surface of the aluminum, and a 
resulting porous surface structure, as shown in FIG. 2. Dimensions "A" and 
"B" are used in FIG. 2 to depict, respectively, the porous aluminum oxide 
portion 103 and the non-porous aluminum portion 102 below. In the 
preferred embodiment, the aluminum is anodized to provide a porous 
aluminum oxide region 103 which is approximately 0.002 inch thick, while 
the non-porous aluminum region 102 is approximately 0.004 inch. Aluminum 
oxide has the benefits of being a good electrical insulator while having a 
relatively high thermal conductivity. It is noted that the pores shown in 
the drawing are not to scale and that, actually, the pore size is 
microscopic. 
After anodizing, a sealant material 104 is applied to the aluminum oxide 
layer 103, as shown in FIG. 3. In the preferred embodiment, the sealant is 
fluorinated ethylene propylene (FEP), commonly referred to as Teflon.RTM. 
FEP ("Teflon" is a registered trademark of E.I. du Pont de Nemours & Co., 
Inc.). The Teflon FEP is heated to its melting temperature of 
approximately 300.degree. C. It is then forced at a pressure of 275 P.S.I. 
into the porous surface of the aluminum oxide 103. The natural capillary 
action of the porous surface may assist in drawing the sealant 104 into 
the pores of the anodized aluminum layer. In one embodiment, a vacuum is 
maintained in the vicinity of the porous layer before and during the 
application of the sealant. The vacuum is later removed after the sealant 
104 is applied, but while it is still in a liquid form. This enhances the 
drawing of the sealant into the pores of the aluminum oxide. 
Teflon FEP is a good electrical insulator. The Teflon-filled aluminum oxide 
has a dielectric strength on the order of 2000 V/mil thickness. A 0.002 
inch thick layer of this material thus has a total dielectric strength on 
the order of 4000 volts, which is more than sufficient for most 
electronics applications. The thermal conductivity of this layer is 
approximately one-third of pure aluminum oxide. Due to its thinness, it 
provides a minimal thermal resistance to the heat flowing through it. 
Referring again to FIG. 3, a layer 105 of metal foil is laminated to the 
sealant layer 104 after its application. The sealant 104 may be used as an 
adhesive to secure the foil layer in place. In the preferred embodiment, 
the foil layer 105 is copper, but other types of electrically conductive 
material may also be used. A typical copper foil used herein would be a 
0.5 ounce to 3 ounce copper. Once the foil 105 is firmly adhered to the 
sealant 104, electrical traces may be formed from the foil material. This 
is demonstrated by FIG. 4, which depicts a top view of a portion of the 
fabricated circuit board according to the present invention. As shown, 
electrical traces 106 are formed from the foil 105 to provide the desired 
circuit pathways. In the present embodiment, the foil 105 is 
photolithographically imaged and etched to expose the adhesive sealant 
104, thus electrically isolating the traces 106. 
As demonstrated by FIG. 3, the sealant material 104 flows into the pores of 
the anodized metal 103. In essence, the anodized metal 103 functions as a 
matrix for the sealant 104, resulting in an anchoring of the sealant to 
the anodized metal, and correspondingly good bond strength between the two 
materials. This matrix structure also provides an array of natural thermal 
vias. The aluminum oxide 103 has much better thermal conductivity than the 
sealant. The portions of the aluminum oxide 103 which surround the 
cavities in its porous structure are particularly close to the foil 105, 
and provide heat conducting channels between the foil and the base 
material 101, with only a small amount of lower-thermal conductivity 
sealant material 104 to cross. Thus, good overall thermal conduction is 
achieved between the foil 105 and the base 101, while maintaining a 
constant adhesive surface between the sealant 104 and the foil. 
FIG. 5 depicts a typical application of the present invention in which an 
electrical component 107 is soldered or epoxied to the electrical traces 
106. Given the finite thickness of the electrical traces 106 and the 
relative length of the leads of component 107, a thermally-conductive 
filler 108 is used between the component 107 and the adhesive sealant 104. 
This allows a high thermal conductivity path between the component 107 and 
the sealant 104 to be maintained. 
FIG. 6 shows another application of the present invention in which the 
electrical component 107 is directly bonded to the adhesive sealant 104. 
Electrical wires 109 are connected between the electrical traces 106 and 
the electrical contacts of component 107, which are located on the 
component's top surface. In this configuration, the component itself is in 
physical contact with the sealant 104, and the need for a conductive 
filler material 108 (as shown in FIG. 5) is eliminated. 
FIG. 7 shows still another application of the present invention in which a 
Kapton tape 110 is used which has integral electrical traces 111. The use 
of this type of Kapton tape is common in the flexible circuit industry. In 
this embodiment, the layer of metal foil (FIG. 3) is eliminated, as the 
Kapton tape 110 is bonded directly to adhesive sealant 104. The electrical 
traces 111 are connected to the electrical device 107 by solder or epoxy 
to provide the desired electrical path. 
The embodiment of FIG. 8 is similar to the surface-mounted embodiment of 
FIG. 6, but has the component mounted on foil 105, rather than directly on 
sealant 105. This allows a lateral spreading of heat from the component 
not only through the base layer 101, but also through the foil layer 105. 
The contacts for component 107 may extend outward from the component to 
patterned conductors (not shown) which surround it. This embodiment may 
also be combined with the embodiment of FIG. 7, such that conductors of 
the Kapton tape (which may be two-sided or multi-layered) connect to both 
the top side and the bottom side of component 107. 
In certain situations, the underside of the component 107 may also have 
electrical contacts which are connected directly to patterned conductors 
beneath the component. In such a case, thermally conductive material could 
optionally be applied to the recesses surrounding the patterned conductors 
beneath the component 107 so as to ensure optimum thermal conduction from 
the component to the sealant layer 104. The arrangement of FIG. 8 is 
particularly well-suited for applications in which thermal contact points, 
or exposed bare die, exist on the underside of the component 107 to 
provide direct thermal contact between the foil and the heat-dissipating 
material within the component package. 
While the invention has been shown and described with reference to a 
preferred embodiment thereof, it will be understood by those skilled in 
the art that various changes in form and detail may be made herein without 
departing from the spirit and scope of the invention as defined by the 
appended claims.