AU747347B2 - Base webs for printed circuit board production using the foam process and aramid fibers - Google Patents

Description

WO 99'491 I PCT/F 99/IlH I BASE WEBS FOR PRINTED CIRCUIT BOARD PRODUCTION USING THE FOAM PROCESS AND ARAMID FIBERS CROSS-REFERENCE TO RELATED APPLICATION This application is based upon US provisional patent application serial no. 60/078,708 filed March 20, 1998.

BACKGROUND AND SUMMARY OF THE INVENTION Printed circuit boards are conventionally made of primarily fiberglass fibers, with electrically non-conductive fillers. However there 10 has been increasing interest in making printed circuit boards from aramid fibers since aramid fibers have a number of advantages over glass. In particular aramid has less electrical conductivity and therefore :.allows construction of a board that can have closer circuit density and is less susceptible to high frequency energy corruption. Also another 15 outstanding characteristic of aramid is it has a better co-efficient of thermal expansion than glass. Aramid non-woven webs also have a smooth surface, and are superior in high temperature applications to glassfiber webs. Because of these advantages, duPont Chemical Company uses its own brand of aramid fiber ('THERMOUNT") in the production of printed circuit boards.

The duPont aramid PCBs are made using the conventional liquid laid process for non-woven web production using a foraminous element, such as a wire. In order to effectively make non-woven webs using aramid fibers by the liquid laid process, duPont uses a blend of different length and diameter aramid fibers, some of which may be fibrillated, in an WO 9/4911 i PCT,/F99/0021 2 attempt to produce versatile and entirely commercially acceptable printed circuit boards. However there are numerous problems associated with the water laid process of production of aramid non-woven sheets or webs using conventional para aramid fibers (which are "straight").

Conventional aramid printed circuit boards, and layers formed of non-woven webs making up such boards, have a significant number of problems including the inability to randomly disperse the a:amid fibers as uniformly as customers would like, and typically the aramid sheets are directional. This directionality creates different co-efficients of thermal expansion in the machine direction and the cross-machine direction in the finished product, and in tear characteristics relating to saturating the sheet. Also such boards are difficult to handle and require a significant amount of handling experience by customers, and they have an affinity to absorb moisture so that some customers must bake each roll in an oven 15 to drive off humidity before it can be used. Also great care must be exercised during manufacturing to avoid chain wrinkles, lay flat, and other undesirable features which can be introduced during the forming, calendering, and rewinding processes. Also there is a recognized 2 problem with electrically conductive particulate contamination, which 20 reduces the electrical properties of the web produced.

*o 3 According to the present invention there is provided a method producing a printed circuit board characterised by the steps of: providing at least aramid fibers, surfactant and water to form a foam slurry having a solids consistency of about 5-50%, producing a non-woven sheet or web comprising at least 10% by weight aramid fibers and the balance substantially electrically non-conductive fibers, filler and binder; calendering the sheet or web from step to form a layer; forming a pre-preg from the layer of step by impregnating the layer with resin or the like; 15 combining the layer from step with other substantially electrically non-conductive layers; providing electrically conductive circuit Selements between at least two layers or on at least one layer from step and 20 curing the pre-preg of steps to produce a printed circuit board.

0.

The method uses a foam process, such as described S: in co-pending US patent applications serial no. 08/923,900 filed September 4, 1997 and serial no. 08/923,250 filed 25 September 4, 1997 (the disclosures of which are hereby incorporated by reference herein).

The non-woven web or sheet may be made utilizing the foam process as described in the above-mentioned copending applications. The foam process is highly efficient in handling fibers like aramid fibers, allowing the formation of a much more uniform web, and allowing fiber blending to a much better extent than webs produced by the water laid process. Fiber blending may be particularly important in the production of printed circuit board layers containing aramid fibers because aramid fibers are much more expensive than glass fibers, /IA- and in order to reduce the cost of aramid non-woven webs \\melb.files\home$\Priyank\Keep\speci\28392-99.doc 13/03/02 4 or sheets, while still obtaining most of the advantageous properties thereof, glass fibers, and conventional nonconductive fillers (such as plastic or glass particles) can be incorporated in the foam and are uniformly distributed in the final web produced. Also by using the foam process the density of the aramid fiber-containing webs or sheets produced may be much more closely regulated than when the water laid process is utilized, and the entire formation process is less expensive and more energy efficient.

