GB2079066A

GB2079066A - Trimmable electrical inductors

Info

Publication number: GB2079066A
Application number: GB8030763A
Authority: GB
Original assignee: Hull Corp
Current assignee: Hull Corp
Priority date: 1980-06-23
Filing date: 1980-09-24
Publication date: 1982-01-13
Also published as: GB2079066B; CA1154111A; JPS5713717A; FR2485247A1; DE3045585A1

Abstract

Trimming either a thin or thick film inductor located on one surface of an electrically non-conductive substrate 10 is achieved by applying to the opposite surface of the substrate a layer 14 of highly conductive metal or of magnetic material of high resistivity. When a on highly conductive metal e.g. Al, Cu or Ag, is used, the inductance is decreased. Progressively reducing either the thickness or the area of the layer, as by grinding, progressively increases the inductance until all of the metal layer has been removed. When a magnetic layer e.g. ferrite is used, the inductance is increased by an amount dependent on the thickness and the area of the magnetic layer. The substrate may be formed from alumina. <IMAGE>

Description

SPECIFICATION Trimmable electrical inductors and method of making the same BACKGROUND OF THE INVENTION This invention relates to electrical inductors, and more particularly to trimmable thick and thin film inductors.

When inductors are employed in tuned circuits, filters, etc., it is essential that the inductance be adjustable to a desired value and be maintained at that value. It is desirable to use microelectronic circuit construction in many applications to provide miniature electronic equipment. One problem associated with such circuits is that miniature components which are available are not adjustable, and it is difficult to work with them to adjust or align the circuit because of the small size.

To the best of my knowledge, there has been no attempt made to adjust the inductance of a thin or thick film inductor of the type exemplified in U. S. Patent No.

3,785,046 without physical alteration of the inductor itself or to the circuit in which the inductor is being applied. It is difficult and impracticable to make an electrical connection to a point on the winding of the inductor other than the start and finish terminals provided. It is obvious that an attempt to make a connection elsewhere on the winding would require breaking through the dielectric layers, thereby breaking the hermetic seal and exposing the winding to the deleterious elements in the environment.

Furthermore, in such a case, the trimming range is limited by the amount contributed by the last turn of the inductor, since only the last turn is accessible, disregarding the breaking of the hermetic seal.

In a spiral inductor however, all the turns of the inductor are accessible, assuming the inductor is not sealed before trimming. The tap can be moved until the desired inductance is obtained, and then the tap connection is made permanent by soldering or bonding. In practice, however, this is a delicate and precise alteration, especially when the inductance value is low. The position and length of the wire at the tap must be maintained from trimming to the final permanent attachment.

Altering the physical characteristics of such inductors is a meticulous and time-consuming function which necessarily adds to the manufacturing time and expense.

SUMMARY OF THE INVENTION In its basic concept, this invention provides for the trimming of thick and thin film inductors mounted on one surface of a substrate by applying to the opposite surface of the substrate a layer of highly conductive metal or of magnetic material of high resistivity.

It is by virtue of the foregoing basic concept that the principal objective of this invention is achieved; namely, to overcome the aforementioned disadvantages and limitations associated with the prior methods of trimming thick and thin film inductors.

Another objective of this invention is the provision of an inductor of the class described which has a high stability of trimmed inductance.

Another important object of this invention is the provision of an inductor of the class described in which the inductance may be increased or decreased as required by specifications simply through the choice of amount and type of coating applied to the surface of the substrate opposite the inductor.

A further objective of this invention is the provision of a method for trimming inductors of the class described in which the seal or passivation of the inductor is not affected by the trimming process.

A still further object of this invention is the provision of a trimmable inductor of the class described which is of simplified construction for economical manufacture.

The foregoing and other objects and advantages of this invention will appear from the following detailed description, taken in connection with the accompanying drawings of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plan view of a trimmable inductor embodying the features of this invention.

Figure 2 is a side elevation of the inductor and substrate shown in Fig. 1 showing the metal coating on the surface of the substrate opposite the inductor.

Figure 3 is a plan view of a second embodiment of a trimmable inductor embodying the features of this invention.

Figure 4 is a side elevation of the inductor and substrate of Fig. 3 showing the magnetic coating on the surface of the substrate opposite the inductor.

DESCRIPTION OF THE PREFERRED EMBODI MENTS An electrically non-conductive substrate 10 is shown in the drawings, and may be formed of any suitable insulating material such as alumina (Al2O3). One surface of the substrate 10 provides a base for a thin or thick film inductor 1 2. It will be understood that any thin or thick film inductor mounted on an electrically non-conductive substrate may be used, including spiral inductors made either by thin film or thick film process. The inductor coil illustrated is described in U. S. Patent No.

