JP4571062B2 - Reflow Sn or Sn alloy plating strip and electronic parts - Google Patents

Description

The present invention relates to a surface treatment material obtained by reflow Sn or Sn alloy plating of copper or copper alloy, and in particular, a surface treatment material of copper or copper alloy that can be used for electronic parts such as connectors, terminals, switches, and lead frames, Furthermore, the present invention relates to an electronic component using the surface treatment material.

In general, copper or copper alloy is used as a base material for electronic parts such as connectors and terminals used in various electronic devices such as automobiles, home appliances, OA equipment, etc., and these include rust prevention, corrosion resistance improvement, and electrical property improvement. Plating treatment is performed for the purpose of functional improvement. There are various types of plating such as Au, Ag, Cu, Sn, Ni, Sn-Pb (solder), and Pd. Especially, Sn or Sn alloy plating is a connector, from the viewpoint of cost, contact reliability, solderability, etc. Widely used for terminals, switches, outer lead portions of lead frames, and the like Sn-Pb (solder) plating has been widely used as Sn or Sn alloy plating in the past, but since the use of Pb (lead) is scheduled to be regulated, Sn, Sn-Cu as an alternative to solder plating In recent years, research on Pb-free plating mainly composed of Sn, such as Sn-Bi and Sn-Ag plating, has been actively carried out.
However, since the Pb-free plating does not contain Pb that suppresses the generation of whiskers, there is a problem that whiskers are likely to occur, and among them, Sn plating tends to generate whiskers.

Whisker is a growth of Sn needle crystals, but in some cases, a whisker-like crystal structure grows to several tens of μm and may cause an electrical short circuit. This whisker phenomenon is caused by Sn recrystallization and is said to be a phenomenon that grows due to compressive stress acting on the plating film. Pb-free is applied to terminals and connectors that tend to concentrate stress on contacts, especially FPC connectors. In the case of plating, the whisker problem becomes more serious. The compressive stress acting on the plating film is the internal stress of plating, the diffusion of Cu ₆ Sn ₅ , the oxidation of Sn, the expansion and contraction of the base material, the stress generated by the product shape, and the external stress received from the contact Two types of stress have been pointed out.
In order to control the occurrence of the whisker phenomenon as described above, techniques such as a method by improving the plating bath and a heat treatment method have been proposed so far.

For example, in Japanese Examined Patent Publication No. 59-15993, whisker is difficult to generate Sn plating, in which a solution of hydroxyethane is used for a bath in which stannous chloride and stannous sulfate are the main components and the bath pH is neutral with caustic soda or phosphoric acid. Baths with added phosphate esters have been proposed.
Also, Toshihisa Hara and Motohiko Suzuki, “Copper Alloy Sheet with Tin Plating”, Kobe Steel Technical Report, April 2004, Vol. 54, no. 1 and p11-12, it is reported that reflow Sn plating relieves internal stress and suppresses the generation of whiskers.
Although not related to whiskers, a technique for improving the pressability by reducing the carbon concentration in the plating film to 0.01% by weight or less has been reported (Patent Application No. S6331891).

Japanese Patent Publication No.59-15993 Patent application number S6331891 Toshihisa Hara and Motohiko Suzuki, “Copper alloy strip with tin plating”, Kobe Steel Technical Report, April 2004, Vol. 54, no. 1, p11-12

Elucidation of the generation mechanism that is the basis for the development of whisker suppression technology is still in progress. The Japan Electronics Industry Association (JEITA), the American Electronics Manufacturers Association (NEMI) Tin Technology's Soldering Technology Center (SOLDERTEC) has just agreed in 2003 to elucidate the mechanism of whisker growth and establish standardization of whisker test methods.
For this reason, the background of the whisker problem exemplified above shows only one aspect of the whisker suppression problem, and there are many difficult aspects to solve the whisker problem. For example, the Sn, Sn-Cu, Sn-Bi, and Sn-Ag platings exemplified above have their merits and demerits, so it is most effective as an alternative to solder plating, including whisker measures. The current situation is that there is no fixed direction.

