JPH06281362A - Air invasion preventive method for inert atmosphere furnace and its apparatus - Google Patents

Abstract

PURPOSE:To prevent the outside air from entering into an inert atmosphere furnace, in which a heat treatment like re-flow takes place, even when works such as printed substrates are continuously charged into the furnace. CONSTITUTION:An inert gas is flowed in the form of film from the upper and lower parts of an entrance 7 and an exit 8 of a furnace 2 toward the entrance 7 and exit 8, and the other gas (air) is flowed in the form of film over the inert gas to form film-like layer gases to seal the entrance 7 and exit 8 of the furnace 2 with the layer gases. First nozzles 9 with slits are provided at the upper and lower parts of the entrance 7 and exit 8 of the furnace 2 and second nozzles 13 with slits are arranged parallel with the first nozzles 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for preventing air from entering the furnace through an inlet / outlet of the furnace in an inert atmosphere furnace in which heat treatment is performed in an inert atmosphere.

[0002]

2. Description of the Related Art If a large amount of flux residue is attached after soldering a printed circuit board, the flux residue may absorb moisture to lower the insulation resistance or generate a corrosion product. Therefore, in a printed circuit board used for a computer, a communication device, or the like that places importance on reliability, cleaning for removing flux residue has been performed after soldering.

For cleaning the flux residue, halogenated hydrocarbon solvents such as CFCs and trichlene, which often dissolve pine resin, which is the main component of the flux, and activators, have been used. Its use is now regulated because it destroys the surrounding ozone layer and causes a large amount of harmful ultraviolet rays to reach, causing human skin cancer.

Therefore, in the soldering of a printed circuit board, the so-called "no-cleaning", in which the flux residue does not have to be cleaned after soldering, is desired from the electronic industry, and a solder consisting of a paste-like flux and a powdered solder is required. As for the paste, a solder paste for non-cleaning has come to be used.

The non-cleaning solder paste is an extremely small amount of solid components such as pine resin, activator and thixotropic agent added to the flux, and the flux residue after soldering is also very small.

By the way, when the no-cleaning solder paste is heat-treated in a reflow oven in the presence of air, the solder does not adhere sufficiently to the soldered portion of the printed circuit board, or a large number of minute solder particles are formed around the soldered portion. Problems such as the generation of solder balls have occurred.

These defects are caused by the fact that when the printed circuit board or the solder paste applied on the printed circuit board is heated, oxygen in the air oxidizes the metal surface of the copper foil or the powder solder of the soldering portion. This is because it ends up. In other words, when the copper foil of the printed circuit board oxidizes, the molten solder becomes difficult to wet and becomes unsoldered, and when the powdered solder oxidizes, even if the oxide film is reduced and removed with a small amount of activator, the surface tension cannot be reduced and it is very small. They remain as solder balls.

In the conventional solder paste containing a large amount of solid components, even if the metal surface is oxidized, the activator added in a large amount in the flux reduces it and cleans it, and melts the cleaned metal surface. Since it was covered with pine resin to prevent reoxidation, soldering failure did not occur even in the presence of air.

However, it has been known that the solder paste for non-cleaning does not oxidize the metal surface even in the heated state and the soldering can be performed without any defect if oxygen is not present. An inert atmosphere furnace filled with active gas was used.

The oxygen concentration of the reflow furnace suitable for the non-cleaning solder paste varies depending on the type of the solder paste, the surface condition of the printed board, etc., but is generally 1.00.
Must be below 0 ppm, preferably 500 pp
It is said that m or less is desirable. Further, in a reflow furnace using a non-cleaning solder paste, no matter how the oxygen concentration in the furnace is lowered, it is not economical because the amount of the inert gas used is large.

In order to reduce the oxygen concentration in the reflow furnace, a reflow furnace in which a mechanical shutter is installed at the entrance and exit of the reflow furnace has also been proposed (see JP-A-62-18).
3960). However, in the reflow furnace equipped with this shutter, the printed circuit board must be put in and taken out by the pusher, and so-called in-line operation that performs continuous work with pre-process and post-process is not possible. It was very unproductive because it had to be. Further, since this reflow furnace is mechanical, its structure is complicated and expensive, and it is not economically preferable.

