Method of producing a hermetic glass to metal seal without metal oxidation

A method for forming a hermetic compression glass to metal seal which leaves the metal free of oxide. A temporary assembly of a metal header having a glass bead in an aperture therethrough and an electrical feedthrough conductor passing through the glass bead is heated in a dehydrated pure hydrogen environment to above the melting temperature of the glass bead, and then cooled to solidify the bead and cause the header to contract around the bead.

BACKGROUND OF THE INVENTION 
The invention disclosed herein relates generally to hermetic electrically 
insulating seals, and more specifically to a method for producing a 
hermetic glass to metal compression seal using a dehydrated pure hydrogen 
environment to avoid metal oxidation. 
Hermetic seals between elements are widely required or advantageous in a 
variety of apparatus, particularly including electrical devices. For 
example, in electrical switches for use in harsh environments, the 
switching elements are located in a sealed chamber and hermetic seals are 
required where feedthrough conductors or terminals pass through the 
chamber wall. Hermetically sealed switches are frequently provided with 
metal cases from which the feedthrough conductors must be electrically 
insulated. It is known to fabricate such switches by utilizing a metal 
header having apertures therethrough for accommodating the feedthrough 
conductors and forming the seals around the feedthrough conductors from 
glass. 
A variety of factors must be considered in forming a glass to metal seal 
for an electrical feedthrough conductor. These include the electrical 
properties of the material from which the conductor is formed, the thermal 
coefficients of expansion of the glass and metal parts, the melting or 
softening temperatures of the glass and metal materials, the suitability 
of the metals from which the feedthrough conductors and other metal parts 
are made for subsequent processing steps, such as soldering, brazing, 
welding, crimping, etc., and the number and complexity of preparatory 
processes required to permit subsequent fabrication and utilization. 
Two general types of hermetic glass to metal seals are known. These are 
bonded seals in which the molten glass wets and adheres to the metal 
surface as the glass solidifies, and compression seals in which sealing is 
accomplished by large compression forces on an inner member, such as a 
glass bead, by an outer member. Hermetic seals may also employ a 
combination of these characteristics. 
One of the problems encountered with bonded seals where the glass and metal 
have different thermal coefficients of expansion, as is true for most 
types of glass and metal, arises from stress concentrations set up in the 
glass. Since, in a bonded seal, the molten glass wets the surfaces of the 
contiguous metal parts, the glass forms a concave meniscus leaving a thin 
glass edge. As the assembly cools and the glass solidifies, stresses are 
created in the glass due to the different thermal coefficients of 
expansion. The thin glass edges may not be able to withstand these 
stresses. The result may be cracks which propagate through the glass 
elements and jeopardize the hermetic seal. 
A concurrent manufacturing disadvantage may accompany the use of bonded 
hermetic glass to metal seal assemblies in that wetting of the metal 
surfaces by the molten glass occurs only if the metal surfaces have an 
oxide formed thereon which occurs normally when the metals from which the 
metal parts are commonly made are subjected to the melting temperature of 
the glass in anything but a highly reducing environment. This oxide must 
be removed for subsequent manufacturing steps, such as brazing or welding 
housing parts together, making either internal or external soldered, 
brazed or welded connections to the feedthrough conductors, etc. Removal 
of the oxidation requires disadvantageous cleaning steps in the 
manufacturing process. 
The applicant has discovered that a satisfactory hermetic glass to metal 
seal can be achieved by selecting glass and metal materials to result in a 
compression seal, and conducting the processes to form the seal in a 
highly reducing environment which avoids the formation of thin glass 
sections subject to stress cracking and the formation of oxide whose 
removal requires disadvantageous manufacturing steps. 
SUMMARY OF THE INVENTION 
The present invention is a method for producing an article having an 
electrical feedthrough conductor extending through a metal header and 
electrically insulated therefrom by a glass hermetic seal, the method 
leaving the feedthrough conductor and header substantially free of oxide. 
