Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
1998-12-16
2001-03-13
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S308100, C361S310000, C338S313000
Reexamination Certificate
active
06201682
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a thin-film component comprising a substrate of an electrically insulating material, which is provided with two U-shaped side contacts as well as with an electrical structure, which is electrically connected to both side contacts. Examples of such electrical structures include resistive films, capacitor structures and coil structures as well as combinations thereof.
A thin-film component of the type mentioned in the opening paragraph is known per se. For example, in U.S. Pat. No. 4,453,199, a description of a thin-film capacitor of this type is given. This capacitor more particularly comprises a substrate of glass on which a capacitor structure is provided by means of vapor-deposition and sputtering. This capacitor structure is composed of electrode layers of aluminum or nickel and of dielectric layers of silicon dioxide. Said capacitor structure is further provided with a passivating or insulating cover layer of a synthetic resin, such as polyimide. The capacitor is constructed such that the capacitor structure is situated between the substrate and the cover layer. Said known component also comprises two side contacts. These side contacts are composed of a three-layer structure of chromium, nickel and silver, which is provided by means of sputtering.
In practice, it has been found that thin-film components of the known type have an important drawback, namely that an unacceptably high percentage of this type of components fails standard bending tests. In these tests, the electrical functioning of the components is monitored while the printed circuit board (PCB) on which the component is soldered is bent through a specific angle. Such bending tests give an indication of the possibility and, if so, the reliability of employing such components on flexible PCBs and on PCBs which are subjected to vibrations.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate the above-mentioned drawback. The invention more particularly aims at providing a thin-film component whose ability to endure standard bending tests is better than that of the known components.
These and other objects of the invention are achieved by means of a thin-film component of the type mentioned in the opening paragraph, which, in accordance with the invention, is characterized in that a flexible layer is situated between the substrate and at least one of both legs of each of the U-shaped side contacts, which flexible layer is provided directly onto the substrate surface, and in that the modulus of elasticity of the material of the layer is smaller than 50 GPa.
The invention is also based on the experimentally gained realization that, after soldering the known component onto a PCB, mechanical stress concentrations may occur between the leg of the side contacts soldered to the substrate and the substrate itself. By means of visual inspection of a number of components which failed the bending tests, it has been established that these stress concentrations cause fracture in the substrate. This frequently results in undesirable changes in the electrical function of the component. The percentage of components which fail standard bending tests can be reduced substantially by providing a flexible layer between the leg to be soldered of both U-shaped side contacts. It has been found that the flexible layer is capable of coping with said stress concentrations.
It is noted that a flexible layer is to be taken to mean a layer of a material whose modulus of elasticity is smaller than 50 GPa. The modulus of elasticity of a material is the force per unit of material surface necessary to bring about a certain degree of deformation in said material. In practice, materials whose modulus of elasticity exceeds 50 GPa prove to be insufficiently flexible to absorb said stress concentrations to a sufficient degree. The best results are achieved with flexible layers of materials whose modulus of elasticity is smaller than 10 GPa.
In principle, all kinds of substrates of electrically insulating material can be used in a component in accordance with the invention, such as substrates of inorganic material, for example sintered substrates, for example, of aluminum oxide. The invention can also successfully be employed in thin-film components having a substrate of silicon. But most advantageously, a substrate of glass is used in the thin-film component. However, the problem of fracture caused by stress concentrations around the soldered side contacts is found to be most prominent in components having a glass substrate. It is noted that the modulus of elasticity of glass is approximately 100 GPa.
An advantageous embodiment of the thin-film component in accordance with the invention is characterized in that the flexible layer is provided on the surface of the substrate facing away from the electrical structure, and in that the layer substantially completely covers this surface. This embodiment offers very substantial production-technical advantages in the manufacture of thin-film components from glass wafers. In the case of the embodiment in which the electrical structure is provided on one main surface of the substrate and the flexible layer is provided on the other main surface, the flexible layer can be applied in a single run, without lithographic steps and etching steps, so as to substantially completely cover said main surface of the wafer. This aspect of the invention will be explained in greater detail in the examples.
DETAILED DESCRIPTION OF THE INVENTION
The invention will now be described in greater detail with reference to the figures of the drawing.
It is noted that the components in accordance with the last-mentioned embodiment can only be soldered on to a PCB in such a way that the electrical structure faces away from said PCB. The present invention can alternatively be used in components of which the electrical structure, after the soldering treatment, faces the PCB. In that case, the flexible layer and the electrical structure have to be provided on the same main surface of the substrate, and the flexible layer is preferably present only at the location of the leg of both U-shaped side contacts. This requires additional lithographic steps and etching steps when the flexible layer is provided. Finally, it is noted that the current invention can also be used in thin-film components which, in both configurations mentioned above, should be solderable onto a PCB. In that case, the flexible layer has to be present on both main surfaces of the substrate.
Another interesting embodiment of the thin-film component is characterized in that the flexible layer consists mainly of an organic synthetic resin. It has been found that, in particular, a large number of organic synthetic resins, especially organic polymers, have the desired modulus of elasticity. A polymer which can very suitably be used for this purpose is polyimide. Apart from the desired flexibility, the adhesion of this polymer to glass substrates is excellent.
It has been found that the thickness of the flexible layer preferably ranges between 5 and 50 micrometers. In the case of layers having a thickness below 5 micrometers, the effect of the invention (improved resistance in bending tests) can hardly be observed. If the layers are thicker than 50 micrometers, problems in the manufacture of the individual components from the wafers occur. An optimum compromise between both adverse effects is achieved when use is made of flexible layers having thicknesses in the range between 10 and 40 micrometers.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 4453199 (1984-06-01), Ritchie et al.
patent: 4847136 (1989-07-01), Lo
patent: 4855872 (1989-08-01), Wojnar et al.
patent: 5111179 (1992-05-01), Flassayer et al.
patent: 5637382 (1997-06-01), Kataoka et al.
Mooij Jan H.
Venema Joghum P.
Wagemans Henricus H.M.
Dinkins Anthony
Spain Norman N.
U.S. Philips Corporation
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