Vortex tube cooling system for solder reflow convection furnaces

A vortex tube cooling system is presented. The cooling system contains one or more vortex tubes connected to a supply of compressed gas, the vortex tubes providing a stream of cold gas which cools product as it emerges from a furnace. The cooling system is particularly useful for cooling circuit boards emerging from a solder reflow convection furnace.

FIELD OF THE INVENTION 
The invention relates generally to soldering of components via furnaces, 
and more particularly to the cooling of reflow soldered product. 
BACKGROUND OF THE INVENTION 
As is known in the art, after product (typically circuit boards) have been 
brought up to a high enough temperature to be reflow soldered, they need 
to be cooled. Typically, this cooling after reflow soldering requires a 
large cooling section including fans, heat exchangers, cooling water, 
cooling towers, and chillers or various combinations of these devices. 
Additionally, as solder is reflowed effluents such as flux contaminates 
are released into the atmosphere of the furnace. These flux contaminates 
can condense and collect on the heat exchangers, fans and product, 
requiring cleaning and maintenance of the parts of the conventional type 
furnaces and coolers. 
Vortex tubes provide one manner of cooling. A vortex tube accepts 
compressed gas at an inlet which is obliquely disposed with respect to the 
tube body. The compressed gas enters the tube body at an angle and rapidly 
rotates helically towards an end. As a result of the rapid helical 
rotation of the gas (for example at approximately one million revolutions 
per minute), a vortex is produced within the tube in which the inner 
region of rotating gas is expanding and compressing the outer region of 
rotating gas. Thus, the outer region of rotating gas is acquiring heat 
from the inner region of rotating gas. At one end of the tube body a 
diaphragm has an opening which allows the hot, outer region of gas to 
escape, thereby providing a hot gas output stream, while redirecting the 
inner region of gas back through the tube. The opposite end of the tube 
body has a diaphragm which has an opening which allows the cold gas to 
escape, thereby providing a cold gas output stream. U.S. Pat. No. 
1,952,281 issued to Joseph Ranque on Mar. 27, 1934 for a "Method and 
Apparatus for Obtaining From a Fluid Under Pressure Two Currents or Fluids 
at Different Temperatures", U.S. Pat. No. 3,208,229 issued to C. D. Fulton 
on Mar. 16, 1965 for a "Vortex Tube" and U.S. Pat. No. 3,173,273 issued to 
C. D. Fulton on Sep. 28, 1965 for a "Vortex Tube" disclose in greater 
detail a vortex tube, and Applicant hereby incorporates by reference these 
disclosures. 
Vortex tubes have been used in variety of devices. U.S. Pat. No. 4,407,134 
to Snaper discloses the use of vortex tubes in an air conditioning system, 
U.S. Pat. No. 5,344,478 to Zurecki et al. discloses the use of a vortex 
tube to cool molten metal, and U.S. Pat. No. 4,714,484 to Kuhl et al. 
discloses the use of a vortex tube in an IR Spectrophotometer. 
SUMMARY OF THE INVENTION 
A solder reflow cooling system incorporating one or more vortex tubes is 
disclosed. The cooling system has a housing located adjacent to the output 
of a solder reflow furnace and receives reflow soldered product as they 
exit the solder reflow furnace. The vortex tubes receive compressed gas as 
an input and discharge a cold temperature gas at one output and a hot 
temperature gas at another output. The cold temperature gas provided by 
the vortex tubes is directed on the product, thus cooling them. The hot 
temperature gas is directed outside the cooling system housing, or 
optionally, the hot gas output of the vortex tubes can be fed back to the 
furnace to supplement the heating action supplied by the furnace. 
The cooling system is of compact size, requires a minimum of moving parts, 
does not require a water supply or drainage system, and is inexpensive to 
implement, operate and maintain. Also the condensation of flux and other 
volatiles on cold components is reduced, since the gas in the housing is 
not recirculated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a furnace cooling system according to the present invention. 
The cooling system forms a cooling section 90 adjacent to and 
communicating with the exit of a furnace 70 such as a solder reflow 
convection furnace. The cooling section 90 has a housing 110 and one or 
more vortex tubes 10 to provide cooling, discussed further below, disposed 
within the housing 110. 
A conveyor assembly 80, which may be a conveyor belt, walking beam or other 
suitable device, is provided for moving a product such as assemblies 40 
having components to be soldered through the furnace 70 and the cooling 
section 90. In this embodiment the cooling section 90 shown in FIG. 1 
employs the same conveyor assembly as the furnace 70, though in an 
alternate embodiment cooling section 90 could have a conveyor assembly 
distinct and separate from the conveyor assembly 80 of the furnace 70. Any 
other suitable type of support assembly for receiving and supporting the 
product may be provided, depending on the application. As assembly 40, 
typically a circuit board or substrate having components, passes through 
the furnace 70 along conveyor assembly 80, the temperature of the assembly 
40 is brought up to a level that causes the solder or solder paste 
deposited on the assembly 40 to melt. Once the solder has been melted it 
needs to be cooled in order to solidify and provide an electrical and/or 
mechanical connection between a component and the circuit board or 
substrate. 