Utilizing the technique printed circuit boards, and layers for printed circuit boards, may be produced containing at least 10% aramid fiber, and preferably at least 30% aramid fiber, and more preferably at least 15 aramid fiber, on up to substantially 100% aramid fiber.

While substantially 100% aramid fiber boards and layers may be produced according to the invention, because of the expense of the aramid fibers typically there will be at least some other non-conductive fibers, like glass fibers, 20 or non-conductive fillers, and aramid pulp fibers may also be blended with conventional para aramid (straight) fibers.

A non-woven sheet or web is provided comprising at least 10% (and preferably at least 30% more preferably S" 25 at least 50%, up to substantially 100%, e.g. at least aramid fibers) by weight aramid fibers, and any balance substantially electrically non-conductive fibers or fillers or binder, made by the foam process. The sheet or web may comprise at least about 10% straight aramid fibers, or glass fibers, in addition to at least about pulp aramid fibers. The web or sheet is typically compressed (as by using conventional calendering rolls) so that it has a density of between about 1 2 grams per cubic centimeter, and a basis weight of between 20-120 grams per square meter. The web or sheet may also comprise at least about 1% by weight of a substantially electrically non-conductive binder, such as polyvinyl \\melb_files\home$\Priyanka\Keep\speci\28392-99.doc 13/03/02 5 alcohol (PVA), epoxy resin, other conventional substantially non-conductive binders, or combinations thereof.

A printed circuit board is provided comprising the following components: A plurality of substantially electrically non-conductive substrate layers. At least one of the layers comprising, prior to pre-preg, a nonwoven layer comprising at least 10% by weight aramid pulp fibers. A pre-preg material, impregnating at least some of the layers. And, electrically conductive circuit elements provided on or between at least one of the substrate layers. Most printed circuit boards are made with between three to six layers, although a significant number of boards are also made using seven to eight 15 layers, and there are also many boards made using nine or more layers. The pre-preg material is entirely conventional, and typically is epoxy resin, and the electrically conductive circuit elements are also completely convention (as is their positioning), typically 20 comprising copper strips, wires or deposits, or like physical structures of other conductive materials such as silver. The at least one layer containing the aramid pulp fibers is produced by the foam process, and may have at least about 50% by weight aramid pulp fibers prior to pre- S 25 preg. Each of the substrate layers may have a density of about 1-2 per cubic centimeter prior to pre-preg, and the *board typically further comprises a plurality of 0* electronic components (such as computer chips, diodes, resistors, etc) connected to the board substrate, and to the electrically conductive circuit elements using entirely conventional techniques.

A method of producing a printed circuit board is also provided comprising the following steps: Using the foam process producing a non-woven sheet or web comprising at least 10% by weight aramid fibers and the balance substantially electrically nonconductive fibers, filler and binder.

\\melbfiles\homeS\priyanka\Keep\speci\28392-99.doc 13/03/02 6 Calendering the sheet or web from step Forming a printed circuit board layer using the sheet or web from step Forming a pre-preg from the layer of step by impregnating the layer with resin or the like.

Combining the layer from step with other substantially electrically non-conductive layers.

Providing electrically conductive circuit elements on or between at least one of the layers from step and Curing the pre-preg of steps to produce a printed circuit board.

Step is conventional, and typically is accomplished utilizing calendering rollers. The layering 15 of the sheets or webs to produce the printed circuit board, of step and the pre-preg formation of step and combining a layer from step with other substantially electrically non-conductive layers as in step and providing the electrically conductive 20 circuit elements as recited in step as well as securing of step are also all conventional. Also o. there preferably are the further conventional steps of (h) S* mechanically acting on the board from step and (i) electrically and physically connecting electronic 25. components to the board from step and to the circuit elements.

Steps and are typically practiced to produce a sheet or web having a density of 1-2 grams per cubic centimeter, and step is typically practiced using at least about 30%, preferably at least about and perhaps at least 90%, by weight, aramid fibers, either all pulp, or all straight, or a combination of pulp and straight aramid fibers. For example step may be practiced using at least 10% by weight aramid pulp fibers, and at least 10% by weight glass fibers, straight aramid fibers, or a mixture of glass and straight aramid fibers.

