[0001] The present invention relates to nonlinear resistive devices, such as varistors,
and more particularly to methods of making such devices using barrel plating techniques
in which only the electrically contactable end terminals of the device are plated.
[0002] Nonlinear resistive devices are disclosed in the specifications of U.S. Patent No.
5,115,221.
[0003] Figure 1 is a typical device 10 that includes plural layers 12 of semiconductor material
with electrically conductive electrodes 14 between adjacent layers. A portion of each
electrode 14 is exposed in a terminal region 16 so that electrical contact may be
made therewith. The electrodes 14 may be exposed at one or both of opposing terminal
regions, and typically the electrodes are exposed at alternating terminal regions
16 as illustrated. The exposed portions of the electrodes 14 are contacted by electrically
conductive end terminals 18 that cover the terminal regions 16.
[0004] The apparently simple structure of such devices belies their manufacturing complexity.
For example, the attachment of the end terminals 18 has proved to be a problem in
search of a solution. The terminal regions may be plated with nickel and tin-lead
metals to increase solderability and decrease solder leaching. One method of affixing
the end terminals 18 is to use a conventional barrel plating method in which the entire
device is immersed in a plating solution. However, the stacked layers are semiconductor
material, such as zinc oxide, that may be conductive during the plating process so
that the plating adheres to the entire surface of the device. Thus, in order to provide
separate end terminals as shown in Figure 1, a portion of the plating must be removed
after immersion, or covered before immersion with a temporary plating resist comprised
of an organic substance insoluble to the plating solution. However, the removal of
the plating or organic plating resist is an extra step in the manufacturing process,
and may involve the use of toxic materials that further complicate the manufacturing
process.
[0005] It has also been suggested that the metal forming the end terminals 18 be flame sprayed
onto the device, with the other portions of the surface of the device being masked.
Flame spraying is not suitable for many manufacturing processes because it is slow
and includes the creation of a special mask, with the additional steps attendant therewith,
as disclosed in the specification of U.S. Patent No. 4,316,171.
[0006] An object of the present invention is to provide a method and device that obviates
the problems of the prior art, and in which an electrically insulating, inorganic
layer is formed on portions of the device before the device is barrel plated.
[0007] Another object is to provide a method and device in which a phosphoric acid is reacted
with the exposed surface of stacked zinc oxide semiconductor layers to form a zinc
phosphate coating, and in which a zinc phosphate coating protects portions of the
device that are not to be plated when the end terminals are formed.
[0008] A further object is to provide a method and nonlinear resistive device having a body
of layers of semiconductor material with an electrode between adjacent layers, in
which the body of the nonlinear resistive device is coated with an inorganic layer
that is electrically insulating, except at a terminal region of the body where an
electrode is exposed for connection to an end terminal, and in which the coated body
is plated with an electrically conductive metal to form the end terminal in a process
in which the body becomes electrically conductive and in which the electrically conductive
metal does not plate the coated portions of the body because the inorganic layer is
not electrically conductive.
[0009] The present invention includes a method of making a nonlinear resistive device comprising
the steps of:
a) providing a body for the nonlinear resistive device, the exterior of the body being
a zinc oxide semiconductor except at a terminal region where an end termination is
provided;
(b) reacting a phosphoric acid with the body to form an electrically insulative zinc
phosphate coating on the exposed zinc oxide semiconductor, the end termination not
being coated with the zinc phosphate; and
(c) barrel plating the body to plate the end termination with an electrically conductive
metal, wherein the electrically conductive metal does not form on the zinc phosphate
coated portions of the body during the barrel plating because the zinc phosphate is
not electrically conductive.
[0010] The invention also includes a method of providing an electrically insulative coating
for a varistor, the method comprising the steps of:
(a) providing an uncoated varistor having plural zinc oxide layers with electrodes
therebetween, the electrodes contacting at least one of two exterior electrically
conductive metal end terminations that are separated by an exposed surface of the
zinc oxide layers;
(b) submerging the uncoated varistor in a phosphoric acid solution for 25 to 35 minutes
at 70°C to 80°C to form an electrically insulative zinc phosphate coating on the exposed
surface of the zinc oxide layers, the end terminations not being coated with the zinc
phosphate coating; and
(c) subjecting the coated varistor to a barrel plating process in which the varistor
is electrically charged and an electrically conductive plating material adheres to
the charged portions of the varistor, the end terminations being plated and the zinc
phosphate coating not being plated.
