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Dokumentenidentifikation EP1116513 23.08.2001
EP-Veröffentlichungsnummer 1116513
Titel Gasmischeinrichtung
Anmelder Praxair Technology, Inc., Danbury, Conn., US
Erfinder Hanley, Jonathan P., Kenmore, New York 14217, US
Vertreter derzeit kein Vertreter bestellt
Vertragsstaaten AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LI, LU, MC, NL, PT, SE, TR
Sprache des Dokument EN
EP-Anmeldetag 12.01.2001
EP-Aktenzeichen 011007671
EP-Offenlegungsdatum 18.07.2001
Veröffentlichungstag im Patentblatt 23.08.2001
IPC-Hauptklasse B01F 3/02

Beschreibung[en]
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 of defects.

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 such characteristics.

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: Dimension 24" high x 24" long x 12" deep

(61 cm high x 61 cm long x 30.5 cm high)
Weight Connections 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
Gas Requirement    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 claims.


Anspruch[en]
  1. A compact gas mixing apparatus for accurately mixing a plurality of gases, said apparatus comprising
    • a) a gas panel;
    • b) pressure control valves to regulate differential pressures;
    • c) flow meters; and
    • d) a static mixing chamber.
  2. The apparatus of claim 1 wherein said apparatus mixes a minor gas and a major gas.
  3. The apparatus of claim 2 wherein said major gas is nitrogen and said minor gas is air.
  4. The apparatus of claim 2 wherein said major gas is nitrogen and said minor gas is oxygen.
  5. The apparatus of claim 2 wherein said apparatus mixes said major gas and said minor gas to achieve a gas mixture of from about 100 ppm to about 10,000 ppm oxygen in nitrogen.
  6. The apparatus of claim 2 wherein said control valves regulate the differential pressure to about ± 5 inches of water column.
  7. The apparatus of claim 1 wherein said apparatus is integrated with a reflow oven.
  8. The apparatus of claim 1 wherein said static mixing chamber comprises fixed in-line baffles.
  9. The apparatus of claim 1 wherein said flow meter is a rotometer.
  10. The apparatus of claim 1 wherein said static mixing chamber is replaced by a surge tank and a diffuser nozzle.






IPC
A Täglicher Lebensbedarf
B Arbeitsverfahren; Transportieren
C Chemie; Hüttenwesen
D Textilien; Papier
E Bauwesen; Erdbohren; Bergbau
F Maschinenbau; Beleuchtung; Heizung; Waffen; Sprengen
G Physik
H Elektrotechnik

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