THERMOSETTING POLYMER SYSTEMS AND ELECTRONIC LAMINATES

Abstract: A resin system including at least one epoxy resin that is essentially free of nitrogen and with essentially no chain extension, and at least hardener that is mixture of at least one phenolic resin and a copolymer of an allyl functional material and maleic anhydride.

Description: TITLE: Thermosetting Polymer Systems and Electronic Laminates BACKGROUND OF THE INVENTION (1) Field of the Invention This invention concerns thermosetting polymers that, when cured, exhibit excellent electrical and thermal performance while absorbing very little moisture. This invention also concerns printed wiring board laminates made from thermosetting polymers. The laminates exhibit superior dielectric constants, dissipation factors, thermal performance, and reduced moisture absorption. (2) Description of the Art Printed wiring boards are seeing increased use as substrates in high frequency microprocessor chip packages and as high-density substrates in high frequency telecommunications. Both end-uses require dielectric materials with: low dissipation factors to avoid signal loss and to maintain signal integrity; low moisture absorption to form stable substrates not requiring bake-out; and high thermal resistance to withstand higher wiring temperatures. Board designers prefer the materials with low dissipation factors so that they can route lines at tighter spacing and to permit thinner laminates with closer inner layer distances. In addition, during the formation of higher density substrates, low moisture absorption is desirable to enhance the laminate stability and prevent mis-registration. Many printed wiring boards processing steps use aqueous baths, which can saturate a substrate causing resin swell. Drying substrates after aqueous exposure is a costly and lengthy process. Finally, where substrates are used in packaging and in some chip on board applications, higher thermal resistance permits wire bonding temperatures increase wiring through put. Circuit board laminate resin systems are well known in the prior art. U.S. Patent No. 3,686,359 describes electronic circuit board laminates based on the curing of cycloaliphatic epoxies with anhydride curing agents. The reference discloses that laminates using the cycloaliphatic materials and anhydrides have attractive moisture absorption, low dielectric constants, and low dissipation factors. However, the formulations also have low Tg's. U.S. Patent No. 4,623,578 discloses laminates based on epoxy crosslinked polyanhydrides. An anhydride based copolymer, such as styrene maleic anhydride is first reacted with an amino aromatic compound forming a phenolic imide. The resulting polyimide is reacted with an epoxy to form a thermosetting network. Laminates made from this process have good thermal properties but suffer from high moisture absorption due to the imide moiety. U.S. Patent No. 5,620,789 discloses a cure inhibited resin composition used in manufacturing electronic laminates. The thermosetting resin systems are cure inhibited using boric acid. Most printed wiring boards laminates are manufactured as fiberglass polymeric substrates with fillers being added to improve electrical properties or enhance board processing. Copper foils are typically clad on the laminate surfaces imparting a conductive medium for circuit formation. In a general glass reinforced laminate, the system contains three phases and two interfaces: phases include the glass fiber, the resin matrix, and the copper foil; two interfaces are the region between the resin and glass and the region between the copper foil and the laminate surface. The interface regions serve as adhesion layers binding the resin to the glass or bonding the copper foil to the laminate surface. Predominantly coupling agents coated on the glass fibers before impregnation and coated on the copper foil prior to lamination controls adhesion. Coupling agents are silanes with a hydrolyzable silicon end group which reacts with the glass or copper surface and a reactive organic end group which bonds to the resin matrix. For controlling the laminate cohesive integrity and for maintaining circuit adhesion, the coupling agents are critical. Providing these interfaces are of high quality with proper adhesion, they effect little the laminate chemical, thermal, moisture or electrical properties. Laminate electrical properties (specifically dielectric constant (Dk) and dissipation factor (Df)) fundamentally depend on the resin matrix and the glass fabric. Glass types such as D-glass or Q-glass can reduce the Dk and Df of a substrate. However, these glass types are more difficult to drill compared to the standard electrical grade, E-glass. Hence, most interposer laminates are based on woven E-glass fabrics and the dielectric and dissipation properties must be controlled by the resin composition. Thermal and moisture absorption characteristics are primarily controlled by the resin formulation. Moisture permeates into printed wiring board substrates by absorption into the resin phase and permeates negligibly into the glass fibers. The resin systems typically have lower thermal performance than glass, and are the weakest phase at elevated temperatures. Using resin systems which resist moisture and have high thermal characteristics reflected in high glass transition temperature will result in improved laminate substrates. Current FR4 based laminates that use dicyandiamide (DICY) as a curative with epoxy resins, show high dielectric constants, very high moisture absorption, and fail to give adequate thermal resistance. Using higher performing resins based on bismaleimide triazine, epoxy novolac, or polyimide, can improve the moisture absorption with an increase in thermal performance, but