[0011] The invention will now be described, by way of example, with reference to the accompanying
drawings in which:
Figure 1 is a pictorial depiction of a varistor typical of the prior art.
Figure 2 is vertical cross section of an embodiment of the device of the present invention.
Figure 3 is a pictorial depiction of a high energy disc varistor with an insulating
layer of the present invention thereon.
Figure 4 is a pictorial depiction of a surface mount device with an insulating layer
of the present invention.
[0012] Figure 2 illustrates an embodiment of a nonlinear resistive element 20 that includes
a body 22 having stacked zinc oxide semiconductor layers 24 with planar electrodes
26 between adjacent pairs of layers 24. Each electrode 26 has a contactable portion
28 that is exposed for electrical connection to electrically conductive metal (preferably
silver, silver-platinum, or silver-palladium) end terminations 30 that cover terminal
regions 32 of the body 22 and contact the electrodes 26. The portions of body 22 not
covered with the end terminations 30 are coated with an electrically insulative zinc
phosphate layer 34. The end terminations 30 may be plated with layers 36 of electrically
conductive metal that form electrically contactable end portions for the resistive
element 20. By way of example, in one embodiment the zinc oxide layers 24 may have
the following composition in mole percent: 94-98% zinc oxide and 2-6% of one or more
of the following additives; bismuth oxide, cobalt oxide, manganese oxide, nickel oxide,
antimony oxide, boric oxide, chromium oxide, silicon oxide, aluminum nitrate, and
other equivalents.
[0013] The body 22 and end terminations 30 are provided conventionally. The zinc phosphate
layer 34 may be formed by reacting phosphoric acid with the zinc oxide semiconductor
layers exposed at the exterior of the body 22. The reaction may take place for 25-35
minutes at 70° to 80°C. By way of example, one part orthophosphoric acid (85 wt%)
may be added to fifty parts deionized water. The solution may be heated to 75°C and
stirred. The body 22 with end terminations 30 affixed may be washed with acetone and
dried at 100°C for ten minutes. The washed device may be submerged in the phosphoric
acid solution at 75°C for thirty minutes to provide the layer 34. After the layer
34 is applied, the body may be cleaned with hot, deionized water and dried at about
100°C for about fifteen minutes. The layer 34 does not adhere to the end terminations
30 because the silver or silver-platinum in the end terminations 30 is not affected
by the phosphoric acid. The phosphoric acid solution may also be applied by spraying,
instead of submerging, the washed device.
[0014] After the zinc phosphate layer 34 has been applied, the device may be barrel plated
with an electrically conductive metal, such as nickel and tin-lead, to provide the
layers 36. A conventional barrel plating process may be used, although the pH of the
plating solution is desirably kept between about 4.0 and 6.0. In the barrel plating
process the device is made electrically conductive and the plating material adheres
to the electrically charged portions of the device. The metal plating of layers 36
does not plate the zinc phosphate layer 34 during the barrel plating because the zinc
phosphate is not electrically conductive.
[0015] The zinc phosphate layer 34 is electrically insulating and may be retained in the
final product to provide additional protection. The layer 34 does not effect the I-V
characteristics of the device.
[0016] In an alternative embodiment, the phosphate layer may be an inorganic oxide layer
formed by the reaction of phosphoric acid with the metal oxide semiconductor in the
device. For example, instead of zinc oxide, the semiconductor may be iron oxide, a
ferrite, etc.
[0017] In another alternative embodiment, the method described above may be used in the
manufacture of other types of electronic devices. For example, a high energy disc
varistor has a glass or polymer insulating layer on its sides. With reference to Figure
3, instead of glass or polymer, the disc varistor 40 may have an insulating layer
42 of phosphate formed in the manner discussed above. The present invention is applicable
to other varistor products such as a surface mount device depicted in Figure 4, radial
parts, arrays, connector pins, discoidal construction, etc.
[0018] A method of providing a semiconductor device with an inorganic electrically insulative
layer, the device having exposed semiconductor surfaces and electrically conductive
metal end terminations. The device is reacted with phosphoric acid to form a phosphate
on the exposed surfaces of the semiconductor but not on the metal end terminations.
