JPH06226437A - Solder reflow method and device - Google Patents

Abstract

PURPOSE:To make printing of substrates finer in pitch and to increase the scale of integration of hybrid ICs, etc., by executing reflow in a vessel internally having a reducing gas atmosphere isolated from the outside. CONSTITUTION:Cream solder is subjected to the reflow in the reducing gas atmosphere in the hermetic vessel 1. The reducing gas mainly consisting of gaseous hydrogen is, thereupon, supplied from a reducing gas supply device 10 into the vessel 1. The cream solder is subjected to the reflow in the vessel 1 internally having the reducing gas atmosphere isolated from the outside. The reducing gas is recovered by a reducing gas recovery device 9 without substantially oxidizing the reducing gas after the reflow and is reutilized for the reflow. As a result, the life of a flux is prolonged and the preservation period of the cream solder is extended. The solder joint parts having the reliability are thus obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for reflowing cream solder in a reducing gas atmosphere. More specifically, the present invention relates to a method of reflowing cream re-solder such as a chip mount or a connection portion of parts and omitting subsequent cleaning or mild cleaning, and a reducing atmosphere for soldering used for this method. Regarding the furnace.

In semiconductor products such as hybrid ICs, active components such as semiconductor devices are first fixed to a printed circuit board by thermosetting resin, and then the components are wire-bonded, and then passive components such as resistors, capacitors and inductors. Is manufactured by soldering to a printed circuit board.
Since the copper portion of the substrate is heated to a high temperature when soldering components, a flux such as rosin with or without an antioxidant is usually used for the purpose of preventing the oxidation of the solder.

[0003]

2. Description of the Related Art Conventionally, as cream solders, there are flon cleaning cream solders, water cleaning cream solders and the like, and these have been reflowed by an air reflow device. However, such a manufacturing method has a problem that after the soldering is finished, a cleaning operation and a drying operation must be performed to remove the residue. The most effective way to clean the remaining residue is to use chlorofluorocarbon, but due to pollution problems, chlorofluorocarbon cannot be cleaned.
Although the water washing method has come into use, there is concern about pollution in the future even with water washing.

Therefore, recently, it has been proposed to reflow the cream solder, in which the activator is removed from the flux, in order to improve the cleanability by using a reducing atmosphere furnace.
Japanese Examined Patent Publication No. 18950/1990 relates to such a reducing reflow apparatus, and is characterized by a direct cooling bed continuously provided to the direct heating bed and an internal portion installed above the direct heating bed. Is a furnace structure having a reduction box filled with a reducing atmosphere and a pusher for directly pushing the product from the heating bed inlet side through the inside of the reduction box toward the cooling bed outlet side. In the present invention, the heating at the time of bonding has conventionally been indirect heating by an atmosphere, so that it is difficult to control the heating temperature, and a large furnace is required, whereas the direct heating furnace has a temperature control and a furnace size. There are many advantages.

In the method of reflowing the solder on the substrate by filling the reducing box with hydrogen gas as disclosed in Japanese Patent Publication No. 18950/1990, hydrogen gas is used as an inert gas such as nitrogen gas at the inlet / outlet portion on the substrate. Is shielded by
Further, since it has an extrusion pusher, reflow is not performed in a closed container.

The invention of JP-A-60-136395 (Japanese Examined Patent Publication No. 62-46272) heats an object such as an electronic circuit chip to be solder-bonded on a substrate to a desired temperature by applying heat. Then, in order to maintain the object isothermally in the oxygen-free gas at its desired temperature, the heat exchanger in which the object to be heated is placed on the upper surface of the heat exchanger during heating. , At least a part of which is formed of a material such as copper that serves as a catalyst for a chemical reaction of oxygen and hydrogen, and which has an internal bypass path of gas formed with at least a part of the catalytic material as an inner wall The internal passage is characterized in that it is connected to a gas supply source, and as a result, oxygen in the gas is almost removed by the effect of the catalytic substance while passing through the internal gas passage. Therefore, the present invention has the advantage that a heating furnace that is small, inexpensive, and easy to manufacture can be used. In addition, JP-A-59-17427
A rotary table type reflow furnace is also known from JP-A-2.

[0007]

In recent years, there has been a tendency toward finer pitches of soldered portions in connection with the higher density of elements mounted on a substrate in semiconductor products and the higher integration of LSI and the like. In order to achieve a fine pitch, it is necessary to improve the printability by refining the solder alloy powder. However, along with this, the surface area of the powder increases, and it is necessary to increase the oxidizing power of the activator in order to remove the oxide film on the surface that affects the solderability.

