WO1998050218A1

WO1998050218A1 - Extruded core laminates for circuit boards

Info

Publication number: WO1998050218A1
Application number: PCT/US1998/008872
Authority: WO
Inventors: Fred J. Gefri; Frank Mares; Jeffrey D. Pratt; Larry D. Olson
Original assignee: Alliedsignal, Inc.
Priority date: 1997-05-02
Filing date: 1998-05-01
Publication date: 1998-11-12
Also published as: JP2000512232A; KR20010012177A; AU7276498A; CN1261844A

Abstract

A method for fabrication of a circuit board laminate (10) in a continuous process employs an extruded laminate core (12). Streams of a fibrous material, microspheres or fused silica, a catalyst and a thermosetting resin are added to an extruder and the core extruded as a film; outer layers (18, 19) can be applied to the extruded core.

Description

EXTRUDED CORE LAMINATES FOR CIRCUIT BOARDS

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority from provisional patent application Serial No. 60/045,387, filed May 2, 1997.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT Not applicable.

BACKGROUND OF THE INVENTION Circuit boards used in electronic circuitry typically comprise a multi-layer composite made of laminations comprising a thermosetting polymer and a suitable reinforcing material. Typically, the reinforcing material is a woven glass material, such as fiberglass, and the thermosetting polymer is an epoxy resin. One limitation that must be considered in the manufacture of multi-layer composites is reduced dimensional stability of the laminates, particularly in response to temperature. Thermocycling during circuit board processing can cause the laminates to expand and contract, which can result in a loss of the intended adhesion or interconnections between the inner layers. Because effective functioning of electronic equipment depends on proper registration of these interconnections, a failure in interconnect layers is cause for rejection of a multilayer circuit board. The inherent costs associated with producing laminate board, together with the necessity of having to reject a composite that fails to meet the requisite high quality assurance standards, makes the manufacture of prior art laminate relatively expensive for an upstream component part .

U.S. Patent No. 5,273,816 discloses a laminate with improved dimensional stability, in which the laminate is prepared by impregnating reinforcing fibers, such as a glass cloth, with an epoxy resin of a relatively high molecular weight, i.e., in excess of 850, and pressing the material.

Although the laminate has improved dimensional stability, the relatively high cost of production of the laminate reduces profitability. The use of organic solvents in the manufacture of the laminate of U.S. Patent No. 5,273,816 and of other laminates raises safety and environmental concerns.

What is needed in the art is a method for producing a high quality extrudable laminate for use in printed circuit boards with low dielectric constant (D_κ) and dissipation factor (D_F) and with good dimensional stability at a lower cost.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide an extruded core laminate having good dimensional stability together with improved dielectric properties. It is a further object of this invention to provide a method for producing an extruded core laminate having good dimensional stability at a lower cost than laminates currently available. The present invention eliminates the use of organic solvents in the manufacture of the laminate .

This invention relates to a novel continuous process for fabrication of an extruded core laminate. Optionally, the extruded core laminate may be laminated in a continuous process with copper on one or both sides, with epoxy prepregs on one or both sides, or with prepreg on one side and copper on the other side . The extruded core laminate of this invention comprises chopped glass, fused silica or hollow microspheres as the substrate. The inclusion of this material has the advantage of allowing selection of a desired dielectric constant and dissipation factor of the laminate by varying the relative concentrations of the materials in the laminate. The use of inexpensive chopped glass, fused silica and microspheres as raw materials in the manufacture of laminates reduces the cost of manufacture.

The laminate extruder is a co-rotating twin screw extruder that features self-cleaning screw elements, an optimized feed arrangement, and a die design that allows uniform flow and orients the glass fibers in a manner that contributes to the dimensional stability and flex strength of the laminate.

Another aspect of this method for producing extruded core laminate is that the extruded core can be fed directly from the extruder to the b- staging oven by means of calendar rolls placed between the extruder and the b- staging oven. In addition, the core can be co-fed into the b-staging oven with prepreg and/or copper and laminated in the oven.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Figure 1 is a cross-sectional illustration of a laminate constructed in accordance with the present invention.

Figure 2 is a schematic flow illustration of the overall process design for the construction of the extruded core laminate of the present invention. Figure 3 is a side plan elevation of the twin screw extruder used in the process of Figure 1.

