JPH07283525A - Migration preventing method - Google Patents

Abstract

PURPOSE:To restrain migration which is to be generated on an electronic circuit board. CONSTITUTION:All of the soldering parts 2 on an electronic circuit board 1 are electrically connected, and a voltage of 12V is so applied that the soldering parts 2 on the electronic circuit board 1 are made a positive pole and a platinum board 3 is made a negative pole. After that, both of the poles are dipped in 10% nitric acid aqueous solution 4. A non-conductor coating film is formed on the surface of solder, as the result of electrochemical reaction of solder composition. Since the non-conductor coating film is formed on the whole surface of the solder, generation of migration can be restrained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to soldering for assembling electronic circuits, and more particularly to a method for preventing migration by surface treatment after soldering.

[0002]

2. Description of the Related Art Generally, an electronic circuit board manufacturing process is a process of mounting electronic parts on a printed wiring of an electronic circuit board,
The steps of soldering electronic components, cleaning the surface of the substrate, and coating the surface of the substrate are performed in this order. During the process of soldering electronic components, flux is used to facilitate the wetting and joining of the printed wiring on the electronic circuit board and the solder. As its usage, after applying flux to the solder joint surface of the printed wiring, soldering is used, or a mixture of solder and flux to form a cream is applied to the solder joint surface of the printed wiring and heated. There is a method of melting and soldering the solder. By using flux, it is possible to dissolve and remove the dirt and oxides that intervene on the solder joint surface of the printed wiring and deteriorate the wettability and interfere with soldering, and then oxidize again. Can be prevented. The types of the flux are roughly classified into an oil-soluble flux easily soluble in an organic solvent and a water-soluble flux easily soluble in water.

Since flux is generally corrosive,
After soldering, it is necessary to remove the flux remaining on the substrate. Since the oil-soluble flux is easily dissolved in the organic solvent, the oil-soluble flux can be washed off by washing the electronic circuit board with the organic solvent. Among organic solvents, chlorofluorocarbon, which is particularly excellent in detergency, has been often used so far, but cleaning with chlorofluorocarbon is being changed to cleaning with water.

[0004]

However, since water has a weak detergency property, even if a water-soluble flux is used, the water-soluble flux and the like that have entered between the electronic circuit board and the electronic component cannot be sufficiently washed, Some water-soluble flux may remain on the electronic circuit board. Since the water-soluble flux easily absorbs moisture and swells and becomes an electrolyte by containing water, if the water-soluble flux remains on the electronic circuit board after the soldering process, the Therefore, the soldered electrodes may be connected to each other. In addition, even if the flux residue is thoroughly washed, water has a weak detergency, so it is not sufficiently washed up to dirt and moisture in the air adheres, and various dirt components dissolve in it and become an electrolyte. Sometimes. In this way, when the electrolyte 9 in which the soldered electrodes are short-circuited is present, as shown in FIG. 3, when a charge is applied between the electrodes, the metal element M on the surface of the circuit that becomes the positive electrode 7 is ionized and metal The ions become 5 to cause migration, which is a phenomenon of migration to the negative electrode 8 side (a). Then, a part of the moved metal ions 5 is crystallized on the surface of the negative electrode to become crystals 6 (b). When the crystallization further progresses, it reaches the positive electrode,
Finally, the electrodes may be short-circuited by the metal element M and a large current may flow (c).

In view of the above problems, it is an object of the present invention to prevent migration from occurring by forming a passivation film, which is an insulating material and hardly soluble in water, on the solder surface on an electronic circuit board. .

[0006]

The migration prevention method of the present invention forms a passivation film on the surface of solder after or at the same time as cleaning for cleaning flux residue after soldering with flux. It is characterized by

[0007]

[Function] Before soldering, flux is applied to the solder joint surface, or solder and flux are mixed to perform soldering. Then, after or at the same time as the cleaning for cleaning the flux residue, a passive film that is an insulator and hardly soluble in water is formed on the solder surface. In this way, since the entire solder is covered with the passivation film, the metal element that is a component of the solder is not exposed on the surface and does not come into contact with the electrolyte, so that the metal element hardly becomes a metal ion. Therefore, the generation of metal ions from the solder surface is suppressed, and the migration is suppressed.

