The present invention relates to a process for producing an inorganic fiber-based prepreg sheet, as well as to a process for producing a laminate sheet using the inorganic fiber-based prepreg sheet. More particularly, the present invention relates to a process for producing an inorganic fiber-based prepreg sheet superior in electrical properties (e.g. insulation) and mechanical properties (e.g. heat resistance), as well as to a process for producing a highly insulating laminate sheet suitable for use in printed wiring substrates of high-density mounting.
Laminate sheets of phenolic resin-impregnated paper have heretofore been used for printed substrates widely utilized in household electronic appliances, etc. for their low production cost and good punchability.
The phenolic resin-impregnated paper is obtained by dissolving a phenolic resin in a solvent, impregnating a paper with the resulting solution, and drying the impregnated paper to remove the solvent. The laminate sheet is obtained by laminating necessary sheets of the phenolic resin-impregnated paper and then heating the resulting laminate under pressure.
However, using an organic solvent, the production of the phenolic resin-impregnated paper has had problems of operational danger during production process, bad working environment, odor pollution due to organic solvent and cost for solvent recovery.
Moreover, the phenolic resin-impregnated paper per se has had drawbacks in that it is inferior in insulation, heat resistance, moisture resistance, dimensional stability in thickness direction and through hole ability (e.g. through hole formability).
Moreover, since the wiring and mounting in printed substrates are becoming to be made in a higher density, sheets and laminate sheets for printed substrates have been required to be superior in heat resistance, moisture resistance and dimensional stability in thickness direction, etc. and have high insulation.
Hence, as the sheet for printed substrates, improved in insulation and other properties, there were proposed various inorganic sheets obtained by fixing a thermosetting resin to an inorganic fiber, or various laminate sheets obtained by laminating such inorganic sheets.
Japanese Patent Application Kokai (Laid-Open) No. 41399/1987 describes an inorganic sheet obtained by compounding 5% by weight or more of a small-diameter fiber (average fiber diameter: 3 .mu.m or less) and fixing a large amount (50-300% by weight) of a filler to an inorgnaic fiber (e.g. glass fiber) by the combined use of cationic and nonionic high-molecular fixing agents, which is said to be suitable also for circuit substrates.
This inorganic sheet, however, contains electric charges and therefore shows reduced insulation and cannot retain insulation such as required in current printed substrates.
Japanese Patent Application Kokai (Laid-Open) No. 160998/1990 describes a paper for insulating laminate sheet, obtained by subjecting a fibrous base material (only paper is used in Examples) and 2 to less than 35% of a thermosetting resin fine powder (0.3-300 .mu.m) to sheeting. The fibrous base material may be a glass fiber, according to the document.
Japanese Patent Application Kokai (Laid-Open) No. 160998/1990, however, describes, in Examples, only a paper obtained by fixing a phenolic resin powder to a wood pulp fiber, and mentions no specific detail as to how fixing could be made to a glass fiber (fixing to glass fiber is generally difficult).
The above document also describes "a coupling agent can be compounded as necessary". However, since there is generally no clear basis for selection of a particular coupling agent from among many coupling agents available, it is not clear only from the above description what kind of a coupling agent should be selected to increase both the amount of phenolic resin powder fixed and the insulation of sheet.
So far as the present inventor knows, the correlation between thermosetting resin, fixing agent, inorganic fiber and coupling agent has been unclear both qualitatively and quantitatively.
Accordingly, it has been considered difficult to produce a phenolic resin-containing fiber-based prepreg sheet suitable for use in laminate sheets for printed substrates wherein wiring, mounting, etc. are made at a high density, which sheet has high insulation and retains high insulation even after moisture absorption, is superior in heat resistance, moisture resistance, dimensional stability in thickness direction and through hole ability, and gives rise to no whitening even when made into a laminate sheet.
Prepreg sheets obtained by impregnating an inorganic fiber base material with an epoxy resin have been used heretofore but the epoxy resin is expensive. Hence, also from a cost standpoint, there has been desired a phenolic resin-impregnated inorganic fiber-based prepreg sheet of high insulation.