Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1999-07-26
2001-12-25
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S551000
Reexamination Certificate
active
06333633
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for inspecting a flexible printed circuit.
The present application is based on Japanese Patent Application No. Hei. 10-221970, which is incorporated herein by reference.
2. Description of the Related Art
Flexible printed circuits (FPC) are used in various electric or electronic appliances. Each of the flexible printed circuits is constituted by an electrically insulating film of a polyimide resin or the like, and wiring patterns of metal foil of copper or the like formed on the electrically insulating film. Such a flexible printed circuit can be provided in a narrow space because it is flexible and thin. Accordingly, reduction in size and thickness of the appliances can be attained.
With the requirement of reducing the size and increasing the degree of integration in electric or electronic appliances, the inter-wiring pitch in the flexible printed circuit has been also narrowed in recent years. Accordingly, when elution of metal ions (ion migration) from a wiring material in the flexible printed circuit occurs, there is a tendency that inter-wiring short-circuiting occurs easily. For example, the ion migration causes the case where the life is shortened to 100 hours in a test under a condition of a temperature of 85° C. and a humidity of 85%.
Accordingly, flexible printed circuits being short-lived because of ion migration are required to be removed as defective products in an inspection stage before shipping. In the short-lived flexible printed circuits, it has been found that foreign matters such as ion impurities, fiber, etc. are present between wiring portions. Accordingly, defective products can be detected when inter-wiring foreign matters in each of the flexible printed circuits are detected by using a microscope.
It is, however, very inefficient to detect inter-wiring foreign matters in the flexible printed circuits individually by microscopic observation. This method is impracticable as an inspection method before shipping.
A method for performing inspection efficiently to detect short-lived flexible printed circuits before shipping has been, therefore, required.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an inspection method in which discrimination between good flexible printed circuits and poor ones can be made efficiently.
The present inventor has made various experiments and investigations. As a result, there has been obtained a knowledge that the quantity of change of an inter-wiring insulation resistance value during application of a voltage in a flexible printed circuit after moisture absorption has a specific relation to the life of the flexible printed circuit. The following invention has been designed on the basis of this relation.
According to the present invention, the method for inspecting a flexible printed circuit comprises the steps of: making moisture absorbed into a flexible printed circuit which is a subject of inspection; and making a judgment as to whether the flexible printed circuit is good or not on the basis of a quantity of change of an inter-wiring insulation resistance value under a condition of application of a predetermined voltage.
In the method for inspecting a flexible printed circuit according to the present invention, discrimination between good flexible printed circuits and poor ones can be made by electrical measurement because the discrimination between good flexible printed circuits and poor ones is made on the basis of the quantity of the change of an inter-wiring insulation resistance value during application of a voltage in a flexible printed circuit after moisture absorption. Accordingly, flexible printed circuits can be inspected efficiently.
Preferably, the method for inspecting a flexible printed circuit, comprising the steps of: making moisture absorbed into a flexible printed circuit which is a subject of inspection; and measuring a quantity of change of an inter-wiring insulation resistance value measured at regular intervals under a condition of application of a predetermined voltage to thereby conclude that the flexible printed circuit is good when a maximum value of the quantity of change of the insulation resistance value is not larger than a predetermined reference value.
In this case, discrimination between good flexible printed circuits and poor ones can be made easily and evenly by comparison of the maximum of the quantity of the change of the insulation resistance value with a predetermined reference value.
Preferably, the quantity of change of the insulation resistance value is expressed as a difference between common logarithms of the insulation resistance values at two points of time. In this case, the predetermined reference value may be 0.05.
When the quantity of the change of the insulation resistance value is expressed in a difference between common logarithms of insulation resistance values at two points of time, the bad influence of variations in inter-wiring distance and in inter-wiring applied-voltage is suppressed so that an accurate discrimination result is obtained.
Preferably, the flexible printed circuit as a subject of inspection is left under a condition of high temperature and high humidity for a predetermined time so as to absorb moisture. When the flexible printed circuit is left under a condition of constant temperature and constant humidity for a predetermined time so as to absorb moisture, the quantity of water absorption of the flexible printed circuit can be kept constant.
According to the present invention, there is provided a flexible printed circuit wherein a maximum value of a quantity of change of an inter-wiring insulation resistance value measured at regular intervals under a condition of application of a predetermined voltage after moisture absorption is not larger than a predetermined reference value.
When the maximum of the quantity of the change of an inter-wiring insulation resistance value measured at regular intervals under a condition of application of a predetermined voltage after moisture absorption is not larger than a predetermined reference value, the flexible printed circuit is long-lived because inter-wiring short-circuiting due to ion migration is not caused.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.
REFERENCES:
patent: 4183609 (1980-01-01), Luna
patent: 5744964 (1998-04-01), Sudo et al.
patent: 6-43194 (1994-02-01), None
Honjo Mitsuru
Kawamoto Ikuo
Miyake Yasufumi
Sugimoto Toshihiko
Kerveros J.
Metjahic Safet
Nitto Denko Corporation
Sughrue & Mion, PLLC
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