Field of the Invention
This invention is generally related to an apparatus for producing
a gaseous mixture. More specifically, this invention relates to a gas mixing panel
which enables the creation of a controlled atmosphere for enhancing the performance
of the electronic assembly process in reflow ovens.
Background of the Invention
In general, a controlled atmosphere containing a specific gas characteristic
is required to improve the performance of the electronic assembly process in reflow
ovens. It is desirable to provide an economical and compact apparatus which can
produce a controlled atmosphere.
The specific gas characteristic in the atmosphere for optimum reflow
performance is preferably from between about 100 ppm and about 10,000 ppm of a
minor gas in a major gas, and more preferably from between about 500 ppm to about
2,000 ppm of a minor gas in a major gas. For purposes of this invention, the minor
gas may be oxygen or air, and the major gas may be nitrogen. It is believed that
the concentration of oxygen or air in nitrogen between this particular range helps
to reduce defects such as tombstoning and solder splatter and improves solderability
during the electronic assembly process, particularly in the manufacture of circuit
boards. Below about 100 ppm oxygen or air in nitrogen, certain low oxygen defects
may occur, and above about 10,000 ppm oxygen or air in nitrogen, certain high
oxygen defects occur. The desired concentration of oxygen or air (typically between
about 100 ppm and about 10,000 ppm) in nitrogen which effectuates the optimum performance
of reflow ovens is known as the "sweet spot".
It is therefore desirable to provide a gas mixing apparatus which
will aid in providing a controlled atmosphere having a gaseous characteristic of
between about 100 ppm and about 10,000 ppm oxygen or air in nitrogen so that the
electronic assembly process can be conducted with lower amount or fewer occurrence
Traditional gas mixer technology generally uses expensive mass flow
controllers and large surge tanks. These traditional mixers have been used to produce
gas mixtures in the 1% to 50% range of oxygen in nitrogen, and have not been available
in the ppm range. Recent developments have led to the commercial availability of
larger size gas mixers which are significantly more expensive. More recent improvements
have led to the development of less expensive gas mixers using fixed orifice plates
to measure the gas flow, and then mix in a significantly smaller surge tank. Additionally,
other traditional mixers have developed which are aimed at accurately providing
certain gas compositions.
Other gas mixing devices also exists in the art which also provides
for mixed gas compositions. U.S. Patent No. 6,614,655 discloses the combination
of a test gas with a zero gas. The zero gas could be either nitrogen or artificial
oxygen; and the test gas (calibration gas) is unspecified. The '655 patent discloses
a reduction in the number of critical nozzles necessary to produce different concentrations
of the feed streams in the product. The mixing chamber of the gas mixing device
in the '655 patent is connected to the outputs of the critical nozzles, which ensure
that the outlet pressures are above a certain minimum pressure. Each critical
nozzle increases in proportion of 1:2 from one to the next. Since the outlet pressure
is always above the critical pressure, the gas volume is always constant, relying
on the cross-section of the nozzle only. The critical nozzle receives either the
test or zero gas. Valves, having two inlets, are placed upstream from the critical
nozzles and the two inlets are capable of being alternatively open. The four nozzles
have cross sections with openings of 1:2:4:8. The largest nozzle is placed opposite
the output of the mixer. A microprocessor activates the valves and monitors and
regulates the gas pressures. The gas mixing device in the '655 patent obtains
sixteen linearly graduated concentrations of the test gas in the zero gas.
Both U.S. Patent No. 5,671,767 and No. 5,544,674 disclose mixing
two gases together in controlled proportions. Since the mixing apparatus is for
a respirator, the gases in these patents are directed only to oxygen and air.
The mixing apparatus achieves high mixing accuracy regardless of flow rate, so
the apparatus can be used for high or low flow rates. The first gas stream is
controlled by an orifice of a variable area, which supplies the gas to the mixing
chamber. This is also true for the second gas. When the flow rate decreases, the
piston moves towards the valve members. This reduces the size of each orifice
while maintaining the same ratio between the orifice flow areas. The passage way
or orifice area for each of the two gases is increased or decreased in the same
amount in each valve so that the proportions of the gases are maintained.
Japanese Patent No. 11,033,382 discloses a miniaturized and simplified
gas mixing unit. This patent uses a pressure equalizing device to adjust two gases
to the same pressure. The two gases are mixed in a gas passing area with a ratio
that corresponds to a predetermined mixing ratio. The flow rate can be regulated
by an adjusting valve.
Although there are a number of apparatuses that mixes gas, none of
which is believed to be able to produce an economical and compact apparatus which
can mix gases to produce a nitrogen product having between about 100 ppm and 10,000
ppm of oxygen. It is therefore desirable to have a gas mixing apparatus having
Summary of the Invention
An aspect of this invention is directed to a compact gas mixing apparatus
for accurately mixing a plurality of gases. This apparatus comprises a gas panel,
pressure control valves to regulate differential pressures, flow meters and a static
mixing chamber. The plurality of gases may be a minor gas and a major gas, where,
for example, the major gas is nitrogen and the minor gas is air or oxygen. The
major gas and the minor gas are preferably mixed to achieve a gas mixture of from
about 100 ppm to about 10,000 ppm oxygen in nitrogen, more preferably in a proportion
of from about 500 ppm to about 2,000 ppm oxygen in nitrogen.
The pressure control valves regulate the differential pressure to
about ± 5 inches of water column. The apparatus is integrated with a reflow oven.