The method comprises providing a metal header having an aperture 
therethrough through which a feedthrough conductor extends and a glass 
bead positioned in the aperture surrounding the conductor, the material of 
the glass bead having a lower coefficient of thermal expansion than the 
material of the metal header. The header, glass bead and feedthrough 
conductor are heated in a dehydrated pure hydrogen environment to a 
temperature above the melting temperature of the glass bead. The header, 
glass bead and feedthrough conductor are then cooled in the dehydrated 
pure hydrogen environment to solidify the glass bead and contract the 
header around the solidified bead to form a hermetic compression seal. The 
glass bead is preferably formed of a lead free glass and the metal header 
may be formed of stainless steel. The header, glass bead and feed-through 
conductor may be heated at a rate of approximately 15.degree. C. per 
second, soaked at a temperature of approximately 1900.degree. F. for a 
period of approximately 7.5 minutes and then cooled at a rate of 
approximately 25.degree. C. per second.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the conventional glass to metal electrical feedthrough seal illustrated 
in FIG. 1, reference numeral 11 identifies a metal header, which may be 
part of an electrical switch. Header 11 is formed with an aperture 12 
therethrough for accommodating an electrical feedthrough conductor 13 
which extends through aperture 12. Reference numeral 14 identifies a glass 
seal 14 surrounding conductor 13 in aperture 12. 
Glass seal 14 is initially in the form of a bead loosely surrounding 
conductor 13 and loosely fitting into aperture 12. Using conventional 
processing techniques, this temporary assembly of parts is heated to above 
the melting temperature of the glass bead, and then cooled to solidify the 
glass in the form of a seal. The glass material of seal 14 is selected to 
have substantially no lead content to minimize out gassing. This glass 
material has a relatively high softening temperature and a relatively low 
thermal coefficient of expansion. 
Header 11 is typically formed of cold rolled steel or stainless steel, both 
of which have a significantly higher thermal coefficient of thermal 
expansion than glass seal 14. Conductor 13 is formed of a 
chromium-nickel-iron composition which has a coefficient of thermal 
expansion which is relatively low, but higher than the coefficient of 
thermal expansion of seal 14. 
As the assembly is heated, header 11 expands along with aperture 12 
therethrough. The glass bead melts, and fills the additional volume of the 
aperture. As the assembly is cooled, header 11 and aperture 12 contract at 
a faster rate than seal 14 and subject seal 14 to heavy compression forces 
which, because of the elasticity of seal 14, are partially transmitted to 
conductor 13, thus forming a compression hermetic seal. 
Heating of the parts has typically been performed in a neutral to slightly 
reducing environment. In such an environment, the metal compositions of 
which header 11 and conductor 13 are formed oxidize readily. This surface 
oxide causes the molten glass to wet the surfaces of the metal parts, thus 
resulting in bonding of the glass to the metal surfaces to form a bonded 
hermetic seal. 
Because the molten glass wets the surfaces of the metal parts, its surface 
tension causes a concave meniscus as shown at 15 and thin glass sections 
where the glass surface meets the metal surfaces as shown at 16. 
Particularly as header 11 and conductor 13 are subjected to soldering, 
brazing or welding operations, the thin glass sections may become stressed 
to the extent that cracks are formed which may propagate through the 
remainder of the glass body and jeopardize the hermetic seal. 
The surfaces of header 11 and conductor 13 must have the oxide removed 
therefrom in order to permit subsequent soldering, brazing and/or welding 
operations. Removal of the oxide entails additional steps whose avoidance 
would be advantageous in a manufacturing process. 
FIG. 2 illustrates a header and electrical feedthrough assembly similar to 
that of FIG. 1, but formed using the applicant's method. The header, 
aperture therethrough, electrical feedthrough conductor and glass seal are 
identified by reference numerals 21, 22, 23 and 24 respectively. However, 
the process of heating the header, glass bead and conductor is done in a 
highly reducing dehydrated pure hydrogen environment which does not result 
in oxidation of the surfaces of header 21 or conductor 23. Satisfactory 
melting of the glass material requires that the temperature of the parts 
be raised to between 1700.degree. F. and 2100.degree. F., and preferably 
1900.degree. F..-+.50.degree. F., for a period of from 5 to 10 minutes. In 
order to avoid oxidation of the surfaces of the metal parts, the hydrogen 
must be kept at a dew point of at least -50.degree. F., and preferably as 
low as -70.degree. F. This has been found to necessitate a hydrogen flow 
rate of approximately 15 cubic feet per hour. 