the level of moisture absorption still requires costly drying and handling steps, and the electrical properties do not meet the high frequency applications. Thus, there still remains a need for resin systems combining low dielectric constant and low dissipation factor, with high thermal resistance and low moisture absorption. SUMMARY OF THE INVENTION This invention includes resin systems that, upon curing, have a low moisture affinity. This invention also includes resin systems that, upon curing, produce cured materials with high performing electrical and thermal performance. This invention further includes electromc materials and laminates produced using the resins systems of this invention which include good electric and thermal performance and low moisture absorption. In one embodiment, this invention is a resin system comprising at least one epoxy resin with essentially no chain extension, and at least one hardener that is a mixture of at least one phenolic resin and a copolymer of an allyl functional material and maleic anhydride wherein the resin system is essentially nitrogen free. In another embodiment, this invention is a laminate including a reinforcing material impregnated with one or more resin systems of this invention. In the laminate, the reinforcing material may be unwoven or woven paper, cloth, glass, or any other material that are useful in manufacturing reinforced prepregs and cores used in the manufacture of circuit boards and circuitized substrates. DESCRIPTION OF THE CURRENT EMBODIMENT The present invention relates to high Tg, low moisture absorbing resin systems and laminates, encapsulants, and underfills manufactured using the resin systems. The resin systems of this invention use epoxy resins cured with phenolic and anhydride based resins and may include optional ingredients such as flexibilizing or toughening components or fillers. An important aspect of this invention is the use of resin systems that do not include nitrogen-containing groups. Resins including nitrogen containing groups will usually produce products with undesirable electrical properties, low Tg, and high moisture absorption. Eliminating resin nitrogen and selectively using a defined class of epoxy resins with phenolic and anhydride hardeners will produce useful resin systems. The epoxy resins used in this invention should not be formed by chain extension reactions or contain nitrogen. Chain extended epoxies suffer from low Tg do to high epoxy equivalent weights. When the epoxy contains nitrogen (for example aniline moieties) the Tg tends to be high, but moisture absoφtion also dramatically increases. Furthermore, nitrogen containing epoxies are usually toxic and present processing difficulties. Preferred resins are those that are essentially nitrogen free, those that have low hydrolizable chloride, low epoxy equivalent weights and combinations thereof. The epoxy resins used in the resin systems of this invention should be essentially nitrogen free. It has been determined that epoxy resins including nitrogen-containing groups produce undesirable electric properties, high moisture absoφtion properties in cured products. By eliminating nitrogen groups from the epoxy resin and from the other resin system ingredients, improved products can be achieved. The resin systems of this invention should be essentially nitrogen free and are preferably nitrogen free. The term "essentially nitrogen free" as used herein means that the resin system of this invention includes less than 1 wt% nitrogen. The term "nitrogen free" as used herein means that the resin system of this invention includes less than 0.1 wt% nitrogen. Preferred epoxy resins useful in the resin systems of this invention are chosen from multifunctional types. These epoxy resins can be of the cresol novolac or phenol novolac variety. Preferably the epoxy resins are those derived from trisphenol resins prepared with low hydrolizable chloride content, low hydroxyl moieties, and with minimal chain extension reactions. These resins can be prepared in accordance with U.S. Patents 4,468,508 and 4,876, 371 and 5,008.350, the specifications of each of which are incoφorated herein by referenced. Preferred epoxies useful in the resin systems of this invention have the formulas: and and and mixtures thereof. In the resin formulations above, N is an integer from 1 to 300; X 1 and X 2 are each individually selected from hydrogen or from the halogens Cl, F and Br; R„ R 2 , R 3 , R 4 and R 5 are each individually selected from hydrogen and alkyl groups wherein the term "alkyl" refers to a straight or branched alkyl moiety having from 1 to 20 carbon atoms; and G refers to: Non-exclusive examples of epoxy resins having the above-identified formulas are available as Quatrex 6410 (Ciba-Geigy), Bren 304 (Nippon Kayaku), ESCN-195X (Sumitomo), TNM574 (Sumitomo Chemical), EPPN 502H (Nippon Kayaku), and NC6000 (Nippon Kayaku). The resin system of this invention includes one or more hardeners. Those skilled in the art will know the hardener purpose is to react with the pendent oxirane rings in the epoxy resins creating a polymer network. The epoxy resins and hardeners are combined such that the epoxy equivalent to hardener equivalent ratio ranges from 0.7 to about 1.3. It is preferred that the epoxy to hardener equivalency ratios range from about 0.9 to 1.1. Examples of hardeners include phenol novolacs, cresol novolacs, aromatic amines, cyanate esters such as polycresol cyanates, styrene maleic anhydride copolymers, aromatic anhydrides such as 4,4'- (2-acetoxy-l,3-glycerol)-bisandhydrotrimellitate and vinyl alkyl maleic anhydride co- polymers such as poly(maleic anhyride-α/t-1-octadecene) and mixtures thereof. Preferred hardeners are phenolic resins in combination with a copolymer