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47490/RAG/S968 



LIGHTWEIGHT CIRCUIT BOARD WITH 
CONDUCTIVE CONSTRAINING CORES 

CROSS-REFERENCE TO RELATED APPLICATION(S) 

5 The present application claims priority on the basis of the provisional patent application, 

Serial No. 60/254,997, filed December 12, 2000, and entitled "LIGHTWEIGHT MULTIPLE 

LAYER PRINTED WIRING BOARD." 



0 STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR 

gio DEVELOPMENT 

Vfl 

Certain portions of the background technology to the present invention were made under 

01 

,y one or more of the following United States Government Contracts: NAS 3-27743; NAS 
m 3-97040; and F 29601 -94'C-0093. Accordingly, the United States Government may have some 
rights. 

O 

BACKGROUND OF THE INVENTION 

Multiple-layer printed circuit boards or printed wiring boards (PWBs) are used for 

mounting integrated circuits (ICs) and other components. The push to decrease circuit size and 

weight and to operate at higher frequencies and clock speeds has led to smaller components 

20 generating greater heat and being placed more closely together on the PWB. Additional size and 

speed improvements have also been achieved by reducing the footprints of the components by 

using leadless chip carriers. 

The greater density of components on the PWBs and hotter components resulted in 

thermal management problems. The Coefficient of Thermal Expansion ("CTE") mismatch 

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47490/RAG/S968 



between the PWBs and the components becomes more important when greater temperatures are 
generated. CTE mismatch between the PWBs and components can result in fracture or fatigue 
during the thermal cycling caused by powering on and off of electronic devices. Leadless chip 
carriers are especially susceptible to disengagement Jfrom the PWB when there are CTE 
5 mismatches. Solder joints and connections tend to pull apart in the "tug-of-war'* introduced by 
the CTE mismatch. 

H: Prior PWB designs have used metal constraining layers or cores, such as copper-invar- 

P copper, aluminum or steel, to lower the board's CTE, However, these materials add undesirable 

fU 
■PI 

TZ weight. US Patent 4,318,954 to Jensen provides an example of a PWB design for use in cycling 

jjio thermal environments that uses lightweight carbon based constraining layers to lower the board's 

£ 

CTE. US Patent 4,591,659 to Leibowitz also demonstrates that carbon constraining layers can 

hi 

H serve as thermal conductors for carrying heat away from the components mounted on the PWB 
Q in addition to lowering the board's CTE. US Patent 4,318,954 to Jensen and US Patent 
4,591,659 to Leibowitz are incorporated by reference in their entirety to the present disclosure, 
15 The ability of previous PWBs to conduct heat away from the components mounted on 

their surfaces is limited by the prepreg used to prevent electrical conductivity between the 
functional layers of the PWB. The materials used in prepreg have poor thermal conduction 
properties. Therefore, the abiUty of the carbon constraining layer to conduct heat away from the 
surface of the board was limited by the amount of prepreg between it and the surface of the 
20 board. The carbon material used in the carbon constraining layers is electrically conductive, 
which required the functional layers of the PWB in prior structures to be electrically insulated 
from the carbon constraining layers in order to prevent short circuits and cross talk. In previous 

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47490/RAG/S968 

designs, this requirement places a lower limit on the distance between the carbon constraining 
layers and the surface of the board equivalent to the minimum amount of prepreg required to 
insulate the functional layers of the board from each other and from the carbon constraining 
layers. This lower limit translated into an upper limit on the amount of heat that could be 
5 conducted away from the surface of the PWB. Accordingly there was a need for a PWB that 
possessed mechanical strength with a low CTE and that exceeds the upper limit on the amount of 

U heat that can be conducted away from the surface of the PWB which was inherent in previous 

C'; designs. 

}ilO SUMMARY OF THE INVENTION 

s In one aspect, the invention relates to a structure and method in which a thermally 

conductive layer is provided in a PWB or a portion thereof. For example, the invention may 
C inchide a prepreg layer made of a substitute impregnated with a resin which is thermally 

conductive, and possibly also electrically conductive. A laminate may be formed from such a 
15 prepreg layer, the laminate having first and second metallic layers positioned above and below 

the prepreg. Alternatively, the laminate may itself be thermally and/or electrically conductive, 

enabling its use in a high performance printed wiring board, 

BRIEF DESCRIPTION OF THE DRAWINGS 
20 FIG. 1 is a semi-schematic cross-sectional view showing a PWB in accordance with the 

present invention including an electrically and thermally conductive laminate; 



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47490/RAG/S968 

FIG. 2A is a flow chart illustrating a process for constructing a PWB in accordance with 
the present invention; 

FIG. 2B is a flow chart illustrating a process for impregnating a substrate with resin in 
accordance with the present invention; 

FIG. 3 is a semi-schematic cross-sectional view showing a laminate incorporating four 
layers of unidirectional carbon fibers; 

FIG. 4 is a semi-schematic cross-sectional view showing another embodiment of a 
laminate incorporating four layers of unidirectional carbon fibers; 

FIG. 5 is a semi-schematic cross-sectional view showing a laminate incorporating three 
layers of unidirectional carbon fibers; 

FIG. 6 is a semi-schematic cross-sectional view showing a lanfiinate incorporating foxir 
layers of unidirectional carbon fibers in an isotropic configuration; 

FIG. 7 is a semi-schematic cross-sectional view showing a PWB in accordance with the 
present invention including a laminate incorporating prepreg layers; 

FIG. 8 is a semi-schematic cross-sectional view showing a PWB in accordance with the 
present invention including an electrically and thermally conductive laminate including a fiber 
glass layer impregnated with electrically and thermally conductive resin; 

