Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-06-04
2002-03-26
Paladini, Albert W. (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S256000, C174S259000
Reexamination Certificate
active
06362433
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a flexible printed circuit board for establishing the electrical connection in, for example, an electronic apparatus. More particularly, the present invention relates to a flexible printed circuit board which incorporates a conductor, such as copper foil which is directly applied with polyimide resin, and which has an insulating protective layer made of the polyimide resin and formed thereon.
2. Description of the Related Art
To reduce the size and cost of a so-called portable electric product, such as a portable recording/reproducing apparatus, the flexible printed circuit board is usually employed. The flexible printed circuit board establishes the connection among the electric circuits to one another, the flexible printed circuit board being a relatively low cost board which permits space saving.
In the industrial field of the flexible printed circuit boards, there are requirements for reducing the sizes of the electronic apparatuses and permitting a multiplicity of functions to be provided. Therefore, techniques have rapidly been progressed, the techniques including raising of the mounting density of the circuits, surface mounting which uses wire bonding, direct mounting of a semiconductor chip and packaging. Therefore, the shape of the flexible printed circuit board becomes more complicated. Moreover, small circuits for use in the dense mounting process have been required.
The conventional flexible printed circuit board incorporates a circuit formed by etching a flexible printed board which is a base board. Then, an insulating protective layer (a cover layer) is formed to protect the circuit.
The flexible printed circuit board, which is the base board, for use in the flexible printed circuit board usually incorporates polyimide films because of their satisfactory flexibility and heat resistance. The flexible printed board is exemplified by a triple-layer flexible printed board constituted by bonding a polyimide film and a copper foil, which is a conductor, to each other through a heat-resisting adhesive.
However, the foregoing triple-layer flexible printed board suffers from unsatisfactory heat resistance of the adhesive. Thus, the adhesive is easily deformed owning to heat. Therefore, heat hysteresis, such as heat crimping, which is performed when a bonding process is performed by using the adhesive causes warp or curl of the board to occur. As a result, there arises problems in that a small circuit pattern cannot easily be formed and that surface mounting which uses wire bonding cannot easily be performed.
To overcome the problems experienced with the triple-layer flexible printed board, a double-layer flexible printed board
53
has been suggested and put into practical use. As shown in
FIG. 1
, no adhesive is employed and polyamic acid, which is a precursor for the polyimide, is directly applied to the surface of metal foil
51
made of copper or the like. Then, the polyamic acid is dried, and then it is turned into an imide so that a polyimide resin layer
52
is laminated so that the double-layer flexible printed board
53
is manufactured. A flexible printed circuit board
50
incorporating the double-layer flexible printed board
53
, as shown in
FIG. 2
, has a circuit formed thereon. Moreover, an insulating protective layer
54
is formed on the circuit. In
FIG. 2
, a portion of the metal foil
51
which is not coated with the insulating protective layer
54
and which is exposed to the outside is a portion which will be formed into a terminal.
Also the double-layer flexible printed board
53
must be subjected to a high-temperature process in which the polyamic acid applied to the surface of the metal foil
51
is turned into an imide. Although the polyimide resin layer
52
having heat resistance superior to that of the foregoing adhesive is employed, the difference between the coefficient of linear thermal expansion of the metal foil
51
and that of the polyimide resin layer
52
causes the following difference. That is, the thermal contraction ratio becomes different between the metal foil
51
and the polyimide resin layer
52
after the temperature has been lowered to room temperature. Thus, a curl is undesirably formed. If the double-layer flexible printed board
53
is curled, an accuracy of the interval between conductors of the circuit deteriorates after the etching process has been performed. What is worse, mounting of elements cannot easily be performed.
Therefore, a method of preventing the curl of the double-layer flexible printed board has been suggested. The method has the step of specifying the chemical structure of the polyimide to be formed. To minimize the coefficient of linear thermal expansion of the polyimide resin, a method has been suggested with which the structure of the polyamic acid, which is the precursor, is specified.
However, either of the methods of preventing the curl of the double-layer flexible printed board cannot easily completely prevent the curl. When the circuit has been formed by etching the metal foil, the curl cannot be removed.
The foregoing conventional methods have been adapted to the double-layer flexible printed board
53
, which is the base board on which the insulating protective layer
54
has not been formed. Therefore, no investigation has been made about the flexible printed circuit board
50
which incorporates the double-layer flexible printed board
53
and the insulating protective layer
54
for the circuit which are integrated with each other.
The industrial field of the flexible printed circuit boards have recently be required to be technically progressed to realize, for example, dense mounting. Since the semiconductor chip is mounted by using soldering, the required performance of the flexible printed circuit board can satisfactorily be realized when the flexible printed board and the insulating protective layer for the circuit are considered as one device. Therefore, prevention of the curl of the double-layer flexible printed board
53
is insufficient to realize the performance. As a result, a state of the flexible printed circuit board
50
incorporating the insulating protective layer
54
mounted integrally must be investigated.
The insulating protective layer
54
(the cover layer) for use in the flexible printed circuit board
50
must have heat resistance. Therefore, the insulating protective layer
54
is usually made of the polyimide material. The insulating protective layer
54
can be formed by a printing method or a film method.
The printing method employs a step similar to a step which is performed with a hard printing plate to print resist ink by using a silk screen. Since the resist ink is mainly composed of epoxy resin, the foregoing method has a problem of unsatisfactory flexibility. If the resist is not selected in sufficient consideration of the thermal expansion ratio of the resist, the flexible printed circuit board undesirably encounters a curl. Since solvent having a high polarity and contained in the polyimide resist ink absorbs water in the atmosphere, satisfactory printing workability cannot be realized. Thus, there arises a problem in that the thicknesses of the films cannot easily be administered.
On the other hand, the film method has the steps of forming apertures in portions corresponding to the land portions and terminal portions by using a die or a punch. Then, a polyimide film applied with an adhesive and a flexible printed board having a circuit formed thereon are bonded to each other by heat crimping or the like. Thus, a flexible printed circuit board is manufactured.
The foregoing method, however, encounters difficulty in forming fine land portions and terminal portions though the accuracy of the die or the like is improved. Moreover, the method has a problem in that leakage of the adhesive contaminates the fine circuit. When the flexible printed circuit board is mounted on the semiconductor chip or the like, the flexible printed circuit board must have flatness. When the
Arai Akihiro
Arai Koji
Ichihara Osamu
Kudo Noriaki
Oaku Satoshi
Alcalá José H.
Maioli Jay H.
Paladini Albert W.
Sony Chemicals Corporation
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