Note: Descriptions are shown in the official language in which they were submitted.
TRIMMABLE ELECTRICAL INDUCTORS
AND METHOD OF MAKING THE SAME
,
BACKGROUND OF THE INVENTION
This inve~tion relates to electrical induct-
ors, and more particularly to trimmable thick and thin
~ilm inductors. ;~
:.
When inductors are employed in tuned circuits,
filters, etc.,~ it is essential that the inductance be ad-
:
; ~ justable to a desired value~and~be maintained at that
: : :
value. It is des;irable to use microelectronic clrcuit
construction in many applications to provid~ miniature
~ electronic equipment. One~problem as~sociated with such
; ~ ~lO ~c~ircuits~is that miniature components which are avai~able
; ar~e 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 ~ithout physical alteration of the inductor
itself or to the circuit in which the inductor is being
:.
~LlS~lil
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 ko 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, dis-
regarding 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, especial-
ly 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 manufactur-
ing time and expense.
In its basic concept, this disclosure provides
for the trimming of thick and thin film inductors mounted on
one surface of a substrate by applying to the opposite
- 2 -
- ~ . . .
., . , : .
` ` 115~
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 is achieved; namely, to overcome the afore-
mentioned disadvantages and limitatlons associated with the prior
methods of trimming thick and thin film inductors.
Another objective is the provision of an inductor of the
class described which has a high stability of trimmed inductance.
Another important objective is the provision of an in-
ductor of the class described ln which the inductance may be in-
creased 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 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 objective is the provision of a trimm-
able inductor of the class described which is of simplified
construction for economical manufacture.
More particularly in accordance with one aspect of the
invention there is provided a trimmable inductor, comprising:
a) an electrically non-conductive substrate,
b~ an electrical inductor mounted on one surface of sald
substrate and having a certain value of
inductance, and
. ..
..''~,,
.: ,,
.
,: ~ , , . . . :
115~
c) mounted on the surface o~ the substrate opposite
said inductor a layer of material characterized by
being capable of being changed in thickness or area
to adjust said inductor to a predetermined value of
inductance different rom said certain value. The
layer may be a highly conductive metal or a magnetic
material of high resistivity~
In accordance with another aspect of the invention there
is provided the method of making a trimmable thin or thick film
inductor comprising:
a) mounting on one surface of an electrically non-
conductive substrate a thick or thin film inductor ~`
: having a certain value of industance, and
b) mounting on the surface of the substrate opposite
the inductor a layer of material characterized by
being capable of being changed in thickness or area to
adjust said inductor to a predetermined value of in- .:
ductance different from said certain value. If the
layer is a hlghly electrically conductive material,
reducing its thickness or area increases the inductance.
If:the layer is a magnetic material of high resistivity
: ~ reducing the thickness or area o~ the layer reduces the
inductance.
In accordance with another aspect of the invention there
:: is provided the method of adjusting the inductance of a thin or
thick film inductor having a certain value of inductance and
mounted on one sur~ac~e of a substrate, comprising mounting on the
^-,
;- .. :- :
,: :: : -: , ~ ~
.. . . . .
~lS4~
surface of said substrate opposite the inductor a layer of material
characterized by being capable of being changed in thickness or
area to change said certain value of inductance, and changing the
thickness or area of said layer to adjust said inductor to a
predetermined value of inductance different from said certain value.
The foregoing and other objects and advantages of this
disclosure will appear from the following detailed description,
taken in connection with the accompanying
'
. - .,
.~ - .
: ~ - . . : -. . . - , ,:
~` llS~
4.
drawings of preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a plan view of a trimmabie inductor
embodying the features of this invention.
Fig. 2 is a side elevation o the inductor
and substrate shown in Figure 1 showing the metal coating
on the surface of the substrate opposite the inductor.
Fig. 3 is a plan view of a second embodiment
of a trimmablP inductor embodying the features of this
10 `invention.
Fig. 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 inductorO
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An eLectrically non~conductive substrate 10 -
is shown in the drawings, and may be formed of any suit-
able insulating material such as alumina (A1203). One ~`
surface of the substrate 10 provides a base for a ~hin
or thick film inductor 12~ It will be understood that
any thin or thick film inductor mounted on an electrically
non-conductive substrate may be used, including spiral in-
duc~ors made either by thin fiLm or thick film processO
The inductor coil illustrated is described in U. S. Patent
No. 3,785,046.
As shown in Fig. 2, the surface of the sub-
strate 10 opposite the inductor l2 is coated with a metal
layer 14 such as aluminum, copper, gold, or other highly
electrically conductive metal. When an alternating current
- . , -,
;
115~
.
Jj
flows through the inductor 12, the varying magnetic fie~d
induces eddy currents in the metal layer 14 in such a dir-
ection that the magnetic field produced by the eddy cur-
rents opposes the magnetic field produced by the current
in the ind~ctor. 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 griAding off var-
ious amounts of its area or thicknessJincrease the induct-
ance untiL all of the metal layer is removed, ~ is desir-
able to use metal of high conductivity in the layer to min-
imize the loss reflected into the inductor circuit. The re-
flected 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 sub-
strate~ the inductance is reduced by about 35% when a cop-
per coating 1 mil. thick is placed on the lower surface
of the substrate. Thus a trimming range of 35% is feasible~
In the~embodiment o Figs. 3-and-4j a magnetic lay-
er 16 of the high resistivity is used in place of the metal
layer 14 described hereinbeore. The inductance of the coil
is increased due to the proximity of the magnetic layer.
When the magnetic layer is partially removed, either in
area or in thickness, the inductance decreases until all of
the magnetic layer is removed. The a~ount of increase in
inductance depends on the permeability of the magnetic mat-
: ;
-. .- - - . ~ . , , . . : -
. - ~ .. : : : ,
: ! : :
:' ` . ' : '' : '''
.. ':
., , , ,, " .
f
6-
erial 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 ~lumina
substrate, the inductance i8 increased by about 17% when
a magnetic alpha ferrite layer of 2 mil. thickness i8
applied to the opposite surface of the substrate. Con-
sequently, the trimming range is about 17% in this case.
Therefore, it will be understood that the funct-
ion of the metal coating is to provide an overall reduct-
ion in inductance from the value provided by the inductorper se. Trimming to increase the inductance by various
amounts is accomplished by removing various amounts of
the thickness or, preferably, the are~ of the metal coating.
Use of the magnetic CoatiQg increases the inductance
~rom the value provided by the inductor per se. Trimming
to dec$ease the inductance is accomplished by removing
various amounts of the thickness or area of the magnetic
coat~ng. The function of the magnetic coating therefore
is to provide for an overall increase in inductance.
The method using a magnetic coating is more des-
irable, since the increa&e in inductance ~ d much smaller
reflected loss result in higher Q-factor ~ the lnductor.
The metal coating, however, can be easily re-
placed when over-trimmedg by painting conductive paint
or epoxy to replace the trimmed areas.
.
., ,. . ~, . .. ~
~1S411 ~
The trimming of inductance may be accomplished
by grinding off the metal or magnetic coating on the sub-
strate 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 disclosure 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.
It will also be apparent to those skilled in the
art that various changes may be made in the size, shape,
and arrangement of parts described hereinbefore without
departing from the spirit of this invention and the scope
of the a~pended claims.
-- 7