of an allyl functional material and maleic anhydride. Phenolic resins that are especially useful in the preferred hardeners are compounds have the following formulas: or or mixtures thereof wherein R„ R 2 , and R 3 , are each individually selected from hydrogen and alkyl groups wherein the term "alkyl" refers to a straight or branched alkyl moiety having from 1 to 20 carbon atoms and wherein n= 1 to 300. Non-exclusive examples of useful phenolic resins having the above-identified structures include SDI711 (Borden Chemical), MU- 12700 (Schenectady International), and MEH7500 (Meiwa Plastics). The second component of the preferred hardner is a copolymer of an allyl functional material and maleic anhydride. A most preferred allyl functional material is styrene. Generally the allyl functional material and the maleic anhydride will be combined in a weight ratio amount ranging from about 15:1 to about 1:1. Preferably, when the allylfunctional material is styrene, the styrene to maleic anhydride weight ratio is about 5 to 1. Preferred copolymers useful in the hardness of this invention have the following formula: wherein R^ is selected from and branched and straight chain alkyl group having from 1 to 20 carbon atoms and combinations thereof, R, is as described above, and R b and R < . are each individually selected from polymerization initiation or termination groups. Appropriate polymerization initiators are free radical type initiators. Such initiators are available from Du Pont and sold under the NAZO® product line. The Du Pont initiators are substituted azonitrile compounds that thermally decompose and generate free radicals. The most common azonitrile is 2,2-azobis(butyronitrile) (AIBΝ). AIBΝ produces two 2- cyanoisopropyl radicals which constitute the end groups Rb and Re. Other useful initiators include VAZO 64 and NAZO 68 manufactured by Du Pont. For the present invention, inappropriate epoxy hardeners are those based on dicyandiamide (DICY). The DICY based systems have significant nitrogen containing moieties, and produce a cured epoxy resin with much higher moisture contents, inadequate electrical properties, and lower Tg than produced by the hardeners of the present invention. Besides the catalysts, all resin components should be essentially nitrogen free as defined above. The resin compositions of the present invention may contain ingredients in addition to the epoxy resins and hardeners discussed above. Optional ingredients include inorganic fillers such as a silica powder fine silicon oxide powders, talc and clay. Other optional ingredients may include flame retardants such as antimony trioxide phosphates and aluminum trihydrate; flow control agents such as phenol terminated polyether sulfone, polyvinyl butyrate, magnesium hydroxide and polyether imide; and dyes or pigments to control color, and surfactants to control surface appearance. The following examples illustrate preferred embodiments of this invention as well as preferred methods for using compositions of this invention. The Examples are not intended to limit the scope of the inventions which are set forth in the claims appended hereto. EXAMPLE 1 This Example compares properties of laminates prepared using resin compositions of this invention with a commercial FR-4 laminates prepared with a DICY hardener system. Each laminate was prepared by coating and drying the resins on 7628 woven glass and curing to an intermittent gel point (b-stage prepreg). The prepregs were layered to four layers between copper foil and fed under appropriate temperature and pressure into a press until cured. Press conditions followed loading at 180°F, bringing to pressure at 200 psi, ramping the temperature to 392°F at a rate of 10°F/min. under pressure. The temperature is maintained at 392°F for 1 hour after which the laminate stack was cooled to room temperature before opening. The outer copper foil layers were etched in a conventional etchant to remove the copper layers to form an unclad laminate. All laminates contained resin fractions on the order of about 40% by weight. The resin systems used to prepare the prepregs are set forth in Tables 1 and 2, below. Table 1 Resin Formulations Table 3, below sets forth the compositions and functions of the ingredients identified in Tables 1 and 2. Before determimng laminate properties, all laminate samples were brought to equilibrium at 24°C and 50% relative humidity (RH). Moisture absoφtion at 85°C and 85% RH was measured to the point, of saturation. Table 4 below, summarizes the results of the FR- 4 DICY system compared against laminates of the current invention. Table 4 The resin systems of this invention produce resins with significantly higher Tg values and with much lower moisture absoφtion in comparison to FR4 resin. Dielectric constants (Dk) are typically 0.3 to 0.6 lower than FR4 materials. Dissipation factors (Df) are low, with some formulations being under 0.0090. EXAMPLE 2 This Example evaluates whether or not resin curing properties and the resulting laminate thermal characteristics can be improved by altering the resin catalyst package. Laminates using resin systems of this invention were prepared according to Example 1. The resin systems used included various catalyst combinations. The laminate properties were evaluated according to the method described in Example 1 and the results are reported in Table 5, below. In the current invention a boric acid - imidazole catalyst package was used with good success. However, removing the boric acid to give a purely imidazole catalyzed system or using a phosphonium catalyst had no detrimental impact on the laminate properties. Furthermore, removing the boric acid improved the laminate properties through the lowering of the dielectric and dissipation factors.

