Note: Descriptions are shown in the official language in which they were submitted.
B~O
. FIELD OF INVENTION
~This invention relates to metal foi] res:istors, more specif- !
ically, a method of making such resistors.
DESCRIPTION OF T~IE PRIOR ART
~tal foil resistors, per se, as well as methods of making such
resistors are well known in the art. Generally speaking, th~s~
resistors comprise an insulating substrate (usually of glass or
ceramic material); a thin metallic foil bonded to the substrate,
the foil having a circui-t path thereon usually formed by
photographic-acid etch -techniques; connector leads attached to the
thin foil at each end of the circuit path; and a protective
coating surxounding the entire structure.
Typically the photographic-acid etch technique of forming the
circuit path comprises the steps of photographing the desired cir-
cuit path and reducing -the artwork in size to correspond to the
desired size of the final resistor; coating the thinr~tal foil
with a photosensitive masking medium; exposing the coated side of
the foil to the photographed circuit; and, subjecting the exposed
foil to an etching process wherein all foil not corresponding to
c~O the desired circuit is removed. The etching process may be under-
taken either before or after the foil has been bonded to the sub-
strate.
- Resistors formed by me-thods analogous to -the aforedescribed
p^ocesses are exhibited by the following U.S. Patents:
2,899,658 Bean August 11, 1959
3,071,749 Starn January 1 1963
3,405,381 Zandman et al October 8, 1968
3,517,436 ~andman et al June 30, 1970
- -2- ~ `.
Despite the general acceptance of thls basic method by the
electronics industry, the resistors Eormed thereby have exhibited
several deficiencies. Among the more prominent prohlems has been
the use of a welding process to attach the connector leads to the
terminals of the foil circui-t pa-th. Due -to the small size of the
terminals and the thinness of the circuit foil (on the order of
0.0001") the welds attaching the relatively thick connector leads
to the circuit terminals have exhibited very poor strength. Normal
usage often causes a breakage in the welds and, consequently, a
catastrophic failure in the resistor due to the open circuit.
A typical foil circuit has a serpentine current path deEined
by a series of closely spaced foil "legs". The value of the re-
sistance may be adjusted, to overcome inaccuracies inherent in the
manufacturing process, by cutting through specifically designated
portions of the circuit to alter the path of current travel. This
method of adjustment`requires a number of circuit portions for fine
adjustment, since the acljustment must be made in discrete steps
In the prior art resistors, the required adjustment causes
serious deficie~cies, notably the extremely fine lines used to
c~ adjust circuit patterns by the scratch and break method are often
much finer than the basic pattern and are, therefore, sensitive to
the tiniest defects during manufacture. Obviously, this decreases
the reliability of the resistor.
Also, in the prior art methods of adjusting by cutting through
the foil to alter the current path, i-t is necessary to carry out
the adjustment manually, usually by an opera-tor with the aid of a
microscope. The manual adjustment necessitates a large amount of
time in the production process and results in a higher priced pro-
duct t~hich is subject to human error.
Many prior art resistors devote a portion of the foil area to
3~3~0
a trimmer circuit pattern l.e. a pa~tern used solely in the adjust-
ing operation to adjust the value of the resistor. I'his portion
serves no other purpose than adjus-tment, and results in a resistor
somewhat larger in size than is absolutely necessary.
, The standard adjustment technique of cutting the foil to the
current flow path also contributes to unnecessary size since, us-
ually, a number of conductor lines do not carry any current. In
this age of miniaturization, it is a serious product deficiency to
have unused or partially used space which results in a resistor
/0 larger than necessary.
SUM~'IARY OF THE INVENTION
The present invention relates to thin me-tal foil resistors.
The method comprises the steps of bondin~ an annealed
thin metallic foil to an insulating substrate using an epoxy
glue; coating the foil surface of the laminate with a photosensitive
masking medium; exposing the coated surface to photographic artwork
of the desired circuit pattern or patterns; etchina away the metallic foil not
required for the circuit; removing the masking medium; etching the
thickness of the circuit pattern to roughly adjust the value of the
resistor; recoating the circuit with a masking medium except for
the terminal lands to which the connecting leads are to be attached;
electroplating the terminal lands; removing the masking medium;
soldering the connecting leads to the terminal lands; coating the
resistor with varnish; laser adjusting the resistor to its final
value; coating the resistor with rubber for strain relief; and en-
capsulating the completed resistor structure.