The lack of metal oxide prevents the molten glass from wetting the metal 
surfaces. The surface tension of the molten glass thus causes a convex 
meniscus as shown at 25. Also, thin glass sections where the glass meets 
the metal, as at reference numeral 26, are avoided. As the assembly is 
cooled, seal 24 solidifies and header 21 exerts heavy compression forces 
thereon which are partially transmitted to conductor 23. The compression 
seal thus formed has been found to provide satisfactory hermetic sealing. 
In addition, glass seal 24 is not subject to stress fracturing upon 
subsequent soldering, brazing and/or welding operations, and oxide removal 
steps prior to such soldering, brazing or welding operations are reduced 
or eliminated. 
The applicant's process may be carried out in a sintering oven as shown in 
FIG. 3 in which reference numeral 30 identifies a mesh belt which passes 
around drums 31 and 32 journalled in bearings 33 and 34 respectively, and 
of which drum 31 is driven by a motor and speed regulator 35. 
The furnace includes a preheat chamber, a high heat chamber and a cooling 
chamber generally identified by reference numerals 36, 37 and 38 
respectively through which belt 30 passes in sequence. Reference numeral 
40 identifies an entrance door to preheat chamber 36 which may be lifted 
by an operator 41. Preheat chamber 36 contains preheat elements 42 which, 
according to the applicant's process, are capable of raising the 
temperature of the parts in the preheat chamber at a rate of 25.degree. C. 
per second. 
Parts on conveyor 30 pass from preheat chamber 36 through an intermediate 
door 44 controlled by an operator 45 into high heat chamber 37. Reference 
numeral 46 identifies heating elements in chamber 37 which are capable of 
maintaining the temperature of parts in the chamber at 2100.degree. F. 
According to the applicant's process, the parts remain in chamber 37 at 
from 1700.degree. F. to 2100.degree. F. for a period of from 5 to 10 
minutes. 
Parts in chamber 37 then pass through an intermediate door 48 controlled by 
an operator 49 into cooling chamber 38. Cooling chamber 38 includes means 
(not shown) capable of cooling the parts therein at a rate of 15.degree. 
C. per second. The parts then pass from cooling chamber 38 through an exit 
door 50 controlled by an operator 51. 
During the preheating, heat soaking and cooling operations, a dehydrated 
pure hydrogen environment is maintained within the furnace. This is 
accomplished by introducing hydrogen having a dew point at least as low as 
-50.degree. F., and preferably 70.degree. F., into high heat and cooling 
chambers 37 and 38 through inlet tubes 54 and 55 under the control of 
metering device 56. Hydrogen is removed from preheat and high heat 
chambers 36 and 37 through exhaust tubes 58 and 59. This arrangement is 
designed to provide a flow of dehydrated hydrogen of approximately 15 
cubic feet per hour through the furnace in a direction opposite to the 
direction of travel of parts through the furnace. The heating and cooling 
rates, as well as the travel speed are maintained such that the parts are 
subjected to glass to metal sealing and brazing temperatures for a minimum 
of 5 minutes. 
In accordance with the foregoing description, the applicant has provided a 
unique method for producing glass to metal seals which reduces or 
eliminates stress fractures in the glass which may otherwise occur as a 
result of subsequent soldering, brazing or welding operations, and which 
also reduces or eliminates the necessity for oxide removal and other 
cleaning operations of the metal parts prior to such soldering, brazing or 
welding operations. Although the applicant's method has been shown and 
described with particularlity, variations of the method which do not 
depart from the applicant's contemplation and teaching will be apparent to 
those of ordinary skill in the art. It is not intended that coverage be 
limited to the disclosed embodiment, but only by the terms of the 
following claims.