Resistive electrical components

Improved resistive electrical components are disclosed comprising an insulating or insulated substrate, a resistive foil bonded to the substrate having photoetched therein a pair of terminal pads for making electrical connections to the component and a system of resistive paths interconnecting the terminal pads, said system including an unadjustable section or sections and a plurality of adjustable sections, each having an adjustment tab associated therewith, said tab being removable to modify said section resistance and thereby altering the total resistance presented by the component between its terminal pads, the configurations of the sections differing from each other in a modified geometric progression so that the total resistance of said component is altered by a differing amount depending on which of the sections is modified by removal of its associated adjustment tab, whereby the total resistance of the component may be systematically varied in a sequence of successive steps to achieve a desired ultimate value of the total resistance of the component to the desired degree of accuracy with the least number of steps, the most effective use of substrate surface area, the least contribution to reliability degradation, and the lowest cost. Adjustment of the total resistance of the component to its desired ultimate value and precision is assisted by monitoring the component resistance while making successive adjustments. Placement of said adjustment tabs along one edge of said substrate simplifies manual adjustment and makes practical the use of automatic apparatus controlled by a monitor.

This invention relates to improvements in resistive electrical components. 
More particularly it relates to the construction details that permit 
production of a useful range of resistance values to a high degree of 
accuracy with a minimum of adjustment time. 
It is known to construct resistive electrical components comprising an 
insulating cylinder having a conductive material deposited thereon between 
a pair of terminal pad portions, the magnitude of the resistance of such a 
component being determined by the resistivity of the material of which it 
is formed and the length and width of a pattern, having numerous 
convolutions that are developed in the resistive material at adjustments 
to achieve a desired value of the total resistance of the component to an 
accuracy within the capabilities of this adjustment method. 
Also, to achieve more accurate adjustment, it is known as in the manner 
described in U.S. Pat. No. 3,405,381 granted Oct. 8, 1968, to Zandman et 
al to provide a foil of a resistive alloy cemented to a substrate, said 
foil and substrate having physical characteristics such that when cemented 
together the temperature coefficient of resistance of the component is 
close to zero. The pattern is composed of a system of gross and fine 
adjustment, the gross adjustment being facilitated by an arithmetic 
progression of significant resistance sections, each section being 
comprised of resistive lines in parallel connection to be changed to 
series connection at the time of adjustment by the removal of shorting 
bars and the fine adjustment facilitated by a series of digressively less 
sectional resistance contribution, thus permitting adjustment of the total 
resistance to final value with a very high order of accuracy. 
The present invention is directed to an improved resistive component, also 
employing a flat substrate having resistance paths formed thereon between 
a pair of terminal pad portions, in which the resistive paths are so 
formed and configured as to make possible the systematic variation of the 
total resistance in a series of successive steps so that the ultimate 
value of total resistance of the component to a high order to accuracy may 
be arrived at but now with the minimum number of successive incremental 
adjustment operations through the use of a modified geometric progression 
of sectional resistance contribution. 
Another object of the invention is to modify the geometric progression in 
such a way that resistive errors in the progression due to manufacturing 
variations are permitted. 
A further object of the invention is to modify the geometric progression in 
such a way that adjustment errors in incorrectly skipping one step are 
permitted. 
Yet another object of the invention is to collect all the adjustment points 
along one edge of the pattern for ease of manual adjustment and 
facilitation of automatic adjustment. 
Yet another object of the invention is to obtain a higher order of 
reliability through the minimizing of resistance adjustment steps and 
maximizing the use of available surface area with active resistance lines. 
