JPS63104777A - Soldering method - Google Patents

Abstract

PURPOSE:To uniformly perform nonoxidized heating with good heat conductivity and to execute a soldering of good quality with high efficiency by soldering the printed board on which a flux containing solder is placed with its heating under helium gas atm. inside a tunnel kiln. CONSTITUTION:The printed board 13 placing a flux contg. solder on is placed on a heat transfer belt 12 and fed into the tunnel type reflow furnace 10 covered by a stainless steel cover 15. The board 13 is subjected to heating by the heater block 11 of a subordinate heating wire, but the fed 18 helium gas is filled up inside the reflow furnace 10, the board 13 is heated uniformly at high speed with the high heat conductivity of helium gas and subjected to nonoxidized soldering with high efficiency. An infrared heater may be used for heating.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is suitable for a soldering method, particularly for heating and melting cream solder applied to a printed circuit board or flux-cored molded solder placed on a printed circuit board. This article relates to a soldering method using a flow furnace.

(Prior Art) Recently, in electronic devices, solder, such as cream solder and flux-cored molded solder, has come to be widely used to connect printed circuit boards and electronic components.

Hereinafter, in this specification, cream solder will be explained as an example of solder for convenience.

"Cream solder" is obtained by mixing powdered solder and liquid flux, and by using a screen printing device or dispenser, a constant amount of solder can be applied only to desired areas. It has unique and excellent features. Generally, printed circuit boards coated with cream solder are heated, that is, reflowed with cream solder, using a heating device called a 1-no-flow furnace, which is a long tunnel furnace with heaters installed on the top and bottom.

Furthermore, in the reflow oven, the solder paste is heated to a temperature higher than the melting temperature of the cream solder using infrared rays and electric wire heaters, and the solder paste flows in air or in a non-oxidizing atmosphere such as nitrogen gas or carbon dioxide gas. was.

However, when using the soldering method described above to heat the objects to a specified temperature in air or in an atmosphere of nitrogen gas, carbon dioxide gas, etc., it may take a long time for the temperature of the atmosphere to reach the solder melting temperature. It takes. Therefore, in order to shorten the heating time by rapid heating or the like, it was necessary to increase the heating temperature of the heating device. Therefore, the heating temperature of the reflow oven is set to be 40 to 50° C. higher than the solder melting temperature. However, the nitrogen gas poured into the furnace by four people lowered the temperature inside the furnace, making it unstable. Therefore, in Japanese Patent Application Laid-Open No. 59-22028, the nitrogen gas introduced into the furnace is preheated to the solder melting temperature, and then the temperature inside the furnace is suppressed by suppressing the temperature drop in the atmosphere inside the furnace. Atmospheric furnaces were proposed for stabilization. However, even when using the atmospheric furnace as described above, it is difficult to support rapid soldering by rapid heating.

(Problems to be Solved by the Invention) An object of the present invention is to provide a soldering method that eliminates the drawbacks of the prior art.

(Means for Solving the Problems) Therefore, the inventor of the present invention focused on the thermal conductivity of the soldering atmosphere gas in a flow furnace, which had not been considered in the past, and found that By using helium gas in the furnace atmosphere when heating with an infrared heater etc., heat transfer in the furnace atmosphere is carried out smoothly.
The present invention was completed based on the discovery that the heating temperature in the reflow oven becomes uniform and good soldering can be performed.

Here, the gist of the present invention is, for example, a tunnel furnace extending in the longitudinal direction, a heating device provided above and below in the tunnel furnace, and an endless conveyor for transporting printed circuit boards that runs inside the tunnel furnace. This is a soldering method using a reflow oven consisting of a device that uses solder containing flux i5! This is a soldering method characterized by performing soldering in a helium gas atmosphere while heating a printed circuit board with the heating device.In the present invention, the heating device is not particularly limited. Any known heating device such as the above-mentioned infrared heater or heating wire heater may be used.