In addition to the above-mentioned technologies for suppressing whiskers, in addition to those described above, technologies such as formation of diffusion barriers with Ni or Ag underlayers, flash plating of Au, Pd or Ag, and organic coating treatments with heat-resistant preflux, etc. were also included. Because of the wide variety of possibilities, it is quite difficult to focus on the development of whisker suppression technology.
In spite of the current situation as described above, the advanced functionality and downsizing of information equipment associated with the rapid development of IT has inevitably urged further improvement of whisker suppression technology. Development is required. If whisker suppression technology that can be implemented by a simple method with little new capital investment is provided, it will contribute to the development of the industry.
Therefore, the main problem of the present invention is a surface treatment method for whisker suppression that can be carried out easily and relatively inexpensively for copper or copper alloy that can be used for electronic parts such as connectors, terminals, switches, and lead frames, among others. It is providing the Sn or Sn alloy plating strip which gave, and an electronic component using the same.

  In order to develop a technique for suppressing the whisker generated by the complicated mechanism as described above, the present inventor has conducted extensive research and found that when a reflow Sn or Sn alloy plating strip and an electronic component are manufactured, The present inventors have found that whisker generation can be suppressed if the residual stress is produced in a specific range.

This invention is completed based on the said knowledge, and is specified by the following.
(1) Low whisker characterized in that the residual stress in the reflow Sn or Sn alloy plating film applied to the surface of copper or copper alloy strip is a tensile stress, and the magnitude thereof is 0.1 MPa or more and 50 MPa or less Reflow Sn or Sn alloy plating strip.
(2) Low whisker reflow Sn according to (1) above, wherein the carbon concentration in the reflow Sn or Sn alloy plating film applied to the surface of the copper or copper alloy strip is 0.01% by mass or less. Or Sn alloy plating strip.
(3) An electronic component using the low whisker reflow Sn or Sn alloy plating strip described in (1) or (2) above.
In the semi-invention, the reflow Sn plating film refers to the pure Sn layer remaining after reflow, and the reflow Sn alloy plating film refers to the Sn alloy layer remaining after reflow. The Sn alloy layer does not include an Sn alloy layer formed of base plating or a base material element. Electronic parts mean parts such as connectors, terminals, switches, and lead frames.

  According to the present invention, whiskers are hardly generated and the length of the generated whiskers is short. And since it does not require the large remodeling with respect to the existing Sn plating equipment, it is a whisker suppression technique which can be implemented simply and at a relatively low cost.

The reason for the limitation of the present invention will be described below.
Plating Step In the plating method according to the present invention, Sn or Sn alloy plating is performed after applying a base plating to a part or the whole of the surface of a metal such as copper or a copper alloy or without a base plating, and then reflowing, Cooling process is performed. The area to be plated may be determined as desired. For example, the entire surface of copper or copper alloy in various forms (including the form of terminals) may be plated, or only one side or a partial plating may be applied. Also good.
Moreover, in this invention, there is no restriction | limiting regarding base plating, Cu base plating, Ni base plating, etc. can be selected arbitrarily. The base plating can be carried out by a method known per se. For example, in the case of Cu base plating, a sulfuric acid bath containing sulfuric acid and copper sulfate is used as a plating bath, and in the case of Ni base plating, nickel sulfate or the like is used. A watt bath as a component can be used.

  By the way, the whisker which is a problem in the industrial world is a type of whisker that is generated particularly when an external stress received from a contact acts on Sn-based plating. It is said that when an external stress is applied to the Sn or Sn alloy plating, a compressive stress is generated in the film, which promotes diffusion of Sn in the plating, and as a result, whiskers are more likely to occur. That is, reducing the external stress acting on the plating film is an effective measure in terms of whisker prevention.