[0008]

EXAMPLE An example of the present invention will be described with reference to FIG.
Due to the electrochemical reaction on the solder surface on the substrate, lead dioxide (PbO ₂ ) and tin dioxide (S
form a film with nO ₂ ). This will be described in more detail. Applying water-soluble flux on the printed wiring of the electric circuit board, mounting electronic parts, soldering,
After that, the water-soluble flux on the printed wiring is washed with water. Next, the solder portion 2 (Pb and S
All alloys with n) are electrically connected by wiring. This is for making all the solder parts 2 into a positive electrode by the electrochemical reaction described later, and the wiring is cut off after the electrochemical reaction described later. A voltage of 12 V is applied so that the solder portion 2 on the electric circuit board 1 becomes the positive electrode and the platinum plate 3 becomes the negative electrode. Separately, a 10% aqueous nitric acid solution (pH value <1) is prepared as an electrolyte.
Then, both the positive electrode and the negative electrode were immersed in a 10% aqueous solution of nitric acid with a voltage applied to carry out an electrochemical reaction. At this time, the positive electrode and the negative electrode are arranged so as not to come into contact with each other. The reaction was started at an applied voltage of 12 V and an applied current of 2.4 mA and allowed to react for 5 minutes. As the above conditions were followed, the silver-white solder gradually lost its luster and the surface turned dark gray. Initially, bubbles were generated from the positive electrode, but as the color changed to dark gray, the generation of bubbles decreased, and finally the generation of bubbles stopped. Bubbles are mainly hydrogen gas. The main chemical reaction formulas for the positive electrode are shown below.

[Chemical 1] Through the above reaction, lead dioxide and tin dioxide, which are insulative and hardly soluble in water, were formed on the solder surface of the positive electrode.

[Comparative Test Example] [0009] As described above, a solder film on which a passivation film that is poorly soluble in water is formed on the solder surface and a conventional solder film on which a passivation film is not formed, The time until short-circuiting between the electrodes due to migration was compared. The test piece has the same shape on the positive electrode side and the negative electrode side, and has a length of 18 mm and a width of 30 on the electric circuit board.
An electrode with solder of 0 μm and thickness of several 10 μm
Those arranged in parallel with the length direction at intervals of 900 μm are prepared. The substrate is soldered with a water-soluble flux and then washed with water. Further, a passivation film is formed on the surface of the solder on the test piece by the same method as in the above embodiment. On the other hand, a test piece for comparison on which a passive film is not formed is prepared. Each test piece is fixed on a container containing pure water at 60 ° C., and steam is applied to apply a voltage of 12 V in a state where the electrode surface is condensed. In this way, a test for promoting the occurrence of migration was conducted. Table 1 shows the time to short circuit due to each migration.

[Table 1] From Table 1, it can be seen that the test piece having a passivation film, which is an insulator and hardly soluble in water, formed on the solder surface is 3 times larger than the comparative test piece having no passivation film formed on the solder surface.
It can be seen that there is no short circuit between the electrodes for more than twice the time, and the life is extended.

In the above-mentioned embodiment, a surface treatment step is separately provided after the water-soluble flux cleaning, and lead and tin on the solder surface are chemically changed in the aqueous solution to form a passive film. A passivation film is easily formed on the exposed portion of the solder surface, but a passivation film is formed even if there is a flux residue left without being cleaned. Since the water-soluble flux has hygroscopicity, even if the water-soluble flux remains on the solder surface and covers the solder surface, the aqueous solution enters the water-soluble flux and becomes an electrolyte, and a passivation film is formed on the solder surface. Because. Further, by using a cleaning solution for water-soluble flux on the electronic circuit board, surface treatment for forming a passivation film on the solder surface may be performed at the same time as cleaning the water-soluble flux.