In accordance with the invention, the foregoing objects and others which 
will appear are achieved, in a resistive electrical component comprising 
an insulating substrate and a pair of conductive pads formed thereon for 
making electrical connections to the component by providing a system of 
resistive paths formed on said substrate and interconnecting said terminal 
pads, said system including a plurality of sections, each having an 
adjustment tab associated therewith, said tab being removable to modify 
the resistance in said section and thereby alter the total resistance 
presented by said component between said terminal pads, the configurations 
of said sections differing from each other in a modified geometric 
progression so that the total resistance of said component is altered by a 
different amount depending on which of said sections is modified by 
removal of its associated adjustment tab, whereby the total resistance of 
said component may be systematically varied in a series of successive 
steps to achieve a desired ultimate value of said total resistance to an 
ultimate accuracy by a minimum number of incremental adjustment 
operations. A range of resistance values is achieved in one common 
finished device size by employing various patterns differing only in the 
line widths and lengths in each section but always with the same number of 
sections and associated adjustment tabs. 
In an idealized form of the invention, the geometric progression of 
adjustment contribution starts, for example, with a 16.384% addition to 
the total resistance in the most significant section and through fifteen 
additional sections, each being one-half of the previous, provides for a 
final adjustment to value with the least significant section contributing 
0.0005%. Adjustment of all sections increases the total resistance by 
32.7675% and adjustment to any intervening value to say .+-.0.001% is 
achievable by skipping sections that when added to previous sections 
already adjusted would cause an adjustment to a value above the desired 
value. For example, adjustment of a resistive component in accordance with 
the invention to a final value of 10,000 ohms from an initial value of 
8,000 ohms is accomplished in accordance with the following sequence. 
Step number 1 will increase the total resistance by 16.384% of 8,000 ohms 
and since this change, when added to 8,000 ohms, will bring the total 
value to something less than 10,000 ohms, the adjustment is made and the 
new value is now 9,310.72 ohms. 
Step number 2 will increase the total resistance by 8,192% of 8,000 ohms 
which is one-half the previous step of 16.384%, and since this change when 
added to 9,310.72 ohms will bring the total value to something less than 
10,000 ohms, the adjustment is made and the new value is now 9,966.08 
ohms. 
Steps number 3, 4, 5 and 6 will increase the total resistance by various 
amounts but since any of these changes, when added to 9,966.08 ohms, will 
bring the total value to something greater than 10,000 ohms, the 
adjustments 3, 4, 5 and 6 are skipped and the value remains 9,966.08 ohms. 
Step number 7 will increase the total resistance by 0.256% of 8,000 ohms 
and since this change, when added to 9,966.08 ohms, will bring the total 
value to something less than 10,000 ohms, the adjustment is made and the 
new value is now 9,986.56 ohms, and so on in the process of selecting and 
skipping until the final total resistance of 10,000 ohms to the final 
precision is achieved. 
The idealized form of the invention provides no margin of error in the 
value of each adjustment or in the performance of the operator in 
selecting the adjustments to be made. 
In a preferred form of the invention, the geometric progression is modified 
from the ideal to allow for manufacturing error in the accuracy of each 
section and to permit an inadvertent skip. Starting with a 12.1% addition 
to the total resistance in the most significant section and through twenty 
additional sections, each being approximately 60% of the previous section, 
adjustment of all sections increases the total resistance by approximately 
31.74% and adjustment to any intervening value to any accuracy down to say 
.+-.0.001% is achievable by adjusting and skipping appropriate sections as 
in the idealized form. With this modification of sectional value from 50% 
of the value of the preceding section in the idealized form to 60% of the 
value of the preceding section in this preferred form, small errors within 
a section or the false skipping of any section are corrected by the proper 
selection for adjustment and skipping in subsequent sections. 
To cover resistance ranges for practical use from 1 ohm to 100,000 ohms for 
example whereby all said values could be produced on the same size 
substrate, one has to produce different photoetched patterns since every 
pattern is adjustable by for example 30% only. To cover the range from 1 
ohm to 2 ohms, one would need about 3 patterns, the first one covering the 
range from 1 ohm to 1.3 ohms (a 30% adjustability), the second pattern 
from 1.3 ohms to 1.7 ohms (another approximately 30% adjustability), the 
third pattern from 1.7 ohms to over 2 ohms. From 2 ohms to 4 ohms, another 
3 different patterns would have to be produced. From 4 ohms to 8 ohms, 
another 3 patterns. Hence, approximately 10 patterns per decade or about 
50 patterns for the range of 1 ohms to 100,000 ohms. Obviously if the 
adjustability is 50% rather than 30%, then fewer patterns are required. 