The gas atmosphere in the present invention is a helium gas atmosphere, but the helium gas atmosphere may be formed at least in the heating zone region of the furnace.For this purpose, for example, a helium gas outlet may be provided at an appropriate position in the tunnel furnace. Just that is enough. For example, a helium gas outlet may be provided at each entrance and exit of the furnace. Although there are no particular restrictions on the helium gas flow rate or other matters, it is preferable to purge the inside of the furnace with helium gas before starting operation.

Alternatively, the helium gas may be guided into the furnace while being preheated using an auxiliary heating device. Note that the helium gas atmosphere may contain a small amount of oxygen gas, etc., but preferably gas other than helium such as oxygen is 10% (by volume) or less,
More preferably, it is limited to 5% (capacity) or less.

As described above, the helium gas used in the present invention forms an inert gas atmosphere and is stable.

Soldering apparatuses that can be used in the present invention include, in addition to the above-mentioned tunnel furnace type reflow furnace, a blower type soldering heating apparatus using heater heating.

(Function) Next, the present invention will be explained in further detail. Hereinafter, the case where a reflow oven is used will be explained as an example.

The soldering flow furnace that can be used in the present invention consists of a tunnel furnace extending in the longitudinal direction, heating devices provided above and below within the tunnel furnace, and an endless conveyor device for conveying printed circuit boards that runs within the tunnel furnace. It is configured.

A plurality of the heating devices can be provided in the upper and lower portions of the tunnel furnace depending on the type of heater used.

Further, the endless feeding device is provided with a thermally conductive belt conveyor so as to be able to run between heating devices provided above and below in the tunnel furnace. The transfer speed of this belt conveyor is changed as appropriate depending on heating conditions and the like.

The cream solder in the present invention usually includes a liquid flux mainly composed of pine resin and 250 mesh of Sn-40Pb.
A suitable example is one in which solder powder is mixed.

Further, the maximum heater substrate heating temperature of a furnace using a heating wire heater is generally 230° C., but the maximum heating temperature may be changed as appropriate depending on the type of heater used. It is sufficient to provide two helium gas outlets, one at the outlet and the other at the inlet of the tunnel furnace, but there is one more outlet in the center of the tunnel furnace.
) An air outlet may be provided as a nozzle. and,
A helium gas atmosphere is created by blowing out the air from the blowing port at a ratio of 21/vain to preliminarily evaporate the air inside the furnace. Continue to blow helium gas into the reflow oven during the soldering operation.

In this way, soldering with cream solder is done.
The soldering process is completed in 2 minutes, and the resulting solder surface has a uniform solder luster, resulting in good soldering. This is because the helium gas used in the atmosphere furnace has a high thermal conductivity, so the temperature increase rate of the atmosphere in the furnace is accelerated, and rapid uniform heating is possible.

In order to compare with such a helium gas atmosphere, the above operation was repeated using nitrogen gas, but the heating time to reach the melting temperature was too long, and the difference in heating temperature increased as the heating time progressed. As a result, the temperature inside the furnace became uneven and some connection failures occurred, resulting in a product that did not meet the required characteristics.

As described above, in a flow furnace using nitrogen gas as an atmosphere, the low thermal conductivity of the nitrogen gas forming the atmosphere is the reason why uniform heating is not performed and the soldering characteristics are not very good.

Hereinafter, the present invention will be explained based on examples.

Embodiment 1 FIG. 1 is a schematic cross-sectional diagram showing how objects to be joined are heated using a heating wire heater block or a flow furnace. 1st
As shown in the figure, in the reflow oven 10, a heat transfer belt 12 moves on a heating wire heater block 11 to heat the objects to be bonded on the heat transfer belt. Stainless steel cover 1 on the outside of the reflow oven
Covered by 5. Using the reflow oven, the temperature increase rate of the objects to be bonded was compared in nitrogen gas or helium gas.

FIG. 2 shows a one-hour temperature rise curve of the object to be bonded under the above-mentioned atmospheric gas. In FIG. 1, the heating temperature of the heating wire heater block of the reflow oven was adjusted to 230° C., and the feeding speed of the heat transfer heald was 0.5 m/min. In order to know the temperature status of the atmosphere inside the furnace, a temperature measuring plate (Glare Epo) having temperature measuring points at two locations was used as the object to be bonded during 25 tests. The flow rate of nitrogen gas was 217 m1n. The one-hour temperature rise curve 1 thus obtained is shown in the shaded area in FIG.