\\.elbf iles\home\Priyanka\Keep\speci\28392-99 .doc 13/03102 7 BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic view illustrating an exemplary method according to the present invention, resulting in the production of a printed circuit board; and FIGURE 2 is an exploded schematic view of a circuit board according to the present invention without electronic components mounted thereon.

FIGURE 3 is a schematic illustration of a foamlaid process system for practicing a foam-laid process with which the present invention is desirably utilized.

DETAILED DESCRIPTION OF THE DRAWINGS 15 FIGURE 1 schematically illustrates a preferred method 10 of producing printed circuit boards, which have .at least one layer containing aramid fibers. The first step according to the invention is the production of a web or a sheet using the foam process, as illustrated 20 schematically at 11 in FIGURE 1. Aramid fibers from source 12, other fibers or fillers from source 112, surfactant and water from source 14, and the like are provided and the foam process is practiced preferably as described in US patent applications serial no. 08/923,900 25 filed September 4, 1997 and serial no. 08/923,250 filed September 4, 1997, or the prior art mentioned therein.

Typically the slurry has a consistency of at least about e.g. about 5-50%. Typically some binder will be added to the web, either prior to formation, as indicated schematically at 15 in FIGURE 1, and/or after formation, as indicated schematically at 16 in FIGURE 1. The binder may comprise at least about 1% by weight of a substantially electrically non-conductive binder, examples of known binders for that purpose being PVA, epoxy resin, and combinations thereof. By practicing the invention, however, because the web formation is much better (being more uniform, less directional, and having better strength \\relb-files\homeS\Priyanka\Keep\seci\28392-99 .doc 13/03/02 8 properties) than webs of identical composition made by the water laid process, less binder is necessary (whether added at 15, 16, or both places) than is conventional, e.g. at least 25% less binder is necessary, in fact often less than 5% After web or sheet formation, the web or sheet is dried as indicated schematically at 17 in FIGURE 1 using conventional drying equipment (such as a drying oven), and the web is calendered as indicated schematically at 18, e.g. using conventional calendering rolls. Typically steps 11, 15 and 16, 17 and 18 will take place at one location, and then the final web or sheet produced (if a web is produced it is wound using conventional techniques, and if sheets are produced they are typically stacked for 15 transport) is transported to another location where the other conventional steps for printed circuit board production take place.

S- The webs of sheets produced by steps 11 and through 18 typically have a density of between about 1-2 20 grams per cubic \\melb-files\homeS\Priyanka\Keep\speci\28392-99.doc 13/03/02 WO 9Y/49118 PCT/F199/00215 9 centimeter, and a basis weight of between about 20-120 grams per square meter.

The step schematically illustrated at 20 in FIGURE 1 is a pre-preg step, where the web or sheets from 18 are impregnated with epoxy resin from source 21 or the like, the impregnating resin being substantially electrically non-conductive. After pre-preg formation, the board is layered that is various layers are utilized (either the layers from step 18, or other layers produced by conventional techniques and of more conventional materials, such as glass fibers) are assembled together and circuit elements added, as schematically illustrated at 22. Circuit elements may be added in any conventional manner screen printing, cladding, mechanical laydown and attachment, etc.) Then the layered intermediate board, with circuit elements, is cured in a conventional manner as in a curing oven, as illustrated schematically at 23 in FIGURE 1.

After curing at 23, the intermediate board is acted on mechanically as illustrated schematically at 24 in FIGURE 1 as is conventional, e.g.

various holes being formed therein, shaping, shaving, texturing, enhancing exposure of circuit elements, or the like. Then the electronic components are added as schematically illustrated at 25 in FIGURE 1 to produce the final circuit board illustrated schematically at 26 in FIGURE 1. The electronic component addition step 25 is also conventional, various electronic elements that are to be utilized on the final board 26 being mechanically connected to the board and electrically connected to each other and/or circuit elements.

