Patent Publication Number: US-2007111010-A1

Title: Flame retardant prepregs and laminates for printed circuit boards

Description:
This application claims benefit under 35 USC 119(e) of U.S. provisional application No. 60/737,302, filed Nov. 16, 2005, the disclosure of which is hereby incorporated by reference. 
    
    
      This invention relates to prepreg and laminate compositions for use in forming printed circuit boards having excellent flame retardant properties, good thermal stability and thermal expansion characteristics, uniform appearance, low density and good drillability.  
     BACKGROUND  
      Printed circuit boards are typically produced by impregnating a fabric, such as an electronics grade fiber glass (E-glass) with a liquid thermosetting epoxy resin. The impregnated fabric is heated to partially cure the resin and to form a dry, flexible sheet in which the resin is in an intermediate cure state, sometimes referred to as the “B” stage or a “pre-preg (prepreg).” Prepreg sheets are then stacked together to a desired thickness and subjected to heat and pressure that fully cures the resin. This forms a laminated composite in which the resin is sometimes said to be in the “C”-stage.  
      Typically, printed circuit boards consist of about 50% by weight epoxy resin and 50% by weight electronics grade fiber glass (E-glass). It is necessary that the prepregs and laminates have acceptable flame retardant properties. For example, the flame retardance standard for electronic compositions is the UL-94 V-0 rating.  
      Published U.S. app. No. 2004/0002559 teaches flame retardant coating formulations.  
      U.S. Pat. No. 6,632,511 teaches prepregs and laminates for printed circuit boards.  
      U.S. Pat. No. 6,255,371 teaches a flame retardant combination of organic phosphinates and condensation products of melamine with phosphoric acid.  
      U.S. Pat. No. 5,739,187 discloses epoxy resin compositions as semiconductor encapsulants which include a certain phosphorus containing flame retardant to eliminate the use of antimony trioxide and brominated compounds.  
      U.S. Pat. No. 5,434,199 discloses a low stress epoxy molding composition which includes a tris-phenolmethane multifunctional epoxy resin in combination with a tris-phenolmethane multifunctional phenolic hardener, along with silicone rubber powder and an organofunctional silicone fluid. The organofunctional silicone fluid is provided to provide flowability to the molding compound.  
      U.S. published app. Nos. 2004/0166241 and 2004/0166325 teach electronic epoxy molding compositions comprising melamine cyanurate as a flame retardant. The disclosures of these references are incorporated by reference.  
      It is desirable to provide flame retardant prepregs and laminates prepared with epoxy resins that meet UL-94 V-0 rating without halogenated flame retardants and without antimony compounds.  
     SUMMARY  
      Disclosed is a prepreg or a laminate for a printed circuit board, which prepreg or laminate comprises  
      an epoxy resin and  
      a flame retardant which is a fine particle size melamine cyanurate or a mixture of fine particle size melamine cyanurate and fine particle size melamine polyphosphate, where about 99% of the particles have a diameter of less than or equal to about 15 microns.  
      Further disclosed is a prepreg or a laminate for a printed circuit board, which prepreg or laminate comprises  
      an epoxy resin and  
      a flame retardant combination comprising 
          a fine particle size melamine cyanurate or a mixture of fine particle size melamine cyanurate and fine particle size melamine polyphosphate, where about 99% or the particles have a diameter of less than or equal to about 15 microns and     a phosphinate and/or a diphosphinate.        

      The present epoxy prepregs and laminates and the resultant coated circuit boards exhibit excellent flame retardancy as measured by UL-94, meeting V-0, have a uniform dielectric constant, good thermal stability and thermal expansion characteristics, uniform appearance, low density and good drillability. 
    