FIG. 9 is a semi-schematic cross-sectional view showing a PWB in accordance with the 
present invention including an electrically and thermally conductive laminate having a fiber glass 
layer impregnated with electrically and thermally conductive resin contained within layers of 
prepreg; 



47490/RAG/S968 

FIG. 10 is a semi-schematic cross-sectional view showing a PWB in accordance with the 
present invention including two electrically and thermally conductive laminates and a number of 
chimneys and plated through holes; 

FIG. IIA is a flow chart illustrating a process for manufacturing a PWB in accordance 
5 with the present invention including multiple electrically and thermally conductive laminates, 
chimney holes and plated through holes; 
'rz FIG. 1 IB is a flow chart illustrating a process for determining locations in which to drill 

chimney holes in a PWB; 

: FIG. 1 IC is a flow chart illustrating a process for determining locations in which to drill 

tTlO filled clearaace holes in electrically and thermally conductive laminates during the construction 
^ of a PWB in accordance with the present invention; and 

■ FIG. 12 is a semi-schematic cross-sectional view showing a PWB in accordance with the 
:^ present invention including two electrically and thermally conductive laminates and an 
electrically isolated carbon support layer. 

15 

DETAILED DESCRIPTION OF THE INVENTION 

Referring now to the drawings, FIGURE 1 illustrates a lightweight multiple-layer PWB 
in accordance with the present invention. The PWB 10 includes a laminate 12 comprising a 
carbon containing layer 14 sandwiched between a first layer of metal or other electrically 
20 conductive material 16 and a second layer of metal or other electrically conductive material 18. 
The laminate is sandwiched between a first layer of prepreg 20 and a second layer of prepreg 22. 
The top layer of the PWB is constructed fi-om a third layer of metal or other electrically 

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47490/RAG/S968 



conductive material 24. The bottom layer of the PWB is constructed using a fourth layer of 
metal or other electrically conductive material 26, As set forth below, the electrically conductive 
layers 16, 18, 24 and 26, and the corresponding layers of the other embodiments described 
herein, may be made of metal or any of a variety of metal-containing compositions having 
5 suitable properties of electrical conduction. For convenience, however, these layers often will be 
referred to herein simply as "metal" layers. 
1^ The laminate 12 is electrically conductive, which enables the laminate to be used as a 

C3 ground plane within the PWB, a power plane within a PWB or both a ground and power plane in 
y the PWB where routing is used to electrically isolate portions of the laminate. Use of the 
iJlO laminate 12 in a PWB results in the PWB being thinner and having less weight than previous 
PWB designs that utilize electrically insulated carbon containing layers to lower the CTE. 

fU 

|p4 Reducing the thickness of the PWB 10 also enables the carbon containing layers 14 to be located 

Ml 

Q closer to the surface of the board than in PWBs that utilize electrically insulated carbon 
constraining layers. An advantage of this configuration is that it gives the PWB increased ability 
15 to transfer heat away from its surface compared to previous designs. Another advantage of this 
configuration is that it provides low surface CTE which is important in applications such as 
semiconductor applications. 

A prepreg is a composite layer that includes a substrate or supporting material composed 
of fibrous material that is impregnated with resin. A prepreg may also be a film. A film is a type 
20 of prepreg that does not include a substrate but is instead a composite that only includes resins. 
The first prepreg layer 20 and the second prepreg layer 22 electrically insulate the electrically 
conductive laminate 12 fi:-om the third layer of metal 24 and the fourth layer of metal 26. In one 

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47490/RAG/S968 

preferred embodiment, the third and fourth layers of metal or other electrically conductive 
material are patterned with electrical circuits. For example, electrical contact between the third 
layer of metal, the electrically conductive laminate or the fourth layer of metal can result in the 
functions of the electrical circuits patterned onto the third and fourth layers of metal being 
interrupted. In other embodiments, only one of the third and fourth layers of metal are patterned 
with electrical circuits. 

In one preferred embodiment of the PWB in accordance with the present invention, the 
layer containing carbon used in the construction of the laminate 12 is made from woven carbon 
fibers such as woven K13C2U manufactured by Mitsubishi Chemical America, Inc. of 
Sunnyvale, California and having a thickness of 0.006 inches. In another embodiment, the layer 
containing carbon can be constructed from carbon fibers having a tensile modulus of 1 10 msi, a 
tensile strength of 540 ksi, a thermal conductivity of 610 W/m.K, a fiber density of 2.15 g/cc and 
a fiber elongation of 0.5% and that are woven with a balanced weave. In other embodiments, the 
layer containing carbon can be constructed from any woven carbon fibers having a thickness 
greater than 0.002 inches, a thermal conductivity greater than 10 W/m.K, a co-efficient of 
thermal expansion in the range -3,0 to 3.0 ppm/C, a stiffiiess greater than 20 msi, a tensile greater 
than 250 ksi, a density less than 2.25 gm/cc. Preferably, the layer containing carbon is 
constructed from woven carbon fibers having a thermal conductivity greater than 75 W/m.K, a 
co-efficient of thermal expansion in the range -1.25 to 1.0 ppm/C, a stiffiiess greater than 35 msi, 
a tensile strength greater than 350 ksi, a density less than 2.22 gm/cc. More preferably, the layer 
containing carbon is constructed from woven carbon fibers having a co-efficient of theraial 
expansion of 0.0 ppm/C. In other embodiments, the layer containing carbon is constructed from 



47490/RAG/S968 

any woven carbon fibers capable of dissipating the required amount of heat from the surface of 
the PWB 10, to support the CTE requirements of the PWB and to achieve the desired stiffness of 
the PWB, 