Thereafter barrel plated in a conventional electrical barrel plating process and the
plating is provided only on the end terminations.
1. A method of making a nonlinear resistive device comprising the steps of:
(a) providing a body for the nonlinear resistive device, the exterior of the body
being a zinc oxide semiconductor except at a terminal region where an end termination
is provided;
(b) reacting a phosphoric acid with the body to form an electrically insulative zinc
phosphate coating on the exposed zinc oxide semiconductor, the end termination not
being coated with the zinc phosphate; and
(c) barrel plating the body to plate the end termination with an electrically conductive
metal,
wherein the electrically conductive metal does not form on the zinc phosphate coated
portions of the body during the barrel plating because the zinc phosphate is not electrically
conductive.
2. A method as claimed in claim 1 wherein the end termination comprises a layer of a
metal selected from the group consisting of silver, silver-platinum, and silver-palladium.
3. A method as claimed in claim 1 or 2 wherein the body comprises in mole percent, 94-98%
zinc oxide and 2-6% of one or more of the additives selected from the group of additives
consisting of bismuth oxide, cobalt oxide, manganese oxide, nickel oxide, antimony
oxide, boric oxide, chromium oxide, silicon oxide, and aluminum nitrate.
4. A method as claimed in any one of claims 1 to 3 wherein the step of reacting phosphoric
acid comprises the step of submerging the body in the phosphoric acid, with the step
of submerging the body comprising the step of submerging the body in a orthophosphoric
acid solution for 25 to 35 minutes at 70° to 80°C.
5. A method as claimed in any one of claims 1 to 4 wherein the electrically conductive
metal comprises at least one of nickel and tin-lead, and the body is a varistor.
6. A method of providing a semiconductor device as claimed in any claims 1 to 5 including
an inorganic electrically insulative layer, the semiconductor device having an exposed
semiconductor surface and electrically conductive metal end terminations, the method
comprising the steps of:
(a) exposing the semiconductor device to a phosphoric acid solution to form a phosphate
coating on the exposed semiconductor surfaces, and not on the end terminations; and
(b) barrel plating the semiconductor device with an electrically conductive metal
plating in a process in which the device is electrically charged and submerged in
a plating solution, the plating being formed on the end terminations and not on the
phosphate coating because the phosphate coating is not electrically conductive.
7. A method as claimed in claim 6 wherein the exposed semiconductor surfaces comprise
one of zinc oxide and iron oxide.
8. A method as claimed in claims 6 or 7 wherein the phosphoric acid solution comprises
orthophosphoric acid and deionized water.
9. A nonlinear resistive element made by the process of any one of claims 6 to 8.
10. A method of providing an electrically insulative coating for a varistor, the method
comprising the steps of:
(a) providing an uncoated varistor having plural zinc oxide layers with electrodes
therebetween, the electrodes contacting at least one of two exterior electrically
conductive metal end terminations that are separated by an exposed surface of the
zinc oxide layers;
(b) submerging the uncoated varistor in a phosphoric acid solution for 25 to 35 minutes
at 70°C to 80°C to form an electrically insulative zinc phosphate coating on the exposed
surface of the zinc oxide layers, the end terminations not being coated with the zinc
phosphate coating; and
(c) subjecting the coated varistor to a barrel plating process in which the varistor
is electrically charged and an electrically conductive plating material adheres to
the charged portions of the varistor, the end terminations being plated and the zinc
phosphate coating not being plated.
11. A nonlinear resistive element comprising:
a body constituted by a varistor having stacked zinc oxide semiconductor layers;
a planar electrode between each pair of said layers, each said electrode having a
contactable portion that is exposed for electrical connection;
plural spaced electrically conductive metal end terminations, each of said end terminations
being on an end portion of said body for contacting at least one said contactable
portion;
an electrically insulative zinc phosphate coating covering said body between said
end terminations;
a metal plating covering said end terminations, and in which the body comprises in
mole percent, 94-98% zinc oxide and 2-6% of one or more of the additives selected
from the group of additives consisting of bismuth oxide, cobalt oxide, manganese oxide,
nickel oxide, antimony oxide, boric oxide, chromium oxide, silicon oxide, and aluminum
nitrate.