As used herein, oxygen may at times be interchangeable with air.
It is noted that oxygen constitutes about 20% content in air.
Brief Description of the Drawings
Further advantages of the present invention will become apparent
from the following detailed description of the invention when read in conjunction
with the accompanying drawing of which Fig. 1 is a general schematic diagram of
the gas mixing apparatus utilizing a differential pressure gauge, flow meters and
a static mixing chamber.
Detailed Description of the Invention
This invention provides a compact gas mixing apparatus which can
produce a mixture of a minor gas and a major gas, preferably oxygen or air as the
minor gas, and nitrogen as the major gas, in a concentration of from about 100
ppm to about 10,000 ppm of oxygen or air in nitrogen, and more preferably, from
between about 500 ppm to about 2,000 ppm of oxygen or air in nitrogen using a
static mixing chamber. This apparatus consists of a fixed, in-line baffle, which
causes the gases to shear and swirl, thus providing a homogeneous output.
This present invention provides a stable oxygen concentration in
a gas mixture that is obtained by mixing nitrogen, and air or oxygen in a mixing
panel where the pressure differential between the nitrogen supply and the oxygen
supply is measured and controlled to vary by no more than about 5 inches water
column. The mixture is homogenized by passing it through a static spiral mixer.
The technique is particularly advantageous where the amount of oxygen or air mixed
into the nitrogen is sufficient to give a final nitrogen concentration of preferably
between about 100 ppm and about 10,000 ppm oxygen or air in nitrogen, more preferably
between about 500 ppm and about 2,000 ppm oxygen or air in nitrogen.
This invention significantly differs from the prior art design which
measures two gas streams, and mixes the gas streams in a surge tank.
Small pressure fluctuations between the major and minor gases adversely
affects the accuracy of the mix. To contain costs, inexpensive flow meters were
left in the design, but a differential pressure gauge was incorporated to measure
and control the differential pressure between the major and minor gases to less
than about 5 inches of water column.
The present invention uses a static mixing chamber. Such static mixing
chambers consist of fixed, in-line baffles which cause the gases to shear and
swirl, providing a homogeneous output. The use of the static mixing chamber offsets
the cost of the differential pressure gauge, and contributes greatly to the compact
size of the present gas mixing apparatus.
Here, mechanical regulators are used to control the nitrogen and
oxygen streams, and a differential pressure indicator is used to ensure that the
two gas streams are regulated within 5 inches of water column pressure. The regulated
gas stream flows are controlled using a flow meter, such as a rotameter, and are
adjusted to deliver the desired measured gas mixture. The two gas streams are then
mixed in a static mixing chamber, and the final gas mixture is then delivered
to a reflow oven.
The present invention produces a stable gas mixture with an accuracy
of ± 1 ppm with a gas mixture of 100 ppm. A simple differential pressure gauge
produces a accuracy of a wide variety of flow ranges and gas mixtures of up to
about ± 1 ppm. The accuracy was improved by assuring identical delivery pressure
of the two gas streams to ± 5 inches of water column. The delivery pressure in
this invention has been measured to up to about ± 300 inches of water column.
As an example of the gas mixing panel of this invention, a "flat
plate" design was constructed to facilitate an in-situ installation in the reflow
oven, which allows for a free standing installation. The specification of the
gas mixing panel of this invention is as follows:
24" high x 24" long x 12" deep
(61 cm high x 61 cm long x 30.5 cm high)
65 lbs (29.2 kg)
Nitrogen Inlet: S" FPT
Air Inlet: R" FPT
N2/Air Mix Outlet: S" FPT
N2/Air Mix Sample: R" quick-connect
Clean Dry Air
50 scfh (100 psig) minimum
Praxair Commercial Grade Nitrogen
3000 scfh (100 psig) minimum
Fig. 1 provides one embodiment of the flow scheme of this invention.
Nitrogen stream 12 (280-2800 scfh, 5 to 50 scfm) and air stream 14 (2 to 20 scfh)
enter at 75 psig. The pressure differential between the streams is measured by
differential pressure gauge 24. If the pressure difference is too high, pressure
regulator 20 and 22 will adjust until the pressure differences are within about
± 5 inches column water. The streams continue through pressure release valves 16
and 18 (set at 100 psi) prior to passing through flow meters 26 or 28. Flow valves
30 and 32 monitor the flow of the streams depending on the particular composition
chosen for the final stream. Check valves 36 and 38 regulate the air and nitrogen
gases prior to entering gas mixer 34, which is a static mixing chamber. The gases
are mixed and product stream 16 exits, having oxygen content of between about
10 ppm and about 10,000 ppm, depending on the chosen composition. Sample valve
40 leads to flowing sample gases.
Various aspects of the present invention may be interchanged. For
example, the differential pressure gauge with a pilot/slave regulator arrangement
may be placed on the pure nitrogen and air supplies. In this design, the nitrogen
regulator would control the delivered nitrogen pressure, and provide a signal to
pre-set the air regulator to substantially the same pressure as the nitrogen pressure.
Another variation contemplated by this invention is replacing the
static mixing chamber with a compact surge tank and a diffuser nozzle on the inlet
of the surge tank. The diffuser nozzle would provide the necessary shear and swirl
to properly mix the nitrogen and oxygen streams.
Specific features of the invention are shown in one or more of the
drawings for convenience only, as each feature may be combined with other features
in accordance with the invention. Alternative embodiments will be recognized by
those skilled in the art and are intended to be included within the scope of the