Instead of this mechanical shutter, a gas shutter having a simple structure and capable of being inlined,
That is, many reflow furnaces using an air curtain or a gas curtain that shuts in the outside air with a gas have been proposed (see JP-A-1-118369 and 4-30536).
1, No. 3-31087, No. 3-36361, No. 4-1255056)

[0013]

However, the conventional gas-type shutter has the effect of lowering the oxygen concentration in the furnace when the number of printed circuit boards to be conveyed is small, but when the printed boards are continuously conveyed, the gas in the furnace will be reduced. There was a problem that the oxygen concentration would drop.

According to the present invention, even if a gas shutter having many advantages as compared with a mechanical shutter is used, the oxygen concentration in the furnace can be lowered to an oxygen concentration suitable for using a solder paste for non-cleaning. It is to provide a prevention method and device.

[0015]

The inventors of the present invention have made extensive studies as to the cause of the air outside the furnace entering the furnace in a reflow furnace having a gas shutter. As a result, the conventional gas shutter is shown in FIG. As shown in FIG. 5, when the inert gas (N ₂ ) blown from the up and down direction hits the printed circuit board 3, a turbulent flow is generated, and the air (Air) outside the furnace is entrained and enters into the furnace. It has been found. This is because the gas flow after flowing out from the nozzle is unstable and is in a turbulent state. That is, if the gas flowing out from the nozzle is in a turbulent state, the turbulence becomes large even when the gas hits the printed circuit board, and the air is entrained.

Therefore, the present inventor pays attention to the fact that if the gas flowing out from the nozzle is always kept in a stable state, the outside air is not entrained even when the gas hits the printed circuit board, and the gas flowing out from the nozzle is stabilized. To let
The present invention has been completed by finding that the gas is very stable when the film-like gas is formed into several layers.

According to the present invention, an inert gas is made to flow downward and upward in the form of a film from the upper and lower portions of the inlet and outlet of the furnace, and another gas is provided above and below the film-like inert gas. From above in the form of a film, these two film gases are formed into a layer, and the inlet and outlet of the furnace are sealed with the layer gas to prevent air intrusion in the furnace. In the method, a first nozzle is provided at the upper and lower parts of the inlet and outlet of the furnace, which allows gas to flow downward and upward, respectively, and another nozzle is provided in parallel with the film gas. An air intrusion prevention device for an inert atmosphere furnace, characterized in that a second nozzle capable of discharging two gases in a film shape is installed.

In the method for preventing air intrusion in an inert atmosphere furnace according to the present invention, the other gas flowing out from the outside of the inert gas flowing in a film form from the vertical direction of the furnace may be air, or the above-mentioned. The same inert gas as the inert gas may be used.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. 1 is a front sectional view of a reflow furnace in which the air intrusion prevention device of the present invention is installed, FIG. 2 is a perspective view of the air intrusion prevention device of the present invention, and FIG. 3 is a diagram illustrating an air intrusion prevention method of the present invention. .

The reflow furnace 1 is a tunnel-shaped furnace 2, and a conveyor 4 for carrying a printed circuit board 3 is provided in the furnace 2.
Is traveling in the direction of arrow A. Further, the furnace 2 has a pre-heating zone P, a main heating zone R, and a cooling zone C from the printed circuit board running direction. 2-194
The surface blowing type heater 5 proposed in 385 is installed,
Further, cooling devices 6, 6 are installed in the cooling zone.

An inert gas is supplied into the furnace 2 from an inert gas supply source (not shown), and the inert gas is allowed to flow between the upper and lower surface blowing type heaters 5 and between the coolers 6 and 6. ing.

At the upper and lower parts of the inlet 7 and the outlet 8 of the furnace 2, there are installed first nozzles 9 ... For discharging an inert gas (N ₂ ). The first nozzle installed at the upper part of the furnace inlet / outlet allows the inert gas to flow out downward, and the first nozzle installed at the lower part of the furnace inlet / outlet causes the inert gas to flow out upward, causing the inert gas flowing out from above and below. The active gas collides with the upper and lower substantially central portions. In the first nozzle 9, the outflow port 10 forms a slit, and the inert gas exiting the outflow port is in the form of a film.

The inert gas is supplied to the first nozzles 9 ...
Alternatively, it may be directly supplied from an inert gas supply source.
As shown in, the inert gas (N ₂ ) may be taken out from the furnace 2 by the pump 11 and sent to the first nozzle. A valve 12 is installed during the supply to the first nozzle 9 to adjust the outflow amount of the inert gas.