In another form of the invention, fewer sections are employed reducing the 
amount of adjustability available for the same end accuracy and thereby 
however increasing the number of different patterns necessary to cover the 
full range of resistance values, thereby reducing the number of adjustment 
steps. 
In another form of the invention, more sections are employed increasing the 
number of adjustment steps but reducing the number of patterns necessary 
to cover the full range of resistance values. 
In yet another form of the invention, the terminal pad portions may be 
plated with metals such as copper and gold to facilitate electrical 
connection. 
Preferably, in accordance with the invention, the adjustment tab portions 
are positioned adjacent an edge of the substrate. For example, the 
substrate may be of rectangular configuration and the adjustment tabs may 
be positioned adjacent one edge thereof. If desired, the system of 
resistive paths may occupy more than one surface of the substrate. Also, 
if desired, the substrate may be made of metal having an insulating layer 
applied to one or more surfaces thereof, the conductive terminal pads and 
the system of resistive paths being cemented to said insulating layer, or 
the cement itself could serve as an insulator. 
Preferably also, the individual sections are designed and constructed so as 
to provide for changes in the total resistance of the component by 
predetermined differing amounts such that, if a particular adjustment step 
is inadvertently omitted, its effect can be made up by subsequently making 
a plurality of adjustments of smaller magnitude. 
Obviously, the same approach applied to resistor networks where many of the 
above described patterns can be interconnected on one or several 
substrates to form any desired network such as voltage dividers, ladder 
networks, etc.

The same reference numerals designate similar elements in the different 
figures. 
Referring to FIG. 1, there is shown a completed electrical component in 
accordance with the invention comprising external leads 1 and 1' with 
flattened and extended tabs 6 and 6' brought up and over substrate 2 to 
engage terminal pad portions 7 and 7'. A stress isolation coating of 
rubber 3 is applied to the entire surface of the subassembly of leads and 
substrate and the coated subassembly is molded or encapsulated into outer 
plastic housing 5. 
Referring now to FIG. 2, there is shown a resistive element of the 
electrical component in accordance with the invention comprising a 
substrate 2 on the upper surface of which is cemented a nickel chromium 
alloy film layer 4 into which has been photoetched one of various 
available patterns depending on final desired resistance always consisting 
of terminal pad portions 7 and 7' and serially-connected between the 
terminal pad portions a series of resistive path sections. Referring to 
FIG. 2a, this shows in enlarged cross-section one possible construction of 
the resistive element, employing a substrate 2 which is made of a metal 
portion 2a insulated from film layer 4 by a layer of electrical insulation 
2b. 
