Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
1999-11-30
2001-08-14
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C174S254000, C174S258000, C174S259000
Reexamination Certificate
active
06274225
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a self-shape retaining rigid circuit member which has been bent into a predetermined shape.
The present invention also relates to a circuit board adapted to be connected to a rigid substrate having a conductor printed thereon, e.g., glass substrate having a transparent electrode printed thereon.
The present invention further relates to a printed circuit board, particularly a flexible printed circuit board, as a functional part mainly for use in the art of electric and electronic apparatus.
BACKGROUND TECHNIQUE
As a circuit member in electric and electronic apparatus there is normally used a flexible circuit board. In some cases, a self-shape retaining circuit member obtained by bending a rigid circuit board into a predetermined shape may be used.
For example, as a circuit member provided between a battery and an apparatus driving portion there has heretofore been used a flexible printed circuit board. It has been a common practice that a metal tab as a contact point to apparatus is mounted on the terminal of the flexible circuit substrate on which electronic parts are mounted and the metal tab is connected to the apparatus driving portion. However, the mounting of the metal tab on the flexible circuit board can cause thermal shock that deteriorates the reliability of the circuit board. In recent years, an attempt has been made to use a self-shape retaining circuit board obtained by stamping a conductor circuit integrated with a metal tab out of a metal foil, interposing the circuit board between rigid plastic films with an adhesive layer to form a composite laminate, and then bending the composite laminate under press.
Such a self-shape retaining circuit member needs to comprise a tough rigid plastic film, i.e., plastic film having an elastic modulus of not less than 450 kg/mm to withstand press molding and assure stable self-shape retention.
As well known, when heated, a plastic film tends to shrink due to residual stress or recrystallization during film forming.
A circuit member is a composite of a metal foil with a plastic film. Assuming that the percent thermal shrinkage, elastic modulus and thickness of the plastic film are &zgr;, Ep and tp, respectively, and the elastic modulus and thickness of the metal foil are Em and tm, respectively, the percent thermal shrinkage X of the circuit member is given by the following equation:
X
=&zgr;/(1
+Em·tm/Ep·tp
)
Since a conventional flexible printed circuit board comprises a plastic film having a small elastic modulus of Ep and a small thickness of tp, its entire percent thermal shrinkage is small as evident from the foregoing equation.
However, the foregoing self-shape retaining circuit member needs to comprise a rigid plastic film having a high elastic modulus and a large thickness of tp to withstand press molding and assure stable self-shape retention. Thus, Ep and tp cannot be reduced, and the resulting X value is increased. Therefore, the thermal expansion and shrinkage of the foregoing self-shape retaining circuit board itself cannot be neglected. The resulting thermal stress can break the point at which it is connected to apparatus or battery or the point on which an electronic part is mounted. Accordingly, assured excellent reliability can hardly be given to the foregoing self-shape retaining circuit member under severe thermal conditions.
It is therefore a first object of the present invention to provide a self-shape retaining rigid circuit member shaped into a predetermined shape by bending under press, which exhibits assured press-moldability and self-shape retention as well as assured reliability against severe thermal use.
In the assembly of electric and electronic apparatus, the connection of a circuit portion comprising a rigid circuit board to other circuit portions or power supply may be made via a flexible circuit board.
As such a flexible circuit board there has heretofore been normally used one comprising a polyethylene terephthalate film or polyimide film as a substrate or cover.
The connection of a rigid circuit board to a flexible circuit board has traditionally been accomplished by soldering. In recent years, in order to meet the demand for high precision in conductor on circuit board, it has been attempted to use an anisotropic electrically-conductive film.
An attempt has been made as follows. In some detail, an anisotropic electrically-conductive film, which comprises a film-shaped adhesive having a thickness of scores of microns and containing electrically-conductive particles incorporated therein such that both ends of the particle are exposed out of both sides of the film and the adhesive is interposed among the particles, is interposed between surfaces to be connected. The laminate is then heated under pressure so that the film-shaped adhesive undergoes flow deformation to connect the conductors on the surfaces to be connected through the electrically-conductive particles and fill the gap between the surfaces to be connected with the adhesive.
This connection process makes it possible to connect high precision circuits having, e.g., 20 conductors per mm, that is, conductor pitch of 50 &mgr;m.
In the case where two members a, b are superposed and connected to each other as mentioned above, if the two members a, b shrink at different expansion and shrinkage rates, stress is produced at the interface of the two members. In this case, taking the expansion and shrinkage rate, thickness and Young's modulus of the member a as Ka, Ta and Ea, respectively, and taking the expansion and shrinkage rate, thickness and Young's modulus of the member b as Kb, Tb and Eb, respectively, the resulting X is given by the following equation (1):
X
=(
Ka−Kb
)/[1/(
Ta·Ea
)+1/(
Tb·Eb
)] (1)
In order to reduce (Ka−Kb) in the equation (1) and hence reduce the resulting thermal stress with respect to heat cycle applied to the point at which the foregoing glass circuit board b and plastic flexible circuit board a are connected, a plastic substrate having a small thermal expansion coefficient can be used for the flexible circuit board to advantage.
A plastic expands or shrinks either when it absorbs moisture or it drys. Thus, stress is generated at the foregoing connecting portion either when the plastic substrate in the flexible circuit board absorbs moisture or it drys as evident from the equation (1)
Fatigue failure of the connecting portion on circuit board due to stress caused by thermal expansion and shrinkage has heretofore been considered problematical. However, stress caused by moisture absorption or drying has not too much been considered problematical from the standpoint of fatigue failure.
However, if the glass circuit board exhibits substantially zero hygroscopic and drying expansion and shrinkage rate and the rigid circuit board is such a glass circuit board, assuming that Kb in the equation (1), which is the hygroscopic and drying expansion and shrinkage rate of the glass circuit board, is 0 and Ka is the hygroscopic and drying expansion rate of the plastic flexible circuit board, the resulting stress Y is given by the following equation (2):
Y=Ka/[
1/(
Ta·Ea
)+1/(
Tb·Eb
)] (2)
Thus, the hygroscopic and drying expansion and shrinkage rate Ka of the plastic flexible circuit board is straightly reflected in the stress. The resulting stress cannot be neglected. The fatigue and damage caused by the stress cannot be neglected.
Among the foregoing flexible circuit boards, the circuit board comprising a polyethylene terephthalate film as a substrate exhibits a high thermal expansion and shrinkage rate and thus is not appropriate from the standpoint of prevention of fatigue and damage on the foregoing connecting portion due to stress. The circuit board comprising a polyimide film as a substrate exhibits a small thermal expansion rate but a high hygroscopic and drying expansion and shrinkage rate and thus is not appropriate from the standpoint of prevention of fatigue and damage
Miyaake Chiharu
Miyake Yasufumi
Sugimoto Toshihiko
Terada Tetsuya
Yagura Kenkichi
Lam Cathy
McGuireWoods LLP
Nitto Denko Corporation
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