The board 26, being only very schematically illustrated in FIGURE 1, comprise the substrate 27 formed of multiple (typically between three and nine, but most typically between three and six) layers, illustrated schematically at 28 in FIGURE 1. According to the invention each of the wo 99/49118 PCT/F199/00215 layers 28 may comprise at least 10% by weight (prior to pre-preg) aramid fibers (either straight or pulp), and preferably contain at least 30% aramid fibers, more preferably at least 50% fibers, and even more preferably at least about 90% aramid fibers (up to substantially 100% aramid fibers).

However the layers 28 may have different percentages and types of aramid fibers therein, or some of the layers 28 may be conventional glass layers, or have other conventional constructions. However about 55-90% (by weight) or more aramid pulp fibers may be used 60-90%).

The final circuit board 26 illustrated in FIGURE 1 also has electrically conductive circuit elements 29, which are strips, wires, or deposits of electrically conductive material, such as copper, silver, or other conventional conductive materials or blends thereof. The elements 29 connect electronic components together, and connect the board 26 to a power source, other boards, or other external components. FIGURE 1 schematically illustrates conventional chips 30 as electronic components, as well as diodes or resistors or capacitors 31, or the like. Any conventional electronic components can be utilized in the construction of the board 26 according to the invention.

The board 26 according to the invention will have less electrical conductivity than conventional glass boards, therefore can have closer circuit density and is less susceptible to high frequency energy corruption.

Also because of a better co-efficient of thermal expansion, the board 26 can be expected to have longer life than an otherwise conventional board, can be used in higher temperature environments, and is otherwise advantageous.

In the web formation step 11, the appropriate type and percentage of fibers will be added to get the desired results. Preferably the aramid fibers added at 12 are at least 10% by weight pulp aramid fibers, e.g. at WO 991719118 PCT/FI99/00215 11 least about 30%, more preferably at least about 60%, up to substantially 100%. Conventional straight aramid fibers may also be added instead of the pulp fibers at 12, or in addition to the aramid pulp fibers at 12, as indicated at line 13. For example at least about 10% straight aramid fibers may also be added, and/or at least about 10% glass fibers.

Conventional fillers may also be utilized, as long as they are substantially electrically non-conductive, such as known glass and plastic particulate fillers.

FIGURE 2 schematically illustrates the board 26 before the mechanical activity at 24 and the electrical component addition at 25 from FIGURE 1, showing the components in an exploded view. Each of.the layers 28 are preferably produced by the steps 11 and 15 through 18 (as well as by pre-preg at 20) and can have varying fiber compositions, but preferably each have at least 10% aramid fibers. The electrically conductive circuit elements are shown disposed between the layers 28, and may overlap the edges of the layers 28 for connection to external components, or to facilitate connection to components that will ultimately be mounted on the substrate 27. As is conventional, one or more of the layers 28 may be etched, mechanically sanded or handled, or otherwise acted upon to expose circuit elements 29 where necessary or desirable.

When aramid pulp fibers are utilized according to the present invention, preferably they are those sold under the trademark COSMORON, from Kolon Industries, Inc. of Seoul, South Korea. While fiber length is not necessarily significant, normally available fiber lengths that can be used are .5-2.5 mm. The pulp fibers illustrated highly schematically at 32 in FIGURE 2 for the upper layer 28 are curly, so that they readily fill the area between other fibers and/or fillers, and/or provide many cross-over points for contact between themselves or other 12 fibers, increasing web or sheet strength, and allowing reduction by about 25% or more) of the amount of binder used for web formation.

It will thus be seen that according to the present invention a highly advantageous non-woven sheet or web for use in a printed circuit board construction, a printed circuit board, and a method of producing a printed circuit board, have been provided. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the 15 appended claims so as to encompass all equivalent structures and methods.

Figure 3 schematically illustrates the foam-laid S process system of US 08/923,250 for practicing a foam laid process with which the invention is desirably utilized.

20 The system 11 includes a mixing tank or pulper 33 having a fiber input 12, a surfactant input 14, and an input 34 for other additives, such as pH adjustment chemicals like calcium carbonate or acids, stabilizers, etc. The :particular nature of the fibers, surfactant, and additives is not critical and they may be varied widely depending upon the exact details of the product being produced (including its basis weight). It is desirable to use a e5 surfactant that can be fairly readily washed out since a surfactant reduces the surface tension of the final web if it is still present, and that is an undesirable feature for some products. The exact surfactant used, from the thousands that are commercially available, is not part of the present invention.