    
     DETAILED DESCRIPTION  
      U.S. published app. Nos. 2004/0166241 and 2004/0166325 teach electronic epoxy molding compositions comprising melamine cyanurate as a flame retardant. The disclosures of these references are incorporated by reference.  
      There is no restriction on the type of epoxy resin that can be used in the prepregs and laminates of this invention. It generally contains two or more reactive oxirane groups. For example, the epoxy resin may be selected from bisphenol A type epoxy resins, novolac type epoxy resins such as epoxy cresol novolac resin and phenolic novolac epoxy resin, alicyclic epoxy resins, glycidyl type epoxy resins, biphenyl epoxy resins, naphthalene ring-containing epoxy resins, cyclopentadiene containing epoxy resins, polyfunctional epoxy resins, hydroquinone epoxy resins, and stilbene epoxy resins. The prepregs and laminates can include more than one epoxy resin, for example, a combination of epoxy cresol novolac resin and biphenyl epoxy resin.  
      As noted, bisphenol and biphenyl epoxy resins, which are traditionally referenced as di-epoxies, and epoxy cresol novalac resins, which are traditionally referenced as multifunctional epoxies, are useful in the present invention. Such epoxies have a degree of branching of two, in that two phenolic groups having pendant epoxies are linked through the same carbon atom. For example, diglycidyl ether of bisphenol A is difunctional, including two phenolic groups with pendant epoxies extending from a central carbon atom. It therefore has a degree of branching of two. Epoxy cresol novolac resins are often times referenced as “multifunctional”, in that they are polymeric compounds with a plurality of pendant epoxy moieties which may extend from the polymeric chain. For example, epoxy cresol novolac resins include the following structure:  
                 
 
      When n=0, the functionality if this structure would be 2. If n=1, the functionality, is 3, if n=4, the functionality is 4, etc. As such, this compound is traditionally referred to as a multifunctional epoxy resin. However, since only two phenolic groups extend from the same carbon or small cluster of carbons, the degree of branching of this type of resin would be equal to two.  
      In a particularly desirable embodiment, the epoxy resin is a multifunctional epoxy resin having a degree of branching within the resin backbone of at least three. Thus, particularly desirable multifunctional epoxy resins are those derived from phenol and which include at least three phenolic groups branching directly from the same central carbon atom or central cluster of carbons, with a pendant oxirane group linked to each of the at least three phenolic groups.  
      Non-limiting examples of useful multifunctional epoxy resins having a degree of branching of at least three include:  
                 
 
 triphenylol methane triglycidyl ether (having a degree of branching of three, represented by three terminal glycidyl ether moieties branching from a central carbon atom);  
                 
 
 tetra glycidyl ether of tetra phenol ethane (having a degree of branching of four, represented by four terminal glycidyl ether moieties branching from a central two carbon cluster ethyl moiety). 
 