Next, under the same operating conditions as above, helium gas was added instead of nitrogen gas to the temperature measuring plate from the gas outlet in a countercurrent direction of 217 m1.
FIG. 2 shows a one-hour temperature rise curve 2 obtained by using the temperature measuring plate at a flow rate of n. Figure 2 summarizes the temperature rise time to 204℃ in nitrogen gas and helium gas and the temperature difference between temperature measurement points as the temperature rises from 30 to 204℃. This is table 1.

As can be seen from Table 1, when helium gas is used in the atmosphere inside the reflow oven, the temperature difference due to temperature rise is 115 or less compared to when nitrogen gas is used, and the temperature rise time until reaching 204 °C is can be shortened to 273 or less.

From the above, it has been found that using helium gas as the reflow furnace atmosphere has the effect of improving the temperature increase rate of the reflow furnace atmosphere and making the temperature inside the furnace uniform.

Embodiment 2 FIG. 3 shows a cross-sectional view of the infrared heating device 16. An infrared heater 17 (
S? ll1-tarLOOV) Heat to 230℃,
The temperature increase rate of the objects to be bonded was compared in nitrogen gas and helium gas. The flow rate of nitrogen gas and helium gas is 21t.
/llll1n. Figure 4 shows the temperature measurement plate 1 in the device.
3 (Gara Evo) shows a 1-hour temperature rise line at 2G+ temperature point 14. The one-hour temperature increase line in nitrogen gas is a straight line marked by Δ and O in FIG.

It can be seen that the one-hour line of temperature rise in nitrogen gas shows a tendency for the temperature difference to increase with time due to infrared irradiation heating.

In addition, the straight line marked with an x in FIG.
4! This is a one-hour temperature rise line when helium of /1 inch was flowed. As can be seen from the figure, the heating time straight line with helium gas is on a single line at ±1°C, showing an extremely small tendency for temperature difference. Table 2 summarizes the numerical values showing the heating temperature after 5 minutes and 7 minutes of heating time and the temperature difference between the two side temperature points.

Table 2 As can be seen from Table 2 above, in the case of using an infrared heater or a flow furnace in a helium gas atmosphere, the effect of uniform heating can be obtained by improving the heating rate.

(Effects of the Invention) As described above, according to the present invention, by using helium gas, which has high thermal conductivity and safety, as the atmospheric gas of the reflow oven, heat transfer during heating in the reflow oven is accelerated. In addition to uniformizing the temperature inside the furnace more quickly, it also has the effect of preventing oxidation as an inert gas.
High-quality soldering can be performed quickly.

Therefore, in soldering operations that require high quality and speed, such as the soldering of precision electronic components, efficient and high quality soldering can be achieved, so the present invention brings great benefits to the industry.

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional view showing how objects to be welded are heated using the electric wire heater block according to the present invention or in a flow furnace; A graph showing a one-hour temperature rise curve of the objects to be welded; Figure 3 is a schematic cross-sectional view showing how the objects to be welded are heated using an infrared heater or a flow furnace; and Figure 4 is a diagram showing the flowchart of the third diagram. 1 is a graph showing a straight line of temperature increase over one hour of a workpiece to be welded under a nitrogen or helium gas atmosphere in a furnace. 10: Reflow oven 15 stainless steel cover 11: heating wire heater block 16: infrared heating device

Claims (3)

[Claims] (1) A soldering method characterized by performing soldering in a helium gas atmosphere while heating a printed circuit board on which solder containing flux is mounted using the heating device. (2) A patent claim based on a reflow furnace consisting of a tunnel furnace extending in the longitudinal direction, heating devices provided above and below within the tunnel furnace, and an endless transport device for transporting printed circuit boards that runs within the tunnel furnace. Soldering method described in Scope 1. (3) The soldering method according to claim 1 or 2, wherein the helium gas atmosphere is formed by providing a helium gas outlet in the tunnel furnace.