Infrared furnace with muffle

An infrared furnace has a firing chamber in which a source of infrared energy is disposed and an elongated envelope transparent to the infrared energy extending through the firing chamber. The envelope has first and second open ends outside the firing chamber. First and second baffle chambers surround the respective first and second ends of the envelope. A product conveyor travels through the furnace via the baffle chambers and the envelope. Gas flow is prevented from the exterior of the furnace into the baffle chambers and from the firing chamber into the baffle chambers. Non-atmospheric gas is supplied to the baffle chambers so as to create therein a superatmospheric pressure which prevents gas flow into the baffle chambers from the exterior of the furnace. The non-atmospheric gas is exhausted from one of the baffle chambers, thereby inducing flow of the non-atmospheric gas from the other baffle chamber through the envelope. A seal chamber is disposed between each baffle chamber and the firing chamber such that gas leakage from the seal chamber to the baffle chamber and to firing chamber occurs around the periphery of the envelope. A packing is compressed against the periphery of the envelope and the walls of the seal chamber to minimize such leakage and the seal chamber is pressurized with non-atmospheric gas to prevent gas flow from the firing chamber to the baffle chamber.

BACKGROUND OF THE INVENTION 
This invention relates to infrared furnaces and, more particularly, to an 
infrared furnace suitable for firing electronic components in a 
non-reactive environment. 
Application Ser. No. 306,200, filed Sept. 28, 1981, the disclosure of which 
is incorporated fully herein by reference, describes a method for firing 
thick film electronic circuits and an infrared furnace in which such 
method can be carried out. A plurality of infrared lamps are disposed in a 
firing chamber of the furnace such that the ends of the lamps extend 
through the walls of the firing chamber for electrical connection to a 
power source outside the furnace. The interface between the walls of the 
firing chamber and the lamps cannot be completely sealed from the 
atmosphere outside the furnace. As explained in this application, the 
described furnace very effectively fires thick film electronic circuits. 
Layers of a number of materials, such as Ruthenium oxide, silver, gold, 
glass, and dielectric can be fired in an atmospheric environment. In such 
case, the furnace described in the referenced application adequately 
controls the environment during component firing. Layers of other 
materials, such as copper, need to be fired in a non-atmospheric 
environment in order to prevent reaction, e.g., oxidation, of the material 
with the air. In such case, the furnace described in the referenced 
application is not totally satisfactory because atmospheric air can leak 
into the firing chamber, where it reacts with the material being fired. 
SUMMARY OF THE INVENTION 
According to the invention, an infrared furnace has a firing chamber in 
which an elongated envelope transparent to the infrared energy extends 
through the firing chamber and a source of infrared energy is disposed 
outside the envelope; The envelope has first and second open ends outside 
the firing chamber. First and second baffle chambers surround the 
respective first and second ends of the envelope. A product conveyor 
travels through the furnace via the baffle chambers and the envelope. Gas 
flow is prevented from the exterior of the furnace into the baffle 
chambers and from the firing chamber into the baffle chambers. Thus, 
products such as thick film circuits can be fired in a controlled 
environments, essentially in the absence of atmospheric air because 
atmospheric air is kept from the interior of the envelope where firing 
occurs. 
Specifically, in the preferred embodiment, non-atmospheric gas is supplied 
to the baffle chambers so as to create therein a superatmospheric pressure 
which prevents gas flow into the baffle chambers from the exterior of the 
furnace. The non-atmospheric gas is exhausted from one of the baffle 
chambers, thereby inducing flow of the non-atmospheric gas from the other 
baffle chamber through the envelope. A seal chamber is disposed between 
each baffle chamber and the firing chamber such that gas leakage from the 
seal chamber to the baffle chamber and to firing chamber occurs around the 
periphery of the envelope. A packing is compressed against the periphery 
of the envelope and the walls of the seal chamber to minimize such leakage 
and the seal chamber in pressurized with non-atmospheric gas to prevent 
gas flow from the firing chamber to the baffle chamber. 
