Abstrict
An electric heater plate is disclosed comprising a sheet of tempered
glass with a metallized aluminum circuit extending over the face
thereof for carrying an electrical heating current. At the terminals
of the metallized aluminum circuit there is provided a terminal
area of silver between the glass and the metallized aluminum coating,
which silver terminals are each exposed through a small opening
in the metallized aluminum coating such that lead wires may be soldered
to the silver through said openings.
Claims
I claim:
1. The method of manufacturing an electrical heater plate having
an electrically conductive grid and terminals disposed in terminal
areas comprising bonding a layer of silver to one surface of a sheet
of tempered glass, said silver extending only over the terminal
areas of the glass sheet, coating the one surface of said sheet
including all of said terminal areas with a metal coating, applying
an enamel resist to said coating in the pattern of an electrical
grid including said terminal areas, while providing an opening through
said enamel resist at the location of each of said terminal areas
exposing part of the metal coating therebeneath, and removing said
metal coating in all areas not covered by said resist including
the metal coating exposed by said openings at said terminal areas
whereby to expose the silver through the metal coating.
2. The method of claim 1 including finally applying to the entire
surface of the panel including the terminal areas a heat resistant
and atmospheric resistant coating.
3. The method of claim 2 including subsequently soldering a wire
lead to the silver of said terminals, the heat of said soldering
removing the heat and atmospheric resistant coating over said silver,
and soldering in such manner as to substantially fill the opening
through said metal coating to protect the silver from atmosphere.
Description BACKGROUND OF THE INVENTION
In conventional electric heater plates the terminals for the metal,
preferably aluminum, circuit present a unique problem in that lead
wires cannot be satisfactorily soldered to the aluminum terminal
areas since solder will not wet the thin aluminum oxide layer and
adhere thereto, Conventional heater plates therefore, are first
provided with an area of metallized bronze on the aluminum circuit
at the terminal area and then a layer of molten solder is applied
to the metallized bronze. While lead wires can easily be soldered
to such a built-up terminal the constant manufacture of quality
terminals is extremely difficult if not impossible. This is due
to a number of factors, including among others, the fact that after
the application of a metallized aluminum coating with an added bronze
layer at the terminals and molten solder over the bronze, the circuit
pattern must then be etched in the aluminum coating. To do this,
the circuit pattern is silk-screened with a resist usually an enamel
resist, that is resistant to the etching bath and a subsequent neutralizing
bath. The terminal area is thicker than the circuit area due to
these additional layers. Also, the terminal area tends not to be
perfectly flat. For these reasons the resist often does not completely
coat the terminal area thus exposing it to the etching and neutralizing
baths. Further, when the resistant paint is baked in the oven the
solder softens causing the paint over and near the terminal to blister.
Accordingly, when the plate is finally etched, the blistered paint
frequently leaks causing open areas to be etched at and around the
edges of the terminals. The terminals of this invention are not
as thick and are almost completely flat thus avoiding those problems.
SUMMARY OF THE INVENTION
In accordance with this invention the glass sheet base for the
electric heater plate is provided with a small area of silver in
the location to be occupied by the circuit terminals. The silver
being provided as a low resistance conductive silver paste and silk-screened
onto the terminal area before tempering. During the subsequent heating
cycle at an elevated temperature of 600.degree.-625.degree. C. the
silver is effectively bonded permanently to the glass. Subsequently,
the entire surface of the glass is metallized with the coating of
aluminum. The circuit is then silk-screened with a suitable etching
bath resistant paint, the paint dried, and then the aluminum is
etched away from the glass in the exposed areas in an etching bath
in a known and conventional way. Over the terminal area a small
portion of the aluminum is etched away to expose the silver surface.
As a final step the plate is sprayed with a coating of heat resistant
lacquer to protect the silver from atmospheric attack and to render
the clear areas between the grid lines opaque. During manufacture
lead wires can be soldered to the silver terminals in a conventional
manner which operation by its heat removes the lacquer film over
the silver exposed through the aluminum.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a sheet of glass having the silver applied
to the terminal areas,
FIG. 2 is a similar view of the plate of FIG. 1 with the electrical
circuit completed thereon; and
FIG. 3 is taken along the lines 3--3 of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 1 a sheet of glass 10 has had applied thereto
terminal areas 12 and 14 of silver. These terminal areas 12 and
14 may be of any desired size with a dimension of approximately
1/2 inch by 1/2 inch being convenient for most such plates.
The silver is first applied through a silk-screen in a conventional
manner, which silk-screen has the pattern of the terminals 12 and
14 provided therein, such that when a low resistance conductive
silver paste is applied therethrough it passes through the screen
and adheres to the glass 10, the pattern being dictated by the silk-screen
such as the squares shown at 12 and 14. Typical of the low resistance
conductive silver paste that may be used are Drakenfeld Silver Paste
A953 or Englehard Hanovia Silver 9124, both commercially available.
The former, for example, may be obtained from the Drakenfeld Division,
Imperial Color and Chemical Dept. of Hercules Incorporated, Washington,
Pennsylvania. This conductive Silver Paste No. A953 is finely divided
particles of silver milled in squeegee oil and has other organic
additives to facilitate silk-screening of the paste. The additives
burn out during the subsequent firing.
The glass sheet with the silver paste applied is heated to a temperature
range of about 600.degree.-625.degree. C for from 4-5 minutes in
a tempering furnace. This is above the strain point of the glass
but below its softening point and this heating treatment tempers
the glass. Additionally, during this tempering treatment of the
glass, the fine particles of silver melt and bond to the glass by
a bond believed to be in part chemical and in part mechanical.
Subsequent to the firing of the silver to the glass sheet the entire
surface of the glass including the silver terminal areas is metallized
with a coating of aluminum. This metallizing process is well known
and need not be described in complete detail here. However, briefly,
an aluminum wire is atomized in a oxygen/propane flame and deposited
upon the heated surface of the glass. The glass for this purpose
is heated in a range from about 550.degree. to about 730.degree.
F. The heated glass is usually coated with the atomized aluminum
while on a conveyor belt and the thickness of the coating depends
in large part on the speed of the conveyor belt with the conveyor
being slowed for thicker coatings. The thickness of the aluminum
is carefully controlled to obtain the desired resistance on the
finished plate. From 2 to 3 mils is the presently preferred thickness
of the aluminum coating. After metallizing the plate with an aluminum
coating and cooling the same, the grid pattern of the circuit is
silk-screened with a suitable etch resistant paint of which many
are known with several enamel resists being particularly useful.
The silk-screen pattern corresponds to that of the desired grid
design such as the grid 16 shown in FIG. 2. As shown the pattern
for grid 16 includes terminal areas 18 and 20 which overly the silver
terminal areas 12 and 14 respectively. The terminal areas 18 and
20 of the grid 16 are larger than the corresponding silver terminal
areas 12 and 14. A currently preferred size for the silver layer
for the terminal area 12 and 14 is approximately 1/2 inch by 1/2
inch. The size of the overlying aluminum areas 18 and 20 is somewhat
larger with 5/8 inch by 3/4 inch being a satisfactory size when
using silver layers of 1/2 inch by 1/2 inch. The terminal area size
is at least 5/8 inch wide and is of an area sufficiently large to
properly conduct the electrical load of the heater plate when operating,
for example, below about 175.degree. F.
In addition to protecting the silver, the large terminal area offers
very little electrical resistance. This allows the terminal area
to remain relatively cold during operation. The center of the heater
plate may reach 400.degree. F during operation which temperature
would weaken the solder bond used to connect the lead wire to the
terminal should the terminal ever reach this temperature. The large
terminal area prevents such from occurring.
The grid lines, other than at the terminals run upwards from a
minimum 1/16 inch width. There is no upper limit to the width of
the grid lines other than the overall resistance required to produce
a given wattage.
The etching bath resist is not applied to those areas outside of
the boundries of the electrical grid circuit shown since in those
areas the aluminum is to be removed by etching.
Also not covered with resist are smaller areas 22 and 24 within
the terminal areas 18 and 20 respectively. Accordingly, in the subsequent
etching operation the small areas 22 and 24 of aluminum will be
etched away exposing the underlying silver. The open areas 22 and
24 are preferably, but not necessarily, circles 3/8 inch in diameter.
Subsequent to the application of the resist, the grid pattern is
retained and the other aluminum on the plate etched away in a conventional
etching bath followed by a suitable neutralizing bath. The heater
place will then have the appearance as shown in FIG. 2 with the
silver areas 12 and 14 similarly bonded to the underlying glass
10 and to the overlying aluminum terminal areas 18 and 20, but with
exposure of the silver through the aluminum terminal areas in the
openings 22 and 24. In FIG. 2 the full grid pattern is not shown
merely as a convenience. It will be understood that the pattern
repeats throughout the surface in the same manner as the top and
bottom portions of the pattern shown.
The plate is then sprayed with a coating of heat resistant lacquer
which renders the clear areas of glass between the grid lines opaque
and at the same time covers the exposed silver in the areas 22 and
24 of the terminals 18 and 20. This lacquer serves to protect the
silver from any possible attack from sulphur in the atmosphere which
can cause the formation of silver sulphate on the surface thereof.
A number of suitable heat resistant lacquers are known and commercially
available. One typical lacquer of this type is White Nubelon Lacquer
-- AGL-3815 obtainable from the Glidden-Durkee Division of SCM Corporation,
Coatings and Resins Group, Atlanta, Georgia. The manufacturer describes
this lacquer as being as alkyd-based silicone modified heat-resisting
lacquer. It contains no lead and is designed for high temperature
work. When the electric heater plate is to be incorporated into
a heating system the lead wires are then soldered to the silver
terminals 12 and 14, through the openings 22 and 24. Ordinary resin
core 60 - 40 grade solder is suitable. During soldering the heating
thereof removes the thin lacquer film from the silver permitting
the solder to adhere thereto. The solder should fill in the openings
22 and 24 in order to continue to protect the silver from corrosion
by the atmosphere.
It has been found that terminals of this type can be more consistently
produced to acceptable quality standards then the built-up type
previously referred to. Additionally, silver being an excellent
conductor, the terminals tend to operate cooler than the conventional
terminals.
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