  It is an object of the present invention to produce a residual stress in the Sn or Sn alloy plating film after reflowing as a tensile stress and to have a magnitude of 0.1 MPa to 50 MPa. The reason why the residual stress of the Sn or Sn alloy plating strip is set to the tensile stress in advance is that even if an external stress is applied in the state of the electronic component, it is offset by the tensile stress and the compressive stress is not easily generated in the plating film. Because. When the tensile stress in the film is less than 0.1 MPa, compressive stress is generated in the film due to external stress, and whiskers are likely to be generated. When the tensile stress exceeds 50 MPa, the whisker suppressing effect is saturated, Problems occur, such as warpage of the plated material.

  In addition, as a means of measuring the residual stress of the plating film, a plating film residual stress measurement method using a test strip (plating on one side of a thin metal plate, measuring the amount of warpage of the plate after plating, and measuring the residual stress) There is a plating film residual stress measurement method by an X-ray diffraction method.

In order to obtain the residual stress of plating defined in the present invention, it is necessary to control the plating conditions of Sn or Sn alloy plating and the reflow and cooling conditions described later.
Sn or Sn alloy plating can be performed by a method known per se, but the addition amount of an organic compound used as a brightener is reduced, or an organic compound that is difficult to be adsorbed on Sn or Sn alloy is used. Must. The reason for this is that when an organic compound is taken into Sn or Sn alloy plating, an organic compound having a size larger than Sn atoms enters between Sn atoms in the plating film, so that the residual stress in the plating film becomes compressive stress. Because it is easy to become. The amount of the organic compound adsorbed in the plating film can be evaluated by the carbon concentration in the plating film. Although the organic compound in the plating film changes in the reflow process, the carbon concentration in the plating film is finally 0.01% by mass or less, and more preferably 0.006% by mass or less.

Examples of the Sn or Sn alloy plating solution include an organic acid bath such as a phenol sulfonic acid bath, an alkane sulfonic acid bath, and an alkanol sulfonic acid bath, an acid bath such as a borofluoric acid bath, a halogen bath, a sulfuric acid bath, and a pyrophosphoric acid bath, Alternatively, electroplating can be performed using an alkaline bath such as a potassium bath or a sodium bath. As the organic compound to be added to the plating solution, a nonionic surfactant is preferable, and an aldehyde-based or carboxylic acid-based organic compound is adsorbed on Sn or Sn alloy plating and is easily taken into the plating, so it should not be used. Good.
The thickness of the Sn plating is usually 0.5 to 5.0 μm, preferably 0.7 to 2.0 μm, for reasons of appearance and plating cost.

Reflow and cooling step In the present invention, surface plating of Sn or Sn alloy plating is performed on the base plating, and then reflow (heating, melting) treatment and cooling treatment are performed to obtain reflow Sn or Sn alloy plating.
In the reflow process, the plating material is heated to a temperature equal to or higher than the melting point of Sn or Sn alloy, that is, 250 to 600 ° C., preferably 300 to 500 ° C., more preferably 350 to 450 ° C. The Sn or Sn alloy is melted for a moment by applying a heat treatment for -40 seconds, preferably 5-30 seconds, more preferably 5-20 seconds. Heating is preferably performed in a reducing atmosphere or an inert atmosphere condition. In the reflow process, since the organic compound taken in the plating film is gasified by heating and goes out of the plating, the organic compound in the plating film decreases and the carbon concentration decreases. As described above, the carbon concentration in the plating film is preferably 0.01% by mass or less, and more preferably 0.006% by mass or less.

  In the cooling step, the Sn or Sn alloy plating material heated in the reflow step is immediately cooled, but the cooling is preferably rapid cooling. This is because the rapid cooling causes a tensile stress larger than the residual stress in the Sn or Sn alloy film. Although cooling conditions are performed by immersion in a water tank immediately after the reflow process, the temperature of the water tank is preferably about 40 ° C. to 70 ° C. in consideration of plating residual stress and plating appearance.

  As described above, a material plated on copper or a copper alloy strip by the method of the present invention, or a terminal obtained by pressing the plated strip has excellent whisker resistance (low whisker property). On the other hand, an electronic component such as a plated terminal obtained by applying reflow Sn plating to a terminal obtained by pressing copper or a copper alloy strip by the method of the present invention also has low whisker properties.