In one preferred embodiment, the woven carbon fibers are impregnated with an 
electrically and thermally conductive resin such as an epoxy pyrolitic carbon resin in accordance 
with the process described above in relation to FIG. 2B, Electrical conductivity is defined as 
having a dielectric constant greater than 6,0 at 1 MHz. Thermal conductivity is defined as having 
a thermal conductivity of greater than 1.25 W/m.K. Preferably, a material that is thermally 
conductive will have a thermal conductivity greater than 2.5 W/m,K. In other embodiments, the 
woven carbon fibers are impregnated with a resin such as polyimide (cyanate ester) based 
pyrolitic carbon resin, epoxy or polyimide based silver oxide resin, epoxy or polyimide based 
carbon powder resin or any other resin having a glass transition temperature greater than 100 °F, 
low moisture absorption, high resistance to chemical corrosion, high resistance to microcracking, 
high structural durability, controlled flow, good adhesion, a thermal conductivity greater than 0,2 
W/m.k and a dielectric constant greater than 6.0 at 1 Mhz. Preferably, the woven carbon fibers 
are impregnated with a resin having a glass transition temperature greater than 250 °F and a 
thermal conductivity greater than 2,0 W/m,k. 

In one preferred embodiment, the first and second layers of metal are constructed from a 
1/4 oz copper foil such as NT-TW-HTE manufactured by Circuit Foil Trading, Inc of Glenside, 
Philadelphia. In other embodiments, other electrically conductive materials such as Cu, 
Palladium, Ag, Al, Au, Ni and Sn, or alloys or other compositions thereof, having thicknesses 
from 0.00003 inches to 0.021 inches can be used in the construction of the first and second layers 



47490/RAG/S968 

of metal In other embodiments, an electrically conductive material of any thickness can be used 
in the construction of the first and second layers of metal provided that the overall conductivity 
of the electrically conductive laminate 12 is sufficient to carry the electrical load in the laminate. 

In one preferred embodiment, the first prepreg layer and the second prepreg layer are 
constructed from thermally conductive dielectric material such as the prepreg 44N0680 
manufactured by Arlon Materials for Electronics of Rancho Cucamonga, California having a 
thickness of 0.0015 inches, a resin content of approximately 80%, a resin flow of approximately 
50% and a gel time in the range of 90 to 1 10 seconds. In other embodiments, other prepregs such 
as FR-4, polyimide, teflon, ceramics, GIL, Gtek or high frequency circuit materials 
SlO manufactured by Rogers Corporation that include additives such as aluminum oxide, diamond 
particles or boron nitride or any other prepreg having dielectric constants less than 6.0 at 1 MHz 
H and a thermal conductivity of greater than L25 W/m.K can be used in the construction of the 
?f first and second prepreg layers. More preferably, the first and second prepreg layers are 
constructed from a dielectric material having a dielectric constant less 4.0 at 1 Mhz and a thermal 
15 conductivity greater than 2.0 W/m.K, In other embodiments, prepregs that have thermal 
conductivity less than 1 .25 W/m.K can be used in the construction of the first and second prepreg 
layers. Use of prepreg layers that have a thermal conductivity that is less than 1.25 W/m.K can 
reduce the ability of the PWB to conduct heat away fi^om its surface. 

In one preferred embodiment the top and bottom layers of conductive material are 
20 constructed fi^om materials similar to those used in the construction of the furst and second layers 
of metal as described above. 



fli 



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f'1 



47490/RAG/S968 

One preferred embodiment of a method of manufacturing PWBs in accordance with the 
present invention is illustrated in FIG. 2A. A first lamination is performed in the step 32, The 
first lamination involves placing a 1/4 oz layer of copper foil on one side of a layer of woven 
carbon fibers impregnated with the epoxy based pyrolitic carbon resin, as described above, and 
5 placing a second 1/4 oz layer of copper foil on the other side of the layer of woven carbon fibers. 
The layers are then placed in a vacuum and heated fi:om room temperature to 350°F. The 
H temperature increase is controlled so that the temperature rise is maintained within the range of 

5 

Jr! 8-12 "^F/min as the temperature rises firom 150^F to SOO^F. When the temperature is in the 
range 150°F - 165°F, the pressure on the layers is increased to 250 PSL Once a temperature of 
CP 10 350°F has been reached, the temperature is maintained at that temperature for 70 minutes. After 

Si 

^ the completion of the 70 minute time period, the layers are exposed to room temperature and a 
il pressure greater than atmospheric pressure for a period of 30 minutes. The first lamination cycle 

Q 

5 produces the electrically conductive laminate 12 described above. Preferably, the electrically 

conductive laminate is manufactured to be as flat as possible. 

15 The first lamination cycle is followed by a second lamination cycle in the step 34. The 

second lamination cycle involves placing a layer 44N0680 prepreg on one side of the electrically 

conductive laminate produced in the first lamination cycle and a second layer of 44N0680 

prepreg on the other side of the electrically conductive laminate. In addition, layers of 1/2 oz 

copper foil are placed on the outside surfaces of the two layers of 44N0680 prepreg. The layers 

20 are then placed in a vacuum and heated fi"om room temperature to 350°F. The temperature 

increase is controlled so that the temperature rise is maintained within the range of 8 - 12 °F/min 

as the temperature rises from 150°F to 300°F. When the temperature is in the range 

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47490/RAG/S968 



ISO'^F - 165°F, the pressure on the layers is increased to 250 PSL Once a temperature of 350°F 
has been reached, the temperature is maintained at that temperature for 90 minutes. After the 
completion of the 90 minute time period, the layers are exposed to room temperature and a 
pressure greater than atmospheric pressure for a period of 30 minutes. The first lamination cycle 
produces the electrically conductive laminate 12 described above. The second lamination cycle 
produces the PWB 10 shown in FIG, 1. The third and fourth layers of metal of the PWB 10 
shown in FIG. 1 are then patterned with electrical circuits in the step 36. 