Claims: What I claim is: 1. A resin system comprising at least one epoxy resin with essentially no chain extension, and at least one hardener that is a mixture of at least one phenolic resin and a copolymer of an allyl functional material and maleic anhydride wherein the resin system is essentially nitrogen free. 2. The resin system of claim 1 wherein the epoxy resin is selected from and and and mixtures thereof wherein n=l to 300; X l and X 2 are each individually selected from hydrogen, Cl, F and Br; R l9 R 2 , R 3 , R 4 and R 5 are each individually selected from hydrogen and straight and branched alkyl moieties having from 1 to 20 carbon atoms, and wherein G 3. The resin system of claim 2 wherein the epoxy resin is and wherein R, is a tert-butyl group, and R 2 , R 3 and X, are each hydrogen and wherein n=l to 300. The resin system of claim 2 wherein the epoxy resin is wherein R„ R 2 , R 3 , R 5 and X 2 are each hydrogen and wherein X, is bromine. 5. The resin system of claim 1 wherein the phenolic resin is selected from compounds having the following formulas: and and mixtures thereof wherein R„ R 2 , and R 3 , are each individually selected from hydrogen and straight or branched alkyl moieties having from 1 to 20 carbon atoms, wherein n=l to 300, and wherein X, and X 2 are each individually selected from hydrogen, Cl, F and Br. 6. The resin system of claim 1 wherein the phenolic resin is wherein R l5 R 2 , R 3 and X, are each hydrogen and wherein n=l to 300. 7. The resin system of claim 1 wherein the copolymer of an allyl functional material and maleic anhydride has the formula: wherein R,, is selected from - Λ © | -* and straight and branched alkyl moieties having from 1 to 20 carbon atoms and combinations thereof, R ! is selected from hydrogen and straight and branched alkyl moieties having from 1 to 20 carbon atoms, and R t , and R,. are each individually selected from polymerization initiation or termination groups. 8. The resin system of claim 1 wherein the copolymer of an allyl functional material and maleic anhydride is styrene maleic anhydride. 9. The resin system of claim 1 wherein the equivalence ratio of epoxy resins to hardeners ranges from about 0.9 to about 1.1. 10. The resin system of claim 1 wherein the allyl functional material and the maleic anhydride will be combined in a weight ratio amount ranging from about 15:1 to about 1:1. 11. A laminate including a woven material impregnated with the resin system of claim 1. 12. The laminate of claim 11 wherein the woven material is woven glass and the laminate is selected from a prepreg, a core, or a combination thereof. 13. An essentially nitrogen free resin system comprising an epoxy resin having the formula: wherein G is , R,, R 2 , R 3 , Rs and X 2 are each hydrogen, X, is bromine, and n= 1-300; at least one hardener that is a mixture of at least one phenolic resin and a copolymer of an allyl functional material and maleic anhydride wherein the phenolic resin has the formula: wherein R l5 R 2 , R 3 and X ! are each hydrogen and wherein n=l to 300; and wherein the copolymer of an allyl functional material and maleic anhydride is styrene maleic anhydride having the formula: wherein R,, is selected from *- Λ !£ | « . R, is selected from hydrogen and straight and branched alkyl moieties having from 1 to 20 carbon atoms, and -- , and R,. are each individually selected from polymerization initiation or termination groups.

Inventors: SMITH GORDON C, SMITH, GORDON, C.
Applicants: ISOLA LAMINATE SYSTEMS CORP, ISOLA LAMINATE SYSTEMS CORP.
Application year: 2000
Main classification: H05K1
IPC codes: C08G59, C08J5, C08L63, H01L23, H05K1