Cooling section 90 has one or more vortex tubes 10 disposed adjacent to the 
conveyor assembly 80. For simplicity, only a single tube is illustrated in 
FIG. 1. The particular arrangement, number, and spacing of the vortex 
tubes and the distance from and orientation with respect to the product 
are dependent upon the type of product and the amount of cooling needed, 
as would be apparent to one of ordinary skill in the art. The vortex tubes 
10 (see FIG. 2) receive compressed gas from a compressed gas source 50, 
such as a compressor or tank of compressed gas, through an inlet 15, which 
enters the tubular body 25 of the vortex tube 10 tangentially to impart a 
vortex to the flow of gas through the tubular body 25. Cold gas discharges 
from one output 20 into the interior of the housing 110, hot gas 
discharges from another output 60 and is directed to ambient outside the 
housing 110. Optionally, the hot gas can be provided to furnace 70 via 
conduit 120 to supplement the heating action provided by furnace 70. 
Typically, the gas is air or N.sub.2. 
As the product 40 move along conveyor assembly 80 within cooling section 
90, they are brought into proximity with the cold gas provided by the 
vortex tubes 10. The cold gas provided by the vortex tubes blows upon the 
product, thus cooling them. The temperature and flow rate of the cold gas 
provided by the vortex tubes 10 are controlled by the cold fraction 
adjustments found on the vortex tubes 10 or by adjustment of the 
compressed gas source 50. In such a manner the molten solder becomes 
solidified and provides electrical and/or mechanical connections between 
the circuit board and the components. 
The cooling section 90 may also have barrier curtains 100 disposed about 
the exit of cooling section 90 as shown in FIG. 5. When N.sub.2 is used as 
the gas for cooling, it is important to prevent mixing of the outside air 
with the N.sub.2 atmosphere within the cooling section. Physical curtains 
100 may be used to isolate the atmosphere inside the cooling section 90 
from the ambient air. These curtains are disposed about the exit of the 
cooling section 90, and the product 40 exiting the cooling section 90 
brush aside the curtains 100 as they exit. Alternatively, as shown in FIG. 
4, a gaseous curtain 130 may be provided by an apertured tube 140 disposed 
across the upper opening of the cooling section exit perpendicularly to 
the direction of movement of conveyor assembly 80. A compressed gas is 
supplied to the tube 140 from gas supply 150, and exits the apertures 
forming a sheet of gas 130 across the exit of cooling section 90, thus 
keeping the N.sub.2 atmosphere of cooling section 90 on one side of the 
gaseous curtain 130, and the atmosphere of the surrounding environment on 
the opposite side of the gaseous curtain 130. 
In a particular embodiment for cooling solder reflowed product, four vortex 
tubes 10 are disposed in a line perpendicular to the movement of the 
conveyor assembly 80, the tubes 10 being spaced five inches apart center 
to center. Alternatively, the vortex tubes could be disposed parallel to 
the movement of conveyor assembly 80 or in an array. The vortex tubes 10 
receive compressed gas at a minimum pressure of 50 psig, and preferably at 
100 psig from a compressor or tank. Commercially available vortex tubes 
can provide temperatures as low as -40.degree. F. at the cold outlet, and 
as high as 200.degree. F. at the hot gas outlet at high input pressures. 
For cooling reflow soldered product, vortex tubes such as the Exair model 
number 3215, available from Exair of Cincinnati, Ohio, are suitable. The 
3215 vortex tube provides cold gas at a temperature of approximately 
40.degree. F. and a flow rate of approximately 56 liters/minute from the 
cold gas outlet, and provides hot gas at a temperature of approximately 
85.degree. F. at the hot gas outlet from a compressed air source providing 
air at a range of approximately 50-70 psig, which is generally 
satisfactory for cooling reflow soldered assemblies. The speed of the 
product 40 through the cooling section 90, the vortex tube input pressure 
and the cold fraction adjustment can be adjusted to ensure that the 
product 40 are cooled at a maximum ramp rate of 3.degree. C./second. 
The vortex tubes 10 within cooling section 90 take in fresh, clean gas as 
opposed to conventional approaches which recirculate air that typically 
contains flux contaminates. These flux contaminates can condense and 
collect on the heat exchangers, fans and product, requiring cleaning and 
maintenance of the parts of the conventional type coolers. By use of the 
vortex tubes and their supply of compressed gas, there is no recirculation 
of the air and thus no condensation of flux contaminants inside the 
cooling section. Accordingly, cleaning and maintenance are minimal. 
Referring now to FIG. 3. the vortex tubes can also be used in combination 
with conventional cooling elements, such as fans 210 and heat exchangers 
230. For example, the vortex tubes may be used to cool the soldered 
product to below the condensation temperatures of the solder and other 
volatiles. The conventional cooling elements are disposed downstream in 
the cooler environment where they may provide additional cooling without 
accumulating condensation products. Heat exchangers 230 are provided with 
chilled water by chilled water supply 200, and the water exiting the heat 
exchangers 230 is delivered to a drainage system via conduit 220. 
In accordance with the present invention, the product 40 are cooled in a 
minimum amount of space by a cooling section 90 which has no moving parts, 
does not require a cold water source or drainage system and is inexpensive 
to implement, operate and maintain. 
Having described preferred embodiments of the invention it will now become 
apparent to those of ordinary skill in the art that other embodiments 
incorporating these concepts may be used. For example, although the 
invention has been described with respect to a solder reflow convection 
furnace, the present cooling system can be used with other types of 
furnaces where it is necessary or desired to provide cooling to a product 
exiting the furnace. For example, the furnace 70 can be an infra-red 
furnace, a microwave furnace, a vapor-phase system, a hot air furnace or 
other conventional solder reflow system. Accordingly, it is submitted that 
the invention should not be limited to the described embodiments but 
rather should be limited only by the spirit and scope of the appended 
claims.