The present invention relates to a multilayer printed wiring board.
It is invariably necessary to make thinner the copper foil on the substrate of a printed wiring board due to current pattern formation technology, in order to realize a super high density circuit by using a conventional printed wiring board. When the copper foil is made thinner, however, the thickness of land on which a through hole plating is provided must be as much thinner. The junction area of the through hole plating and the land becomes small therefore, so that plating is easily peeled off. As a result, the reliability of connection cannot be insured.
The reliability of connection between the through hole plating and the land may be assured by making the thickness of copper foil of each laminate thick. However, such a configuration makes the miniaturization of a high density circuit difficult and the overall thickness of the super multilayer substrate becomes very thick. In addition, the drilling of through holes for establishing electrical connection between laminates becomes very difficult due to accuracy requirements. Furthermore, soldering for mounting electrical parts also becomes very difficult.
The printed wiring board of the invention permits the accurate forming of a miniaturized high density circuit by using a copper clad laminate in which the thickness of the copper foil at the land formation area is made thicker than the remaining area, as the laminates. This assures high reliability of connection between the through hole plating and the land.
The multilayer printed wiring board of the invention is often used in the electrical wiring of electronics devices. The manufacture of a multilayer printed wiring board may be roughly classified into the Subtractive Method and the Additive Method. The Subtractive method is generally utilized for a printed wiring board of which reliability and a high density characteristic are especially required for use in computer and communications equipment. A multilayer printed wiring board manufactured by the Subtractive Method is formed as follows.
A predetermined circuit pattern is formed on a copper clad board by an etching process for copper foil. The copper clad board is then sequentially laminated up to the specified layers with pre-preg as a bonding agent in order to form an intermediate layer. The copper clad board is then laminated again with the pre-preg as the surface layer. Thus, a through hole is drilled through the lands of the intermediate layer and the surface layer. Thereafter, the surface circuit pattern is formed and the through hole is plated in order to provide continuation between the surface circuit pattern and the intermediate circuit pattern.
The copper clad laminate used for the multilayer printed wiring board may generally be provided by bonding copper foil on a plate substrate of epoxy resin utilizing glass fiber as the core material. Therefore, the circuit pattern is formed on the intermediate layer by providing a photo resist pattern corresponding to a circuit pattern consisting of the land and the circuit conductor on the copper foil of the copper clad laminate and then etching the copper foil.
The integration of the multilayer printed wiring board becomes high in density and the miniaturization of the circuit becomes as high in density, so that the clearance of the circuit conductor becomes narrow. As a result, the pattern accuracy of the mask film, produced by art work, requires considerable improvement. Furthermore, a considerable improvement is required in the pattern generation accuracy of the etching process.
An ultra-thin copper clad laminate which is just suitable for high density and circuit miniaturization has been often used, recently. The ultra-thin copper clad laminate uses a copper foil having a thickness of 15 micrometers or .mu.m, or less. This is considerably thinner than the existing thickness of 35 .mu.m. This laminate therefore may prevent an overhang phenomenon in the pattern etching process, which occurs in a thick copper foil. Furthermore, since the copper foil itself is very thin, the final thickness after lamination may be considerably thinner than the existing thick copper clad laminate. In the manufacture of an ultra-thin multilayer printed wiring board with 20 to 30 layers by using the existing thick type copper clad laminate, for example, the final thickness becomes as thick as 3 to 5 millimeters or mm. A thick lamination produces a bad effect in the process after laminations. In other words, the through hole drilling process and soldering for mounting parts become difficult.
The foregoing explains why ultra-thin copper clad laminate has often been used recently. However, a problem has recently arisen due to the use of ultra-thin copper clad laminate. The problem concerns the reliability of the connection between the land and the through hole, and is emphasized by the thinness of the copper foil. That is, when an ultra-thin copper clad laminate is used, the reliability of the connection between the area exposed to the through hole on the land formed by the ultra-thin copper foil and the through hole plating is drastically deteriorated, since the plating is likely to peel off because the bonding area is very very narrow.
The principal object of the invention is to provide a multilayer printed wiring board which is suitable for manufacturing a high density miniaturized circuit pattern and insures high reliability.
An object of the invention is to provide a multilayer printed wiring board which assures excellent workability in the through hole drilling process and in the soldering process for mounting parts.
Another object of the invention is to provide a multilayer printed wiring board of satisfactorily high density.
Still another object of the invention is to provide a multilayer printed wiring board having a miniaturized circuit pattern formed with satisfactory accuracy.