This invention relates to ladder copolymers. More specifically, the invention is concerned with a novel high temperature ladder copolymer and to a method for the preparation thereof. The ladder copolymers of the present invention, which constitute novel compositions of matter, may be used to coat and/or impregnate a substrate which is thereafter cured and utilized in circuit board laminates and dielectric coatings, the use thereof being attributable to the desirable characteristics which are possessed by these polymeric compositions of matter. The particular characteristics of the polymer dielectric and reinforcing components which go to make up the circuit boards contribute to the efficiency and stability of the final electronic equipment in which the circuit boards are used. For example, a lowering of the dielectric constant in the polymer matrix reduces the signal delay time or "crosstalk" and line capacitance. This results in faster PWB circuitry and, in addition, provides the potential to increase the number of functions per board. The polymeric matrix of the present invention possesses a lower dielectric constant than that which is possessed by thermosetting polyimide or epoxy matrices which are used as the standards by the industry for electrical laminates.

Another desirable characteristic of a polymer matrix for use in circuit boards is that the coefficient of thermal expansion should be relatively low in order to avoid a mismatch of thermal expansions with the electronic components and the fiberglass reinforcement with which the polymeric matrix is composited. It has been found that the coefficient of expansion of the novel ladder copolymer of the present invention is comparable to a polyimide matrix. Furthermore, the thermal stability of the polymer matrix must be relatively high ' in nature inasmuch as the matrix must possess the ability to withstand soldering temperatures without melting or degrading. A desirable characteristic of the ladder copolymer of the present invention is that the thermal stability of the polymer is comparable to a polyimide matrix. The ladder copolymer of the present invention, which results from the reaction between the poly(vinyl benzyl ether) of a polyphenol and a bismaleimide, results in a high degree of cross-linking which renders the polymer resistant to dissolution or decomposition due to the action of a solvent on the board. This is of particular importance when a solvent such as methylene chloride is employed in the subsequent vapor degreasing process during the soldering stage in copper clad or multilayer boards.

An object of this invention is to prepare novel ladder copolymers which are useful as a component in circuit board laminates.

In one aspect, an embodiment of this invention resides in a ladder copolymer formed by the reaction between a poly(vinyl benzyl ether) of a polyphenol and a bismaleimide or a chain extended bismaleimide.

These ladder copolymers will comprise a mixture of a poly(vinyl benzyl ether) of a polyphenol and a bismaleimide or a chain extended bismaleimide, examples of these compounds being hereinafter set forth in greater detail. The alternating ladder copolymers would result from a free radical reaction of maleimide groups or the chain extended bismaleimide with the styryl groups of the poly(vinyl benzyl ether) of a polyphenol. By utilizing such a ladder copolymer, it is possible to obtain a solid composition of matter which is useful in electronic circuitry, said solid matrix possessing a relatively low dielectric constant which is of particular advantage for use in circuit boards.

The poly(vinyl benzyl ether) which forms one component of the ladder copolymer of the present invention will possess the generic structure:

in which X is selected from the group consisting of CH3C-CH3, SO2, 0, CH2-, and S radicals, R is selected from the group consisting of -CH ₂ -C ₆ H4-CH2-,(CH ₂ ) in which b ranges from 1 to about 6,-CH ₂ -CH=CH- CH2- and radicals, A is independently selected from the group consisting of hydrogen, chlorine, bromine, fluorine, alkyl, alkoxy and phenyl radicals, and n has an average value in the range of from 0 to about 20.

. The second component of the ladder copolymer of the present invention will comprise a bismaleimide or a chain extended bismaleimide which has the generic structure:

or

in which Ar is selected from the group consisting of C^H^,

CH^-CgH^-CI^-CgH^, -CgH^-O-CgH^-CgH^S-CgH^- and -CgH^-S-C^H^-S- gl _j .- radicals and n has an average value in the range of from 0 to about 20. A common material used to chain extend bismaleimide is methylene dianiline, i.e.,

NH ₂ -C ₆ H ₄ -CH2-C ₆ H ₄ -NH ₂ .

The desired and novel ladder copolymer of the present invention may be prepared in any suitable manner of operation which is known in the art. For example, one method of preparing the copolymer is to blend the poly(vinyl benzyl ether) of a polyphenol with a bismaleimide or a chain extended bismaleimide to form a homogenous melt, said blending being effected at a temperature which may range from about 100 to about 200°C. The homogenous melt is then poured into a mold and cured at an elevated temperature in the range of from about 150° to about 200°C for a period of time which may range from about 0.1 to about 1.0 hours and thereafter may be post-cured at a temperature ranging from about 200° to about 260°C for a period of time which may range from about 1 to about 12 hours in duration.