A feature of the invention is the use of one or more hollow conveyor 
support rods to supply more non-atmospheric gas to the interior of the 
envelope.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT 
With reference to FIG. 1, an infrared furnace incorporating principles of 
the invention comprises a plurality of interconnected chambers as follows: 
an entrance chamber 10 leading to a baffle chamber 11; a seal chamber 12 
at the interface between baffle chamber 11 and a firing chamber 13 with 
muffle; a seal chamber 14 at the interface between firing chamber 13 and a 
baffle chamber 15; and an exit chamber 16 to which baffle chamber 15 is 
connected by a cooling chamber 17. As described in more detail below, a 
product conveyor constructed in the manner disclosed in the referenced 
application, travels through the described chambers including the muffle 
in firing chamber 13, which extends through seal chambers 12 and 14 into 
baffle chambers 11 and 15. The ends of the muffle, which are open, are 
surrounded by baffle chambers 11 and 15, respectively. 
Entrance chamber 10, baffle chamber 11, and seal chamber 12 are shown in 
FIG. 2. A porous conveyor belt 18 travels through a horizontally elongated 
passage 20 through the furnace. Products to be fired, such as copper 
layers on thick film circuits, travel in carrier trays (not shown) through 
passage 20 from left to right on conveyor belt 18. Partitions 22 pivotally 
mounted on a horizontal bracket 43 serve to impede the flow of gas through 
passage 20 to the exterior of the furnace. As a carrier passes under a 
partition 22, it contacts the partition and pivots it in a 
counterclockwise direction as viewed in FIG. 2. Alternatively, stationary 
partitions having a clearance with respect to the carrier trays could be 
provided. Near the end of entrance chamber 10, a vertical upwardly 
extending exhaust duct 25 communicates with passage 20. A blower not shown 
in duct 25 draws gas out of passage 20. One or more dampers 26 serve to 
control the flow rate of exhaust gas passing through duct 25. Except for 
the absence of an exhaust duct, exit chamber 16 is the same as entrance 
chamber 10 and therefore is not shown in detail. A tray 45 extends 
horizontally across the full width of passage 20 under duct 25 to catch 
volatiles that may condense in passage 26 formed between tray 45 and the 
top of chamber 10. Tray 24 extends the full length of chamber 10 to 
chamber 11. 
FIG. 3 shows baffle chamber 11, seal chamber 12, and part of firing chamber 
13. A horizontal row of infrared lamps 30 and a horizontal row of infrared 
lamps 32 are mounted in firing chamber 13 in the manner disclosed in the 
referenced application so their ends extend outside firing chamber 13. 
Specifically, lamps 30 and 32 are disposed in firing chamber 13 such that 
their ends extend through the walls of firing chamber 13 for connection to 
an electrical power source outside the furnace. Lamp holders at the 
interface between the walls of the firing chamber and the lamps cannot 
adequately seal the interior of the firing chamber from the atmosphere to 
fire satisfactorily some materials such as for example, copper. The outer 
surface of the walls of baffle chamber 11, seal chamber 12, and firing 
chamber 13 are all preferably made of sheet metal as specified in the 
referenced application. A tubular muffle 34, which is a straight elongated 
envelope of fused silica quartz or other material transparent to the 
infrared energy radiated by lamps 30 and 32, extends through firing 
chamber 13 between lamps 30 and lamps 32. Muffle 34 has an open end 36 
outside of firing chamber 13 which is surrounded by baffle chamber 11 and 
an open end not shown outside of firing chamber 13 which is surrounded by 
baffle chamber 15. As shown in FIG. 3, conveyor belt 18 passes through 
muffle 34. A plurality of hollow rods 38 underlie conveyor belt 18 
throughout the furnace so as to provide support thereto. A portion of one 
or more of rods 38 within muffle 34 has holes 40 to provide communication 
between the interior of rods 38 and the exterior. 
Firing chamber 13 is constructed in the manner disclosed in the referenced 
application. An insulative horizontally extending layer 42 covers the top 
of passage 20 throughout the length of chambers 10 and 11. A porous 
insulative horizontally extending layer 44 covers the bottom of passage 20 
throughout the length of chambers 10 and 11. A non-atmospheric gas, 
preferably nitrogen, under pressure is supplied through a fitting 48 to 
layer 44, which has a series of channels not shown to facilitate gas 
distribution to all parts of layer 44. Nitrogen under pressure seeps 
through the pores of layer 44 to provide a low velocity non-atmospheric 
environment in passage 20 and around open end 36 of muffle 34. This 
nitrogen gas establishes a superatmospheric pressure around open end 36 of 
muffle 34. Gas thus flows slowly but continuously and unidirectionally 
toward exhaust duct 25, and to a lesser extent to the atmosphere through 
entrance chamber 10, thereby preventing gas flow in the other direction, 
i.e., toward open end 36 of muffle 34. This eliminates the possibility of 
contamination by atmospheric air. 
