JPS6056800B2 - Method for removing protruding parts of base copper plating in partial silver plating and method for finishing surface of silver plating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for obtaining partial silver plating on a required part of a material, such as applying partial silver plating to a lead frame for a semiconductor device corresponding to its element mounting area or wire bonding surface, etc. A method for removing the protruding part of the base copper plating in selective silver plating that can selectively and efficiently remove the protruding part of the copper plating as the base, and a method for peeling off the protruding part of the base copper plating and peeling off the silver plated surface to the desired appearance. This invention relates to a silver plating surface finishing method in selective silver plating.

In order to obtain partial silver plating on lead frames for semiconductor devices, if the material is iron-nickel alloy, iron-nickel-cobalt alloy, etc., a copper base plating is required to obtain adhesion. The base copper plating is applied to the entire surface of the material or to a portion slightly larger than the desired silver plating area.

After the necessary partial silver plating is applied to the base copper plating, the protruding base copper plating is peeled off because it causes various problems as described later. At this time, there is a need for a method that selectively and efficiently removes only the protruding portions of the base copper plating without affecting the silver plating in any way. Chemical treatment and electrolytic treatment are known as methods for removing copper plating and silver plating.

For the former, various stripping solutions such as Enstrip 108 (Japan Metal Finishing Co., Ltd.) and Enstrip S (EEJA) are used, and stripping is performed by immersing the lead frame.

In this case, in order to continuously process the strip lead frame, the immersion time is regulated to be constant because the lead frame is continuously fed into the stripping solution. However, with partial plating, the mask area may be uneven. Therefore, it is difficult to make the area and plating thickness of the base copper plating constant due to variations in plating efficiency or the like.

For this reason, in the above-mentioned continuous process, undesired copper plating remains unremoved, which can cause problems with the resin mold or when re-plating the leads, and conversely, excessive peeling can cause partial silver plating and parts under the partial silver plating to be removed. There are disadvantages in that even the copper plating is peeled off, adversely affecting wire bonding properties and causing electrical failures due to the falling off of the silver plating film. Furthermore, the plated surface is attacked by the stripping solution, causing uneven stripping. In the latter electrolytic treatment, stripping is performed using a constant current in a stripping solution.

However, in this method, the silver plating is also peeled off as well as the copper plating. As a result, the silver-plated surface becomes matte or peels unevenly, making it extremely difficult to achieve the desired plating appearance. The present invention was proposed to solve the above-mentioned problems, and by regulating the potential using a reference electrode, only the protruding portion of the base copper plating can be selectively removed without affecting the silver plating. By accurately regulating the potential using a removal method and a reference electrode, it is possible to remove the protruding portion of the base copper plating and finish the silver plating surface to the desired appearance. The purpose is to provide a finishing method.

The present invention utilizes the difference in elution potential between silver and copper to selectively remove only the copper plating.

Many metals elute at potentials more positive than their elution potential in a suitable electrolyte.

Moreover, this elution potential differs depending on the type of metal in the same solution. It was found that in a cyanide solution, the elution potential of silver is more positive than the elution potential of copper, and at a potential between the elution potential of copper and the elution potential of silver, only copper is eluted and no silver is eluted. With this, by setting a potential between the elution potentials of silver and copper and performing stripping, only the copper plating outside the desired area can be selectively removed.

FIG. 1 shows the measurement results of the current-voltage curves of silver and copper.

Using KCN5Oy'e stripping solution, attach a platinum electrode (1.5
cTi), copper and silver (both 1c1t) were used as the anode, and a saturated calomel electrode was used as the reference electrode. The liquid temperature is 25℃,
For both silver and copper, the potential was swept in the positive potential direction at a rate of 240 sec 1 V from the natural potential, and from +1.5 ■, the natural potential was swept in the negative potential direction. For silver electrodes, the elution potential is approximately -
The voltage is 0.7 V, and at a more positive potential, silver is removed.
The copper electrode starts to peel off at about -1.2V. Therefore, it has been found that if the potential is set between -0.7 and -1.2 .mu., only the copper can be peeled off. This was confirmed by the following example. Four crucibles of 2G x 2C7n were pretreated, the entire surface was plated with copper to a thickness of about 2μml, and then the entire surface was plated with copper to a thickness of about 10μm and an area of about 0.4c1.
A sample with f partial silver plating was made.

