Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-02-17
2003-12-16
Cuneo, Kamand (Department: 2827)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S737000, C174S050510
Reexamination Certificate
active
06665192
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of providing a synthetic resin capping layer on a printed circuit, said printed circuit comprising a printed circuit board which is provided with at least one electric component, and the capping layer exhibiting a variation in mechanical properties in a direction at right angles to said capping layer. The invention also relates to a printed circuit which is provided with a synthetic resin capping layer, said printed circuit comprising a printed circuit board having at least one electric component, and the capping layer exhibiting a variation in mechanical properties in a direction at right angles to said capping layer. The invention further relates to a portable apparatus, in particular a mobile telephone, comprising a printed circuit obtained by means of such a method.
A method of the type mentioned in the opening paragraph is known. For example, in European Patent Application EP-A-0650315, inter alia, a description is given of a method in which a printed circuit is provided with a capping layer composed of a stack of various layers having different mechanical properties. The capping layer comprises a relatively thin innermost insulation layer, which is contiguous to the printed circuit and accurately follows its contours, which insulation layer is provided with a relatively soft supporting layer which, in turn, is covered with an outermost layer which, in comparison with the supporting layer, is relatively hard and stiff, so that the stack of layers provides for a satisfactory mechanical protection of the printed circuit. To enhance the recyclability, the layers are made from the same kind of synthetic resin, for example a polyurethane. Differences in mechanical properties are obtained by varying both the type and the quantities of the starting materials. Both the supporting layer and the outermost layer are obtained by accommodating the printed circuit in a suitable mould, whereafter a two-component polyurethane casting resin is poured into the mould and the whole is cured for some time, for example 15 minutes. This method of providing a capping layer exhibiting a variation in mechanical properties in a direction at right angles to said layer is labor-intensive because each layer requires a separate moulding process. In addition, the process in which a moulding resin is moulded so as to form a layer is characterized by a long residence time in the mould which, from a practical point of view, leads to an unattractive process, in particular if the mould does not form part of the finished product. In addition, said method leads to a capping layer in which the mechanical properties differ discontinuously from layer to layer. Owing to differences between the coefficients of expansion, mechanical stresses and temperature-induced stresses will, in particular, strain the interfaces between the layers. These stresses could be reduced by using a larger number of layers, however, this leads to a proportionally longer duration of the manufacturing process.
SUMMARY OF THE INVENTION
One of the objects of the invention is to obviate these drawbacks. The invention particularly aims at providing a method of applying a synthetic resin capping layer to a printed circuit, said capping layer being applied, by means of a single-layer process, in a short period of time of, typically, less than 5 or better still 1 minute, and said capping layer exhibiting a variation in mechanical properties in a direction at right angles to the capping layer, so that the circuit is properly protected against mechanically and thermally induced stresses. However, the application of this method must not cause damage to the printed circuit.
This object is achieved by a method of the type mentioned in the opening paragraph, which is characterized, in accordance with the invention, in that the capping layer is provided on the printed circuit by injection moulding of a foam-forming reactive injection-moulding material.
Reactive injection moulding (abbr. RIM) proves to be a technique which can very suitably be used to provide printed circuits with a capping layer. The use of a foam-forming reactive injection-moulding material results in a capping layer whose mechanical properties, viewed from the side facing the printed circuit to the side facing away from the circuit, change from the properties belonging to a foam, that is, relatively soft and elastically compressible, to the properties of a solid material, that is, hard and stiff (in torsion). In fact, due to the proximity of the printed circuit, the regions of the capping layer immediately adjacent to the surface facing the printed circuit are somewhat more dense and stiff than the regions of the capping layer which are somewhat further removed from said surface. However, in the context of the present invention this slightly more intricate pattern of variation is relevant. Such a construction of the capping layer provides the parts of the circuit thus covered with an excellent mechanical protection. For example, shocks and vibrations are effectively damped by the soft interior and the hard outer portion prevents damage to the connections between the components and the printed circuit board, for example, by applied torsional forces. The mechanical properties change continuously, which leads to a uniform spread of the mechanical strain. Since the side facing the circuit is elastic and compressible, any expansion or shrinkage caused by changes in temperature during operation of the circuit can be effectively dealt with. As the capping layer will exhibit an improved heat conduction relative to still air, said capping layer stimulates heat dissipation and, as a result, the thermal strain on the circuit is reduced. Another advantage resides in that, if the thickness of the capping layer provided is sufficient, there is a great freedom of design regarding the side of the capping layer facing away from the circuit, so that said capping layer can also be used for constructive or decorative purposes. In a particularly advantageous embodiment, the capping layer also forms the housing of a device which comprises the circuit. In another, equally advantageous embodiment of the method, (a part of) the housing, which is formed, for example, in a preceding process step by means of injection moulding of a thermoplastic material, is used as the injection mould for applying the capping layer.
The provision of the capping layer by means of reactive injection moulding also has the advantage that the process is economically attractive. By virtue of the short residence times in the injection mould, said process is particularly suited for mass production. The inventors have found that the low viscosity and the foam-formation of the reactive injection-moulding material cause the thermal and the mechanical load on the circuit, caused by filling the mould and curing the reactive injection-moulding material, to be very small, so that the circuit is not damaged by the implementation of the method. In the context of the invention, a clear distinction should be drawn between reactive injection moulding and injection moulding of, whether or not elastically compressible, thermoplastic materials. The inventors have found that, in the latter technique, the thermal and mechanical loads are so high during filling the mould that the connections between the components and the printed circuit board can be damaged or even broken, which results in a defective circuit.
Reactive injection moulding is a technique which is known per se. In said technique, a reactive injection-moulding material is injected into an injection mould, whereafter said material reacts, often at an elevated temperature, while curing in the desired shape. As the reactive injection-moulding material is not prepared until just before the injection by intensively mixing, in general, two mutually reactive starting materials in a cavity which is isolated from the mould, highly reactive injection-moulding materials can be used which enable short cycle times to be achieved. The provis
Broeksema Egbert
Deeben Henricus A.C.
Friedl Reinhold Wimberger
Biren Steven R.
Cuneo Kamand
Dinh Tuan
Koninklijke Philips Electronics , N.V.
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