JP4986141B2 - Method for suppressing tin-plated needle whiskers - Google Patents

Description

  The present invention relates to a method for suppressing the occurrence of tin-plated needle-like whiskers generated from a connector or a terminal, which is an electric / electronic component to which tin or a metal plating mainly composed of tin is applied.

When tin plating (including solder plating) that does not contain lead (specific hazardous substances) is applied to mounted parts of electrical and electronic equipment, tin whisker is generated from the plated part, and even if it is not recognized at the initial stage of plating, time passes The phenomenon that occurs and grows with it. When this tin plating is used for connectors and terminals, whiskers that grow in the shape of needles may cause a serious failure such as short circuit of the electric circuit due to contact between the electrodes, and this prediction is important. . Against this background, the Japan Electronics and Information Technology Association (JEITA) is standardizing electronic device whisker test methods, and drafting JIS drafts and international proposals for the International Electrotechnical Commission (IEC) are planned.
Research institutions and companies in Japan, the United States, and Europe are working on elucidation and suppression methods of this occurrence mechanism, but none of them are international urgent issues that no fundamental solution has been found.

Until now, it has been known that acicular whiskers are hardly generated from the above-described parts plated with tin containing lead.
However, in recent years, as interest in global environmental issues has increased, there has been an increasing demand for reducing the use of lead.
From such a background, it is an urgent need to use a tin-plated component that does not contain lead, but a method that can completely prevent the occurrence of the above whiskers has not yet been disclosed.

Therefore, as a method for suppressing the whisker, a method of using nickel plating on the plating base and using tin plating containing bismuth on the surface layer (see Patent Document 1), a tin alloy component content ratio in the film thickness direction of the plating film A method of forming a tin alloy plating film by giving a concentration gradient to the alloy component so that the amount of the alloy increases (refer to Patent Document 2), surface treatment agent for tin plating surface treatment agent containing benzimidazole compound and salt thereof (patent Reference 3), plating solution for producing tin or tin alloy plating film contains sodium saccharin, current density at the time of plating of 15 mA / cm ² or more, and cathode potential with respect to saturated calomel electrode (SCE) A method of controlling to 900 mV or more (see Patent Document 4), as an additive for tin or tin alloy plating solution, bisphenol A ethylene oxide adduct, How to use those containing at least one selected from the group of phenol A propylene oxide adduct and bisphenol F propylene oxide adduct (see Patent Document 5) are disclosed.
Moreover, when manufacturing a film carrier having copper or the like as a conductor pattern, the film carrier in which whiskers are generated after plating is irradiated with ultrasonic waves for 15 seconds in purified water or while being heated, and the whiskers are cut and removed from the root. Furthermore, a method for drying and annealing the film carrier (see Patent Document 6) is disclosed.

Although the method as described above is disclosed, whisker generation has not been completely prevented even in tin plating containing bismuth or silver, which is supposed to suppress whisker generation, as well as pure tin plating not containing lead. The fact that there is nothing is as well known by the JEITA / Tin Whisker Control Technology Research Committee mentioned above.
This fact is known as a product developed by Panasonic Shikoku Electronics Co., Ltd. by optimizing the silver content and optimizing the plating film thickness (about 1 μm), but whiskers with a length of 10 μm or less are generated. It is supposed to be.

  Further, the method of adding various adducts as an additive for the plating solution has a problem that the generation of the whisker cannot be completely suppressed in a part having a plating film thickness exceeding 2 μm.

  As described above, any of the above-described methods is difficult to adopt as a common whisker suppressing method for plating parts of connectors and terminals, which are various types of general-purpose electronic components.

As a method similar to the present invention, there is a “film carrier whisker removal method and apparatus” shown in Patent Document 6.
Although not disclosed in the patent, whiskers generated and grown after the plating process are to be removed by low-frequency ultrasonic vibration. Further, it is assumed that annealing heat treatment is subsequently performed as a supplementary method for suppressing the generation of slow-growing whiskers.
However, even if the grown whiskers are completely removed by other methods, for example, it has been confirmed by the inventors that a long time elapses and new whiskers are generated and grown.
In addition, although the annealing heat treatment temperature is not disclosed, it is predicted from the report result of the “Whisker Test Method Subcommittee” of the Japan Electronics and Information Technology Association (JEITA) that the generation of new whiskers cannot be completely suppressed by the heat treatment. .
JP 2006-319288 A JP 2000-174191 A Japanese Patent Laid-Open No. 2004-156094 JP 2006-322014 A JP 2002-275678 A Japanese Patent Laid-Open No. 06-77291

  It is an object of the present invention to provide a method for suppressing the occurrence of tin-plated needle-like whiskers, particularly in connectors and terminals of electrical and electronic parts that have undergone pure tin plating, semiconductor lead frames, and the like.

