JPH0123554B2 - - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for removing an oxide film formed on the surface of a thin nickel layer formed on an electronic component.

(Prior Art) Conventionally, leads derived from packages of electronic components such as ICs (Integrated Circuits), receivers, light emitting elements, and reed switches have been plated with gold. This is because gold plating has high corrosion resistance and good wettability with solder. However, in recent years,
To reduce costs, solder plating has come to be used instead of gold plating. Both gold plating and solder plating usually use a nickel thin film layer as the base plating layer, but solder plating cannot withstand the heat treatment used to manufacture these electronic components.
Therefore, solder plating is performed after these heat treatment steps, and the heat treatment is performed with the nickel thin film layer exposed.

Heat treatment processes include, for example, the process of die-bonding an IC chip to a package, or the process of covering the package with a cap, a so-called sealing process. During these heat treatment steps, a thin oxide film is formed on the nickel thin film layer. The oxide film has undesirable properties for electronic components, such as poor solder wettability and poor conductivity, and must be removed.

Conventional methods for removing nickel oxide films are listed in Table 3 and 236 of the Metal Handbook (edited by the Japan Institute of Metals), page 967.
The pickling solution described in is used. For example, in the case of oxalic acid, the reaction when the nickel oxide film is removed by acid is NiO + (COOH) ₂ →2CO ₂ +
_H2O +Ni. In addition, it has many pinholes, and acid that enters through these pinholes causes underetching and separates the oxide film from the remaining nickel layer. In this way, the oxide film on the surface of the nickel layer is etched with acid and chemically removed. However, since the metal layer is also etched by the acid, the nickel surface becomes rough.

To remove the nickel oxide film in this way,
There is no problem if the nickel layer is thick, but problems arise if the nickel layer is thin, such as the thin nickel film layer applied to the leads of electronic components. That is, the oxide film cannot be completely removed by the method of removing the oxide film using only the pickling solution or organic acid such as carboxylic acid. On the other hand, if the oxide film is completely removed by increasing the etching time, the nickel layer becomes too thin, or the nickel thin film layer is peeled off, exposing the surface of the underlying layer. Consequently, in any case, they have had drawbacks such as poor solder wettability.

(Purpose of the Invention) The present invention has been made to eliminate the above-mentioned drawbacks, and its purpose is to reduce the etching depth of the nickel layer by removing the oxide film formed on the surface of the nickel layer adhered to the electronic component. The objective is to find a way to remove it while keeping it small.

(Structure of the Invention) The present invention provides a method for removing an oxide film from a nickel thin film layer that is attached to a lead led out from a package of an electronic component and has an oxide film formed on the surface. A method for removing a nickel oxide film, comprising the steps of immersing the lead in an organic acid solution, and then immersing the lead of the electronic component in a chemical polishing solution containing hydrogen peroxide and ammonium fluoride as main components. .

(Example) Examples of the present invention will be described below by taking as an example an oxide film formed on the surface of a nickel plating layer formed as a lead underlayer of an IC. FIG. 1 is a sectional view of an embodiment of the present invention.

FIG. 1 is an enlarged cross-sectional view of a lead 2 on which a nickel plating layer 1 with a thickness of 2 to 3 μm is formed. An oxide film 3 is formed on the nickel plating layer 1 due to oxygen in the atmosphere. 4 is a metallized layer pulled out from inside the IC package 5;
The leads 2 are bonded here with silver solder 6.

To remove this oxide film 3, the lead 2 is immersed in an oxalic acid solution. The reaction of oxalic acid is slow at room temperature, so it is best to heat it close to its boiling point and keep it at a temperature of about 80°C.

Here, when heating from below, a flow is generated by convection, so there is no need to stir the liquid, which is convenient.

Since the organic acid such as oxalic acid etches the nickel plating layer 1 slowly, the etched state of the nickel plating layer 1 can be easily controlled. That is,
Etching of the nickel plating layer 1 can be stopped when the passive oxide layer peels off.
Note that organic acids that can replace oxalic acid include butyric acid and acetic acid.

The oxide film 3 remains on the nickel plating layer 1 even after the oxalic acid solution treatment step is completed. If the nickel plating layer 1 is continued to be immersed in the oxalic acid solution, the nickel plating layer 1 will continue to be etched, which is not preferable. This is because oxalic acid performs isotropic etching. Therefore, chemical polishing is performed next.