One preferred embodiment of a process 40 for impregnating a layer containing carbon 
constructed from woven carbon fiber with an electrically conductive resin is illustrated in FIG. 
2B. The first step in the process 42 involves adding together ingredients to form a resin. Most 
resins are formed using epoxy or polyimide solid state resin, solvent, acetones, catalysts and 
additives. Typically, the properties of a particular resin are determined by the various additives 
included in the resin and the quantities of these additives. Additives can be used to increase the 
electrical conductivity or the thermal properties of a resin. When an additive is used to increase 
the electrical or thermal conductivity of a resin, the thermal or electrical conductivity of a resin 
increases with the amount of the additive mixed through the resin. In one preferred embodiment, 
an amount of pyrolitic carbon in powder form equal to 10% by weight of the resin is added as an 
ingredient to increase the electrical conductivity and thermal properties of the resin. In other 
embodiments, any amount of pyrolitic carbon can be added to improve the thermal and electrical 
properties of the resin. Preferably, the amount of pyrolitic carbon added to the resin is between 
5% to 50% by weight of the resin. The various resin ingredients are then mixed in the step 44 to 
form a substantially homogenous resin. 



47490/RAG/S968 

Once a resin is formed, the resin is placed in a prepreg treater in the step 46. The prepreg 
treater is used to impregnate a substrate with resin. In the next step 48, the substrate to be 
impregnated is passed through the prepreg treater. In one preferred embodiment of the process, 
the substrate material is woven carbon fibers such as the woven carbon fiber materials described 
above. In one preferred embodiment using a woven carbon fiber substrate, the substrate is 
impregnated with 45% by weight resin. In other embodiments, the substrate is impregnated with 
between 5% to 80% by weight resin. 

Once the substrate has been passed through the prepreg treater, the B-stage curing cycle 
is performed in the step 50. The B-stage curing cycle involves exposing the substrate and resin to 
a temperature of between 250''F to SOC^F. The amount of time that the substrate and resin are 
exposed to this temperature is determined by the amount of resui loaded onto the substrate and 
the extent of curing required. In one preferred embodiment, a time period of 15 minutes is 
required for the impregnation of a woven carbon fiber substrate with 45% resin cured to B-stage 
so that it is suitable for use in the process described above in relation to FIG. 2A. Upon the 
completion of the B-stage curing cycle, the resin is stored in a controlled environment prior to 
use in the step 52. 

In other embodiments, silver oxide particles are used as a resin additive to increase the 
electrical conductivity and thermal properties of the resin. In one preferred embodiment, an 
amount of silver oxide equal to 40% by weight of the resin is added. In other embodiments, any 
amount of silver oxide can be added to increase the thermal properties of the resin. Preferably, 
the amount of silver oxide added to the resin will be between 5% and 70% by weight of the resin. 

-12- 



47490/RAG/S968 

In other embodiments, boron nitride particles are used as a resin additive to increase the 
thermal properties of the resin. In one preferred embodiment, an amount of boron nitride equal 
to 40% by weight of the resin is added. In other embodiments, any amount of boron nitride can 
be added to increase the thermal properties of the resin. Preferably, the amount of boron nitride 
5 added to the resin will be between 5% and 70% by weight of the resin. 

In other embodiments, diamond particles are used as a resin additive to increase the 
thermal properties of the resin. In one preferred embodiment, an amount of diamond particles 

O equal to 15% by weight of the resin is added. In other embodiments, any amount of diamond 

fll 

£3 particles can be added to increase the thermal properties of the resin. Preferably, the amount of 

m 

^10 diamond particles added to the resin will be between 2% to 50% by weight of the resin. 

Ijl 

^ , In other embodiments, aluminum oxide particles are used as a resin additive to increase 

il the thermal properties of the resin. In one preferred embodiment, an amount of aluminum oxide 
y equal to 40% by weight of the resin is added. In other embodiments, any amount of aluminum 

oxide can be added to increase the thermal properties of the resin. Preferably, the amount of 
15 aluminum oxide added to the resin will be between 5% to 70% by weight of the resin. In other 

embodiments, two or more of the additives described above can be used as additives to form a 

resin. 

In other embodiments, prepregs can be manufactured using the above process by using 
substrate materials that have dielectric constants less than 6.0 at 1 MHz. In one preferred 
20 embodiment, a fiberglass substrate is impregnated with a resin containing boron nitride to 
produce a thermally conductive prepreg with a dielectric constant less than 6.0 at 1 MHz. 



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47490/RAG/S968 

Preferably, the fiberglass is impregnated with 70% by weight resin. In other embodiments, the 

fiberglass is impregnated with between 20% and 80% by weight resin. 