Figure 4A is a top plan view of the extruder die used in the process of Figure 1.

Figure 4B is a side view of the extruder die used in the process of Figure 1.

Figure 5 is a graphical representation of the viscosity of the extruded laminate as a function of time.

Figure 6 illustrates certain details of the b-stage oven constructed in accordance with the present invention. Figure 7 is a cross-sectional view of the belt used in the oven of Figure 5.

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an extruded core laminate board for use in electronic packaging connection materials such as circuit boards or chip carriers. The extruded core laminate board is formed of an extruded core onto which are laminated on either side either with the circuit board laminate material, known as prepreg, or with copper layers, or both, or a combination of the two. Shown in Figure 1 is a circuit board laminate 10 made in accordance with the present invention. The laminate 10 is made up of a laminate core 12 to which are adhered one to four additional laminate layers. In Figure 1, the maximum number of four layers are shown, the inner two layers 14 and 16 being thin laminations of partial cured woven fiber and epoxy based circuit board laminates known in the trade as prepregs. The outer layers 18 and 19 are layers of copper foil.

The circuit board laminate 10 differs from prior art circuit board laminates principally because of the make up of the laminate core 12. The laminate core 12 is a non-woven, extruded sheet of material rather than the usual woven core. The core is formed from four principal ingredients, a fibrous material component, a thermosetting resin and a catalyst, and may include other materials to improved dielectric performance such as microspheres or fused silica. The fibers can be any fibrous material that is durable and that adds strength and stability, such as cellulose fibers from shredded paper, but is preferably a shredded fiberglass. The optional microspheres are hollow air-filled spheres which reduce the weight and increase the dielectric properties of the core. The preferred microspheres are in the range of 1 micron to 100 microns in size and are also formed of hollow glass. The fused silica is also in the size range of about 1 micron to 100 microns. The thermosetting resin can be any of the thermosetting resins used in circuit board products and is typically an epoxy resin. The catalyst is a catalyst selected for use with the selected thermosetting resin to catalyze its curing, although it is envisioned also that resins may be used which are auto catalytic and then do not require a catalyst. The use of these materials in the no-woven extruded core 12 permits the finished laminate material to have a dielectric constant (D_κ) in the range of about 2.5 to 3.5 and a dissipation factor (D_F) in the range of .001 to .008. In relative amounts, the contribution of the four components to the volume of the extruded core are about in the following proportions. Fibrous material 10% to 50%

Fused silica 0% to 40%

Microspheres 0% to 40%

Thermosetting resin 25% to 60% Catalyst 0% to 5% Other materials with good dielectric properties may be substituted or added, such as, for example, quartz crystals.

Shown in Figure 2 is a schematic illustration of the overall process which is used to produce the extruded core laminate board described herein. It is envisioned that other variations may be used for the outer layers 18 and 19 other than conventional copper foils. Any other metal conductive layers may also be used. Conductive layers can be applied to the extruded core after fabrication by sputtering or vacuum deposition. In Figure 2, there is located an extruder 22. Feeding into the extruder 22 are suitable supply reservoirs for the four basic ingredients of the extruded core of the board. Shown at 24 is the reservoir for the fibrous material such as glass fiber. Shown at 26 is the reservoir for microspheres. Shown at 28 is the feed reservoir for catalyst. Shown at 30 is the feed reservoir for the thermosetting resin. The outputs from each of the feed reservoirs 24, 26, 28, and 30 are each fed into the extruder 22. At the output of the extruder is the extruder die 32 from which the output of the extruder issues. The output of the extruder is an extruded web of laminate core material, indicated at 34. That web is then passed into a staging zone 36. In the staging zone 36 are up to two feed rolls 38 of prepreg sheet material and up to two feed rolls 40 of copper foil sheet material. The appropriate sheet materials to be laminated to the core, either or both of prepreg or copper foil, is rolled off the appropriate feed rolls and pressed adjacent to one or both surfaces of the extruded laminate core by a pair of rollers 42. It is to be understood that on any given application of the use of the laminate core, there may be from zero to two layers of prepreg and zero to two layers of copper foil, with the selection of the laminates being as needed for a particular application.

The laminate now consisting of the extruded laminate core, plus whichever layers of prepreg and copper have been applied, to it then passes through a b-stage curing oven 44. When the laminate material exits from the oven it is cut by shears at 46 and then placed in a press 48 for final pressing and curing. These two stages are well known in the art and will not be described in greater detail here.