3,785,046.

As shown in Fig. 2, the surface of the substrate 10 opposite the inductor 1 2 is coated with a metal layer 1 4 such as aluminum, copper, gold, or other highly electrically conductive metal. When an alternating current flows through the inductor 12, the varying magnetic field induces eddy currents in the metal layer 14 in such a direction that the magnetic field produced by the eddy currents opposes the magnetic field produced by the current in the inductor As a result, the application of the metal layer reduces the inductance of the inductor. The amount of reduction depends on the diameter of the inductor coil, the thickness of the substrate, and the thickness or the area of the metal layer.

Removing the metal layer 14, as by grinding off various amounts of its area or thickness increases the inductance until all of the metal layer is removed, it is desirable ta use metal of high conductivity in the layer to minimize the loss reflected into the inductor circuit. The reflected loss reduces the Q-factor of the inductor.

For example, in an inductor having a coil with an outside diameter of .100" on a 10 mil, thick alumina substrate, the inductance is reduced by about 35% when a copper coating 1 mil. thick is placed on the lower surface of the substrate. Thus a trimming range of 35% is feasible.

In the embodiment of Figs. 3 and 4, a magnetic layer 1 6 of the high resistivity is used in place of the metal layer 14 described hereinbefore. The inductance of the coil is increased due to the proximity of the magnetic layer. When the magnetic layer is par tally removed, either in area or in thickness, the inductance decreases until all of the magnetic layer is removed. The amount of increase in inductance depends on the permeability of the magnetic material used and the thickness or area of the layer.

For example, in an inductor having a coil with an outside diameter of .055" on a 10 mil. thick alumina substrate, the inductance is increased by about 17% when a magnetic alpha ferrite layer of 2 mil. thickness is applied to the opposite surface of the substrate.

Consequently, the trimming range is about 17% in this case.

Therefore, it will be understood that the function of the metal coating is to provide an overall reduction in inductance from the value provided by the inductor per se. Trimming to increase the inductance by various amounts is accomplished by removing various amounts of the thickness or, preferably, the area of the metal coating. Use of the magnetic coating increases the inductance from the value provided by the inductor per se. Trimming to decrease the inductance is accomplished by removing various amounts of the thickness or area of the magnetic coating. The function of the magnetic coating therefore is to provide for an overall increase in inductance.

The method using a magnetic coating is more desirable, since the increase in inductance and much smaller reflected loss result in higher Q-factor or the inductor.

The metal coating, however, can be easily replaced when over-trimmed, by painting con ductive paint or epoxy to replace the trimmed areas.

The trimming of inductance may be accom-,I plished by grinding off the metal or magnetic coating on the substrate after it is mounted in a circuit package with the coated surface exposed. Trimming by laser is also feasible.

Therefore, it will be apparent to those skilled in the art that this invention provides a thin film or thick film inductor which is trimmed without affecting the hermetic seal or altering the physical characteristics of the inductor or the circuit in which the inductor is being applied. It will also be appreciated that the inductor may be trimmed during manufacture or after being incorporated in a circuit package.

Claims

1. A trimmable inductor, comprising an electrically non-conductive substrate, an electrical inductor mounted on one surface of the substrate and a layer of electrically conductive or magnetic material mounted on the surface of the substrate opposite the inductor.

2. An inductor as claimed in Claim 1, in which the layer is a highly conductive metal.

3. An inductor as claimed in Claim 1, in which the layer is a magnetic material of high resistivity.

4. An inductor as claimed in Claim 1, in which the thickness or the area of the layer is selected to provide a predetermined magnitude of inductance.

5. A method making a trimmable thin or thick film inductor, comprising mounting a thick or thin film inductor on one surface of an electrically non-conductive substrate and mounting a layer of electrically conductive or magnetic material on the surface of the substrate opposite the inductor.

6. A method as claimed in Claim 5, in which the layer is a highly conductive metal and includes the step of reducing the thickness or the area of the layer to increase the inductance to a predetermined magnitude.

7. A method as claimed in Claim 5, in which the layer is a magnetic material of high resistivity and includes the step of reducing the thickness or the area of the layer to decrease the inductance to a predetermined magnitude.

8. A method as claimed in Claim 5, in which the layer is a highly electrically conductive metal.

9. A method as claimed in Claim 5, in which the layer is a magnetic material of high resistivity.

10. A trimmable inductor constructed and arranged substantially as herein described with reference to and as illustrated in the accompanying drawings.

11. A method of making a trimmable thin or thick film inductor substantially as herein described.