Alternatively, another method of preparing the desired copolymer of the present invention is to dissolve the poly(vinyl benzyl ether) of a polyphenol and the bismaleimide or chain extended bismaleimide in an appropriate solvent such as dimethyl formamide, N-methyl pyrrolidinone, dimethyl acetamide, acetone, benzene, toluene, etc., in amounts so that the resulting solution will contain from about 30 to about 70% by weight of the components of the copolymer. The resulting solution may then be coated and/or impregnated on an appropriate substrate such as various resins, glass cloth, etc., and treated at an elevated temperature of from about 150° to about 180°C for a relatively short period of time, which may range from about 1 to about 10 minutes, to obtain a prepreg. The resulting prepreg may then be stacked by pressing a predetermined number of sheets of the prepreg and pressing the stack in a heated press to form a desired laminate. The pressing of the prepreg may be effected for a period of time ranging from about 1 to about 4 hours in duration at an elevated temperature ranging from about 150 to about 190°C, at a pressure in the range of from about 200 to about 1,000 pounds per square inch gauge (1379 to 6895 k Pa). Following the pressing, the laminate is then subjected to a post-cure which is effected at a temperature in the range of from about 200° to about 260°C for a period of time which may range from about 3 to about 6 hours in duration.

It is also contemplated within the scope of this invention that the ladder copolymers may be prepared in a continuous manner of operation. When this type of operation is employed, the predetermined amounts of the poly(vinyl benzyl ether) of a polyphenol and the bismaleimide or chain extended bismaleimide which have been dissolved in an appropriate solvent of the type hereinbefore set forth in greater detail, are continuously charged to a zone which is maintained at the proper operating conditions of temperature and pressure.

Those skilled in the art will recognize that a continuous reactant charge is necessary, with amounts depending upon the individual components, to provide a high yield of product which contains the desired percentage of each component in the finished ladder copolymer. After passage through this zone, the mixture resulting therefrom may be continuously withdrawn and utilized to coat and/or impregnate a substrate or reinforcement. The coated or impregnated substrate or reinforcement may thereafter be continuously charged to a curing zone where it is subjected to a partial cure by passage through this zone which is maintained at varying operating temperatures for a predetermined period of time. After passage through the zone, the resulting prepreg material is continuously withdrawn and passed to storage. The prepreg can then be layed up as sheets with or without a metal such as copper foil as an electrical or thermal conductor, and pressed in a predetermined number of sheets to form the desired laminate or circuit board matrix.

It is also within the scope of this invention that the circuit board precursor may be prepared by a solventless continuation lamination process. When this type of process is employed, the solid resin blend comprising at least one poly(vinyl benzyl ether) of a polyphenol and a bismaleimide or chain extended bismaleimide is used to impregnate a reinforcement such as glass cloth which is continuously fed through an appropriate apparatus. The reinforcement such as the glass cloth may pass through this apparatus in a single ply or, if so desired, in a predetermined number of plies, one criteria being that each ply is impregnated with the resin blend. As an alternative, it is also contemplated that, if the laminate is to be used as a circuit board, one or both sides of the laminate may be covered with a metallic coating such as copper foil. The laminate is then passed through the apparatus under predetermined conditions of temperature and pressure so as to provide a finished and cured laminate which emerges from the apparatus. This metal-covered laminate or uncovered laminate may then be cut into desired sizes and utilized, as hereinbefore set forth, as a circuit board in various electric or electronic devices.

While the above discussion has been limited to the copolymerization of a poly(vinyl benzyl ether) of a polyphenol and a bismaleimide or chain extended bismaleimide, it is also contemplated within the scope of this invention that the desired final polymer which possesses a ladder configuration may be formed from a combination of more than one poly(vinyl benzyl ether) of a polyphenol. For example, the desired polymeric composition may be prepared by admixing styrene terminated Bisphenol A (i.e. a vinyl benzyl ether of Bisphenol A) and styrene terminated tetrabromosubstituted-Bisphenol

A (i.e. the same compound in which each aromatic ring contains two bromine atoms) with a bismaleimide to obtain a laminate which will exhibit improved properties and characteristics when compared to other laminates prepared from other resins, such as epoxy resins or blends of epoxy resins and bismaleimide. The presence of the ladder copolymer resin blend in substrates or reinforcements will attempt a desired property enhancement to the substrate or reinforcement in that the laminate will meet certain flammability requirements such as, U.L. 94 flammability tests, the laminate having a value of 0. This is in contrast to other resins such as polyimide resins which cannot meet this standard. EXAMPLE I:

In this example, 1 gram of styrene terminated Bisphenol A and one gram of a chain extended bismaleimide sold in the trade as Kerimid 601 by the Rhone-Poulenc Company, were blended at a temperature of 150°C to form a homogenous melt. The melt was then poured into an aluminum mold and cured at a temperature of 200°C for a period of 30 minutes. Following this the solid casting was then post-cured at a temperature of 230°C for a period of 3 hours. The post-cured resin which was recovered possessed the following properties:

(Thermal Expansivity)

3 . 05

(Dielectric Constant)

Tan δ 0.001 (Dissipation Factor)