The passivation film can be formed not only by the method of forming the passivation film in the electrolyte as in the above embodiment but also by spraying the electrolyte. For example, there is a method in which a solder portion on an electronic circuit board is used as a positive electrode, a nozzle for spraying an electrolyte is used as a negative electrode, and an electrolyte is sprayed while applying a voltage to both electrodes to form a passivation film on the solder surface.

The passivation film formed on the surface of the solder may be an insulator, is hardly soluble in water, and has a chemical component change of the solder. For example, in the case of lead, which is one of the components of solder, lead monoxide, trilead tetraoxide, dilead trioxide, lead dioxide and other oxides, lead sulfide and other sulfides, and lead chloride and other chlorides. Then it becomes a passive film. Also,
As a condition for forming the passivation film, P from the diagram shown in FIG.
If the potential of b and pH are adjusted so that a passive state is formed,
Form lead oxide which is a passive film. Also, in the case of Sn, tin oxide is similarly formed.

When a corrosive gas is exposed to solder, a passivation film can be formed on the solder surface. However, when the solder surface is covered with flux, it is difficult for corrosive gas to pass through the flux and reach the solder surface.
The passivation film is hard to form where the flux is covered. However, the position to attach the solder on the electronic circuit board is
Since it is preferable that the position where the heat is applied to the solder is easy, the solder is often applied to a position where the surface of the electronic circuit board can be easily seen. Therefore, the flux on the solder surface is easily removed in the cleaning process. In this way, if the flux on the solder surface is removed in the cleaning step, the cleaned electronic circuit board can be exposed to a corrosive gas atmosphere, and a passivation film can be formed on the solder surface. When sulfurous acid is used as the corrosive gas, a passivation film of mainly lead sulfide and tin sulfide is formed on the solder surface.

After soldering with an oil-soluble flux and washing with chlorofluorocarbon having excellent cleaning properties, most of the oil-soluble flux on the electronic circuit board is removed, so there is almost no possibility that the flux will connect the electrodes. Absent. However, when the electrode surface is condensed, the condensed water may become an electrolyte to cause migration. For this reason, it is better to form a passivation film by washing with chlorofluorocarbon after soldering with an oil-soluble flux.

On the solder surface where the conventional passive film is not formed, when the surface of the electronic circuit board is moisture-proof coated with a resin, the coating material is easy to flip and difficult to ride. However, since the passivation film is formed on the solder surface, many irregularities are formed on the surface, so that the coating material permeates the solder surface and the coating becomes firm.

[0016]

EFFECTS OF THE INVENTION By forming a passivation film, which is an insulator and is hardly soluble in water, on the surface of the solder, the metal element, which is a component of the solder, is not exposed on the surface and does not touch the electrolyte.
Metal elements rarely become metal ions. Therefore, the generation of metal ions from the solder surface can be suppressed, and thus the migration can be suppressed.

[Brief description of drawings]

FIG. 1 is a diagram showing a method of forming a passivation film on the solder surface of the present embodiment.

FIG. 2 is a diagram showing a lead compound generated in the Pb—H ₂ O system from the relationship between the Pb potential and pH.

FIG. 3 is a schematic diagram showing movement of a metal element due to migration. (A) is a diagram simulating a state in which a metal element on the surface of a positive electrode is ionized and moves to a negative electrode,
(B) is a diagram simulating a state in which an ionized metal element is crystallized on the surface of the negative electrode, and (c) is a state in which the metal element crystallized on the surface of the negative electrode grows to the positive electrode, and the negative electrode and the positive electrode are short-circuited. It is the figure which imitated.

[Explanation of symbols]

 1 ... Electronic circuit board 2 ... Solder 3 ... Platinum plate 4 ... 10% nitric acid aqueous solution 5 ... Ionized metal element 6 ... Metal element crystallized on the negative electrode surface 7 ... Positive electrode 8 ... Negative electrode 9 ... Electrolyte M ... Metal element

Claims (1)

[Claims] 1. A method for preventing migration, which comprises forming a passivation film on the solder surface after soldering with flux and after or at the same time as cleaning for cleaning flux residues.