      Particularly desirable are epoxy resins derived from tris-phenolmethane, such as triphenylol methane triglycidyl ether.  
      The multifunctional resin having a degree of branching of at least three may be used alone, or in combination with conventional resins such as those described above.  
      The epoxy resin typically has a theoretical epoxy equivalent weight of about 150 to 250. The epoxy resin is for example present in the composition of the present invention in an amount of about 1 to 25 percent by weight, often 4 to about 12 percent by weight, and more often, from about 5.5 to about 8.5 percent by weight, based on the total weight of the composition.  
      The prepregs and laminates typically contain a curing agent (hardener). The curing agent promotes crosslinking of the resin to form a polymer composition upon heating of the composition to a temperature of at least about 135° C. Some suitable curing agents that can be included in the present invention are phenol novolac type hardener, cresol novolac type hardener, dicyclopentadiene phenol type hardener, limonene type hardener, and anhydrides. Flexible hardeners having a hydroxyl equivalent weight greater than about 150 are often desirable, such as xylock novalac type hardener. Non-limiting examples of flexible hardeners include bisphenol M commercially available from Borden Chemical, and DEH 85, commercially available from Dow Chemical. Similar to the epoxy resin component, more than one type of curing agent can be included in the present compositions.  
      As with the epoxy resin component, multifunctional hardeners having a degree of branching of at least three are particularly desirable in one embodiment of the present invention. Particularly desirable are those derived from tris-phenol and which contain at least three functional groups that are reactive with epoxide groups.  
      The curing agent is for example present in the composition of the present invention in an amount of about 1 percent by weight to about 10 percent by weight, often from about 1.5 percent by weight to about 6 percent by weight, based on the total weight of the composition.  
      The composition may further include a catalyst for promoting reaction of the epoxy resin and the hardener. Traditionally, such epoxy compositions incorporate catalysts such as tertiary amines, substituted phosphines, imidazoles, and the like, with compounds such as 1,8-diazabicyclo[5.4.0]undec-7-ene (“DBU”), dicyandiamide (“DICY”) and triphenylphosphine (“TPP”) being particularly well known for use as catalysts.  
      The catalyst is present at least in an amount sufficient to catalytically effect crosslinking of the epoxy resin and curing agent when the composition is heated to a temperature of at least about 135° C.  
      The epoxy resin convieniently comprises a filler. The filler may be for example polymeric microspheres or traditional glass microspheres as disclosed in U.S. Pat. No. 6,632,511, the contents of which are hereby incorporated by reference.  
      Other resins may be suitable for use in printed circuit board applications. For example, resins may comprise thermosetting resins such as epoxy, phenolic resin, benzoxazine, polyimide, cyanate ester, bismaleimide triazine, polyester, polyphenylene ether resins, polystyrene, polyphenylene oxide, polyphenylene sulfide, polysulfone, polyethersulfone, polyetherimide, polyacetal, polycarbonate and the co-polymers and blends thereof.  
      In addition, the resin may additionally comprise a variety of additives, individually or in the various combinations and permutations thereof. For example, the resin may optionally comprise an ultraviolet light blocking dye, a pigment (such as TiO 2 , Fe 2 O 3 ) or resin to increase the opacity of the prepreg, laminate or printed circuit board to ultraviolet light. The resin may also optionally comprise a further flame retardant, for example, a halogen compound such as a brominated epoxy or brominated filler, or a halogen-free compound such as a phosphorus, nitrogen, or boron containing compound to increase the fire or flame-resistance. Preferably, no further flame retardants are employed. The resin may optionally comprise a surfactant such as Chemie BYK 322, an inorganic flow modifier such as hydrophobic fumed silica, and/or a thixotropy agent. Typically, these additive(s) will, in combination, comprise about 3 wt. % to about 20 wt. % of the resin. If the halogen substance is an epoxy resin, the bromine comprises about 15% to about 60% by weight of said resin, and about 5% to about 30% by weight of the total solid content. If the halogen substance is a filler, the bromine comprises about 20% to about 85% by weight of said filler, and about 5% to about 30% by weight of the total solid content.  
      Prepregs of the present invention are formed by impregnating a reinforcing material with a varnish comprising (i) solvent and (ii) a polymeric resin (typically partially cured) or resin monomer. The reinforcing material selected generally depends upon the desired properties for the finished laminate. These include thickness, dielectric constant (Dk), coefficient of thermal expansion (“CTE”), and the intended product application. In general, the reinforcing material may be a woven or non-woven mat comprising a fibrous material such as ceramic, glass, or polymeric fibers. Low dielectric constant materials such as electronics grade glass, D-glass, aramids such as Kevlar® and Nomex®, both registered trademarks of E. I. Dupont de Nemours and Company, poly p-phenylene benzobisthiazole, poly p-phenylene benzobisoxazole, polyetheretherketone, PTFE, aromatic polyesters, quartz, S-glass, paper, and the like, or combinations thereof may be used to form the mat of fibrous material. The reinforcing material may be in a cowoven or comingled form.  