Instead of the chain extended bismaleimide (Kerimid 601), a bismaleimide such as _u 0

may also be used. D- EXAMPLE II:

A laminate was prepared by forming a copolymer comprising a mixture of 25% styrene terminated bisphenol A, 25% of styrene terminated tetrabromo-substituted-Bisphenol A, and 50% of a chain extended bismaleimide dissolved in a solvent comprising dimethyl formamide. The resulting solution was coated on a glass cloth in a treater at a temperature of 166°C for a period of 3.5 minutes to obtain a prepreg which contained 50% by weight of the copolymer. The prepreg was then stacked and pressed at a temperature of 170°C for a period of 2.5 hours at a pressure of 800 pounds per square inch gauge (5516 k Pa). At the end of this time, the laminate was then post-cured at a temperature of 220°C for a period of 3 hours to obtain the desired laminate. The laminate was then analyzed to determine the various characteristics or properties of the laminate. These properties were contrasted in Table 1 below with the properties possessed by other laminates which would not contain the ladder copolymer of the present invention.

Other laminates which were used in comparison were prepared in a similar manner. One laminate comprised a glass cloth reinforced with an epoxy resin, while the second laminate comprised a glass cloth impregnated with Kerimid 601. For purposes of comparison, the laminate containing the

copolymer of the present invention was labeled "A", the glass cloth impregnated with an epoxy resin was labeled "B" and the glass cloth impregnated with Kerimid

601 was labeled "C".

TABLE 1

PROPERTIES B

(Example II) (Epoxy) (Kerimid 601) Dielectric Constant 3.8 4.4-4.5 4.2-4.3

Dissipation Factor 0.011 0.03 0.007

Solder Float (Sec) 301 125-200 300

Tσ by DSC ( ^ό C) 240 125-135 260-300

Methylene Chloride (% Gain) 0.05 1-3% 0.05

Sulfuric Acid

Etch

(% Loss) 0.0 1-1.5%

CTE Z-Axis 74 70-80 35-45

Flammability Reading Underwriters Laboratory Test 94 V0 V0 VI

Retained Resin, % 49 45-47 45-47

EXAMPLE III:

A laminate was prepared by forming a copolymer comprising a mixture of 30 wt.% of styrene terminated tetrabromo-substituted-Bisphenol A, and 70 wt.% of a bismaleimide sold as Kerimid 601 by Rhone Poulonc dissolved in a solvent comprising 60% toluene and 40% dimethyl formamide. The resulting solution was coated on a glass cloth in a treater at a temperature of

149°C for a period of 3 minutes to obtain a prepreg which contained 50% by weight of the copolymer. The prepreg was then stacked and pressed at a temperature of 177°C for a period of 1 hour at a pressure of 200 pounds per square inch gauge (1379 k Pa). At the end of this time, the laminate was then post-cured at a temperature of 220°C for a period of 3 hours to obtain the desired laminate. The laminate was then analyzed to determine the various characteristics or properties of the laminate. These properties are listed in Table 2 as "D" and can be compared with those in Table 1. It should be noted that the dielectric constant for the laminate appears relatively high, the value has been affected by the relatively smaller amount of resin in the laminate. The glass cloth contributes a higher dielectric constant which is counteracted by the lower dielectric constant characteristic of polymers of the invention. EXAMPLE IV: A laminate was prepared by forming a copolymer comprising a mixture of 50 wt.% of styrene terminated tetrabromo-substituted-Bisphenol A, and 50 wt.% of a bismaleimide which has been chain advanced with an aromatic polyamine sold as AD 94374 by Reichold Chemical dissolved in a solvent comprising dimethyl formamide. The resulting solution was coated on a glass cloth prepregged and cured in a manner similar to Example HI to produce a laminate. The laminate was then analyzed to determine the various characteristics or properties of the laminate. These properties are listed in Table 2 as "E" and can be contrasted in Table 1.

Table 2

PROPERTIES f30% STTBBPA) (50% STTBBPA) (70% K 601) (50% AD 94347)

Dielectric Constant 4.6 4.1

Dissipation Factor 0.014 0.001

Solder Float (Sec) N/A 135

Tg by DSC ( ^δ C) 249 272 Methylene Chloride (% Gain) 0.06 N/A

Sulfuric Acid Etch

(% Loss) 0.0 0.0

CTE Z-Axis 60 N/A Flammability

Reading

Underwriters

Laboratory

Test 94 V0 V0

Retained

Resin, % 35 47

It is evident from a comparison of the properties possessed by the various laminates that the laminate containing the ladder copolymer of the present invention possesses excellent properties with respect to glass transition temperature, thermal expansivity, dielectric constant, and dissipation factors when compared to conventional laminates. It is therefore possible, by utilizing the novel composites of the present invention as components in laminates which are used for circuit boards, to improve the electrical properties of the laminates without, at the same time, sacrificing the thermal properties of the laminates.