Further, second nozzles 13 ... Are installed outside the first nozzles 9. Outlet 1 of the second nozzle 13
4 is also a slit for letting out the gas in a film shape. From the second nozzle 13, air (Air) or an inert gas is formed into a film and flows out. Second nozzle 1
When the air is to be discharged from 3, the air is preferably sent by the pump 15, and a valve 16 for adjusting the amount of the discharge is preferably installed on the way.

The inert gas (N ₂ ) is made to flow out in a film form from the first nozzles 9 and 9 installed at the upper and lower portions of the furnace inlet 7,
Air (Air) is made to flow out in a film form from the nozzles 13, 13. Then, as shown in FIG. 3, the inert gas and the air flowing out in a film form a stable layer without mixing with each other.

This is because if one gas is formed into a film,
Since the film thickness is thin, it is impossible to maintain the film state by being disturbed by a little wind, but when the two film-like gases are layered, the overall film thickness becomes thicker and stable even if a little wind does not affect it. It will be in the state of doing. Therefore, when the printed circuit board passes through the center of the stratified gas flowing from above and below, it is not disturbed even when it hits the printed circuit board, and the stratified gas is divided into inert gas and air, and the inert gas inside the furnace is directed toward the inside of the furnace. To the outside, and the air outside it flows in the opposite direction. That is, if the inlet and outlet of the furnace are sealed with the layered gas, only the inert gas enters the furnace and the air does not enter even when the printed circuit board is run.

In the reflow furnace adopting the present invention, nitrogen gas of 160 liters per minute is supplied into the furnace to determine the oxygen concentration in the furnace when the printed circuit board is not run and when the printed circuit board is continuously run. As a result of the measurement, the oxygen concentration is 4
It was 00 ppm ± 20 ppm.

On the other hand, in a reflow furnace in which only a gas curtain is installed at the entrance and exit of the furnace, when 160 liters / minute of nitrogen gas is supplied into the furnace and the printed circuit board is not run and when the printed circuit board is continuously run. As a result of measuring the oxygen concentration in the furnace, the oxygen concentration is 500ppm ± 50ppm
Met.

[0029]

As described above, according to the present invention, since the inlet and outlet of the inert atmosphere furnace are sealed with the stable stratified gas, the outside air does not enter the furnace, and the oxygen concentration in the furnace is reduced. Can be increased. Therefore, in the reflow furnace adopting the present invention, even if the solder paste for no-cleaning is used, the generation of unsoldered or minute solder balls is eliminated, and there is an unprecedented excellent effect that a reliable soldered portion can be obtained. Can play.

[Brief description of drawings]

FIG. 1 is a front sectional view of a reflow furnace equipped with an air intrusion prevention device of the present invention.

FIG. 2 is a perspective view of an air intrusion prevention device of the present invention.

FIG. 3 is a diagram illustrating an air intrusion prevention method of the present invention.

FIG. 4 is a diagram illustrating a gas flow state in a conventional air intrusion prevention device.

[Explanation of symbols]

 1 Reflow Furnace 2 Furnace 3 Printed Circuit Board 4 Conveyor 7 Furnace Inlet 8 Furnace Outlet 9 First Nozzle 10 First Nozzle Outlet 13 Second Nozzle 14 Second Nozzle Outlet

Claims (4)

[Claims] 1. An inert gas is caused to flow downward and upward from the upper and lower portions of the inlet and outlet of the furnace, respectively, and another gas is supplied to the outside of the film-shaped inert gas from the upper and lower portions, respectively. A method for preventing air intrusion in an inert atmosphere furnace, which comprises causing the two film-like gases to flow downward and upward to form a layered structure of these two film-like gases and sealing the inlet and outlet of the furnace with the layered gas. 2. The method for preventing air intrusion in an inert atmosphere furnace according to claim 1, wherein the other gas is air. 3. The method for preventing air intrusion in an inert atmosphere furnace according to claim 1, wherein the other gas is an inert gas. 4. A first nozzle capable of discharging a gas in the form of a film downward and upward respectively is installed at the upper and lower portions of the inlet and outlet of the furnace, and another nozzle is provided in parallel with the film-like gas. An air intrusion prevention device for an inert atmosphere furnace, comprising a second nozzle capable of discharging two gases in a film shape.