The tank 33 is per se entirely conventional, being the same type of tank that is used as a pulper in conventional paper making systems using the water-laid process. The only differences are that the side walls of \\melb.files\home\Priyanka\Keep\speci\28392-99.doc 13/03/02 13 the mixer/pulper 33 are extended upwardly about three times the height in the water-laid process since the foam has a density about a third that of water. The rpm and blade configuration of the conventional mechanical mixer in the tank 33 is varied depending upon the particular properties of the product being produced, but is not particularly critical, and a wide variety of different components and variables may be employed. Brakers may also be provided on the walls. There is a vortex at the bottom of the tank 33 from which the foam drains, but the vortex is not visible once start up occurs because the tank 33 is filled with foam and fiber.

The tank 33 also preferably includes therein a large number of pH meters 35 for measuring the pH at a 15 number of different points. pH affects surface tension, and thus desirably is accurately determined. The pH meters are calibrated daily.

At initial start up, water is added with the fiber from line 12, the surfactant from line 14, and other 20 additives in line 34; however, once operation commences no additional water is necessary and there is merely foam maintenance in the tank 33, not merely foam generation.

The foam exits the bottom of the tank 33, in a C vortex, into line 36 under the influence of the pump 37.

The pump 37, like all other pumps in the system 11, '006 preferably is a degassing centrifugal pump. The foam discharged from the pump 17 passes in line 38 to further components.

FIG. 3 illustrates an optional holding tank 39 in dotted line. The holding tank 39 is not necessary but may be desirable to ensure a relatively even distribution of the fiber in the foam in case there is some variation that is introduced into the mixer 33. That is, the holding tank 39 (which is small, typically only on the order of five cubic meters) acts more or less like a "surge tank" for evening out fiber distribution. Because the total Stime from mixer 33 to the headbox (50) is typically only \\melb-files\homeS\Priyanka\Keep\speci\28392-99.doc 13/03/02 14 about 45 seconds in the practice of the process, the holding tank 39 if used provides time for variations to even out.

When the holding tank 39 is used foam is fed from the pump 37 in line 40 to the top of the tank 39, and exits the bottom of the tank in line 41 under the influence of centrifugal pump 42, then leading to line 38.

That is, when the holding tank 39 is used the pump 37 is not directly connected to the line 38, but only through the tank 39.

The line 38 extends to the wire pit 43. The wire pit 43 is per se a conventional tank, again the same as in the conventional water-laid paper process system, but with higher side walls. It is important to make the wire pit 15 43 so that there are no dead corners and therefore the tank 43 should not be too large. The conventional structure 44 which allows the foam and fiber mixture in line 38 to be introduced into the pump 45 (which is *operatively connected adjacent the bottom of the wire pit 20 43) is described further with respect to FIG. 2 of US 08/923,250. In any event, the pump 45 pumps the foam/fiber mixture in line 38, introduced by mechanism 44, and additional foam from the wire pit 43, into the line 46. Because a fairly large amount of foam is drawn into 25 the pump 45 from the wire pit 43, typically the 0*4 consistency in line 46 is significantly less than that in line 38. The consistency in line 38 is typically between S: 2-5% solids (fibers), while that in line 46 is typically between about 0.5 although the consistency in each case may be as high as about 12%.

In the wire pit 43, there is no significant separation of the foam into layers of different density.

While there is a minimal increase toward the bottom, the degree of increase is small and does not affect operation of the system.

From the line 46 the foam/fiber passes to the manifold 47 which has foam generating nozzles 48 \\melb-files\home$\Priyanka\Keep\speci\28392-99doc 13/03/02 15 associated therewith. Preferably the nozzles 48 which are conventional foam generating nozzles (which agitate the foam greatly)--are mounted on the manifold 47, and a large number of the nozzles 48 are mounted on the manifold 47. Extending from each nozzle 48 is a conduit 49 which leads to the headbox 50, through which one or more conventional paper making wires (foraminous elements) pass.