  The present invention is a post-treatment method for electrical / electronic parts, in particular, tin-based metal plating that does not contain lead, especially pure tin plating, and whisker after tin plating / processing is still present. It is characterized in that generation of tin-plated needle-like whiskers is suppressed by irradiating the plated part with ultrasonic waves in a solution at a non-existing stage (latent period). In other words, the cause of whiskers is that tin atoms are pushed out of the plating film in an oxidizing atmosphere and grow as a single crystal due to the release of stress generated in the plating film during the plating process and the subsequent processing process. It is considered a thing. Since there is a certain latent period before the metal is pushed out of the plating film, ultrasonic irradiation is performed during this latent period.

  In the present invention, as a reaction field for ultrasonic irradiation, in a solution containing a polyol (compound containing two or more hydroxyl groups) that forms a reduction reaction field in a solution in which a reducing agent is dissolved or a chemical reaction by ultrasonic irradiation. It is characterized by ultrasonic irradiation.

The present invention is a method for suppressing the occurrence of tin-plated needle-like whiskers from the surface of a metal-plated film mainly containing tin and not containing lead. Immediately after external stress due to metal plating treatment, cutting / bending processing of electrical / electronic parts is applied, any one of citric acid, oxalic acid or its salts, glycerin as a reducing agent, or a mixed solution of these in an appropriate amount By irradiating with ultrasonic waves, it is possible to suppress the generation of whiskers that are generated and grow with the passage of time from the surface of the plating film.
Similarly, it is possible to suppress the generation of whiskers from the semiconductor lead frame, which is said to generate needle-like whiskers due to the internal stress of the plating film.
Although all of this phenomenon has not been clarified, by irradiating the liquid with ultrasonic waves, bubbles are generated, compressed, and collapsed, called cavitation, resulting in a high-temperature reaction field of several thousand degrees and several thousand atmospheres. The phenomenon that is formed is a known fact.
It is presumed that, due to this phenomenon, the stress concentration remaining on the plating film is dispersed / relaxed in the reduction reaction field, thereby preventing the growth of needle-like whiskers.
The reduction reaction field by glycerin is presumed that a reducing atmosphere is formed by hydrogen generated by decomposing a polyol by ultrasonic irradiation.
In addition, it has been confirmed by the inventors that the generation of acicular whiskers is promoted in an oxidizing atmosphere.
Therefore, it is a necessary condition to irradiate ultrasonic waves in the reduction reaction field.

According to the present invention, it becomes possible to prevent the whisker from being generated without adding a rare metal such as bismuth as in the prior art and without going through a complicated production process.
Also, connector terminals or FSC (Flexible Printed Circuit / FFC (Flexible Flat Cable)) that are said to be caused by external stress of parts, whiskers that are caused by semiconductor lead frame that is said to be caused by internal stress, etc. It is possible to deal with whiskers generated in various forms.

Hereinafter, the best mode for carrying out the method for suppressing the occurrence of tin-plated needle whiskers of the present invention will be described with reference to the drawings.
As a method for suppressing the occurrence of needle-like whiskers generated from the surface of the plating film of the electric / electronic component according to the present invention, a connector terminal will be described as an example with reference to the ultrasonic irradiation apparatus of FIG.
DESCRIPTION OF SYMBOLS 1 is an ultrasonic irradiation apparatus, 2 is a connector terminal of an electrical / electronic component, 3 is a water tank, 4 is an ultrasonic vibrator, 5 is water, 6 is a storage tank, 7 is a solution or polyol in which a reducing agent is dissolved It is a liquid containing.
First, the connector terminal 2 of an electrical / electronic component that has been subjected to cutting or bending after the plating process is housed in the ultrasonic irradiation device 1 and irradiated with ultrasonic waves for a predetermined time. In this ultrasonic irradiation apparatus 1, an ultrasonic vibrator 4 is installed on the lower surface of a water tank 3, the water tank 3 is filled with water 5, and a storage tank 6 for housing a connector terminal 2 is the ultrasonic vibrator. 4 so as to be adjacent to 4.
The storage tank 6 is filled with a solution 7 in which a reducing agent is dissolved or a liquid 7 containing a polyol, and the connector terminal 2 is stored in the storage tank 6.
In this state, the ultrasonic transducer 4 is operated, and ultrasonic waves are irradiated into the liquid 7 to generate, compress, and collapse bubbles called cavitation, resulting in a high temperature of several thousand degrees and several thousand atmospheres. The phenomenon that the reaction field is formed is a known fact.
It is presumed that, due to this phenomenon, the stress concentration remaining on the plating film of the connector terminal 2 is dispersed / relaxed in the reduction reaction field, thereby preventing the growth of needle-like whiskers.