By etching the thickness of the circuit pattern to achieve the
rough adjustment of the resistance value, the present invention
does not require a resistor to have a separate portion of the cir-
cuit used only for value adjustment. Thus, a resistor formed by
--4--
Lo
this method allows the maximurn amount oE the substrate area to be
utilized for the actual resistor circuit. This allows a qiven
value resistor to be smaller in size or, conversely, a larger cir-
cuit path to be incorporated onto a given substrate area.
The coarse adjust may be carried out by automated methods,
thereby eliminating the manual adjust required by prior art techni-
ques.
The present invention also eliminates the problem of the poor
strength welds attaching the connecting leads to the terminal lands
~D by plating the lands and soldering the connecting leads thereto.
The strength of the connection is improved since the solder holds
over the entire area of the terminal lands, not just in one spot
like the welds.
BRIEF DESCRIPTION OF THE D~INGS
Figure 1 is a perspective view of a resistor made according to the
invention, with -the coatings and encapsulation ornitted for clarity.
Figure 2 is a top view of a substrate having a plurality of resistor
circuits thereon according to the invention.
Figure 3 is an exploded perspective view showing the laminating
process of the invention.
Figu~es4A and 4B are side and top views, respectively, o~ the con-
necting leads according to the invention.
0
Figures 5~ and 5s are top and side views, respectively, of a
resistor made according to the invention.
Figure 6 is a diagrammatic representation of the process for
etching the thickness of the resis-tor according to the invention.
~ .
,DETAILED DESCRIPTION O~ PREFERRED EM~ODI~IENT
!l
¦¦ A resistor made by the preferred embodiment of this invention ,
is illustrated in Figure 1 generally as 10 and comprises a substrate
12 of insulative material,such as glass or ceramic, a thin metallic
~foil resistor circuit pattern 14 with lntegral terminal lands16, and
jfonnecting leads 18 having flattened portions 20 attached to the
terminal lands 16 by soldering. The resistor as just described may
pe encapsulated by molding an insulating material around the entire
¦Istructure after first coating with varnish (such as Dow ~orning
G P 77 NP) and rubber or by potting in a case, as is well known in i
the art. The encapuslation and coatinas are omitted from Fiaure 1 j
for purposes of clairity. The individual resistor 10 is cut fro~
a series of resistor patterns la applied to a substrate as sllown
in Figure 2. These resistor patterns are formed by a thin metallic
foil which has been bonded to the substrate by a laminating process
to be described hereinafter, and subsequently subjected to a photo-
¦graphic artwork-etching process which removes all of the foil ex-
¦¦cept that which forms the circuit patterns.
'I -6-
Prior to bonding to the substrate, the thin metallic
foil is ~irst allnealed. I'he metal foil may be made from
any resistive alloy, such as Evanohm alloy made by Wilbur
B. Driver Co. and i9 on the order of 0.0001" thick. The
foil is annealed by heating it in an inert atmosphere at
1000F for a sufficient time to provide a temperature
coefficient of resistance of the completed resistor in
combination with the particular substrate of approximately
zero. The requisite time will, of course, vary with the
particular alloy metal ~ilm being used, but for Evanohm
metal foil, it has been found that heating the foil at
1000F for a period of approximately 15 minutes will
produce the desired temperature coefficient of resistance
when laminated to a Soda Lime glass substrate.
A thin layer of epoxy glue, on the order of o.ono2~
thick, is applied to one surface of the substrate. A thin,
even layer may be applied by spinning the substrate about
a central axis perpendicular to the plane of the substrate
and applying a small drop of epoxy to the center of rot-
ation. The centrifugal force generated by the spinningwill spread the epoxy evenly over the substrate surface.
of course, any other method of applying the epoxy, which
results in a thin, even layer, may also be used.
After the epoxy has been applied to the surface, the
annealed thin metal foil is laminated to the substrate.