(1) Preparation of reflow Sn or Sn alloy plating sample A phosphor bronze plate having a width of 50 mm, a length of 100 mm, and a thickness of 0.1 mm was used as it was when no base plating was used, and when a base plating was used, a sulfuric acid bath was used. The base of copper plating was performed, and Sn or Sn alloy plating was performed thereon. Based on methanesulfonic acid and methanesulfonic acid, a plating solution in which the addition amount of an organic compound such as a surfactant or an oxide of a metal serving as an alloy in the case of Sn alloy plating was changed was used.
The sample after Sn or Sn alloy plating was reflowed and cooled under the conditions shown in Table 1.
Table 1 shows the samples used in each example.

(2) Measurement method (a) Method for measuring residual stress in plating film The residual stress in the plating film was measured using a plating film residual stress measurement method by an X-ray diffraction method. RINT2500 made by Rigaku was used as a measuring device, an X-ray tube having a cobalt target was used, and the conditions were a collimator diameter of 100 μm, a tube voltage of 30 kV, and a tube current of 100 mA.

(B) Carbon concentration analysis method in plating film The high-frequency induction heating infrared absorption method was used to measure the carbon concentration in the plating film. The plating sample is heated and melted in an oxygen atmosphere, and the carbon in the sample reacts with oxygen in the atmosphere. It is calculated by measuring the carbon dioxide concentration in the atmosphere.

(C) Whisker Evaluation Method As shown in FIG. 1, whisker evaluation is performed by bringing an indenter (a stainless steel ball having a diameter of 1.4 mm) into contact with the plated surface of the Sn-plated sample and applying a load of 1.5 N. The sample was left in an air atmosphere at room temperature for 168 hours, and a sample was taken out and its surface was observed with an SEM.
The average whisker length is taken by SEM at the magnification of 1000 to 2000 times near the center of the sample surface for each sample (see Fig. 2 and Fig. 3), and the three longest whiskers in the photograph are taken. The average value was selected.

(3) Evaluation Results of Each Sample (Example, Comparative Example) Table 2 shows the evaluation results of each sample.

* 1: A positive value indicates tension, and a negative value indicates compression.
Invention Example No. In Nos. 1 to 7, the average whisker length was as short as less than 10 μm, and the whisker resistance was good.

Comparative Example No. No. 8 is Sn plating when not reflowing, the residual stress in the Sn plating is compressed, and the whisker is longer than the example of the present invention.
Comparative Example No. 9 is the case of Sn—Cu alloy plating without reflow, the residual stress is compression, and the carbon concentration in the film is high. The whisker is longer than the example of the present invention.
Comparative Example No. No. 10 is a case of Sn—Cu alloy plating without reflow, and the whisker length was long because the residual stress was compression.

Comparative Example No. 11 is a sample prepared by changing the reflow cooling condition of the reflow Sn-2% Cu plating, but the whisker becomes longer because the residual stress is compression.
Comparative Example No. No. 12 is the case of flow Sn-10% Cu plating, but the whisker length was long because the residual stress was compression due to the large amount of organic compound added.

It is a schematic diagram of the apparatus which presses a stainless steel ball on the plating surface, generates a whisker, and evaluates a whisker generation situation. It is a microscope picture of the whisker which generate | occur | produced from the reflow Sn plating sample (invention example). It is a microscope picture of the whisker which generate | occur | produced from the Sn plating sample without a reflow (comparative example).

Claims (3)

The residual stress in the reflow Sn or Sn alloy plating film applied to the surface of the copper or copper alloy strip is the tensile stress, and the magnitude is 0. 5 MPa or more 50MPa Ri der hereinafter characteristics and to Brighter flow of Der Rukoto less than 10μm average length of whiskers Sn or Sn alloy plating strip. Reflow Sn or Sn alloy plating strip of claim 1 in which copper or carbon concentration in the reflow Sn or Sn alloy plating film which has been subjected to the surface of the copper alloy strips is equal to or less than 0.01 mass%. Electronic component using the reflow Sn or Sn alloy plating strip according to claim 1 or claim 2.