Shown in Figure 3 are some details of one example of the extruder 12. The extruder 12 consists of twin screws 52 and 54 each of which is made up of a series of matching segments labeled 56, 58, 60, 62, 64, 66, 68, 70, and 72. In each of the segments of the screws the pitch and spacing of the two screws are matching. The screw threads are interleaved so that the screws are self-cleaning of each other. It is specifically envisioned that the first two component feed items to the extruded laminate material of the present invention be added at the end of the extruder screw farthest from the extrusion end, that would be in the left of Figure 3. The first material added is the fiber. The fiber is preferably a chopped fiberglass . Also added at the upstream end of the extruder screw are the microspheres from the microsphere feed reservoir 26. The microspheres lightened the overall weight of the extruded laminate and increase its dielectric constant. Further down the extruder screws 52 is the site of entry of the catalyst from the catalyst feed reservoir 28. The catalyst enters the extruder screw at one of the extruder screw elements 60, 62, 64, or 66. The catalyst is injected first from the catalyst feed reservoir 28 followed closely thereafter by the epoxy from the epoxy feed reservoir 30. The epoxy resin is injected about one inch downstream from the catalyst. By doing it in this manner, it is ensured that the fiber and the microspheres are well mixed before the epoxy and catalyst are added. This mixing is achieved in spite of the fact that the feed materials have different weights and solubilities. These differences are handled by relative placement of the input streams. In this way, a laminate material of homogenous character can be created.

The extruder screws 52 and 54 feed the combined material for the laminate to the extruder die 32, shown in greater detail in Figures 4A and 4B. The extruder die greater expands the material laterally and also thins it down to a thin sheet material. The input to the extruder die 74, which receives the material from the extruder screw. The massive material passes through the extruder die to the narrowed end 76 from which a sheet of raw uncured extruded laminate core issues. The extrusion temperature will vary with the epoxy resin.

The most desirable range is defined by dependence of the melt viscosity of the resin on temperature. Any resin which solidifies above room temperature can be utilized within the practice of the present invention. Similarly, the type and amount of catalyst as well as the extrusion temperature will define the processing window. For a preferred embodiment using XUR brominated epoxy (Dow) a plot of the processing window is shown in Figure 4, which indicates that the proper extrusion temperature is about ten minutes at 80°. The extrusion is therefore run at 60 to 90°C in the extruder and the die. The most desirable temperature range is 65 to 73° with the most desirable range being 68 to 70°C. The contact time in the extruder is four minutes and in the die two minutes. This utilizes a catalyst of 2 -ethyl, -methyl imidazole which is compatible with and soluble in the chosen epoxy resin. This calculation and graph is presented for exemplary purposes only, and it is well within the ability of those of skill in the art to determine the temperature window and processing time for laminates of this type. Once the extruded core 34 exits from the extrusion die it is taken up by the calendar rolls 42 with the other sheets to be added to the laminate. The thickness of the final partially cured core, referred to in the trade as a b-stage core, is controlled by screws at the entrance to the oven 48. Uncured laminates are referred to as a-stage while fully cured laminates are referred to as c-stage. In a manner well known to the art, biased rollers are placed at the entrance to the oven and by adjusting the bias of the bias rollers, the thickness of the entire b-stage laminate can be controlled. The oven is shown in Figure 6 which simply illustrates that rollers pass the laminate passed the heating section after which it passes a cooling section. The hot temperature section is controlled to approximately 175°C. The oven has a cooling or cold section as well. The cold stage is used for cooling the b-stage cores. The cooling section may or may not be needed for a particular application, but would be useful if the extruded core is to be maintained as an a-stage material.

Copper foil, prepreg layers, or both can be co-fed with the extruded core into the b-stage oven. In the hot section of the oven all the layers are pre-laminated together.

Examples Standard Procedure for Making b-Staged Cores.

