Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2000-03-03
2002-09-10
Dawson, Robert (Department: 1112)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C523S455000, C525S423000, C525S523000, C525S528000, C525S936000, C156S330000, C156S331100, C428S418000, C428S419000
Reexamination Certificate
active
06447915
ABSTRACT:
The present invention relates to an interlaminar insulating resin adhesive for multilayer printed circuit board. More particularly, the present invention relates to an interlaminar insulating adhesive of epoxy resin type for multilayer printed circuit board, which has flame retardancy without containing halogen or phosphorus, which has excellent thermal property, which can give an interlaminar insulating layer of uniform thickness, and which is suitable for formation of fine pattern; as well as to a copper foil coated with said interlaminar insulating adhesive.
Production of a multilayer printed circuit board has been conducted by a process comprising steps of laminating, on an inner layer circuit substrate having a circuit, at least one prepreg obtained by impregnating a glass cloth with an epoxy resin, followed by semi-curing, laminating a copper foil thereon, and molding the resulting material into one piece by hot plate pressing. In this conventional process, the steps of laminating a prepreg(s) and a copper foil(s) on an inner layer circuit substrate, the cost of prepreg(s), etc. incur a high cost; further, obtainment of an interlaminar insulating resin layer of uniform thickness between circuit layers is difficult because, during molding, the resin is allowed to flow by heat and pressure so as to fill the concave portions of the inner-layer circuit substrate and eliminate voids; furthermore, when a glass cloth is present between circuit layers and the impregnatability of resin into the glass cloth is low, there may appear undesirable phenomena such as moisture absorption, copper migration and the like.
In order to solve these problems, attention has been paid in recent years, again to a technique of producing a multilayer printed circuit board using a conventional press but without using any glass cloth in the insulating layer between circuit layers. In using a press but using no glass cloth or the like as a base material of insulating layer, it has been difficult to obtain an interlaminar insulating layer low in thickness variation between circuit layers.
Recently, heat resistance has also become necessary, because bare chips have come to be mounted even on the substrate of a portable telephone or the mother board of a personal computer and the chips mounted thereon have become to possess a higher function, and hence the number of terminals has increased and accordingly circuits have come to have a finer pitch. In addition, use of an environment-friendly material containing no halogen compound or the like has become necessary.
When a film-shaped interlaminar insulating resin layer is used in a multilayer printed circuit board of build-up type, the thickness variation of the interlaminar insulating resin layer after press molding tends to be large because the insulating resin layer contains no glass cloth as a base material; therefore, in such a case, it is necessary to employ strictly controlled molding conditions, making difficult the molding.
In such a process, when a resin of high softening point is coated on the roughened surface of a copper foil in one layer, the resin shows low flowability during molding and does not satisfactorily flow so as to fill the concave portions of inner-layer circuit. When a resin of low softening point and accordingly high flowability is coated in one layer, the flow amount of the resin is too large and it is difficult to secure an insulating resin layer of uniform thickness, although the concave portions of inner-layer circuit can be filled. This problem can be solved by coating an interlaminar insulating adhesive in two layers consisting of a high-flowability layer and a low-flowability layer.
In order to achieve a fine-pitch circuit, the interlaminar insulating adhesive is required to further have heat resistance and low thermal expansion coefficient so that the accuracy during circuit formation and component mounting can be maintained. Many of interlaminar insulating adhesives of conventional type have a glass transition temperature of about 120° C. and therefore give rise to delamination of insulating layer.
Thermosetting resins typified by epoxy resin, etc. are widely used, for their excellent properties, in printed circuit boards and other electric or electronic appliances. They are, in many cases, allowed to have flame retardancy so that they are resistant to fire. Flame retardancy of these resins has generally been achieved by using a halogen-containing compound (e.g. brominated epoxy resin). These halogen-containing compounds have high flame retardancy, but have various problems. For example, brominated aromatic compounds release corrosive bromine or hydrogen bromide when heat-decomposed and, when decomposed in the presence of oxygen, may generate very toxic so-called dioxins such as polybromodibenzofuran and polydibromobenzoxin; further, disposal of bromine-containing waste materials is difficult. For these reasons, phosphorus compounds and nitrogen compounds have recently been studied as a flame retardant replacing bromine-containing flame retardants. For phosphorus compounds, however, there is a fear that when their wastes are used for land reclamation, they may dissolve in water and pollute rivers or soils. When a phosphorus component is taken into the skeleton of resin, a hard but fragile cured material is obtained, and therefore such a cured material often has problems in strength, impact resistance (when dropped), etc. when made into a thin layer of several tens of &mgr;m in thickness as used in the present invention. Further, resin compositions containing a phosphorus compound show high water absorption, which is disadvantageous from the standpoint of reliable insulation.
A study was made on a material which has excellent flame retardancy without containing any of halogen, antimony and phosphorus and which dose not cause the above-mentioned various problems. As a result, the present invention has been completed. The present invention provides a multilayer printed circuit board having a glass cloth-free insulating layer, which has excellent thermal property and which is low in thickness variation of the interlaminar insulating resin layer.
The present invention lies in an interlaminar insulating adhesive for multilayer printed circuit board containing the following components as essential components:
(a) a sulfur-containing thermoplastic resin having a weight-average molecular weight of 10
3
to 10
5
,
(b) a sulfur-containing epoxy or phenoxy resin having a weight-average molecular weight of 10
3
to 10
5
,
(c) a multifunctional epoxy resin having an epoxy equivalent of 500 or less, and
(d) an epoxy-curing agent.
In the present invention, the component (a), i.e. the sulfur-containing thermoplastic resin having a weight-average molecular weight of 10
3
to 10
5
is used so that (1) the resin flowability during press molding becomes low and the insulating layer formed can maintain an intended thickness, (2) the adhesive composition can have flexibility, and (3) the insulating resin layer can have improved heat resistance and reduced heat history. When the weight-average molecular weight is smaller than 10
3
, the flowability during molding is too high and the insulating layer formed is unable to maintain an intended thickness. When the weight-average molecular weight is larger than 10
5
, the component (a) has low compatibility with the epoxy resins and shows inferior flowability. The weight-average molecular weight of the component (a) is preferably 5×10
3
to 10
5
from the standpoint of the flowability. The sulfur-containing thermoplastic resin as component (a) is preferably amorphous because no crystal is formed when subjected to heat history of heating and cooling.
The component (a) includes polysulfone and polyethersulfone. The sulfur-containing thermoplastic resin, when modified with a hydroxyl group, a carboxyl group or an amino group at the terminal(s), has high reactivity with the epoxy resins; as a result, the phase separation between the sulfur-containing thermoplastic resin and the epoxy resins after h
Arai Masataka
Kawaguchi Hitoshi
Komiyatani Toshio
Uesaka Masao
Dawson Robert
Keehan Christopher M.
Smith , Gambrell & Russell, LLP
Sumitomo Bakelite Company Limited
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