      The laminates of the present invention are prepared from prepregs using conventional techniques such as flat bed press or autoclave lamination. For example, prepreg sheets are sandwiched between two sheets of copper and laminated under heat and pressure (e.g., about 188° C. and 200-600 psi (about 13.75-40 Bar). Copper lines providing electrical pathways can be etched on to the resulting laminate. These can be used as a single substrate or pressed with other sheets of laminate, copper, and prepreg to produce a multi-layer laminate or printed circuit board.  
      The resulting laminate preferably has a relatively low dielectric constant. That is, the laminate preferably has a dielectric constant of less than 4.2 at 50% resin content. In some embodiments, the dielectric constant is preferably less than 3.9 at 50% resin content, more preferably less than 3.5 at 50% resin content, and may be no greater than 3.0 at 50% resin content.  
      Prepregs, laminates and printed circuit boards incorporating the present flame retardants may be prepared in accordance with the present invention with existing equipment and methods. For example, a prepreg is most often produced on treaters. The main components of a treater include feeder rollers, a resin impregnation tank, a treater oven, and receiver rollers. The reinforcing fabric (E-glass, for example). is usually rolled into a large spool. The spool is then put on the feeder rollers which turn and slowly roll out the glass. The glass then moves through the resin impregnation tank, which contains the varnish. The varnish wets out the glass. After emerging from the tank, the coated glass moves upward through the vertical treater oven which is typically at a temperature of about 350 to 400° F. (about 175° C. to 200° C.), and the solvent of the varnish is boiled away. The resin begins to polymerize at this time. When the composite comes out of the tower it is sufficiently cured so that the web is not wet or tacky. The cure process, however, is stopped short of completion so that additional curing can occur when laminate is made. The web then rolls the prepreg onto the receiver rolls which can be changed when the run is finished. A new roll is then attached to the treater so that a new run can begin.  
      The laminates prepared in accordance with the present invention have good electrical properties such as a relatively low dielectric constant, good thermal properties such as higher decomposition temperature, good T-260 and T-288 properties, as well as good mechanical properties such as thermal expansion characteristics (CTE and Z axis expansion). The prepreg and laminate prepared by this method also have good machinability, low density, and processability with the existing equipment/methods of prepreg manufacture.  
      This invention can be used with both passive and active components of printed circuit boards. The resulting prepreg and laminate have good electrical, thermal, mechanical, and processable properties as well as homogeneity in drilling and other printed circuit board machining operations.  
      The fine particle size melamine polyphosphate and melamine cyanurate flame retardants have a narrow particle size distribution. For example, about 99% of the particles have a diameter of less than or equal to about 15 microns. For example, about 99% of the particles have a diameter of less than or equal to about 14 or 13 microns. For instance, about 99% of the particles have a diameter of less than or equal to about 12 microns. For instance, about 50% of the particles have a diameter of from about 3.0 to about 3.5 microns.  
      Fine particle size melamine polyphosphate or melamine cyanurate is prepared from commercially available samples that are further milled by known methods.  
      The present fine particle size melamine cyanurate, or combination of melamine cyanurate and melamine polyphosphate, in total, is present in the epoxy resin in a level of up to about 20% by weight, based on the weight of the resin. For example, up to about 17% by weight, or up to about 15% by weight, based on the epoxy resin. For example, the present melamine based flame retardants are present up to about 13% by weight or for example from about 9% to about 13% by weight, based on the weight of the resin.  
      When phosphinate or diphosphinate or their combinations as flame retardants are also present, they are generally present at about 1:1 weight ratios with the melamine based flame retardant(s). For example the phosphinate, diphosphinate or combination thereof is present from about 1:10 to about 10:1 by weight:weight, relative to the weight of the melamine based flame retardants. For example, the weight:weight ratios are from about 1:5 to about 5:1 or from about 1:3 to about 3:1.  
      To be redundant, the phosphinate or diphosphinate flame retardants are also for example present at a level of up to about 20% by weight, based on the weight of the epoxy resin. For example, up to about 17% by weight, or up to about 15% by weight, based on the epoxy resin. For example, the present phosphinate or diphosphinate based flame retardants are present up to about 13% by weight or for example from about 9% to about 13% by weight, based on the weight of the resin.  
      The present phosphinate or diphosphinate flame retardants are disclosed in U.S. Pat. No. 6,255,371, the contents of which are hereby incorporated by reference.  
      The phosphinates or diphosphinates are of the formulae  
                 
