U.S. patent number 3,889,223 [Application Number 05/494,660] was granted by the patent office on 1975-06-10 for resistor trimming technique.
This patent grant is currently assigned to Ing. C. Olivetti & C., S.p.A.. Invention is credited to Claudio Dalmasso, Lino Sella.
United States Patent |
3,889,223 |
Sella , et al. |
June 10, 1975 |
Resistor trimming technique
Abstract
A printed circuit resistor pattern, designed to be trimmed by
electro-erosion, has affixed to the side thereof a highly
conductive strip. The current path passes through a portion of the
resistor pattern and through the conductive strip; as the resistor
is trimmed, a gap is created along the edge of the conductive strip
causing the current path to pass through a greater length of
resistive material and a lesser length of the conductive strip,
thereby causing the overall resistance to current flow to be
increased.
Inventors: |
Sella; Lino (Banchette,
IT), Dalmasso; Claudio (Ivrea, IT) |
Assignee: |
Ing. C. Olivetti & C.,
S.p.A. (Torino, IT)
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Family
ID: |
27273837 |
Appl.
No.: |
05/494,660 |
Filed: |
August 5, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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310729 |
Nov 30, 1972 |
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Foreign Application Priority Data
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Dec 2, 1971 [IT] |
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70956/71 |
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Current U.S.
Class: |
338/195; 29/620;
338/308 |
Current CPC
Class: |
H01C
17/2408 (20130101); H05K 1/167 (20130101); H05K
3/08 (20130101); Y10T 29/49099 (20150115) |
Current International
Class: |
H01C
17/22 (20060101); H01C 17/24 (20060101); H05K
3/08 (20060101); H05K 3/02 (20060101); H05K
1/16 (20060101); H01c 009/00 () |
Field of
Search: |
;338/195,308,309
;29/620 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Tone; David A.
Attorney, Agent or Firm: Schaefer; Ira J.
Parent Case Text
This is a continuation of application Ser. No. 310,729, filed Nov.
30, 1972, now abandoned.
Claims
What we claim is:
1. A resistor trimming arrangement comprising:
a resistor pattern having a first and second end with an initial
resistance between these ends, said pattern including a first
portion of relatively resistive material and a second portion of
relatively conductive material so that an electrical current
passing through said pattern from said first to said second end
will pass through a first length of said resistive material and a
second length of said conductive material;
electrically operable means for removing an amount of said
resistive material; and
circuit means electrically connected to said removing means and
electrically connected to the second end of the resistor pattern
for operating said removing means to effect the removal of an
amount of resistive material to increase the length of resistive
material through which an electrical current passing between said
ends passes to a length of said resistive material which is greater
than said first length and to decrease the length of conductive
material through which the electrical current passes to a length of
said conductive material which is shorter than said second length,
thereby causing the resistance between said ends to be
increased.
2. A resistor trimming arrangement comprising:
a relatively resistive element having a given resistance between
its extremities;
a relatively conductive element positioned in electrical contact
with said resistive element, the path of least resistance between
said extremities including a first length of said conductive
element;
electrically operating means for removing an amount of said
resistive element; and
means electrically connected to said removing means and to one
extremity of said resistive element for operating said removing
means to create an electrical open circuit through a portion of
said resistive element so that the path of least resistance between
said extremities includes a second length of said conductive
element, said second length being shorter than said first
length.
3. A resistor trimming technique for increasing the electrical
resistance between at least two points including the steps of:
providing a relatively resistive element extending between said
points;
providing a relatively conductive element in contact with a portion
of said resistive element so that electrical current passing
between said two points through said resistive element will also
pass through said conductive element;
electro-eroding a gap in said resistive element adjacent and
parallel to a portion of said conductive element so that said
electrical current will not pass through a portion of said
conductive element adjacent to said gap.
4. A combination for increasing the resistance between at least two
points comprising:
a strip of resistive material extending between said two points,
said strip having a shoulder;
a strip of conductive material extending between one of said points
and said shoulder, said conductive strip being contiguous to said
resistive strip between said shoulder and said one of said
points;
means for electro-eroding a gap through said resistive strip
starting at said shoulder and extending parallel to said conductive
strip; and
circuit means causing the electrical energy released by said
electro-erosion to pass to said one of said points through said
conductive strip.
