JPH04220166A - Device for reflow soldering - Google Patents

Abstract

PURPOSE:To prevent a base plate and soft solder from being oxidized and to reduce the residue of flux by making the atmosphere in a reflow body furnace vacuum state and keeping the atmosphere in a preheating furnace in an inert gas state. CONSTITUTION:An inlet shutter 6 is opened to bring the base plate 1 into the preheating furnace 2 and to heat it to a prescribed temperature. The interior of the preheating furnace 2 is kept in an inert gas atmosphere. The shutter 9 is opened, the preheated base plate 1 is received in the reflow body 3 and subjected to soldering. The interior of the reflow body 3 is kept vacuous by a vacuum pump 7. The pressure of the interior is preferred to be 0.7-0.2atm. An outlet shutter 10 is opened to carry away the soldered base plate 1. The flux can be vaporized and removed from the solder by pressure reduction effect. Further, the oxygen remaining volume is so small that the base plate can be prevented from being oxidized.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflow soldering apparatus used in the process of soldering electronic components on printed circuit boards and hybrid ICs.

0002

[Prior Art] In order to mount electronic components on a board such as a printed circuit board, the electronic components are mounted on paste-like solder formed on the board, and then carried into a reflow soldering machine where the paste is soldered. A reflow soldering method is generally used in which electronic components are soldered to a board by heating and melting solder.

[0003] Reflow soldering equipment includes a transport mechanism for loading, transporting, and unloading boards, a preheating furnace that heats the board brought into the equipment to a predetermined temperature below the melting point of the solder, and a board that is sent from the preheating furnace. It is generally equipped with a reflow main body furnace that heats the solder to a temperature higher than its melting point to melt the solder and bond the electronic component to the board.

[0004] As a heat source for heating paste solder,
Generally, infrared rays or hot air are used, but in normal reflow soldering equipment, the board is exposed to the atmosphere during heating, which causes oxidation of the board and solder, resulting in solder balls, unsoldered joints, bridges, etc. A problem arose in that poor soldering occurred. Furthermore, the use of Freon 113 used in the cleaning process performed after the reflow soldering process has become severely restricted, and there is a strong desire to omit the cleaning process. However, in order to omit the cleaning process, it is necessary to reduce the residual flux components contained in the paste solder, and in particular to reduce the chlorine-based active components contained in the paste solder. It is necessary to ensure long-term reliability under humid conditions.

[0005] Therefore, RM with reduced chlorine-based active ingredients
A (Rosin Mild Activated)
A paste-like solder called ``Solder'' was developed, but this solder has low activity and poor wettability, so there was a problem that sufficient wettability could not be ensured when reflowing in the atmosphere.

Under these circumstances, by using an inert gas atmosphere such as nitrogen inside the reflow soldering device, oxidation of the substrate and electronic components during heating can be prevented.
Efforts are being made to improve wettability.

[0007]

[Problem to be Solved by the Invention] When reflowing in an inert gas atmosphere, wettability is improved compared to when reflowing in an air atmosphere, but when using RMA type paste solder, This is not sufficient, and the generation of solder balls in particular becomes a problem. This is considered to be due to the fact that several tens of ppm of oxygen still remain even in the inert gas atmosphere.

Further, even if reflow is performed in an inert gas atmosphere, the flux residue contained in the paste solder cannot be reduced. Although it is possible to further reduce the amount of flux contained in the paste solder, this is not a good idea since it would be difficult to print minute patterns in the solder printing process.

Furthermore, when reflowing is performed in an inert gas atmosphere, there is a problem in that running costs are high because the amount of inert gas consumed is considerably large.

Another problem is that moisture contained in the IC package causes package cracks during reflow. 100% before reflow to prevent this.
There are methods of removing moisture by heating to temperatures above 0.degree. C. and methods of moisture-proof packaging, but these methods have the problem of increasing the number of steps and working time.

The present invention aims to solve the above-mentioned problems.

0012

[Means for Solving the Problems] The reflow soldering apparatus of the present invention includes a transport mechanism for loading, transporting, and unloading a board, and a preheating furnace for heating the board loaded into the equipment to a predetermined temperature below the melting point of the solder. and a reflow body furnace that heats the substrate sent from the preheating furnace to a temperature equal to or higher than the melting point of the solder, and is characterized in that the atmosphere inside the reflow body furnace is kept in a vacuum state.

[0013] It is preferable that the inside of the preheating furnace be provided with an inert gas atmosphere. Further, it is preferable that the pressure inside the preheating furnace is 1 atmosphere or less.