However, when the activity is increased, the life of the cream solder is shortened due to aging and the residue after reflow is increased, so that the cleanability of the flux is deteriorated. Therefore, in order to eliminate or reduce the activator, a reducing gas must be used to prevent the oxidation of the solder during reflow, and the atmosphere must be protected and the reducing effect should be substantially complete. From this point of view, the conventionally proposed method of reflowing the solder excluding the activator in a reducing gas may not be able to simultaneously meet the demands for fine pitch and improved cleanability. Therefore,
The present inventor has conducted research on means for significantly increasing the action of the reducing gas as compared with the prior art.

[0009]

The first object of the present invention is to provide a method for reflowing cream solder in a reducing gas atmosphere, in which a container mainly made of hydrogen gas and having a reducing gas atmosphere independent from the outside is provided inside the container. Is a solder reflow method of reflowing by reflowing, the second is a method of collecting the reducing gas without substantially oxidizing it after the first reflow, and reusing it for reflow, and the third is a cream solder in a reducing gas atmosphere. In a reducing gas reflow device for reflowing, a reduction is characterized by comprising a sealed container having a reducing gas atmosphere mainly consisting of hydrogen gas inside, and a heater provided inside or outside the sealed container. It is a gas reflow device.

The present inventors have found that in the conventional reducing gas reflow, some air is mixed into the reflow furnace from the outside, which unexpectedly deteriorates the solderability. As a result of earnestly studying the preventive measures, the present inventors have found that the action of the reducing gas can be sufficiently exerted by making the reducing atmosphere independent of the atmospheric atmosphere outside the reflow device, the furnace, the container and the like. Therefore, the present invention is characterized by the operation of soldering the substrates one by one or several in a batch process in an independent reducing gas containing hydrogen gas, and the conventional continuous processing method is used. Does not mean

As the reducing gas, it is performed on the order of minutes,
In order to achieve sufficient reduction during the short reflow time, it is necessary to use hydrogen for its main part. The hydrogen concentration in the furnace gas is preferably 5% by volume or more.
The balance is preferably nitrogen, carbon monoxide, an inert gas, or other gas that is harmless to the solder alloy or rosin during reflow, and carbon dioxide, oxygen, water, etc. may be generated as impurities or even by decomposition of rosin. It is desirable to have as few as possible.

The components of the cream solder are not particularly limited,
In addition to normal solder alloys, rosin or its substitutes,
Contains viscosity modifiers, solvents, etc. It is desirable that the activators such as diphenylacetic acid and phthalic acid are not contained at all, or if they are contained, the content thereof should be as small as possible to prolong the life of the cream solder and to reduce the residue after reflow. The upper limit of the activator is preferably 1% by weight or less with respect to the total cream solder. Further, in order to improve printability, the grain size of the solder alloy is preferably 35 μm or less.

As described above, the use of cream solder containing no activator has many advantages in the present invention, but in the present invention, the reflow temperature is lowered by 10 ° C. or more as compared with the conventional method, so that the thermal effect on various elements is reduced. Even when using a cream solder containing an activator, there is still an advantage over the conventional method because it can be reduced.

The reflow apparatus of the present invention is also provided with a heating device inside or outside a closed container that can be opened and closed, and the reflow is performed in an independent reducing atmosphere by the heating. The external heating device is a high frequency or resistance heating device, and the hermetically sealed container is made by processing or casting a copper or aluminum plate and used by making it vertical or horizontal. The internal heating device is usually a resistance heating device and uses a sealed container of a suitable heat resistant material such as steel. After the reflow, opening and closing the closed container allows the gas to be freely taken in and out. Thereby, the hydrogen gas concentration in the container can be arbitrarily set.

Since all of the reflow exhaust gas can be recovered without being released to the outside, when applying cream solder that emits harmful gas, the exhaust gas was emitted to the outside as in the conventional device. It is very effective compared to the device. The recovery is performed by a vacuum device and a recovery chamber provided at the tip of the discharge hole.

[0016]

In general, the solder alloy powder should be as fine as possible in order to improve the printability of the cream solder. However, if the alloy powder is too fine, it is necessary to increase the activity of the activator for removing the oxide film on the surface of the solder alloy powder, which shortens the life of the cream solder and loses the activity during storage of the solder. On the other hand, as described above, when the activator is not used, the residue after the reflow is eliminated or becomes very small, but the conventional method cannot avoid the oxidation during the reflow. However, by making the reflow atmosphere independent from the outside, both the printability and the life of the solder can be improved at the same time, and when the cause was investigated, it was found that a slight amount of air inclusion was prevented. It was also found that the reflow temperature can be lowered by making the reflow atmosphere independent from the outside. Hereinafter, the present invention will be described with reference to examples.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an embodiment of a reflow apparatus according to the present invention. In the figure, reference numeral 1 denotes a hermetically sealed container for performing reflow, which is constituted by a pair of openable and closable lids 1a and 1b being closely attached by a clamp 4.

