Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2002-02-25
2003-09-23
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S212000, C428S215000, C428S220000, C428S461000, C428S447000, C428S704000, C174S250000, C174S251000, C174S258000, C174S259000
Reexamination Certificate
active
06623844
ABSTRACT:
BACKGROUNG OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-layer wiring board suited for a package for holding a semiconductor device and to a method of producing the same.
2. Description of the Related Art
In recent years, electronic equipment have been produced in ever small sizes. Recent development of portable data terminals and widespread use of a so-called mobile computing operated while carrying a computer with the user, are further demanding a multi-layer wiring board fabricated in a further decreased size, in a more reduced thickness and having more highly dense wiring layers.
There have further been widely used electronic equipment that must operate at high speeds as represented by communication equipment. The demand for a high-speed operation includes a variety of requests such as precise switching for the signals of high frequencies. To cope with such electronic equipment, it has been urged to provide a multi-layer printed wiring board suited for high-speed operation.
To carry out a high-speed operation, it is necessary to shorten the length of the wiring thereby to shorten the time required for the electric signals to propagate. In order to shorten the length of the wiring, it has been demanded to provide a small and thin multi-layer wiring board having wirings of a small width and a highly dense wiring layer having a small gap among the wirings.
In order to cope with a demand for such a highly dense wiring layer, there has been employed a production method called build-up method. Under the JPCA standard, the basic structures of the build-up method can be divided into two, i.e., (1) a base+build-up method, and (2) a whole layer build-up method.
(1) In the base+build-up method, the multi-layer wiring board is produced in a manner as described below.
That is, there is prepared a core board which is an insulating board such as a glass epoxy substrate having, on the surfaces thereof, a conductor wiring layer formed of a metal foil like a copper foil and having, as required, via-conductors.
A photosensitive resin is applied onto the surfaces of the core board and is developed by being exposed to light thereby to form a photosensitive insulating layer having through-holes.
Then, a metal layer such as of copper is plated onto the whole surfaces (inclusive of through-holes) of the photosensitive insulating layer, a photosensitive resist is applied onto the plated layer, which is, then, developed and etched by being exposed to light through a mask of the shape of a circuit pattern to fabricate a conductor wiring layer.
Steps are repeated for applying the photosensitive resin and for forming the conductor wiring layer on the thus formed insulating layer having the conductor wiring layer, thereby to obtain a multi-layer wiring board.
(2) A method of producing a multi-layer wiring board by the whole layer build-up has been disclosed in, for example, Japanese Patent No. 2587593. That is, through-holes are formed in the insulating sheet by a laser and are filled with an electrically conducting paste thereby to form via-conductors in the insulating sheet. Then, a conductor wiring layer is formed on the surface of the insulating sheet so as to be electrically connected to the via-conductors, thereby to obtain a wiring sheet. The thus formed wiring sheet is repetitively laminated thereby to obtain a multi-layer wiring board.
According to the base+build-up method (1), however, the photosensitive epoxy resin used for forming the insulating layer has a low glass transition point and is sensitive to light involving such a defect that it absorbs water much. That is, when left to stand under high-temperature and high-humidity conditions, the multi-layer wiring substrate obtained according to this method arouses a problem of decreased insulation. It can be further contrived to form an insulating layer by using a resin which absorbs water little, such as an arylated polyphenylene ether (A-PPE) resin or a BT (bismaleimidetriazine) resin instead of using the epoxy resin. However, the resin which absorbs water little has a low polarity and exhibits poor wettability to the surface of a metal which has a high polarity. That is, with the multi-layer wiring board in which the insulating layer is formed of a resin that absorbs little water, the adhesion or intimate contact is not accomplished between the insulating layer and the conductor wiring layer, very easily permitting the passage of water that is a factor for deteriorating the characteristics of the wiring board. When the pitch is decreased among the via-conductors that are formed in many number in the insulating layer, therefore, the insulating resistance decreases among the via conductors. When the diameter of the through-holes forming the via-conductors is decreased, further, a problem such as an increase in the resistance of the via-conductors appears conspicuously making it very difficult to fabricate a highly dense wiring board.
Further, the surface of the core board is rugged to a degree comparable to the thickness of the conductor wiring layer formed of the copper foil, and the photosensitive resin used for the build-up method is a liquid. Accordingly, ruggedness in the surface of the core board is reflected up to the surface of the multi-layer wiring board that is obtained. Such a multi-layer wiring board is not suited for the applications where flatness is required, such as mounting the silicon chips like flip chips. Through the surface temperature cycle testing and high-temperature high-humidity testing, further, it has been confirmed that the peeling easily occurs on the interface between the core board and the insulating layer that is built up. It is considered that the peeling on the interface stems from the presence of the above-mentioned ruggedness.
A variety of countermeasures have been proposed for solving the above-mentioned problems. The journal “Electronics Mounting Technology”, Vol. 14, No. 1, January 1998 teaches a flat multi-layer wiring board obtained by transferring a circuit pattern formed on a stainless steel by the pattern-plating method onto the surface of an insulating layer that is built up relying upon the laminate pressing method. However, the adhering force has not been improved yet between the core board and the insulating layer that is built up still leaving a problem concerning reliability.
According to the whole layer build-up method (2), the via-conductors are formed by filling the through-holes with the electrically conducting paste. Through the reliability testing such as high-temperature standby testing and pressure cooker testing (PCT), however, it has been confirmed that the via-conductors are oxidized causing an increase in the electric resistance. When the pitch of the via-conductors is decreased, further, the insulation resistance among the via-conductors decreases like that of the above-mentioned base+build-up method (1). That is, since the adhering force is poor on the interface between the insulating layer and the conductor wiring layer or the via-conductors, water which is a factor of deterioration easily infiltrates into the insulating resin layer. Besides, water that has infiltrated into the insulating layer further infiltrates into the via-conductors arousing a problem of rise in the resistance of the via-conductors.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a highly reliable multi-layer wiring board which features an increased adhering force between the conductor wiring layer and the insulating layer, prevents water from infiltrating into the interface between the conductor wiring layer and the insulating layer, prevents water from infiltrating into the via-conductors from the insulating layer, and does not permit characteristics to be deteriorated even after left in a high-temperature and high-humidity environment for extended periods of time.
According to the present invention, there is provided a multi-layer wiring board comprising insulating layers of a thermosetting resin having
Hogan & Hartson LLP
Jones Deborah
Kyocera Corporation
Xu Ling
LandOfFree
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