In other embodiments, other substrates such as kevlar, quart, aramid or any other material 

or mixture of materials having a dielectric constant less than 6.0 at 1 MHz, a glass transition 
5 temperature greater than 250°F, a thermal conductivity greater than 0.1 W/m.K, a CTE between 

"4.5 ppm/'^C and 30ppm/''C, high tensile strength and high thermal endurance can be used in the 

construction of prepreg layers. Preferably, the substrate material has a glass transition 
O temperature greater than 400T, a CTE between -4.5 ppm/°C and 12 ppm/^'C, retains 50% to 
^ 60% of its strength at 700°F and has a dielectric constant less than 3.0 at 1 Mhz. Prepreg 
5^10 manufactured using this process can be used in the construction of the first and second prepreg 
U layers of the PWB 10 in accordance with the present invention illustrated in FIG. 1 . 

u 

U In other embodiments, the layer containing carbon is impregnated with a resin that is 

Q thermally conductive such as a epoxy or polyimide based boron nitride resin, epoxy or polyimide 

based aluminum oxide, epoxy or polyimide based ceramic resin, epoxy or polyimide based 
15 diamond particles resin or any other resin having properties similar to the electrically and 

thermally conductive resins described above except that the dielectric constant of the resin is less 

than 6.0 at 1 Mhz. 

In other embodiments, the layer containing carbon is constructed fi"om a sheet of 
unidirectional carbon fiber such as unidirectional K13C2U manufactured by Mitsubishi 
20 Chemical America, Inc. and having a thickness of 0.001 inches. The unidirectional carbon fiber 
material chosen for use in the construction of the carbon containing layer preferably has 



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47490/RAG/S968 

properties similar to those described above for the woven carbon fiber that can be used in the 
construction of the layer containing carbon. 

In other embodiments, the sheet of unidirectional carbon fiber is impregnated with resin. 
Resins with similar properties to those described above in relation to embodiments of laminates 
5 incorporating sheets of woven carbon fiber can also be used to impregnate sheets of 
unidirectional carbon fiber used in the construction of laminates in accordance with the present 
|::^ invention, 

fast 

O In other embodiments, multiple layers of unidirectional carbon fiber that are aligned such 

HI 

^ that the fibers in each of the layers are substantially parallel can be used in the construction of the 
!^ 1 0 layer containing carbon. 

One preferred embodiment of a laminate 12* constructed in accordance with the present 

fll 

Lh invention using four unidirectional layers of carbon fiber is illustrated in FIG. 3. In this 
C3 embodiment, the laminate is constructed fi-om a first unidirectional layer of carbon fiber 60, a 
second unidirectional layer of carbon fiber 62, a third unidirectional layer of carbon fiber 64 and 
15 a fourth unidirectional layer of carbon fiber 66. Each of the unidirectional layers of carbon fiber 
have the same thickness and fiber area weight. The first and fourth unidirectional layers of 
carbon fiber are constructed so that the carbon fibers in each layer are ahgned to be substantially 
parallel. The second and third unidirectional layers of carbon fiber are constructed fi"om sheets of 
unidirectional carbon fiber, where the fibers are aligned substantially perpendicular to the carbon 
20 fibers in the first and fourth layers. 

Another preferred embodiment of a laminate 12" constructed in accordance with the 
present invention using unidirectional layers of carbon fiber are illustrated in FIG. 4. In this 



47490/RAG/S968 

embodiment the laminate 12" is constructed from a first unidirectional layer of carbon fiber 70, a 
second unidirectional layer of carbon fiber 72, a third unidirectional layer of carbon fiber 74 and 
a fourth unidirectional layer of carbon fiber 76. Each of the unidirectional layers of carbon fiber 
have the same thickness and fiber area weight. The first and third unidirectional layers of carbon 
5 fiber are constructed from sheets of unidirectional carbon fiber having fibers ahgned in 
substantially the same direction. The second and fourth unidirectional layers of carbon fiber are 
^ constructed from sheets of unidirectional carbon having fibers aligned in a direction substantially 

S| perpendicular to the direction in which the fibers in the first and third unidirectional layers of 

O 

ifl carbon fiber are aligned. 

yllO Another preferred embodiment of a laminate 12'" constructed in accordance with the 

present invention using unidirectional layers of carbon fiber are illustrated in FIG. 5. In this 
embodiment the laminate 12'" is constructed from a first unidirectional layer of carbon fiber 80 
having a thickness of 0.002 inches, a second unidirectional layer of carbon fiber 82 having a 
thickness of 0.004 inches and a third unidirectional layer of carbon fiber 84 having a thickness of 
15 0.002 inches. The fiber area weight of the first and third unidirectional layers of carbon fiber 
have the same fiber area weight, which is half the fiber area weight of the second unidirectional 
layer of carbon fiber. The first and third unidirectional layers of carbon fiber are constructed 
from sheets of unidirectional carbon fiber having fibers aligned in the same direction. The 
second unidirectional layer of carbon fiber is constructed from a sheet of unidirectional carbon 
20 having fibers aligned in a direction perpendicular to the direction in which the fibers in the first 
and third unidirectional layers of carbon fiber are aligned. 



-16- 



47490/RAG/S968 

In other embodiments, a number of layers of unidirectional carbon fiber greater than four 
can be used in the construction of the printed circuit board provided that the layer containing 
carbon fiber is balanced. 

In other embodiments, laminates in accordance with the present invention include layers 
5 containing carbon that are substantially isotropic. One embodiment of a laminate in accordance 
with the present invention incorporating an isotropic carbon containing layer is illustrated in FIG. 
'hk 6. The laminate 12"" includes a first unidirectional layer of carbon fiber 90 constructed from a 

9 

U sheet of unidirectional carbon fiber with fibers aligned in a first reference direction, a second 

J''!? unidirectional layer of carbon fiber 92 constructed from a sheet of unidirectional carbon fiber 

ti 

J^io positioned so that its fibers are aligned at an angle of 45 to the first reference direction, a third 

1^. unidirectional layer of carbon fiber 94 constructed from a sheet of unidirectional carbon fiber 

HI 5 
r- w 

)^ positioned so that its fibers are aUgned at an angle of 90° to the first reference direction and a 
Q fourth unidirectional layer of carbon fiber 96 constructed firom a sheet of unidirectional carbon 

S : 

fiber positioned so that its fibers are aligned at an angle of 135° to the first reference direction. 
15 The sheets of unidirectional carbon fiber can be impregnated with resins similar to those resins 
described above. 

A PWB in accordance with the present invention is illustrated in FIG. 7. The PWB 10' 
includes a laminate structure 12""' having a carbon containing layer 14' positioned between a 
first layer of prepreg 100 and a second layer of prepreg 102. A first layer of metal 16* is 
20 positioned above the first prepreg layer and a second layer of metal 18' is positioned beneath the 
second prepreg layer. A third layer of prepreg 20' is positioned above the first layer of metal and 
a second layer of prepreg 22' is positioned below the second layer of metal. A third layer of 

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47490/RAG/S968 

metal 24' is positioned above the third layer of prepreg and a fourth layer of metal 26' is 
positioned below a fourth layer of prepreg. 

In one preferred embodiment, the layer containing carbon 14' is constructed from a 
woven sheet of carbon fiber and the layers of metal are constructed from materials similar to 
those described above in the construction of the layers of metal used in the construction of the 
embodiment of the PWB shown as 10 in FIG. L In addition, third and fourth prepreg layers are 
constructed from materials similar to those described above in the construction of the first and 
second prepreg layers of the embodiment of the PWB shown as 10 in FIG. L 

In one preferred embodiment an electrically and thermally conductive prepreg layer such 
as epoxy based pyrolitic carbon resin prepreg manufactured in accordance with the process 
described above in relation to FIG. 2B, and having properties similar to the pyroUtic carbon 
resin described above, is used in the construction of the first and second prepreg layers. An 
electrically and thermally conductive prepreg is used in the construction of the first and second 
prepreg layers to ensure that an electrically conductive path exists between the layer containing 
carbon and the first and second electrically conductive layers. In other embodiments, other 
electrically and thermally conductive prepregs such as polyimide based pyrolitic carbon resin 
prepreg, epoxy or polyimide based silver oxide resin prepreg or any other prepreg having a glass 
transition temperature greater than 100 °F, low moisture absorption, high resistance to chemical 
corrosion, high resistance to micro cracking, high structural durability, controlled flow, good 
adhesion, a thermal conductivity greater than 0.2 W/m.k and a dielectric constant greater than 6.0 
at 1 Mhz can be used in the construction of the first and second prepreg layers. Preferably, the 



-18- 



47490/RAG/S968 

first and second prepreg layers are constructed from a prepreg having a glass transition 
temperature greater than 250 ""F and a thermal conductivity greater than 2.0 W/m.k. 

The method of manufacturing the PWB 10' illustrated in FIG. 7 is similar to the method 
illustrated in FIG. 2A. A layer of epoxy based pyrolitic carbon resin prepreg is placed on one 
side of a layer of woven carbon fibers and a second layer of epoxy based pyrolitic carbon resin 
prepreg is placed on the other side of the layer of woven carbon fiber. Layers of 1/4 oz copper 
foil are then placed on the outside surfaces of the layers of epoxy based pyrolitic carbon resin 
prepreg. These layers are then subjected to the first lamination cycle as described above in 
relation to FIG, 2 A to produce the laminate 12"'". The second lamination cycle and the 
patterning of the PWB 10' are also similar to processes described above in relation to FIG. 2A. 

In other embodiments, thermally conductive prepreg layers similar to those used in the 
construction of the first and second prepreg layers of the embodiment of the PWB 10 shown in 
FIG, 1, as described above, can be used in the construction of the first and second prepreg layers 
of the embodiment of the PWB 10' shown in FIG. 7. In embodiments of the PWB 10' that use 
thermally conductive prepreg layers that are poor conductors of electricity, electrical contacts are 
made between the first and second layers of metal and the carbon containing layer by plated 
through holes. Plated through holes are holes drilled through the laminate 12""' that are lined 
with electrically conductive material and establish electrical contacts between the first and 
second layers of metal and the layer containing carbon. 

In other embodiments, the laminate 12""' is constructed from a layer containing carbon 
made from layers of unidirectional carbon fibers that have arrangements similar to the 
arrangements described above in relation to the embodiments of laminates in accordance with 

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47490/RAG/S968 

the present invention illustrated in FIGS. 3-6. In other embodiments, the layers of 
unidirectional carbon fibers are impregnated with resins similar to those described above prior to 
the construction of the laminate 12"'". 

In other embodiments, the laminate 12'"" is constructed from a layer containing carbon 
that is made from a carbon composite sheet or plate such as a carbon plate manufactured by 
Mitsubishi Chemical America, Inc having a thickness of 0.001 inches, A carbon composite sheet 
or plate can be constructed using a compressed powder mold. In other embodiments, the layer 

O containing carbon can be constructed from any carbon composite sheet or plate has physical 

fll 

Q properties similar to those described above in relation to woven carbon fiber. 

in 

.Mrs, 

510 For embodiments of the laminate 12""' constructed using layers containing carbon made 



from carbon composite sheets or plates, the first layer of prepreg 100 and the second layer of 
prepreg 102 can be constructed from resins similar to those described above. 

A PCB in accordance with the present invention is illustrated in FIG. 8. The PCB 10" 
includes an electrically and thermally conductive layer 110. A first layer of metal 16" is 
15 positioned above the electrically and thermally conductive layer and a second layer of metal 18" 
is positioned below the electrically and thermally conductive layer. A first prepreg layer 20" is 
positioned above the first layer of metal and a second prepreg layer 22" is positioned below the 
second layer of metal. A third layer of metal 24" is positioned above the first prepreg layer and a 
fourth layer of metal 26" is positioned below the second prepreg layer. The electrically and 
20 thermally conductive layer and the first and second layers of metal form an electrically 
conductive laminate 112. 



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47490/RAG/S968 

In one preferred embodiment, similar materials to those that can be used in the 
construction of the embodiment of the PWB 10 in accordance with the present invention 
illustrated in FIG. 1 can also be used in the construction of the first and second prepreg layers 
and the first, second, third and fourth layers of metal. In one preferred embodiment, the third 
5 and fourth layers of metal are patterned to contain electrical circuits. 

In one preferred embodiment, the electrically and thermally conductive layer is 
1^ constructed from a woven fiberglass substrate impregnated with an electrically conductive epoxy 
pyrolitic carbon resin in accordance with the process described above in relation to FIG. 2B. 
O Preferably, the woven fiberglass used in the construction of the electrically and thermally 

,^^10 conductive layer is E-glass manufactured by JPS Glass located at South Cickering of Ontario in 

y « 

Canada. In other embodiments, other substrate materials such as non-woven fiberglass, kevlar, 

fU 

hk quartz, aramid or other materials having a glass transition temperature greater than 250°F, a 
O thermal conductivity greater than 0.1 W/m.K, a CTE between -4.5 ppm/^'C and 30ppni/°C, high 
tensile strength and high thermal endurance. Preferably, the substrate has a glass transition 
15 temperature greater than 400°F, a CTE between -4.5 ppm/°C and 12 ppm/°C, retains 50% to 
60% of its strength at TOO^'F. Preferably, the fiberglass substrate is impregnated with 70% by 
weight resin of an epoxy resin containing 10% by weight pyrolitic carbon additive. In other 
embodiments, the electrically and thermally conductive layer is formed using a substrate that is 
impregnated with between 5% to 80% of any of the resins described above having a dielectric 
20 constant greater than 6.0 at 1 MHz. In other embodiments, any resin and substrate combination 
can be used that results in the electrically and thermally conductive layer 14' having a dielectric 
constant greater than 6.0 at 1 Mhz. 

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47490/RAG/S968 

The embodiment of the PWB 10" illustrated in FIG. 8 can be manufactured in accordance 
with the processes illustrated in FIG. 2 A and FIG. 2B. 

An embodiment of a PWB in accordance with the present invention is illustrated in FIG. 
9. The PWB 10'" is similar to the PWB 10' illustrated in FIG. 7 except that the carbon 
5 containing layer is replaced with any of the substrate materials described above and that the first 
and second prepreg layers possess dielectric constants greater than 6.0 at 1 MHz. 
M The embodiments of PWBs described above have utilized a single laminate. In other 

yd 

0 embodiments of PWBs in accordance with the present invention, multiple laminates can be used. 

ii 

^ A PWB in accordance with the present invention including two laminates is illustrated in 

y I 

SlO FIG. 10. The PWB 10"" includes a first laminate 120, a second laminate 122, multiple layers 

i^^ of prepreg 124 and multiple layers of metal 126. In one preferred embodiment, the first laminate 

III 

forms a ground plane and the second laminate forms a power plane. In other embodiments, the 
p function of the laminates can be reversed, both laminates can share the same functions or the 
laminates can be utilized for their improved thermal properties only. The use of multiple 
15 laminates can increase the abihty of the PWB to conduct heat away fi:'om its surface, improve the 
CTE of the PWB and can decrease the thickness and weight of the PWB, when compared to 
prior art PWBs. 

In one preferred embodiment, the first laminate 120 and second laminate 122 are 
constructed similarly to the laminate 12 of FIG. 1. In other embodiments any of the laminate 
20 structures described above can be used in the construction of the first or second laminate. 
Preferably, the layers of prepreg 124 and layers of metal are constructed from materials similar 
to those that can be used to construct the prepreg layers and the layers of metal in the PWB 10 

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47490/RAG/S968 

illustrated in FIG. 1 . In other embodiments any of the laminates described above can be used in 
the construction of the first and second laminates. 

A closer inspection of FIG. 10 reveals that the PWB 10"" includes a number of plated 
holes. The PWB 10"" includes chimneys 128 that are holes filled with thermally conductive 
5 material. The chimneys are used to transport heat from the surface of the PWB to the electrically 
and thermally conductive laminates within the PWB. The chimneys do not extend all the way 
^ through the PWB. If the chimneys contacted both the fu-st and second laminates, then the 
si chimneys could short circuit the PWB. The PWB 10"" also includes through holes 130 lined 

rS with electrically conductive material that are used to establish electrical connections between the 

Q 

ffllO functional layers in the PWB. Where connections between the plated through holes and the first 

1=^ or second laminates are not desired, then an annulus of dielectric material 132 such as an epoxy 

^ resin with a dielectric constant less than 6.0 at 1 MHz can be used to ensure that an electrical 

La, 

J=f connection does not exist between the laminate and the electrically conductive lining of the 
through hole. 

15 A process in accordance with the present invention for manufacturing the PWB 10"" 

illustrated in FIG. 10 is shown in FIG. UA. The process 150 commences with the step 152, 
which involves constructing two laminates in accordance with the present invention are formed 
using the process described above in relation to FIG. 2A. Power or ground regions are then 
patterned on the laminates in the step 154. The patterning electrically isolates regions within the 
20 laminate, which can enable laminate to function as a ground or power plane within a PWB. 

Once the patterning is complete, the laminates are subjected to oxide treatment in the step 
156. After oxide treatment, clearance hole drilling is performed in the step 158. Clearance hole 

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47490/RAG/S968 

drilling involves drilling holes in the laminate of a first diameter and filling the resulting holes 
with a dielectric material such as any of the resins described above with a dielectric constant less 
than 6.0 at 1 MHz. Prior to filling the drilled holes, they are inspected and cleaned using high 
pressure dry air. 

Once the clearance holes have been drilled, the second lamination cycle is performed in 
the step 160. The second lamination cycle is similar to the second lamination cycle described 
above in relation to FIG. 2A. After the second lamination cycle, chimney holes are drilled into 
the PWB in the step 162. Once the chimney holes have been drilled, the linings of the chimney 
holes are lined with a thermally conductive material in the step 164. Preferably, the thermally 
conductive material is copper. In other embodiments, any material with a thermal conductivity 
greater than 5 W/m.K can be used. 

After the chimney holes have been lined, circuits are etched onto the layers of metal that 
will be located within the interior of the finished PWB are patterned in the step 166 and then 
subjected to oxide treatment in the step 168. 

Following the oxide treatment, the third lamination is performed in the step 170. The 
third lamination involves ahgning the two structures produced in the second lamination with 
additional prepreg layers to correspond with the layers of the PWB 10"" illustrated in FIG. 10. 
The layers are then exposed to temperatures and pressures similar to those experienced during 
the second lamination cycle. 

After the third lamination cycle, the final through hole drilling is performed in step 172. 
The final through hole drilling involves drilling holes through the entire PWB that have a second 
diameter, which is less than the first diameter described above. The through holes are then lined 

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47490/RAG/S968 

in the step 174. Preferably, the through holes are lined with copper. In other embodiments, the 
through holes can be plated with materials similar to those that can be used in the construction of 
the layers of metal. If a through hole passes through one of the filled clearance holes in a 
laminate, then the lining of the through holes are electrically isolated from the laminate in which 
the clearance hole is drilled. If a through hole does not pass through one of the filled clearance 
holes in a laminate, then the lining of the through hole is in electrical contact with the laminate. 

An embodiment of a process for selecting the locations in which to drill chimney holes in 
a PWB is illustrated in FIG. IIB. The process 190 includes a first step 192, which involves 
creating a model of the structure of the PWB. The second step 194 involves adding a thermally 
conductive material such as copper to the outermost layers of metal on the model. The thermally 
conductive material is added to the model such that the thermally conductive material does not 
create any electrical contacts with the circuits patterned onto the layers of metal on which the 
thermally conductive material is added. 

Once the thermally conductive material has been added, the locations of the chimney 
holes are determined in the step 196. The locations of the chimney holes are determined by 
choosing a location on the surface of the PWB that lies withui an area where thermally 
conductive material was added during step 194. The location is a suitable location for a chimney 
if a hole of a specified diameter corresponding to the diameter of the chimney can be drilled 
through the PWB without intersecting any of the electrical circuits patterned onto layers of metal 
internal to the PWB. Otherwise, the chosen location is unsuitable as a location for drilling a 
chimney hole. The number of locations that must be found is dependent upon the amount of heat 
required to be conducted away from the surface of the board. 

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47490/RAG/S968 



An embodiment of a process for selecting the location of the filled clearance holes in the 
laminates is illustrated in FIG. 1 IC. The process 200 includes a first step 202, which involves 
constructing a model of the PWB. The locations of the through holes in the PWB are used to 
determine the locations in which the through holes intersect the groimd plane laminate or the 
5 power plane laminate in the step 204. Once these locations have been determined, the locations 
of the clearance holes are chosen in the step 206 as the locations where the through holes 

M intersect the ground or power plane laminates and where an electrical connection between the 

P 

lining of the plated through hole and the ground or power plane laminate is undesirable. 

fi A PWB in accordance with the present invention incorporating laminates in accordance 

yi 

SlO with the present invention and an additional carbon containing layer is illustrated in FIG. 12. 
The PWB 10"'" has a first laminate 120', a second laminate 122', an additional carbon 

U containing layer 210, prepreg layers 124' and layers of metal 126'. Preferably, the first laminate 
forms a ground plane and the second laminate forms a power plane. The additional carbon 
containing layer 210, does not act as a ground or power plane and is electrically isolated fi*om the 
15 laminates and the layers of metal. The additional carbon containing layer increases the thermal 
conductivity and stiffness of the PWB and improves the CTE of the PWB. Similar materials to 
those used in the construction of the laminates, prepreg layers and layers of metal of the PWB 10 
illustrated in FIG. 1 can also be used to construct the laminates, prepreg layers and layers of 
metal of the PWB 10"'" illustrated in FIG. 12. The additional carbon containing layer can be 
20 constructed using the similar materials to those that can be used in the construction of the carbon 
containing layers of the laminates illustrated at 12 in FIG. 1, 12^ in FIG. 3, 12" in FIG. 4, 12'" in 
FIG. 5 and 12'"' in FIG. 6. 

-26- 



47490/RAG/S968 

The PWB 10""' in FIG. 12 can be constructed using a processes similar to those 
described above in relation to FIGS 11 A - 1 IC, The only difference is in the arrangement of the 
materials used in the construction of the third lamination cycle and the fact that filled clearance 
holes must also be drilled in the additional carbon containing layer 210 so that the additional 
carbon containing layer is electrically isolated from the linings of any through holes present in 
the PWB, 

Although the embodiments described above have included a single or two laminates in 
accordance with the present invention, one skilled in the art would appreciate that a PWB can be 
constructed including three or more laminates using the processes described above. 



-11-