The substrate 2 on which various elements are mounted and solder is printed is placed on the lower lid 1a via the susceptor 15. A heating element 3 is fixed to the upper lid 1b by an appropriate means, and a power plug 5 is fixed to the upper lid 1b to energize the heating element 3. 16 is a heating element 15
It is a reflection plate whose surface is plated with gold so that the heat from the substrate concentrates on the substrate. Reference numeral 10 is a supply device of a reducing gas such as hydrogen gas or city gas, which is mainly hydrogen, such as a high-pressure gas tank. Reference numeral 6 is a coupler that connects the tubes together, and 7 is a valve that switches the gas flow path and shuts off the inside of the sealed container from the outside air during reflow. 8 is a vacuum suction device, 9 is a reducing gas recovery device, and 11 is an air inlet.

A method of operating the above-mentioned reflow apparatus will be described with reference to the schematic view of FIG. 1 reflow box (1
The board 2 on which the cream solder is printed is inserted into only one of the pair of lids (see FIG. 2A), and the other lid is closed, and then the board is sealed with a clamp tool (see FIG. 2 ( B)). Next, the coupler 6 (see FIG. 1) is connected and the vacuum suction device 8 is communicated with the inside of the sealed container 1 by switching the valve 7.
A vacuum is applied (FIG. 2 (C)).

When the inside of the hermetically sealed container is sufficiently evacuated, the valve 7 is switched to connect the interior of the hermetically sealed container with the reducing gas supply device 10 to fill the reducing gas therein (FIG. 2 (D)). Next, an electric current is passed through the power plug 5 to heat the heating element 3, and the electric power to the heating element 3 is controlled so that the heating condition satisfying a predetermined reflow condition is satisfied to reflow the cream solder of the substrate 1 (see FIG. E)). At this stage, by shutting off the valve 7, the atmosphere in the sealed container 1 becomes independent from the outside,
Gas from decomposition of rosin may enter the atmosphere, but the gas is not mixed in from the outside.

After completing the reflow under the predetermined reflow conditions, the vacuum pump discharges the exhaust gas to the outside of the box (see FIG. 2).
(F)) Next, the box is filled with air through the air intake port 11, the lid is opened, and the substrate inside is taken out. When the exhaust gas in the box is sucked by the vacuum pump 8, for example, if an exhaust gas recovery device 9 is installed via the vacuum pump, the exhaust gas is recovered without dissipating various gases of cream solder generated during reflow. be able to.

To mass-produce various parts, the following steps may be taken. Carrying in parts to the container Ejecting air from the container Filling the container with reducing gas and, if necessary, nitrogen gas, etc. Heating Reducing gas exhausting (may be done after cooling the container) Parts removal It is also possible to carry out each step efficiently while sending them to the place where they are carried out.

The results of evaluation of cream solder reflow properties under various reflow conditions are shown below. The components of the solder were as follows. Solder alloy: 37% Pb, 63% Sn Average particle size 30 μm-90 parts by weight Activator: Organic acid-optional component-4 parts by weight Solvent: Polyethylene glycol-1 part by weight Rosin: Rosin ester-5 parts by weight Reflowability In the description of the evaluation, the components other than the solder alloy are called flux.

The reflow method was performed according to the conventional method using an open type reflow box and the method using the sealed container of the present invention shown in FIG. The reflow atmosphere was air, nitrogen gas and reducing gas in the conventional method, and was reducing gas in the method of the present invention. In both methods, 10% hydrogen and 90% nitrogen were used as the reducing gas. The reflow conditions were: a temperature of 210 to 230 ° C. and a pressure of 1 atm.

The solder ball shape, spread width, and wettability of the substrate after reflow were evaluated. The evaluation criteria are as follows. ○ − In all these tests ○ △ − Only two tests ○ × − In two or more tests ×

The procedure of the test method or the compliant standard will be described below. Solder ball test A sample is prepared by printing cream solder, which is well kneaded, on an alumina substrate in a circle shape with a diameter of 6.5 mm and a thickness of 0.2 mm. The sample is placed on a hot plate set at a temperature of 250 ° C. ± 5 ° C. and melted by heating. Remove the heat-melted cream solder from the hot plate and let it stand until it solidifies. Observe the solidified cream solder with a 20 × graduated magnifying glass and record the solder balls according to the categories in Table 1. Of the three solder balls, the highest category number is the result of the solder balls. If this number is 4 or more, it is judged as ◯.