The lamination is accomplished by placing the thin metal
foil 22 in contact with the epoxy coated substrate surface
12 and placing on top of the foil, as shown in Figure 3,
8~0
a sheet of Mylar* 24, a layer of thin stainless steel 26,
and, finally, a layer of rubber sheet 28. The rubber
sheet may be silicone rubber approximately 1/16" thick.
This entire assembly is placed in a vacuum press. The
press is first closed to form a vacuum seal, withou~
exerting any pressure on the laminate, and a vacuum of
approximately lmm of Hg is maintained for 1 minute.
The press is then closed fully to exert a pressure of
approximately 1000 psi and the laminate is cured for
20 minutes at 335F in the press. The assembly is then
removed from the press and further
-7a-
* T.M. of DuPont for poly(ethylene terephthalate) film.
cured for ~0 minutesclt 350~ in an oven. AEter the cur;rlcJ cycle is c~rlpleted,
the laminate is rel~ved E~m the oven ancl ~e r~r, stainless steel, and
~ylar sheets are re~)ved, leaving the th~ m~tal foil bonded to the substrate.
Since the~lylar does not stick to the epoxy, no mold release is required. The
stainless steel sheet 26 preven-ts the stretching of Mylar sheet24 during com-
pression, thereby preventing damage to metal foil 22.
The eircuit pattern is formed on the foil hy standart photo-
graphic ar-twork-etchins techniques As a practical matter, several
patterns are formed on each substrate, as shown in Figure 2. I-t
is not neeessary to speeifically clelineate -the details of this
proeess, as it is well known to those having reasonable skill in
the art. sriefly, the desired cireuit pattern or a plurality of
sueh patterns are photographed and redueed to the desired size.
The foil is coated wi-th a photosensitive masking medium, such as
Kodak KTFR, and ex~osed to the photographic circuit pattern. The
KTFR is retained on the foil only on the desired eircuit pattern.
The foil and substra-te are then sub~eeted to an acid etehing bath
whieh removes all foil exeept that eovered by the KTFR. The KTF~
is remc,ved, leaving only the thin metallie foil in the dèsired eir-
euit pc~ttern on the substrate. Although Kodak KTFR has been men-
tioned in tlle above deseription, any other suitable photo-resist
material may also be sued sueh as Hunt Se, or Shipley AZIII or AZ
1350J. The aeid bath may consist of hydrochloric aeid, nitrie aeid
and water or hydrochloric acid and stannous chloride.
All of the aforementioned operations have been earried out while the resis-
tors are on a common substrate sueh as shown in Figure 2. At this point, the
individual resistors are separated by diamond saw eutting or other standard,
known teehnique such as seribing & breaking.
me next step in the method of the instant invention is to c~arse adjust
the resistor bo its approxima-te value by subjeeting it to another aeid etehing
proeess to reduee the thiekness of the foil pattern. This e-teh-
inq will adjust the resistor to l 1% or better of its
~38~0
final value. ~he e~chtant usecl to acljust the thickness may be the
same used to form tlle pattern, ~ut ~rther diluted wit,h water.
v The etching ~djust process comprises the steps ofAplacing
the resistor in an etching tank for a predetermined period of time;
removing the resistor from the etching tank and placing into a
rinse solution to rinse off the etcht:ant; dryinq the resistor; and
measuring i-ts value. Prior to etching the thickness, the terminal
lands may be coated with a photo-resist material to prevent the
etching adjust process from reducing their thickness.
/O The aforementioned etching adjust process ~y be carried out by
manually transferring the resistor from etching through measuring,
but is ideally carried out by apparatus which automatically moves
the resistor through the sequence of adjustin~ steps. One form of
apparatus ls shown diagrammatically in Fig. 6 wherein tanks or
troughs 50 and 52 contain the etching solution and rinsing water,
and the drying and measuring apparatus are indicated at 54 and 56.
Extendable or slidable arm 58 has means 60 mounted thereon to grip
a resistor, and is mounted on vertically extensible and retractable
base 62. In use, the resistor is manually loaded onto gripping
means 60, and base 62 is retracted so that the resistor is dipped
into etching tank 50 for a predetermined amount of time. After
expiration of time, base 62 is extended to lift the resistor out of
tank 50 and arm 58 is extended to place the resistor over rinse
tank 52. Base 62 is again retracted to dip the resistor into the
rinsè tank 52. This sequence of operations continue through the
drying and measuring steps. If, after measuring, the value of the
the resistor is not at the desired level, arm 58 may be retracted
0
and the resistor ~ay be cycled through tlle adjust process until
its final value is reasonably approximated.
Control of arm 58 and base 62 may be achieved manually, or
automatically by connection with control means 64. Control means
64 which may be a computer or the like, is connected to measuring
apparatus 56, to sense -the measured value of the etched resistor,
and to base 62 and arm 58. If -the sensed value is not close to
a value preset into con-trol means 64, the con-trol means automatic-
ally recycles the resistor through the adjusting process. No e~plan-
/o ation of the details of con-trol means 64 is deemed necessary since it is well
wi~lin the current knowledge of one having reasonable skill in the art of auto-
matic controls.
After the coarse adjust process, the resistor is coated with
a plating resist material, such as Nazdar 205, except for the area
comprising the terminal lands. The terminal lands 16 are then
electroplated with copper, gold or nickel gold, in order to be cap-
able of forming a good solder joint with the connecting leads 18,
to be described hereinafter. Any particular plating process maybe
used which will effectively coat the terminal lands. After plating,
O the plating resist is removed from the resistor circuit pattern.
The terminal lands 16 are tinned and the connecting leads
are soldered to each end of the circuit path, as shown in Figure
1. A typical connecting lead 18 is shown in detail in Figures 4A
-and 4B. The lead comprises a generally cylindrical portion 30 and
an offset, flattened por-tion 20. Offset flattened portion 20 is
soldered to the terminal lands 16 as shown in Figure 1. Optionally,
talon head 34 is attached to the connecting lead 18 near the flat-
tened portion 20 and cor.~prises radially extending portion 36 and
a plurality of iongitudinai ribs 38. The talon heads 34 are sur-
3 O rounded by the molded coating (to be described hereinafter) appliedto the completed resistor so as to prevent twisting of the leads
about their axes with subsequent weakening of -the soldered connec-
tions to the terminal lands 16. It is also within the scope of
--10--
this invention to omit the talon heads 3~ and use only the connect-
ing leads as shown in Figure 1.
After the connecting leads 18 are soldered -to the terminal
lands 16, the flux is cleaned from the solder joints. The resistor
is -then ready for fine adjustment to its final value. This is
accomplished by cutting a slot 40 (shown in Fig. 1) in a portion of
the circuit pattern. The cu-t al-ters the path of the current to
adjust the resistance value of the circuit. It is env~sioned that
the cut may be made with extreme accuracy by a laser beam as known
/o by those skilled in the resistor fabricating art.
Subsequent to the final adjustment, the resistor is subjected
to an encapsulation treatment. The resistor is fully coated with
varnish as previously described and then with rubber for strain
relief. Both the varnish and rubber (which may be a silicone rubb-
er such as Sylgard 182 or an RTV) may be applied by dipping the
resistor and subsequently rotating it to assure an even coat.
If a t~ranslucent varnish is used, it may be applied before the
step of laser adjusting the resistor. By applying the varnish
prior to the final adjustment, any possible change in resistance
which may be caused by the coating is eliminated by the final ad-
justment. After coating with varnish and rubber, the resistor is
placed in a mold and a hard, protective material in molded so as to
completely surround the resistor such that only the connecting
leads 18 extend from the completed molding. The molding is by the
standard transfer molding process and need not be described in de-
tail. The mold may be a two piece mold split laterally so that the
resistor may be placed therein. The connecting leads, of course,
extend through openings in the mold so as to not be completely
covered. No special means are necessary to hold the resistor cen-
O trally in the mold, since the leads are sufficiently strong to ac-
hieve this purpose. The molded resistor is shown in Figures 5A
and 5B with the substrate 12 foil circuit lg, and connecting leads
18 shown in dotted lines. Other methods of encapuslation, such as
--1 1--
3~3
potting in a pre-fo~rned case, known in the art, may al.so be used
without exceedincJ the scope of this invention.
I