This sample was used as an anode, set in the above-mentioned device, and KC
Stripping was performed in a stripping solution of N5OyIl with the potential set at 1.0 . As a result, only the copper plating was removed cleanly without any change in the silver plating surface. FIG. 2 shows the relationship between current and time during peeling, and the current value becomes 0 in about 4 minutes and no current flows. This indicates that the stripping of the copper is completely completed, indicating that the stripping is complete and the silver is unaffected and does not react.
In this way, the three-electrode potential regulation makes it possible to strip only the copper without affecting the silver. Furthermore, since the KCN concentration is approximately proportional to the current value, if stripping is required in a short time, it can be achieved by increasing the KCN concentration. In this example, KCN was used alone as the stripping solution, but various additives may be added, or stripping solutions other than KCN that exhibit similar properties may be used.

According to experiments, the use of KCN alone resulted in the best peeling. As mentioned above, it is not only possible to selectively strip only the copper plating by regulating the potential, but also by precisely controlling the stripping potential using a reference electrode, it is possible to set the set potential to be 7 points higher than the silver elution potential. Both silver plating and copper plating can be removed by applying a potential, and it is possible to freely and easily set whether the silver plating should be removed a little or more.

Furthermore, the appearance of the peeled surface depends on the electrical potential.

For example, silver has no luster between the natural potential and 0V, and 0V.
From (2) to 1.5V, it becomes glossy, and when it is 1.5V or more, it becomes matte. By regulating the potential in this manner, the silver-plated surface can be easily finished to either matte or glossy finish. Examples are shown below.

Example 1 Electrolytic stripper composition KCN5OyIf Electrolytic tank 50cm x 5cm x 5α Anode SUS43O Temperature 25°C Set potential -1.0VVSS. C. E. A 40-pin lead frame with 0.3 μm copper plating on the entire surface under the above conditions and 3.0 pm partial silver plating within the specified potting area is 5R! 1. Imi
The electrolytic stripping operation was performed continuously by moving at a speed of n.

The appearance of this Yang silver did not change at all, and only the unnecessary parts of the copper were peeled off. Example 2 Electrolytic stripper NaCN5OyIe. ．． NaOH5Oy'e
Setting potential -0.6VVSS. C. E. By this peeling operation, which was otherwise the same as in Example 1, not only was the copper completely removed, but the bright silver-plated surface was also uniformly peeled off and became a moderately glossy surface.

Although the above description has been made using lead frames as an example, it goes without saying that the present invention is not limited to these.

As described above, according to the method of the present invention, the underlying copper plating protruding from the silver-plated area can be selectively and efficiently removed, and the gloss of the silver-plated surface can also be improved by regulating the potential. It has the remarkable effect of being able to be controlled arbitrarily. Although the present invention has been variously explained above with reference to preferred embodiments, the present invention is not limited to these embodiments, and it goes without saying that many modifications can be made without departing from the scope of the invention. be.

[Brief explanation of the drawing]

FIG. 1 shows the cyclic voltammograms of silver and copper.

Claims (1)

[Claims] 1. When partial silver plating is applied to copper plating as a base, in order to peel off the protruding portion of the base copper plating, a reference electrode is used to accurately regulate the potential and the copper plating is eluted. A method for removing protruding portions of base copper plating in partial silver plating, characterized by carrying out anodic electrolysis in a potential range in which silver plating does not elute. 2. In the case where partial silver plating is performed using copper plating as a base, the potential is accurately regulated using a reference electrode in order to peel off the protruding portion of the base copper plating and to peel off the copper plated surface to a desired appearance. A method for raising the surface level of silver plating in partial silver plating, characterized by carrying out anodic electrolysis in a potential range in which copper plating is eluted and silver plating is moderately eluted.