When tin-plated electronic components to which residual stress has been applied in a short heating / cooling process are intermittently irradiated in glycerin at an ultrasonic frequency of 40 kHz for a total of 90 minutes and then left in an oxidizing atmosphere for 40 hours FIG. 2 shows a state where the tin plating surface is observed with a scanning electron microscope. As shown in the photograph, massive tin whiskers are detected, but it is clear that the generation of needle-like whiskers is suppressed and the object is achieved.
On the other hand, when the ultrasonic wave irradiation of the present invention is not performed and left in the atmosphere in the same manner as described above, a large amount of needle whiskers are generated as shown in FIG.
Incidentally, the lead-containing metal plating that has been conventionally used does not generate whiskers, but does not cause a short-circuit failure due to the whisker shape being massive.

Immediately after applying a bending stress to a 0.45 mm diameter wire sample with a pure tin plating on a copper plating base, the sample was left in the atmosphere for 24 hours after irradiating with ultrasonic waves in glycerin at a frequency of 40 kHz for 60 minutes. The results observed with are shown in FIG.
Similarly, FIG. 5 shows the result of observation with a light microscope of a sample that was continuously irradiated with ultrasonic waves in a 0.4 weight percent citric acid aqueous solution (purified water) at a frequency of 40 kHz for 60 minutes and then left for 24 hours. In any case, it has been clarified that the generation of needle-like whiskers is suppressed and the object is achieved.
On the other hand, when the ultrasonic wave irradiation of the present invention is not performed and the apparatus is left in the atmosphere in the same manner as described above, a long needle-like whisker is generated as shown in FIG.

It is explanatory drawing of the ultrasonic irradiation apparatus which enforces the method of suppressing generation | occurrence | production of the acicular whisker of this invention. It is a scanning electron micrograph of the massive whisker generated by heating and cooling for a short time. It is a scanning electron micrograph of a needle-like whisker generated by conventional heating and cooling for a short time. It is the photograph which gave the bending stress to the pure tin plating test material and was observed with the optical microscope after ultrasonic irradiation in glycerin and leaving to air | atmosphere. It is the photograph observed with the optical microscope after giving a bending stress to a pure tin plating test material and irradiating with ultrasonic waves in a citric acid solution and leaving in the atmosphere. It is an optical micrograph of the acicular whisker which generate | occur | produced after giving bending stress to the conventional pure tin plating test material, and leaving to air | atmosphere.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ultrasonic irradiation apparatus 2 Connector terminal 3 Water tank 4 Ultrasonic vibrator 5 Water 6 Storage tank 7 Solution

Claims (3)

A post-treatment method for an electric / electronic component on which tin or tin-based metal plating containing no lead is applied, wherein the metal plating portion of the electric / electronic component is made of citric acid, oxalic acid or salts thereof, or A method of suppressing the generation of needle-like whiskers of metal plating containing no tin or lead mainly containing tin by irradiating ultrasonically in a solution containing a polyol or a mixture thereof . The lead containing tin or tin as a main component according to claim 1, wherein a solution containing glycerin is used as a solution containing citric acid, oxalic acid or salts thereof, polyol or a mixture thereof. To prevent the occurrence of metal-plated needle whiskers. Examples ultrasonic irradiation method, according to claim 1 or 2, characterized in that ultrasonic irradiation during the incubation period of up whiskers after metal plating not containing lead composed mainly of tin or tin appears Of suppressing the generation of needle-like whiskers of metal plating containing no tin or tin-based metal containing lead .

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