Chemical polishing involves applying a 50% aqueous solution of a chemical polishing agent containing hydrogen peroxide and ammonium fluoride as main components at room temperature.
This is done by soaking for about 1.5 minutes. Since this chemical polishing liquid has a higher etching rate in the direction along the surface of the nickel plating layer 1 than in the direction perpendicular to the surface, it is possible to completely remove the oxide film 3 and polish the surface.

Next, a preferred concentration of the oxalic acid solution will be explained. Figure 2 is a graph of oxalic acid concentration (normality) and solder yield, Figure 3 is a graph of oxalic acid concentration (normality)
This is a graph of the etching thickness. These are all
This data is after immersion in 80°C oxalic acid for 5 minutes and chemical polishing.

From FIG. 2, it can be seen that in order to perform solder plating on the nickel plating layer 1, when removing the oxide film 3, about 1 to 3 normal is preferable.

According to FIG. 3, it is approximately 1.1 μm under 2 to 3 standards. Considering FIG. 2, it can be seen that the etching speed is fast and the yield is good before and after the specified value.

By the way, when etching is performed using an inorganic acid containing SO ₄ ^2- , NO ₃ ^-oin , etc. instead of etching using an organic acid such as oxalic acid, the following problems occur. Inside the package 5, an IC chip is mounted and aluminum wiring is made, but the aluminum contains a small amount of SO ₄ ^2- ions and
This is because it is extremely reactive with NO ₃ ^- ions, and reacts with these ions that have entered the interior, making it easy for aluminum wires to break. Also, lead 2
Silver contained in the silver solder 6 that adheres the metallized layer 4 and the metallized layer 4 is likely to undergo migration.

Since the nickel plating layer 1 also plays the role of preventing this migration, the nickel plating layer 1 is peeled off by the inorganic acid, and furthermore, SO ₄ ^2- and
If ions such as NO ₃ ^- remain, the silver contained in the silver solder 6 will be induced by these ions, and migration will be extremely likely to occur. When migration occurs, undesirable phenomena occur in the IC, such as short circuits between leads or between metallized layers.

It is also possible to remove the nickel oxide film using only a chemical polishing solution without etching with oxalic acid, but chemical polishing solutions whose main components are hydrogen peroxide and ammonium fluoride deteriorate quickly. If the oxide film is removed using only this solution, it will often be necessary to replace it. If the number of replacements is as high as this, not only is it troublesome to replace, but the chemical polishing liquid is expensive, resulting in an extremely high cost. Furthermore, it also has the disadvantage that the oxide film is not removed uniformly and becomes uneven.

(Effects of the Invention) As explained above, according to the present invention, in order to remove an oxide film from a nickel thin film layer that is adhered to an electronic component and has an oxide film formed on the surface, immersion in an organic acid solution is performed. Since it is immersed in a chemical polishing liquid whose main components are hydrogen peroxide and ammonium fluoride, it has the following effects.

First, since the oxide film is removed to some extent with the organic acid and the remaining oxide film is completely removed with the chemical polishing liquid, the oxide film can be removed while leaving a predetermined nickel layer thickness. this is,
This maximizes the efficiency of oxide film removal. Second, these etching liquids and polishing liquids do not contain ions that cause image cutting or migration, and therefore do not affect the reliability of electronic components.

Thirdly, it is possible to remove the oxide film formed on the leads of electronic components by simply immersing them, even if the leads of electronic components have mutually insulated leads and cannot be subjected to electrolytic polishing.

Although this embodiment has been explained using an IC lead as an example, it can be widely applied to other devices other than ICs, such as receivers, light emitting diodes, reed switches, etc., as long as they have a nickel thin film layer.

[Brief explanation of drawings]

Fig. 1 is a side view of an embodiment of the nickel oxide film removal method of the present invention, Fig. 2 is a chart showing the relationship between oxalic acid concentration and solder yield, and Fig. 3 is a graph showing the relationship between oxalic acid concentration and etching thickness. This is a diagram showing the relationship. 1... Nickel plating layer, 2... Lead, 3... Nickel oxide film.

Claims (1)

[Scope of Claims] 1. A method for removing an oxide film from a nickel thin film layer that is attached to a lead led out from a package of an electronic component and has an oxide film formed on the surface, comprising: treating the lead of the electronic component with an organic acid. A method for removing a nickel oxide film, comprising the steps of immersing the electronic component in a liquid, and then immersing the lead of the electronic component in a chemical polishing liquid containing hydrogen peroxide and ammonium fluoride as main components. 2. The method for removing a nickel oxide film according to claim 1, wherein the organic acid solution is an oxalic acid aqueous solution heated to approximately the boiling point.