[0014]

[Operation] According to the present invention, since the furnace atmosphere of the reflow main body furnace is kept in a vacuum state, oxidation of the substrate and solder that occurs during heating can be prevented, and the amount of residual oxygen can be lowered than in an inert gas atmosphere. Oxidation can be prevented more effectively than conventional methods using an inert gas atmosphere.

[0015] Also, in the reflow main furnace, the flux in the paste solder is melted and vaporized, and the liquid flux is also vaporized and scattered, so the flux components are sucked out by a vacuum pump that creates a vacuum inside the furnace. Can be removed. This eliminates most of the flux residue on the substrate, making it possible to omit the substrate cleaning process after the reflow process, or to reduce the amount of fluorocarbon 113 used in the substrate cleaning process.
The amount of solvent used can be reduced. Furthermore, since the interior of the reflow main body furnace is in a vacuum state, blowholes are removed from the molten solder.

Furthermore, since the reflow main body furnace does not use an inert gas such as nitrogen, the amount of inert gas used in the entire apparatus is reduced, and running costs can be reduced.

Furthermore, it is possible to remove moisture contained in the IC package in the reflow main body furnace, and package cracks that occur during reflow can be prevented.

When the atmosphere inside the preheating furnace is an inert gas atmosphere, oxidation of the substrate and solder can be more reliably prevented. By setting the pressure of this inert gas atmosphere to below atmospheric pressure, it is possible to remove the solvent component contained in the paste solder and prevent the solder shape from changing (dripping).

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described below with reference to the drawings.

The reflow soldering apparatus shown in FIG. 1 includes a preheating furnace 2 that heats a board 1 carried into the apparatus to a predetermined temperature below the melting point of the solder, and a preheating furnace 2 that heats the board 1 sent from the preheating furnace 2 to a predetermined temperature below the melting point of the solder. The reflow main body furnace 3 heats the solder to the above temperature, melts the solder, and joins the electronic components to the board 1, and the board 1 is carried into the device and sequentially transferred to the preheating furnace 2 and the reflow main body furnace 3, and then removed from the device. Conveyor (transport mechanism) 4
It is equipped with

Before the reflow process, the board 1 is printed with paste solder, electronic components are mounted on the paste solder, and the board 1 is conveyed to the preheating furnace 2 by the conveyor 4.

The preheating furnace 2 is connected to a nitrogen gas cylinder 5 to maintain a nitrogen atmosphere inside.

A highly airtight loading shutter 6 is provided between the outside and the preheating furnace 2, and is opened only when the substrate 1 is loaded. After carrying in the substrate 1, the inside of the preheating furnace 2 is 1
The pressure is reduced to below atm, preferably 0.7 to 0.2 atm.
The pressure is reduced to about m. For this reason, the preheating furnace 2 is operated by the vacuum pump 7.
It is connected to the. Further, the preheating furnace 2 is equipped with a heater 8 such as an infrared heater, and is configured to uniformly heat the substrate 1 to about 150°C. The preheating temperature is preferably set according to the type of substrate 1, and it is preferable that the heating of the heater 8 is adjustable.

Regarding the substrate 1 carried into the preheating furnace 2, a phenomenon occurs in which the solvent is removed from the paste-like solder due to the preheating effect and the pressure reduction effect. A solvent removal phenomenon occurs from about 0.7 atm, and becomes more noticeable as the pressure is reduced. However, if the pressure in the preheating furnace 2 becomes 0.2 atm or less, the solvent will scatter rapidly, causing a phenomenon in which the shape of the solder pattern collapses and leaves bubble-like voids, which is undesirable.

The reflow main furnace 3 is provided adjacent to the preheating furnace 2, and a highly airtight shutter 9 is provided between the two. This shutter 9 opens only when the substrate 1 is transferred from the preheating furnace 2 to the reflow main furnace 3. In addition, a highly airtight unloading shutter 10 is provided between the reflow main furnace 3 and the outside.
is provided. This carry-out shutter 10 opens only when the substrate 1 is carried out to the outside. The reflow main body furnace 3 is constantly vacuumed by a vacuum pump 7, and is configured to have a vacuum atmosphere inside. The vacuum degree of this vacuum atmosphere is 1 to 10-4 Torr, preferably 10-1 to
It is set at 10-3 Torr.

A heater 11 such as an infrared heater is provided in the reflow main body furnace 3 and is configured to heat the substrate 1 to a temperature higher than the solder melting point and bond electronic components to the substrate 1. The appropriate heating temperature is about 230° C. when eutectic solder is used. Note that a filter 12 is arranged in a vacuum suction path connecting the reflow main body furnace 3 and the vacuum pump 7.

Since the substrate 1 in the reflow main body furnace 3 is placed in a vacuum state at a high temperature higher than the melting point of the solder, blowholes are removed from the molten solder, and flux components are vaporized and scattered. do. The vaporized flux component is sucked by the vacuum pump 7 and captured by the filter 12. The filter 12 is provided because the vaporized flux component gradually solidifies as the temperature decreases, so it is not preferable for it to enter the inside of the vacuum pump 7.

The degree of vacuum in the reflow main furnace 3 is 1 Torr.
If it is more than that, the removal of flux components and blowholes in the solder will be insufficient. On the other hand, if the degree of vacuum is less than 10@-4 Torr, the melted solder will scatter, which is not preferable, and the degree of vacuum must be set as described above.

The conveyor 4 moves intermittently, transporting one group of substrates 1 into the preheating furnace 2, simultaneously transporting another group of substrates 1 from the preheating furnace 2 to the reflow main furnace 3, and then transporting another group of substrates 1 from the preheating furnace 2 to the reflow main furnace 3. is carried out from the reflow main body furnace 3. Further, the carry-in shutter 6, the shutter 9, and the carry-out shutter 10 are opened and closed in synchronization with the intermittent movement of the conveyor 4, and the supply of nitrogen gas from the nitrogen gas cylinder 5 to the preheating furnace 2 and the vacuum suction of gas from the preheating furnace 2 are also carried out by the conveyor 4. This is done at a predetermined timing for intermittent motion.

The substrate 1 taken out of the reflow main body furnace 3 is quickly cooled by air blown out from the blower 13.

Table 1 shows a comparison of the amounts of flux residue when reflow was performed using the above-mentioned reflow soldering apparatus and when reflow was performed using the conventional reflow method.

[0032]

[Table 1]

As is clear from Table 1, according to the present invention, the amount of flux residue can be significantly reduced.

Table 2 also shows the results of measuring the number of blowholes in the solder joint using an X-ray transmission inspection device, and the number of package cracks in the case of the present invention and the case of the conventional method, respectively. This is what is shown.

[0035]

[Table 2]

As is clear from Table 2, it can be seen that according to the present invention, remarkable effects are exhibited in removing blowholes and preventing the occurrence of package cracks.

The embodiment shown in FIG. 2 has preliminary chambers 14 and 15 arranged before and after the reflow main furnace 3, and is configured to be advantageous in maintaining the degree of vacuum inside the reflow main furnace 3. be. A heater 16 is provided in the preliminary chamber 14 on the front side to maintain the substrate 1 at a preheating temperature. Further, both the front and rear preliminary chambers 14 and 15 are connected to a vacuum pump 7, and are maintained at a degree of vacuum between that of the reflow main furnace 3 and the atmosphere. Furthermore, a glass muffle 17 is arranged inside the reflow main body furnace 3. In FIG. 2, 18, 19, 20
, 10a are shutters each having airtightness. Since the other configurations are the same as those shown in FIG. 1, common reference numerals are given and explanations thereof will be omitted.

[0038]

Effects of the Invention According to the present invention, oxidation of the substrate and solder can be prevented and flux residue can be reduced, so even paste-like solder with low activity such as RMA type can ensure sufficient wettability, and solder balls can be reduced. It is possible to provide a reflow soldering apparatus that can prevent the occurrence of bridge defects and unsoldered joints, and can also prevent the occurrence of blowholes.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing another embodiment of the present invention.

[Explanation of symbols]

1 Board 2 Preheating furnace 3　Reflow main furnace 4 Conveyance mechanism 5 Nitrogen gas cylinder 7 Vacuum pump 8 Heater 11 Heater

Claims (3)

[Claims] Claim 1: A transport mechanism for loading, transporting, and unloading the board; a preheating furnace for heating the board carried into the device to a predetermined temperature below the melting point of the solder; A reflow soldering apparatus comprising a reflow main body furnace that heats to a temperature above the melting point, and the atmosphere inside the reflow main body furnace is set to a vacuum state. 2. The reflow soldering apparatus according to claim 1, wherein the atmosphere inside the preheating furnace is an inert gas atmosphere. 3. The reflow soldering apparatus according to claim 1, wherein the preheating furnace has an internal pressure of 1 atmosphere or less.