Referring now to FIG. 3, there is shown a plan view of a typical resistance 
pattern comprising the terminal pads 7 and 7', the interconnecting 
resistive paths 8 and the adjustment tabs 9 which may be used in a 
resistive electrical component as described above with reference to FIG. 1 
and FIG. 2. As will be seen from examining this figure, the resistive path 
8 interconnecting the terminal pads 7 and 7', comprises a plurality of 
serially-connected sections, each of said sections comprising a resistive 
path provisionally shorted by the adjustment tab in some instances while 
in others one or more parallel paths are provisionally shorted by the 
adjustment tab. Also, a major section or sections having no provision for 
adjustment are serially-connected with the plurality of serially-connected 
adjustable sections. In each adjustable section there is provided an 
adjustment tab 9 positioned near the outer edge of the resistive pattern 
and having a constricted part immediately bordering on said outer edge 
which can readily be removed by scribing, sand blasting, laser beam 
evaporation, grinding or any of various known methods of material removal 
to interrupt the parallel connection of the adjustment tab of the 
series-connected section with which it is associated. Adjacent the outer 
edge of the pattern in FIG. 3 are numerals indicating the percentage 
change in the total resistance of the component measured between terminal 
pads 7 and 7' which will be produced by cutting through the constricted 
part of the corresponding one of the adjustment tabs 9. It will be seen 
that these percentage changes are of decreasing magnitude proceeding from 
one end to the other. As can be demonstrated, such magnitudes differ by 
unequal amounts in accordance with a modified geometric progression. As 
will readily be seen, given the total resistance of the component measured 
between terminal pads 7 and 7' prior to adjustment, and knowing the 
ultimate value of resistance desired for the component, such desired 
resistance being between the resistance measured prior to adjustment and 
the maximum value attainable by removal of all adjustment tabs, such value 
can be achieved to a high order of precision by successively removing 
parts of appropriately selected adjustment tabs 9 beginning with one of 
relatively large magnitude but less than the total amount of change to be 
effected in the resistance of the component, and then continuing with 
successively smaller values but always less than the total amount of 
remaining change to be effected in the resistance of the component, until 
the desired ultimate component resistance is achieved to the desired 
degree of precision. As hereinafter stated, this may be accomplished by 
monitoring the component resistance and then making the necessary 
adjustments either manually or by means of automatic apparatus controlled 
by a monitor. 
As will be apparent, a resistive component in accordance with the invention 
may be constructed to provide a selection of patterns, with value of total 
resistance between its terminal pads prior to the adjustment operations 
being a function of the total length of the resistive paths 
interconnecting said terminal pads, the widths and thicknesses thereof, 
and the resistivity of the material of which they are formed. Similarly 
the total amount by which the component resistance can be varied during 
the adjustment process will depend on the configuration and construction 
of the individual serially-connected sections. Thus, should the desired 
ultimate value be above the adjustment capability of one pattern, then 
another pattern of appropriately higher initial resistance must be 
selected and if the desired ultimate resistance is below the initial 
resistance value of one pattern, then another pattern of appropriately 
lower initial resistance must be selected. In a preferred embodiment of 
the invention some overlapping of pattern values is provided by 
provisioning the useful range of resistance values with a selection of 
patterns differing in resistance in about 30% increments. 
Further in accordance with the invention, in the pattern according to FIG. 
3, the incremental changes in total resistance provided for are such that 
if, in making the successive adjustments to achieve the desired total 
resistance value of the component, a given adjustment step is 
inadvertently omitted, such omission can be corrected by making several 
successive subsequent adjustments of lesser magnitude, and the ultimate 
desired value of component resistance still can be achieved with the same 
high degree of precision within 0.0005 percent or other as predetermined 
by the pattern design. Assume, for example, that, at a particular point in 
the adjustment process, monitoring of the resistance of the component 
indicates that the next adjustment step should be taken and that a maximum 
of 0.100 percent increase in resistance is indicated, but that this 
adjustment step inadvertently was omitted. Then it still would be possible 
to make up this missed change in component resistance by making four 
smaller adjustments of 0.0562 percent, 0.0316 percent, 0.0100 percent and 
0.0018 percent to produce a total change of 0.0996 percent--i.e., within 
0.0004 percent of 0.100 percent--so as to permit achievement of the 
desired ultimate component resistance value with a precision of 0.0005 
percent. 
From examination of FIG. 3 it will be seen that the resistive paths are in 
a tortuous pattern and such that they comprise a plurality of 
serially-interconnected linear segments, each segment being disposed 
closely adjacent a preceding and/or a succeeding segment 
serially-connected therewith. This arrangement is particularly 
advantageous because it tends to minimize both the self-inductance and the 
inherent capacitance of the component. Self-inductance is minimized 
because the currents in adjacent segments flow in opposite directions, and 
therefore the electromotive forces which they induce in each other tend to 
cancel. Similarly, the inherent capacitance of the component is minimized 
because it is equal to the reciprocal of the sum of the reciprocals of the 
inherent capacitances between adjacent segments. Hence, resistive 
components constructed in the manner described herein are superior to 
conventional wire wound resistors comprising a continuous winding in one 
direction, whose self-inductance and capacitance are inherently high. 
