Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-10-18
2001-11-13
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S259000, C029S830000
Reexamination Certificate
active
06316732
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to printed circuit boards and substrates for chips and chip on board. In particular, the present invention is related to methods of producing cavities in single or multiple layer printed circuit boards or substrates.
BACKGROUND OF THE INVENTION
Printed circuit boards (PCB's), including rigid, flexible, single, double and multilayer boards have found important uses in the semi-conductor and electronic industry. In particular, PCB's with cavities are required to housing electronic devices, and to facilitate wire bonding of chips.
Conventional methods of fabricating a PCB board with a cavity involves cutting out a window in an epoxy or dielectric core layer, and another window from an adhesive or bond film. The two windows are then matched or registered before the bond film and the core layer are laminated onto a second core layer. The bond film is positioned in between the two core layers and cures during the laminating process of heating under pressure such that it acts as a fully cured epoxy-glass layer after lamination.
Two types of adhesive or bond films are available on the market. The traditional or “high flow” film, and the newer “no flow” film. The “high-flow” bond film is in a relatively uncured state before lamination, and tends to seep into the cavity during the lamination process, partially or completely filling the cavity, thus destroying the usefulness of the board. The “no-flow” type bond film will minimize seepage, but is much more expensive, and tends to trap air during the lamination process. Furthermore, due to the pre-cured nature of the material, it does not easily flow around the window edge. Thus the film may not form a good seal at the base of the cavity after lamination, even if the windows are painstakingly registered to high precision.
In order to ensure the integrity of the cavity during the lamination process, and at the same time maintain the quality of the board after lamination, techniques have been proposed which uses the high-flow bond film, such as in U.S. Pat. No. 4,680,075. Another document, U.S. Pat. No. 5,784,782, further describes a method of using a visco-plastic material which is laid over the registered windows to prevent the high flow bond film material from seeping into the cavity during lamination. These techniques, however, are still rather tedious, due to the window cutting and registering steps.
OBJECT OF THE INVENTION
It is therefore an object of the present invention to provide a method to overcome the shortcomings as stated above.
It is another object of the present invention to provide a simple and timesaving method for producing a printer circuit board with a cavity.
It is a further object to provide a printed circuit board which has a well-defined cavity.
SUMMARY OF THE INVENTION
The above objects are generally accomplished by using a low-flow bond film without a window, and to allow this layer to act as the base of the cavity. Thus, according to one feature of the invention, a top core layer having a window is laid on top of the bond film. Since the bond film does not have a window, the tedious step of registering different windows is completely eliminated. According to a further feature of the invention, the thickness of the top core layer from where the cavity will be derived is adjusted to thicker than or the same as the depth of the cavity in the final printed circuit board.
In one embodiment of the invention, the bond film without window is laid on top of a non-stick layer. The top core layer with the window corresponding to the final cavity, is laid on top of the bond film. A non-stick release film is then placed on top of the top core layer, followed by a conforming sheet. This layer structure is then laminated under heat and pressure. As the bond film is cured on top of a non-stick surface, the entire structure can subsequently be released from the non-stick surface to form a board with the cured bond film forming the bottom layer and the base of the cavity.
In another embodiment, the bond film without a window corresponding to the window in the top core layer is laid on top of a bottom core layer. The top core layer, release film and conformer sheet is then place on top of the bond film as in the previous embodiment. The product after lamination is a board with two core layers, with the bottom core layer and the cured bond film forming the base of the cavity.
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Conley & Rose & Tayon P.C.
Gaffin Jeffrey
Gul Technologies Singapore Ltd.
Norris Jeremy
LandOfFree
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