Non-atmospheric gas, preferably nitrogen, is supplied to one or more of 
rods 38 to introduce such gas directly into the interior of muffle 34. 
Gas can leak through the gaps around the periphery of muffle 34 at the 
interface with firing chamber 13 and the interface with baffle chamber 11. 
Seal chamber 12 is designed to minimize the amount and control the 
direction of this leakage flow. Specifically, seal chamber 12 is attached 
to firing chamber 13 by a sealed flange 50 and is attached to baffle 
chamber 11 by a sealed flange 52. A compressible packing 54 surrounds the 
periphery of muffle 34 within sealing chamber 12. Packing 54 is compressed 
against the periphery of muffle 34 and the side walls of seal chamber 12 
adjacent to baffle 11 and firing chamber 13 so as to cover the leakage 
gaps around muffle 34. The space between packing 54 and the outer 
perimeter of seal chamber 12 defines an approximately annular plenum 56 to 
which non-atmospheric gas, preferably nitrogen under pressure, is supplied 
through a fitting 58. 
An end-sectional view of seal chamber 12 is shown in FIG. 4. Preferably, 
packing 54 is made of silicon dioxide/aluminum dioxide fiber sold under 
the trade name Refrasil and is compressed by garter springs 60. The 
pressure in plenum 56 is higher than that in firing chamber 13 or the 
portion of baffle chamber 11 between partitions 42 and 44. Therefore, a 
slow continuous unidirectional flow of nitrogen takes place from seal 
chamber 12 through the leakage gaps to baffle chamber 11 and to firing 
chamber 13 thereby preventing gas flow from firing chamber 13 into baffle 
chamber 11. 
The other end of firing chamber 13, seal chamber 14, and baffle chamber 15 
are identical to baffle chamber 11, seal chamber 12, and the portion of 
firing chamber 13 shown in FIG. 3. A cooling chamber 17 not shown in 
detail, has means such as heat transfer fins for cooling the fired product 
to the desired temperature before leaving exit chamber 16. 
In summary, a non-atmospheric gaseous environment is maintained around the 
product while being fired in chamber 13 because the product is protected 
by muffle 34 from air leaking into firing chamber 13 and the ends of 
muffle 34 are surrounded by baffle chambers 11 and 15; The direction of 
non-atmospheric gas flow is illustrated by the arrows in FIGS. 2 and 3. 
Infrared energy from sources 30 and 32 passes through the walls of muffle 
34 to impinge upon and heat the product passing therethrough on conveyor 
belt 18. Within muffle 34 an essentially pure nitrogen environment is 
maintained. Specifically, the leakage of air from firing chamber 13 into 
the ends of muffle 34 is prevented by seal chamber 12. The leakage of air 
into the ends of muffle 34 through entrance chamber 10 and exit chamber 16 
are prevented by baffle chamber 11 and baffle chamber 15, respectively. A 
continuous slow unidirectional flow of nitrogen takes place through muffle 
34 in a direction counter to conveyor belt 18 by virtue of the position of 
exhaust duct 26. As a result of the non-atmospheric environment, undesired 
chemical reaction between the layer being fired and air can be avoided. 
The described embodiment of the invention is only considered to be 
preferred and illustrative of the inventive concept; the scope of the 
invention is not to be restricted to such embodiment. Various and numerous 
other arrangements may be devised by one skilled in the art without 
departing from the spirit and scope of this invention. For example, 
instead of nitrogen, other non-atmospheric gases can be employed depending 
upon the material being fired. Under circumstances, one or more of the 
described chambers can be eliminated from the furnace; the exhaust duct 
could be eliminated or repositioned or a second exhaust duct could be 
provided at the other end of the firing chamber or direct introduction of 
gas into the muffle could be eliminated.