Moreover, in the construction described, the resistive paths are 
distributed over a relatively large area on the surface of the insulating 
substrate which affords better dissipation of heat generated therein. 
Referring now to FIG. 3a it will be seen that the principles of the 
invention are applicable to rectangular as well as square surfaces or 
other flat forms. 
Reference now is made to the schematic circuit diagram of FIG. 4 to explain 
more fully the principles of operation of the invention. There are 
represented a plurality of three sections 10, 11 and 12 serially-connected 
to the unadjustable resistance 14 and thence between terminals 7 and 7' 
corresponding to the terminal pad portions of the component illustrated in 
FIGS. 1, 2 and 3. Section 10 is shown as comprising a resistive portion 15 
having connected in parallel therewith a portion 16 including serially an 
adjustment tab represented by the block 9. Section 11 likewise comprises a 
somewhat smaller resistive element 17 having a portion 18 connected in 
parallel therewith and also including serially an adjustment tab 9. 
Section 12 comprises a resistive element 19 having connected in parallel 
therewith a resistance element 13 and also in parallel therewith a portion 
20 serially-connected with an adjustment tab portion 9. It will be seen 
that so long as the adjustment tab 9 in section 10 is intact, section 10 
will present a relatively low resistance in the series connection between 
terminals 7 and 7'. When adjustment tab 9 in section 10 is removed, the 
resistance presented by section 10 will be increased substantially and 
will be equal to the resistance of element 15. Similarly in section 11, so 
long as adjustment tab 9 is intact the resistance of the section will be 
relatively low, but when adjustment tab 9 associated with this section is 
removed, the resistance of the section will be increased to be equal to 
the resistance of portion 18 which will be substantially less than that of 
the element 15 in section 10. In section 12, so long as adjustment tab 9 
is intact the resistance presented by this section will be equal to the 
parallel combination of the resistances of portions 19, 13 and 20. 
However, when the connection through adjustment tab 9 is broken, the 
resistance provided by the section will be equal to the parallel 
combination of the portions 19 and 13 alone and therefore greater than 
that previously presented, but less than that of either element 15 or 17. 
Obviously the schematic diagram of FIG. 4 is only illustrative of the 
general principles of operation of a component constructed in accordance 
with the invention and does not purport to portray an actual embodiment of 
the invention or to illustrate all of the various possible configurations 
of the conductive paths in such an embodiment which will be better 
appreciated from a careful examination of the pattern of FIG. 3. 
After their resistance have been adjusted to the desired value, components 
in accordance with the invention may be provided with one or more 
protective coatings as described, for example, in co-pending U.S. patent 
application Ser. No. 742,030 of Leon Resnicow for Attachment of Leads to 
Electrical Components. First, if desired to improve the peel strength of 
the etched pattern, there may be applied one or several coats of an epoxy 
or other resin, which coating may be of the order of 0.0002" to 0.002" in 
thickness. Following this, and over the coating it is highly desirable to 
apply a thicker mechanically protective layer of material such as a 
silicone rubber or other soft material which may be of the order of 0.010" 
in thickness and which provides a pliable cushioning layer for the 
component, which protects it from mechanical strains resulting from 
molding, shrinkage and the like. Also, if desired, following the 
application of the aforementioned protective coatings, the component may 
be encapsulated by molding with any of the well-known compounds commonly 
used in encapsulating electronic devices such as epoxy molding compounds, 
diallylphthalate compounds or silicone molding compounds to provide 
additional protection. The component may also be protected with a plastic 
compound enclosed in a plastic or ceramic case, or in a hermetic metal 
cast with glass bead headers for bringing the connecting leads out through 
the case. 
While the invention has been described with reference to certain preferred 
embodiments and modifications thereof, it will be apparent that numerous 
other modifications may be made, as will be apparent to those skilled in 
the art, within the scope of the invention as defined by the claims which 
follow.