 
 where 
 
      R 1  and R 2  are identical or different and are linear or branched C 1 -C 6 alkyl or C 1 -C 10 aryl;  
      R 3  is linear or branched C 1 -C 10 alkylene, C 6 -C 10 arylene, C 7 -C 14 alkylarylene or C 7 -C 14 arylalkylene;  
      M is magnesium, calcium, aluminum or zinc;  
      m is 2 or 3;  
      n is 1 or 3; and  
      x is 1 or 2.  
      Alkyl is for example methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, t-butyl, n-pentyl or isoamyl.  
      C 6 -C 10 aryl is for example phenyl or naphthyl, but also comprised are C 1 -C 4 alkyl substituted phenyl.  
      Alkylaryl, is, for example, o-, m- or p-methylphenyl, 2,3-dimethylphenyl, 2,4-dimethylphenyl, 2,5-dimethylphenyl, 2,6-dimethylphenyl, 3,4-dimethylphenyl, 3,5-dimethylphenyl, 2-methyl-6-ethylphenyl, 4-tert-butylphenyl, 2-ethylphenyl or 2,6-diethylphenyl.  
      Arylalkyl is for example benzyl, α-methylbenzyl, α,α-dimethylbenzyl or 2-phenylethyl. For example benzyl and α,α-dimethylbenzyl.  
      Arylene, alkylarylene or arylalkylene are divalent versions of the monovalent aryl, arylalkyl or alkylaryl.  
      The present prepregs and laminates and the resultant coated circuit boards exhibit a flammability rating of UL 94 V-1, more preferably, UL 94 V-0.  
      The ratings are determined by measuring the total burn time of a ⅛″ bar according to the UL 94 V flammability test (Underwriters Laboratories UL 94,  Test for Flammability of Plastic Materials UL  94, July, 1997). A UL 94 V-0 and a UL 94 V-1 rating require the total burn time for a single bar to be less than or equal to 10 seconds and 30 seconds, respectively. Ratings according to the UL 94 V test are as compiled in the following table:  
                                                                   Burning   Burn to           Rating   Burn time   drips   clamp                          V-0   &lt;10 s   no   no           V-1   &lt;30 s   no   no           V-2   &lt;30 s   yes   no           Fail   &lt;30 s       yes           Fail   &gt;30 s       no                      
 
     EXAMPLE 1  
      A varnish contains 190 g of epoxy cresol novalac resin, which is mixed with 0.5 g 2-methyl imidazole in 3.6 g methyl ethyl ketone, and 7.2 g 1-methoxy-2-propanol. Also added are 12 weight % of fine particle size melamine cyanurate and 12 weight % of aluminum diethylphosphinate. The mixture is then well stirred for 4 hours, and applied to 7628 style E-glass fabric (supplied by BGF industries). The glass fabric is then brought to the partially cured B-stage by heating at 171° C. for two and a half minutes. A four ply laminate is made, with size 1 oz. STD copper from Gould Foils Inc. on one side and 1 oz. DST copper Gould Foils Inc. on another side, by pressing the prepreg and copper in a book at 188° C. and 140 psi for 90 minutes.  
      The prepregs and laminates exhibit UL 94 rating of V-0.  
      The melamine cyanurate may be replaced with a mixture of melamine cyanurate and melamine polyphosphate. The particle size of each is where 99% of the particles have a diameter of less than or equal to about 12 microns, and about 50% of the particles have a diameter of from about 3.0 to about 3.5 microns.  
      The aluminum diethylphosphinate may be replaced with zinc diethylphosphinate. Excellent results are obtained.