1. Field of the Invention
The present invention is directed to polyimide prepolymers of the addition reacted type and prepregs obtained therefrom as well as to a method of producing laminates by the use of said prepregs, more particularly to such compositions and method suitable for fabricating multi-layer printed circuit boards.
2. Description of the Prior Art
In the field of fabricating printed circuit boards of a fewer number of layers, there have been widely utilized a various epoxy resins having valuable properties including superior adhesive properties, remarkable resistance to chemical agents, high mechanical strength, and excellent dielectric properties. However, the epoxy resins are found to be no longer appropriate for fabricating multi-layer printed boards in use for constructing high density circuits and modules, since they do not always assure enough resistance to heat which is applied repeatedly to such printed boards for module mounting purposes, in addition to that they frequently reduce reliability in conductivity of conductor layers thereof due to possible resin smear or heat expansion in the direction of thickness of the board. To settle the above problems, heat-stable polyimide resins are presently utilized and found to be satisfactory for the fabrication of the multi-layer printed boards. Particularly, polyimide resins of addition reacted type obtained by reacting unsaturated bisimides with diamines are most acceptable for fabricating the multi-layer printed boards, because the use of that material gives rise to remarkable and advantageous features. Included in the above advantageous features expected by the use of that material are to allow the formation of fine conductive paths as well as the drilling operation of minute holes with high accuracy, which are indispensable for high density circuit arrangement; to keep the heat expansion in the direction of thickness at a minimum for providing increased reliability of conductivity at plated through-holes; to eliminate the deposition of resin smear during drilling operation; to give increased bond strength to the conductor layers as well as increased hardness to the bases of the printed board at elevated temperatures for improving module mounting performance; and to well withstand the continuous operation at an elevated temperature of about 200.degree. C.
Such polyimide prepolymers with valuable characteristics have been disclosed in U.S. Pat. No. 3,562,223 granted to Michel Bargain et al, and have been taught therein to be obtained by reacting unsaturated bisimides with diamines.
In the meanwhile, today's increasing miniaturization of large-scaled computers has resulted in the utilization of multi-layer printed boards with high mounting density, which should require much finer circuit pattern arrangement and much more minute through hole drilling. Consequently, the printed boards to be employed for that purpose should have a higher level of dimension stability as well as superior adhesive property. That is, possible variance in the dimensions of boards can seriously affect interlayer or interplane registration. Thus, the dimensional variance should be kept at a minimum and this is particularly important for fabricating the multilayer boards of larger dimensions. Accordingly, the printed boards are preferred to be molded at such a low pressure level as not to lower the dimensional stability. But, there still remains a problem that the molding at such low pressure level may sometimes fail to successfully flow the resin uniformly throughout the boards being cured or fail to eliminate all traces of solvent employed for an impregnation purpose and therefore result in the presence of unacceptable voids in the molded printed boards. With regard to the adhesive property, the formation of high density circuits requires a correspondingly high bond strength not only between the conductor layer and the resin composition but also between the resin composition and the substrate or base material of the board, otherwise minute peels would occur at the interlayer between the cured resin and the base material during the minute hole drilling, such peels being unacceptable and should be of course eliminated.
In view of the above, the multi-layer printed boards for use in the large-scale computers are desired to combine much higher bond strength with much superior dimensional stability. In other words, prepolymers and the prepregs made therefrom used for fabrication of the multi-layer printed boards should exhibit much higher bond strength and as well enable the molding of the boards at such a low pressure level as not to reduce the dimension stability yet without causing any defective void in the resulting boards.
The prior polyimide prepolymers including those disclosed in the above U.S. patent are known to exhibit excellent bond strength together with other remarkable features, but unfortunately are found to be still unsatisfactory for the fabrication of the multi-layer printed board of the type having a larger number of layers which require a higher level of dimension stability and superior adhesive property. That is, the prior polyimide prepolymers are likely to suffer insufficient bond strength when fabricating such multi-layer printed boards and further to cause defective voids when molded at a low pressure in an attempt to ensure the dimension stability.
The investigation into the above unsatisfactory results with the use of prior polyimide prepolymers has revealed that macromolecular components with a molecular weight of more than 15,000 contained in a greater amount may account for the above insufficient bond strength and the occurrence of the voids at the low pressure molding. In fact, the components with a molecular weight of more than 15,000 have inherent high viscosity which requires a more amount of solvent for impregnation purposes and therefore provides more chance of the residual solvent being entrapped in the printed board at the time of low pressure molding, thus leaving the defective voids in the boards. Also characteristic to the above macromolecular components is less sensitivity with a cooperative substrate with which it is in contact for bond therebetween, such lowered sensitivity or activity will be the cause of failing to achieve integral bonding with the substrate, resulting in less bond strength therebetween. As suggested from the above consideration, polyimide prepolymers for the present purpose are expected to contain less amount of the macromolecular components having a molecular weight of exceeding 15,000 than the prior prepolymers. Conversely, prepolymers containing more amount of components with a molecular weight of less than 15,000 are expected to be preferable. With this in mind, the inventors have further studied the adaptability of the polyimide prepolymers to the fabrication of the multi-layer printed boards and other laminate products and found that polyimide prepolymers containing specific proportions of components with a specific range of molecular weight with respect to the other components in the prepolymer composition are responsible for providing satisfactory properties for the fabrication of the multi-layer printed circuit.
To this end, the applicant of the present invention have proposed in their U.S. patent application Ser. No. 715,001 useful prepolymers and prepregs prepared therefrom. However, further investigation into such prepolymers and prepregs reveals that they are still insufficient for practical application purposes. That is, although the prepolymers and prepregs disclosed in the above application have been found to be advantageous in many aspects, the solution of the prepolymers and the prepregs will have a relatively working life, or they will become solidified rapidly so as not to afford enough curing time during which a volatile solvent being used is required to evaporate successfully from the prepreg solutions or the prepregs. Without enough curing time, the solvent is likely to be entrapped in the finished products or laminates when the resin composition is entirely solidified, rendering the cured resin to have defective voids caused by evaporation of the residual solvent. Such voids will certainly lower the bond strength expected by the cured resin and further be the cause of a blister in the finished laminates. Consequently, the prepolymers and the prepregs are mostly desired to have a longer curing time during which the solvent is allowed to successfully evaporated before the resin composition is finally cured.