Table 1 Category Contents 1 Solder balls do not remain unmelted even when heated. 2 When the paste melts, the solder becomes one large sphere, and many fine spheres line up around it in a semi-continuous lump shape, or melt to form spheres of the same size. 3 When the paste melts, the solder becomes one large sphere with three or more small spheres with a diameter of 75 microns or less scattered around it, but they are not arranged in a discontinuous, bulky pattern. 4 When the paste melts, the solder becomes one large sphere with less than three solder balls of 75 microns or less around it. 5 When the paste melts, the solder becomes one large sphere with no solder balls around.

Spreading test A phosphorous deoxidized copper plate C1201P specified in JIS H 3100 is used to wash the surface, and then an oxidation treatment is performed in an electric furnace at about 150 ° C. for 1 hour to prepare a test copper plate. A wire having a diameter of 3.2 mm is wound with a wire solder of 1.6φ, and one wire is cut and used. Approximately 0.05 g of flux and 0.25 g of solder alloy are placed on a copper oxide plate of 0.3 × 30 × 30 mm as a test piece. Three similar test pieces are prepared.

A hot plate is heated to 250 ° C., a test piece is placed on the hot plate for 30 seconds, and then cooled at room temperature. After cooling, the height of the solder alloy is measured with a micrometer, and the spread rate is calculated from the following formula. Spread rate (%) = (D−H) / D × 100 D = Diameter of solder alloy as sphere 1.24V
^1/3 H = height of solder alloy V = specific gravity (S alloy 8.30) The average of three test pieces is taken as the spread rate of the flux. A spread rate of 70% or more was evaluated as ◯, and a spread rate of less than 70% was evaluated as x.

Solder wettability When a connecting pin (cross sectional size 0.4 × 0.4 mm) of an electric element is bent into an L-shape on a substrate (bending radius 1.0 mm) and soldering is performed, the solder becomes a connecting pin. The case where the surface was not covered was evaluated as x, and the case where the solder was raised on the surface of the connection pin was evaluated as o. 8 or more out of 9 connecting pin test pieces
The case was judged to be ○. The evaluation results are shown in Table 2.

Table 2 Activator Left time Air N ₂ gas Open reduction Hr reflow reflow atmosphere reflow 0 ○ ○ ○ ○ Yes 25 × × △ ○ 50 × × △ ○ 0 × △ △ ○ None 25 × × △ ○ 50 × × △ ○ From the results shown in the table, the reflow according to the method of the present invention shows that the activator was added. It can be seen that sufficient reflow is possible while using no cream solder, which is exactly the same as reflowing with normal nitrogen gas of cream solder containing an activator. Table 3 shows the results of performing the above method while changing the reflow temperature.

Table 3 Activator reflow N ₂ gas in air open reduction Temperature ℃ Reflow Reflow atmosphere Reflow 210 × × △ △ Yes 220 △ △ △ ○ 230 ○ ○ ○ ○ 210 × × × △ None 220 × × △ ○ 230 × △ ○ ○ It is possible to lower the reflow temperature in the sealed reflow furnace Table 3 More obvious.

[0033]

According to the present invention, the following advantages are achieved. (A) Fine pitch printing on the substrate: Hybrid I
The degree of integration of C and the like can be increased. (B) Flux life extension: Reliable solder joints can be obtained even when the cream solder has a long storage period. (C) Lower reflow temperature: The reliability of heat-sensitive elements such as semiconductors can be improved. (D) Reuse of reducing gas: Resource saving and environmental pollution prevention can be realized. (E) Omission or simplification of washing: Since the amount of residue is reduced, the need for washing after reflow is eliminated or reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of an apparatus for carrying out the method of the present invention.

FIG. 2 is a diagram illustrating a reflow method by the apparatus of FIG.

[Explanation of symbols]

 1 Sealed Container 2 Substrate 4 Clamp 5 Power Plug 6 Coupler 7 Valve 8 Vacuum Suction Device 9 Reduction Gas Recovery Device 11 Air Inlet

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H05K 3/34 R 7128-4E

Claims (4)

[Claims] 1. A method of reflowing a cream solder in a reducing gas atmosphere, characterized in that the reflow is carried out in a container mainly made of hydrogen gas and having a reducing gas atmosphere independent from the outside inside. Method. 2. The solder reflow method according to claim 1, wherein after the reflow, the reducing gas is recovered without being substantially oxidized and is reused in the reflow. 3. The solder reflow method according to claim 1, wherein the cream solder which does not substantially contain an activator is reflowed. 4. A reducing gas reflow apparatus for reflowing cream solder in a reducing gas atmosphere, comprising a hermetically sealed container having a reducing gas atmosphere mainly composed of hydrogen gas therein, and a heater provided inside or outside the hermetically sealed container. A solder reflow device comprising: