GB2282271A - Chip inductor - Google Patents

Description

2282271 CHIP INDUCTOR The invention relates to a chip inductor, in

particular an RF choke, having a solid core part which has a perpendicular prismatic spatial shape, in particular a block, cube or cylinder, and which is composed of a ferromagnetic or electrically nonconducting material, in particular ferrite, ceramic or plastic, the core part having a winding space which can have single-layer or multilayer winding and is recessed with respect to the parallel butt ends of the core part and having contact elements glued to the butt ends.

Such a chip inductor is disclosed in EP 0 177 759 Al. In the chip inductor disclosed in that document, the end surfaces of these butt ends have mutually parallel cutouts which are open at the edge and extended over the entire narrow side or wide side of the end surfaces and which are suitable for receiving plate-like contact elements which can be glued into the cutouts. At the same time, the plate-like contact elements may initially be designed as detachable parts of a lead frame, the winding ends of the inductor being wound on said contact elements.

In this case, the precise and reproducible gluing of the core parts into the lead frame is the precondition for precision windings of inductors having, for example, less than 10 windings if a narrow inductance tolerance and a high reproducibility of centre inductance value and inductance variation are required.

The core parts which are supported by the lead frame and have cutouts open at the edge are therefore positioned in the lead frame with low lateral play of the contact elements in the cutouts, the spacing of the two contact elements being only slightly larger than the spacing of the two cutouts. The supporting of the core parts on the lead frame at the same time prevents the core parts positioned in the lead frame from falling out of the lead frame during transportation for the purpose of curing the adhesive.

The difficulties in handling the core parts which are disclosed in the prior art and have preferably dovetail cutouts are that the core parts may hook into one another when being automatically fed in a vibratory conveyor and that the breaking-off of the dovetail projections may also lead to manufacturing breakdowns. In addition, the production of said core parts is relatively expensive.

There is furthermore the danger in positioning the core parts which are open at the edge that the core parts may lift off the contact surfaces during transportation vibrations in the lead frame and, if the adhesive runs, may assume altered positions.

Furthermore, the torsional strength of the adhesive joint with core parts open at the edge is low since the maximum diameter of the adhesion surfaces is small. At the same time, the peel strength for the two sides of the cutouts open at the edge is low since a force acting on the contact element perpendicularly to the adhesive joint cannot be braced against the material of the core part.

The invention therefore seeks to provide a chip inductor in which the core parts are more precisely positioned in the lead frame and the adhesive joints have higher mechanical robustness.

According to the invention there is provided a chip inductor, having a solid core part which has a perpendicular prismatic spatial shape, and which is composed of a ferromagnetic or electrically nonconducting material, having a winding space which is recessed with respect to parallel butt ends of the core part wherein the end surfaces of said butt ends have f' 1 projections and in that said projections engage in cutouts disposed on respective contact elements which are glued to the respective butt end.

Advantageously, the projections and the cutouts 5 have a square or rectangular cross section.

For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 shows a core part according to one embodiment of the invention; and Figure 2 shows the core part of Figure 1 with contact elements glued on.

Figure 1 shows a solid core part 1 in block form which is composed, for example, of magnetic material, in particular ferrite, in the case of the manufacture of an RF choke. The core part 1 is designed with a winding space 4 which is recessed with respect to its parallel butt ends 2, 3 and in which the winding of the chip inductor is disposed.

The end surfaces 5, 6 of the butt ends 2, 3 are provided with projections 7, 8 which, according to the exemplary embodiment shown in Figure 1, are disposed over the entire height of the core part 1 and have a rectangular cross section.

Figure 2 shows the core part shown in Figure 1 after contact elements 9, 10 have been pushed over the projections 7, 8 and glued to the end surfaces 5,6. The contact elements 9, 10 have cutouts 11, 12 which are matched to the size of the projections 7, S. The electrical connections of the inductor disposed in the winding space 4 are connected to the contact elements 9, 10.

The advantages of the core parts 1 having butt-end projections 7, 8 over the dovetail core parts disclosed in the prior art include:

(i) invention (ii) fracture and (iii) that they can be produced more simply and, consequently, more inexpensively.

The advantage of the positioning in the cutouts 11, 12 in the lead frame 9, 10 is that the core parts 1 can lift off the support surfaces only within the tolerances of the cutout and are therefore able to alter their position less extensively during transportation in the lead frame.

The advantages of the adhesive joint in the cutouts 11, 12 of the lead frame are that the torsional strength is higher since the chosen maximum diameter of the adhesive surfaces can be, for example, 50% larger. Furthermore, the peel strength is increased in comparison with the hitherto incomplete core support and, finally, a three-dimensional supporting of the core part 1 in the lead frame 9, 10 is produced by the cutouts 11, 12 in the lead frame 9, 10.

The higher torsional strength is desirable, in particular, during the application of the winding in view of the torque acting on the core as a result of the winding tension and in view of the torsional stress during injection- moulding encapsulation with off-centre cavity partitioning in accordance with DE 40 23 141 Al.

The adhesive-joint torque stress is given by: M = (nr4Gy)/2d, where M is the active torque, r is the maximum radius of the adhesive areas, G is the modulus of torsion of the adhesive, (p is the torsion angle and d is the adhesive thickness.

A larger radius r therefore results in a lower torsion angle if M, G and d are unaltered and, consequently, in a lower torsional stress and, given that the core parts 1 of the present cannot hook into one another, that they are less susceptible to 1 the same torsion angle, makes possible the use of a less stable adhesive material. The use of a less stable adhesive material in turn enables a shorter adhesive curing time to be used.

In addition to the exemplary embodiments shown in the figures in which the projections 7, 8 are disposed over the entire height of the core part 1 and have a rectangular cross section, the invention also includes core parts having projections which do not extend over the entire height of the core part or core parts having projections which have different cross sections, for example square, polygonal or noncircular, for example elliptical, cross sections, which engage in correspondingly shaped cutouts in the lead frame.

Claims (7)

1. A chip inductor, having a solid core part which has a perpendicular prismatic spatial shape, and which is composed of a ferromagnetic or electrically nonconducting material, having a winding space which is recessed with respect to parallel butt ends of the core part wherein the end surfaces of said butt ends have projections and in that said projections engage in cutouts disposed on respective contact elements which are glued to the respective butt end.

2. A chip inductor as claimed in claim 1, wherein the projections and the cutouts have a square or rectangular cross section.

3. A chip inductor as claimed in claim 1 or 2 wherein the projections extend over the full height of the end surface.

4. A chip indicator as claimed in any preceding claim wherein the solid core part is made from ferrite, ceramic or plastic. 20

5. A chip inductor as claimed in any preceding claim wherein the solid core is shaped as a block, cube or cylinder.

6. A chip inductor as claimed in any preceding claim wherein the winding space is large enough to accommodate a multilayer winding.

7. A chip inductor substantially as herein described with reference to the accompanying drawings.