The headbox 50 has a plurality of suction boxes (typically about three to five) 51 which withdraw foam from the opposite side of the wire (foraminous element) from the introduction of the foam/fiber mixture, and a final separation box 52 is at the discharge end of the formed web from the headbox 50. The number of suction boxes 51 provided in the suction table to control drainage are increased for denser products, or for higher speed operation. The formed web, which typically has a solids consistency of about 40-60% about is preferably subjected to a washing action as indicated 20 schematically by wash stage 53 in FIG. 3. The wash stage 53 is to remove the surfactant. The high consistency of the web means that a minimum amount of drying equipment need to be utilized.

The web passes from the washer 53 past one or 25 more optional coaters 54, to the conventional drying station 55. In the conventional drying station 55 when synthetic sheath/core fibers (such as Cellbond) are part of the web, the dryer 53 is operated to raise the web above the melting point of the sheath material (typically o 30 polypropylene) while the core material (typically PET) does not melt. For example where a Cellbond fiber is used in the web, the temperature in the dryer is typically about 130 degree C, or slightly more, which is at or slightly above the melting temperature of the sheath fiber, but well below the approximately 250 degree C melting temperature of the core fiber. In that way a 3 binding action is provided by the sheath material, but the \\melb-files\homeS\PriyankaJeep\speci\28392-99.doc 13/03/02 16 integrity of the product (provided by the core fiber) is not compromised.

While it is not always necessary, the process contemplates the addition of pure foam to or immediately adjacent the headbox 50 for a number of advantageous purposes. As seen in FIG. 3, the centrifugal pump 57 draws foam from the wire pit 43 into line 56. The foam in line 56 is pumped to a header 58 which then distributes the foam to a large number of different conduits 59, toward the headbox 50. The foam may be introduced as indicated by line 60 directly underneath the roof of the headbox 50 (where it is an incline wire headbox), and/or via conduits 61 to the lines 49 (or nozzles 48) for introducing foam/fiber mixture into the headbox The suction boxes 51 discharge the foam withdrawn from the headbox 50 in lines 62 into the wire pit 43.

Typically no pumps are necessary, or used, for that purpose.

A significant amount of the foam in the wire pit 20 43 is recirculated to the pulper 33. The foam is withdrawn in line 63 by centrifugal pump 64, and then passes in conduit 63 through the conventional in-line density measurement device 66 for introduction as indicated schematically at 67-- back into the tank 33. In addition to providing density measurement for the foam in Sline 63 at 66, as schematically illustrated in FIG. 3 one :or more density measuring units (such as denseometers) may be mounted directly in the tank 33.

In addition to foam recycle, there is also 30 typically water recycle. The foam withdrawn from the last suction box 52 passes via line 68 to a conventional separator 69, such as a cyclone separator. The separator 69 e.g. by vortex action--separates air and water from the foam introduced into the separator 69 to produce water with very little air in it. The separated water passes in line 70 from the bottom of the separator 69 to the water tank 71. The air separated by the separator 69 passes in \\melbfiles\howe$\Priyaka\Kiep\ seci\283.92-9.doc 13/03/02 17 line 72, with the assistance of the fan 73, from the top of the separator 69 and is discharged to atmosphere or used in a combustion process or otherwise treated.

A liquid level 74 is established in the water tank 71, with some liquid overflowing to sewer or treatment, as indicated schematically at 75. Water is also taken from below the level 74 in the tank 71 via line 76, and under the influence of centrifugal pump 77 is pumped in line 76 through a conventional flow meter 78 (which controls the pump 77). Ultimately, the recycled water is introduced--as indicated schematically at 79 in FIG. 3--to the top of the mixer 33.

Typical flow rates are 4000 liters per minute foam/fiber in line 18, 40,000 liters per minute foam/fiber in line 46, 3500 liters per minute foam in line 63, and 500 liters per minute foam in line 68.

The system 11 also includes a number of control components. A preferred example of various alternatives for controlling the operation of the system comprises 20 first fuzzy controller, 80 controls the level of foam in the tank 33. A second fuzzy controller 81 controls the addition of surfactant in line 14. A third fuzzy controller 82 controls web formation in the headbox area. A fourth fuzzy controller 83 is used with the washer 53. A fifth fuzzy controller 84 controls the pH meters 35, and possibly controls addition of other S..additives in line 34 to the mixer 33. Fuzzy control is also used for surfactant and formation control. A multivariable control system, and a Neuronet control system, 30 also are preferably provided overlaying the other controls. The multi-variable control also is used for controlling the efflux ratio at web formation. The variables can be changed depending upon their effect on desired process regulation, and end result.

