Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2002-03-26
2004-10-12
Zarneke, David A. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S256000, C174S266000, C029S846000, C029S852000
Reexamination Certificate
active
06803528
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a multi-layer double-sided wiring board having wiring layers formed on both sides of an insulating layer using a two-layer tape technology that forms a conductive layer of copper or the like on an insulating layer of a polyimide film or the like without interposing an adhesive layer therebetween, and also having a blind via interconnecting the wiring layers and closed at one end with the conductive layer. The invention also relates to a method of fabricating such a multi-layer double-sided wiring board.
2. Related Art
A multi-layer double-sided wiring board having wiring layers formed on both sides of an insulating layer using a two-layer tape technology, and also having a blind via interconnecting the wiring layers, is fabricated in accordance with the following process.
First, starting with a two-layer tape comprising a film made of an insulating material such as polyimide and a conductive layer formed by sputtering or plating on the upper surface of the film, an opening whose upper end is closed with the upper conductive layer is formed in the insulating layer by selectively etching the lower surface of the film. At this time, a wiring pattern is formed on the upper surface by an additive process or a subtractive process. Next, a conductive layer is formed by sputtering or plating over the entire lower surface including the inside wall of the opening and the portion of the upper conductor which is exposed in the opening. In this way, the blind via is formed with the upper and lower conductive layers interconnected within the opening in a back-to-back configuration.
In the multi-layer double-sided wiring board fabricated in accordance with the above process, at first a sufficient bonding strength is retained between the polyimide and the copper sputtered thereon, but when heat or stress is applied in a subsequent process, the bonding strength decreases. In view of this, when producing a two-layer tape by forming a conductive layer on the upper surface of a polyimide film, it is practiced to deposit an interface layer containing a dissimilar metal such as Cr, Ni, or the like by sputtering on the film prior to the copper sputtering, thereby increasing the peel strength between the base and lead finished product.
On the other hand, careful consideration is required in joining the upper and lower conductive layers in the blind via, since the size of the joining portion is as small as tens of micrometers, the size tending to decrease with decreasing feature size. Japanese Unexamined Patent Publication No. 5-327224, for example, discloses that when sputtering copper, high-purity copper is used in order to prevent disconnection within the via hole.
According to the present invention, there is provided a multi-layer double-sided wiring board which provides excellent adhesion between the conductive layer and the insulating layer because of the provision of an interface layer therebetween, and which exhibits excellent conductivity reliability because the upper and lower conductive layers are joined together within the blind via without interposing a dissimilar metal.
PROBLEMS TO BE SOLVED BY THE INVENTION
To improve the bonding strength between the insulating layer and the lower conductor, it is also possible to interpose an interface layer between the insulating layer and the lower conductor by sputtering a dissimilar metal prior to the formation of the lower conductor, as is done for the upper conductor described above. In that case, however, since the structure is such that the dissimilar metal is sandwiched between the upper and lower conductive layers (copper) in the blind via, there arises the problem that the reliability of conductivity between the upper and lower circuit sides of the finished circuit board greatly degrades.
In the second method, on the other hand, while measures have been taken to ensure good bonding between the copper patterns based on advanced technological studies, no solution has been provided for the adhesion to the base material.
It is, accordingly, an object of the present invention to provide a multi-layer double-sided wiring board and a method of fabricating the same, wherein the above two problems, i.e., the adhesion between the lower conductive layer and the insulating layer and the reliability of conductivity between the upper and lower conductive layers in the blind via, are solved simultaneously.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a multi-layer double-sided wiring board comprising: an insulating layer having an opening formed therein; a first conductive layer formed on an upper surface of the insulating layer; a second conductive layer formed on a lower surface of the insulating layer and covering an inside wall of the opening and a portion of the first conductive layer which is exposed in the opening; and an interface layer interposed between the insulating layer and the first and second conductive layers, wherein the second conductive layer directly contacts the first conductive layer in the opening without interposing the interface layer therebetween.
The interposition of the interface layer provides good adhesion between the insulating layer and the second conductive layer, and at the same time, conductivity reliability is retained since the first and second conductive layers contact each other in the opening.
Preferably, the second conductive layer directly contacts the insulating layer at the inside wall of the opening without interposing the interface layer therebetween.
In this case, while the adhesion between the second conductive layer and the inside wall of the insulating layer is not sufficient, this portion acts as a stress reliever for the conductive layer and thus serves to secure the conductivity reliability of the conductive layer.
Preferably, the interface layer contains at least one metallic element selected from the group consisting of nickel, cobalt, zinc, and chromium.
Also provided is a method of fabricating a multi-layer double-sided wiring board, comprising the steps of: selectively removing a portion of an insulating layer on an upper surface of which is formed a conductive layer, and thereby forming in the insulating layer an opening whose upper end is closed with the conductive layer; forming an interface layer over an entire lower surface; selectively removing at least a portion of the interface layer which contacts the first conductive layer; and forming a conductive layer over the entire lower surface.
Also provided is a method of fabricating a multi-layer double-sided wiring board, comprising the steps of: forming an opening pattern by selectively removing a portion of a conductive layer formed on a lower surface of an insulating layer with an interface layer interposed therebetween, the insulating layer also having a conductive layer formed on an upper surface thereof with an interface layer interposed therebetween; selectively removing a portion of the insulating layer as well as a portion of the interface layers by using the thus patterned lower conductive layer as a mask, and thereby forming in the insulating layer an opening whose upper end is closed with the upper conductive layer; and forming a conductive layer over the entire lower surface including a side wall of the opening and a portion of the upper conductive layer which is exposed in the opening.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1
shows a cross section illustrating the vicinity of the blind via
18
in the previously described multi-layer double-sided wiring board in which the interface layer
14
is simply interposed between the insulating layer
10
and the lower conductive layer
12
. In this case, the reliability of conductivity significantly degrades because the interface layer
14
of a dissimilar metal is formed interposing between the upper conductive layer
16
and lower conductive layer
12
in the blind via
18
where they are interconnected.
FIG. 2
shows a cross section illustrat
3M Innovative Properties Company
Gover Melanie G.
Patel I B
Zarneke David A.
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