A system found to be effective for the continuous production of a b-staged extruded core laminate comprises a Leistritz 34 mm co-rotating twin screw extruder with a screw elements configuration as shown in Figure 2, take-up rolls, and a b-staging oven. The twin screw extruder is equipped with the following components: 1) a loss weight feeder (Ktron, K 2LDS- T20) for metering the chopped glass; 2) a calibrated screw feeder (Ktron, T20-87-035-F1, with a separately designed feeder screw for delivering the microspheres; 3) an injection port connected to a pump and container for the catalyst, both housed within a temperature-controlled oven; 4) an injection port connected to a pump and container for the epoxy resin, both housed within a temperature-controlled oven, and a die for sheet casting. The raw materials used in the production of the extruded core laminate included chopped E-glass (3/16 inch long, Certainteed) , glass hollow microspheres (B38/4000, 3M) , epoxy resin (XUR 1544-53744-6, Dow), and a catalyst (2E4MI, Aldrich) . The catalyst and epoxy resins were placed in their respective containers and the ovens in which the containers were housed were heated to and maintained at 100°C. The catalyst reservoir was pressurized by nitrogen to 10 psig, and the epoxy reservoir was pressurized by nitrogen to 60 psig. The tubings from the pumps to the corresponding injection ports were heated to and maintained at 80°C. The temperature of the extruder and die were set to 70°C. Both feeders were set for 12 g/min. The extruder speed was set at 50 rpms .

Production runs were performed as follows. First, the epoxy pump was started at 5 rpm and slowly increased to 28 rpm. Next, the chopped glass and microsphere feeders were started. After the material flow from the extruder die stabilized, the extruded sheet was placed between the take-up rolls and the rolls closed. After stabilization of sheet thickness and speed, the catalyst pump was started. The amount of catalyst delivered was 1.2% (w/w) , based on the epoxy resin content. The output from the take-up rolls was then fed into the fa- staging oven. The contact time in the b-staging oven was five minutes. Curing of the b-staged cores was done in a press at 175°C and at either 500 or 1000 psig.

The fully cured cores made using the Dow resin had a Tg in the range of approximately 145 to 180°C, depending upon the amount of catalyst used, and a Dk in the range of about 2.5 to 3.5, depending on the ratio of microspheres to chopped glass. The extruded core laminates made in this fashion can be used alone or as a laminate core circuit board, if copper clad, or can be supplied as a laminate component to be incorporated in multiple laminate boards. The extruded core board is dimensionally stable and has a lower dielectric constant and lower cost than comparable thickness laminates made by layering conventional prepregs. It is to be understood that the present invention is not limited to the particular embodiments described here, but embraces all such modified forms thereof as come within the scope of the following claims .

Claims

CLAIMSWe claim:

1. A method of making partially cured circuit board laminates comprising the steps of adding into an extruder continuous streams of (1) a fibrous material, (2) an element selected from the group consisting of hollow microspheres of dielectric material and fused silica, (3) a catalyst and (3) a thermosetting resin; extruding from the extruder a extruded film of laminate core material; placing on at least one surface of the extruded laminate core material at least one layer selected from the group consisting of prepreg laminate layers and metal conductive layers to form an uncured extruded core circuit board laminate; and heating the laminated core under pressure to form an at least partially cured extruded core circuit board laminate material .

2. The method of Claim 1 wherein the chopped glass is chopped E-glass.

3. The method of Claim 1 wherein the microspheres are glass hollow microspheres.

4. The method of Claim 1 wherein both microspheres and fused silica are added in step (a) .

5. The method of Claim 1 wherein the epoxy resin is a brominated epoxy resin, and the catalyst is a catalyst that is miscible in the brominated epoxy resin.

6. The method of Claim 4, wherein the catalyst is added at from about 0 to 5% (v/v) of the resin volume.

7. A laminate for use in a multilayer circuit board comprising an extruded laminate core formed of a fibrous material and a thermosetting epoxy resin, the core also impregnated with a filler material selected form the group consisting of glass microspheres and fused silica to the range of 1% to 40% of the volume of the laminate core; and one to four additional laminated layers applied to at least one surface of the laminate core, the laminated layers selected from the group consisting of prepreg laminate layers and copper foil.

8. A laminate as claimed in Claim 7 wherein the fibrous material is fiber glass.

9. A laminate as claimed in Claim 7 wherein the glass microspheres are in the range of about 1 micron to 100 microns in size.

10. A laminate as claimed in Claim 7 wherein the laminate has a dielectric constant of between about 2.5 and 3.5.

11. A laminate as claimed in Claim 7 wherein the laminate has a dissipation factor of between about .001 and .008.