In order to facilitate control of the various components, typically a scale 85 is associated with the 7 fiber introduction 12 in order to accurately determine the \\melbfiles\home\Priyanka\Keep\speci\28392-99.doc 13/03/.02 18 amount of fiber being added, per unit time. A valve 86 in line 14 may be provided for controlling the introduction of surfactant, as well as a scale 87. A value 88 may also be provided in the line 34.

In the system 11 essentially no valves are provided for intentionally contacting the foam at any point during its handling, with the possible exception of level control valves provided in lines 62.

Also, during the entire practice of the process of the system of FIG. 3 the foam is kept under relatively high shear conditions. Since the higher the shear the lower the viscosity, it is desirable to maintain the foam at high shear. The foam/fiber mixture acts as a pseudoplastic, exhibiting non-Newtonian behaviour.

The use of the foam-laid process has a number of advantages compared to the water-laid process particularly for highly absorbent products. In addition to the reduced dryer capacity because of the high consistency of the web, the foam process allows even distribution of virtually any 20 type of fiber or particle (without excessive "sinking" of high density particles while low density particles do "sink" somewhat--they do not sink at all in water) into the slurry (and ultimately the web) as long as the fibers or particles have a specific gravity between about 0.15 13. The foam process also allows the production of a wide variety of basis weight webs, a product with increased uniformity and higher bulk compared to water-laid process products, and a very high level of uniformity. A plurality of headboxes may be provided in sequence, or two 30 )or more) strata may be made at the same time within a o headbox with a double wire, etc., and/or the simple 0 coaters 54 may be utilized to provide additional layers with great simplicity (like coating).

S\\melb_files\home$\Priyanka\Keep\speci\28392-99.doc 13/03/02 'T 19 It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or in any other country.

00 *0 e g* o \\melb_files\homeS\Priyanka\Keep\speci\28392-99.doc 13/03/02

Claims (7)

1. A method producing a printed circuit board characterised by the steps of: providing at least aramid fibers, sufactant and water to form a foam slurry having a solids consistency of about 5-50%, producing a non-woven sheet or web comprising at least 10% by weight aramid fibers and the balance substantially electrically non-conductive fibers, filler and binder; calendering the sheet or web from step to form a layer; forming a pre-preg from the layer of step by impregnating the layer with resin or the like; combining the layer from step with other substantially electrically non-conductive layers; providing electrically conductive circuit elements between at least two layers or on at least one 20 layer from step and curing the pre-preg of steps to produce a printed circuit board.

2. A method as recited in claim 1 further characterised by the further steps of mechanically acting on the board from step and electrically and physically connecting electronic components to the board from step and to the circuit elements. 30 3. A method as recited in claim 1 further Scharacterised in that step is practiced to produce a sheet or web having a density of 0.1 -0.2 g/cm 3

4. A method as recited in claim 3 further characterised in that step is practiced using at least about 60% by weight aramid pulp fibers. \\melbfils\home$\Priyanka\Keep\speci\28392-99.doc 13/03/02 21 A method as recited in claim 3 further characterised in that step is practiced using at least by weight aramid pulp fibers, and at least about by weight glass fibers, straight aramid fibers, or a mixture of glass and straight aramid fibers.

6. A method as recited in claim 1 further characterised in that step is practiced using at least about 60% by weight aramid pulp fibers.

7. A method as recited in claim 1 further characterised in using, in step at least about 1% by weight of a substantially electrically non-conductive binder.

8. A method as recited in claim 1 further characterised in that said binder comprises PVA or epoxy resin or combinations thereof. 20 9. A method as recited in claim 1, further characterised in that, after step said layer has a basis weight of between 20 120 g/m 2 e

10. A method as claimed in any one of claims 1 to 9 and substantially as herein described with reference to accompanying drawings. Dated this 1 3 th of March 2002 SAHLSTROM GLASSFIBRE OY 30 By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia H:\Priyanka\Keep\speci\28392-99.doc 13/03/02