5. A method of increasing the resistance between selected points on
a circuit board comprising the steps of:
constructing a strip of relatively resistive material between said
points, said strip having a relatively narrow portion and a
relatively wide portion;
constructing a strip of relatively conductive material;
said strips being constructed in electrical contact with each other
with said conductive strip being contiguous to said wide portion
and extending from the area where said wide portion meets said
narrow portion to the end of said wide portion;
forming a gap in said resistive strip, said gap extending parallel
to said conductive strip and starting from said area where said
wide portion meets said narrow portion.
Description
CROSS-REFERENCE
This application discloses the same subject matter as Italian
application No. 70956-A/71 filed on Dec. 2, 1971 by this applicant;
the prior filing date of said Italian application is claimed.
BRIEF DESCRIPTION OF THE INVENTION
1. Field of the Invention
The techniques of trimming resistive elements on printed circuit
boards form the general field of the invention. More particularly,
the invention relates to the geometry and composition of these
resistive patterns.
2. Description of the Prior Art
While resistor trimming methods are well known, a number of
substantial disadvantages are present in the known technology. The
prior art utilizes a resistive pattern which is so constructed so
as to cause the energy which is released through the
electro-erosion device to pass through the length of the resistive
pattern. Heat damage, lost time, improper resistor meter readings,
and difficult operator execution are among the consequences of
this. In order to fully understand the invention, the prior art
techniques will be further commented in conjunction with the
drawing.
SUMMARY OF THE INVENTION
The resistive pattern to be trimmed has affixed thereto a highly
conductive segment; the conductive segment is placed along the side
of the pattern near the region where an electro-erosion probe will
be used to create a gap or cut in the resistive material. The
energy released from the probe tip passes through an
inconsequential length of resistive material in order to reach the
conductive strip. The gap cut by the probe is made parallel to the
conductive strip; therefore, the resistance facing each successive
spark is quite low and remains constant.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1(a) and (b) depict the prior art trimming arrangement;
FIG. 2(a) and (b) depict the trimming arrangement according to the
invention;
FIG. 3 depicts an alternative trimming arrangement constructed in
accordance with the teachings of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In order to more fully understand the advantages of the inventive
concept, it would be helpful to first briefly consider prior art
techniques of resistor trimming. FIG. 1(a) and (b) show a resistive
segment 1 disposed between two relatively highly conductive
segments 2 and 3: these segments 1-3 are disposed on a circuit
board (not shown) of conventional design along with other resistive
and conductive elements which make up the complete circuit board
pattern.
The resistance between end segments 2 and 3 must be a certain,
precise value, and, as the resistive segments 1 are constructed by
mass manufacturing techniques, the actual resistance of each of
these segments will vary a slight amount from the resistance of
other segments. In order to assure that each resistive segment of
precisely the proper resistive value, the conventional technique is
to so design the segments 1 so that their resistance is slightly
lower than actually desired; the segments are then slowly trimmed,
step-by-step, until the desired resistance is achieved.
In FIG. 1(a), the dotted lines i represent the current spread which
would flow if a potential were impressed across end segments 2 and
3; since region 4 is wider than region 5, its resistance is
substantially lower than narrow region 5. In order to slowly raise
the resistance of segment 1, a high voltage probe 6 is lowered to
the surface of a shoulder 15 of region 4, causing a burst of
electrical energy to pass through region 4. This spark causes a
small area of the wide region to be disintegrated; the probe 6 is
continually brought in contact with shoulder 15 causing a gap 7 to
be slowly formed in this region. As can be seen in FIG. 1(b), gap 7
causes the amount of wide region 4 available to current flow to be
shortened while simultaneously elongating narrow region 5. Thus the
resistance of segment 1 is slowly raised each time probe 6 causes
gap 7 to be elongated.
The prior art resistor trimming technique is subject to a number of
important disadvantages. When probe 6 contacts the surface of
shoulder 15, the energy released through the probe passes down the
length of region 4, through end segment 3, and returns to the
opposite terminal of high voltage source 8. These heavy pulses of
current cause highly resistive region 4 to rapidly heat up. If the
gap 7 must be of substantial length, probe 6 must be lowered to the
region 4 a correspondingly substantial number of times. The large
amount of heat generated in region 4 can damage, not only this
area, but also surrounding structure. The reliability of the
resistors so trimmed is accordingly lessened because of this heat
generation.
After each probe produced spark has eroded away a chip of region 4,
ohm meter 9 must be monitored to see if the resistance of segment 1
has reached its desired value. Because of the surge of energy
through region 4 caused by the spark, the operator must wait a
period of time for the noise in region, caused by this energy, to
subside. This waiting period is significant and the consequent lost
time causes an appreciable drop in production. Furthermore, this
noise often causes erroneous readings of meter 9 since the operator
sometimes will not wait long enough for a proper reading.
Another problem which is found in the prior art method is that the
amount of material removed by each spark varies as gap 7 elongates.
This is because the resistance between probe 6 and end segment 3
changes as gap 7 elongates and region 4 correspondingly shortens.
With each successive spark, the resistance lessens, and this, in
turn, causes the energy burst to be released in a shorter time.
These more intense sparks disintegrate more material than the
initial less intense bursts. The result is the creation of a
wedgeshaped gap 7. The problem with this is that the operator never
knows by how much the resistance of segment 1 will be increased
with each succeeding spark. As the resistance of segment 1 closely
approaches the desired value, the operator therefore will not know
whether he should risk another spark or not.
FIG. 2(a) and (b) show the preferred embodiment of this invention;
the arrangement is similar to that of the prior art in that
resistive segment 1 is disposed between highly conductive end
segments 2 and 3. Connected between terminals 10 and 11 is a bridge
(not shown) used to measure the resistance of segment 1 after each
spark produced by probe 6. The resistive segments can be NiCr and
the conductive segments can be gold.
Affixed to the side of segment 1 is a strip 13 of highly conductive
material. Strip 13 can be of the same material as end terminals 2
and 3 (e.g., gold). The current path i now does not traverse the
entire length of resistive segment 1 to reach end terminal 3;
rather the current path i ends at conductive strip 13 as is shown
in the drawing. Each successive spark, produced by probe 6
elongates gap 7, thereby increasing the length of the current path
i through resistive segment 1.
Each successive spark, applied to gap 7, causes a burst of
electrical energy to pass to end segment 3, not through resistive
segment 1, but rather through highly conductive strip 13. For
practical purposes, one can speak of the resistance of highly
conductive strip 13 as being zero since it is so low when compared
with that of segment 1. Each spark, or energy burst, therefore
faces the same resistance no matter how long gap 7 becomes. This
means that each burst will be of the same intensity; therefore,
each spark will remove exactly the same amount of material as the
preceding sparks. The operator can then tell by how much the
resistance will change after each spark and will be able to know
when to stop the cutting.
Furthermore, as the spark energy passes through only a very thin
strip of segment 1 before reaching strip 13, no appreciable heat is
generated and no heat damage to segment 1 or other elements in the
environment is possible. Again because of the fact that the spark
energy traverses an inconsequential amount of high resistance
material 1, there is little noise generated in segment 1 and the
operator need not wait an extra period of time after each spark to
obtain a proper resistance reading.
FIG. 3 shows another embodiment of the invention which utilizes a
plurality of thin strips 14 to connect resistive segment 1 to
conductive strip 13. Spark probe 6 is then used to cut strips 14,
in succession, until the resistance of strip 1 has been raised to
the desired value. Strips 14, which are drawn greatly enlarged for
clarity, are thin enough so that a single spark from probe 6 will
break a single strip. Using this arrangement one can alter the
resistance of strip 1 in precisely defined steps; the amount of
change in resistance after each spark may be set to a desired value
by designing the strips 14 to be at a particular distance from each
other.
Other changes in the geometry of segment 1 and strip 13 which
utilize the concepts of this invention will suggest themselves to
those skilled in the art. The limits of this inventive concept are
defined in the following claims.
* * * * *