Tin whisker-free tin or tin alloy plated article and coating technique thereof

Abstract: A tin or tin alloy plated article, particularly a film carrier which is protected against the generation of tin whiskers characterized in having an indium plated layer on the subsrtrate thereof and a tin or tin alloy plated layer on said indium plated layer. There are also disclosed a coating process therefor as well as a novel and unique electroless indium plating bath therefor, said indium plating bath is an acidic, electroless plating bath characterized by including an indium salt and thiourea or a derivative thereof.

Description: FIELD OF THE INVENTION This invention relates to a tin whisker-free, tin or tin alloy plated article. This invention concerns with ornaments, electrical or electronical parts, structual components etc. which need the protection of a tin or tin alloy plated surface against the generation of tin whiskers. This invention also relates to a coating or production process therefor. BACKGROUND OF THE INVENTION Tin or tin alloy plating or coating which is low in cost, is resistant in corrosion and also is good in appearance and further is excellent in solderability, is in wide use for the surface treatment or production of metal articles such as various ornaments, electrical or electronical parts typified by lead frames, printed circuit boards, contacting elements such as electrical contacts and terminals as well as structual members or parts of ferrous or ferrous alloy sheets or others. Recently, as a method of mounting elements such as semiconductor chips, a packaging system called a film carrier ( called also as a tape carrier ) has been developed. This film carrier system, as illustrated in Fig 1, comprises a continious length of tape 2 having two rows of sproket holes along and adjacent the opposite edges thereof. The tape 2 consists of a base of a polyimide, polyester, polyether sulfone ( PES ), poly-parabanic acid ( PPA ) or other resin and a copper foil bonded onto the base. The copper foil is formed with inner leads 3 ( chip bonding fingers ) and outer leads 4 ( fingers for external connections ) by photoeching. In the specification, micro patterns associated with the inner leads 3 and the outer leads 4 are collectively called " lead portion ". As shown in Fig. 1, around a device hole formed by punching a central portion of the base for mounting a semiconductor chip or the like, inner leads formed from the copper foil are arranged in a high density so as to partially project into the device hole. The individual leads sometimes have a width as narrow as several ten µm. In such fine patterns of the lead portion formed by etching the copper foil laminated on the film base as described above, tin or tin alloy plating is usually provided for the purpose of facilitating the bonding of the inner leads with a semiconductor chip ( bumps ) and of increasing the bonding strength therebetween and further for the purpose of enhancing the solderability of the outer leads. However, tin or tin alloy plated articles have a serious problem that tin whiskers ( tin acicular crystals ) are liable to generate from the plated surfaces which can mar the appearance of ornaments and can cause shortings of electrical parts and can impair structual parts or the like. To prevent this whisker generation, diverse methods have hitherto been proposed. They include, for example, (1) heat treatment after tin plating as described in Japanese Patent Application Publication No. 47995/1981, (2) alloy coating, namely co-deposition of tin with another metal such as lead, nickel, copper, bismuth, antimony etc. and (3) periodic reversal of the polarity of the positive and negative electrodes involved in the plating or the use of ultrasonic energy to minimize the hydrogen occlusion in the plated metal. These prior art approaches for preventing the generation of whiskers still have varied difficulties in practice. For example, the method (1) requires much time for the treatment and has an upper limitation of temperature to be applicable because heating conditions sometimes can melt or soften a plastic base to deform it. Also, in the case where the heating is conducted in air, a tin plated surface is oxidized, leading to decreased corrosion resistance and solderability of the surfaces of electronic parts. The method (2) has the possibilities of causing other problems or troubles due to decreased corrosion resistance and solderability or due to lowered electrical characteristics. The method (3) may be effective only when the whisker occurs by the influence of hydrogen occlusion, but is not effective when the whisker generation is caused by other factors. Furthermore a method for inhibiting tin whisker growth on a substrate material having a tin containing coating is known (US-A-4 749 626) in which an initial tin containing solution is prepared, a metal salt selected from the group consisting of palladium salts, silver salts, nickel salts, iron salts, cadmium salts, platinum salts, gold salts, indium salts, ruthenium salts, and cobalt salts is added to the initial solution in a concentration above the saturation point of the initial solution to form a metal salt saturated solution, and the metal salt saturated solution is applied to the substrate material. Further, the applicant previously succeeded in preventing the whisker generation in connection with thin or tin alloy plated film carriers by plating an indium layer onto a tin or tin alloy plated layer, and have obtained good fruits as described in European Patent Application No. 88 120 838 filed on Dec. 13,1988. However, there are many applications which require by all means that the exposed surface layer is a tin or tin alloy layer. Object of the Invention The present invention is aimed at the provision of a novel article having tin or tin alloy plating and its coating process wherein whisker generation may be prevented by easier and convenient means than in the above-mentioned methods (1) to (3) while maintaining the exposed outermost layer as tin or a tin alloy. Summary of the Invention The applicant has previously made intensive studies as to an undercoat of tin or tin alloy plating for reducing or relieving any internal stress produced at the time of tin or tin alloy plating which may be a primary factor for whisker generation. Unexpectedly, the applicant has now found that the use of indium is effective as undercoat for tin or tin alloy plating which is contrary to the previous applicant's proposal. This is the basis on which the present invention has developed. Based on this finding, the present invention provides a tin or tin alloy plated article protected against the generation of tin whiskers, said article having a substrate selected from the group consisting of iron and iron based alloys as well as copper and copper based alloys, an indium plated layer on said substrate and a tin or tin alloy plated layer on said indium plated layer. The present invention also provides a process for producing a tin or tin alloy plated article protected against the generation of tin whiskers, said article having a substrate selected from the group consisting of iron and iron based alloys as well as copper and copper based alloys, characterized in that indium is plated on said substrate by electroplating or electroless plating and then a tin or a tin alloy is plated on said indium plated layer by electroplating or electroless plating. DETAILED DESCRIPTION OF THE INVENTION The substrates to be used in the present invention are the same as those in conventional tin or tin alloy plated articles and the present invention is particularly suitable for the plating of metallic materials etc. including electrical or electronical components or parts such as film carriers for mounting semiconductor chips, printed circuit boards, lead frames, contacting elements such as contacts or terminals and plated structural members which demand good appearance and high operation reliability. Such substrates include metallic materials such as iron and iron based alloys such as Kovar®, 42 alloy, stainless steel etc. as well as copper and copper based alloy such as brass, red brass, nickel silver, phosphor bronze, zircon copper, titanium copper, tin containing copper etc. as well as composite materials having such metallic material laminated The thickness of each of plated layers is not specially limited, but generally the thickness of the indium plated layer is at least 0.01µm, preferably at least 0.05µm. If the indium layer is less than 0.01µm thick, its effect for preventing the generation of whiskers is inadequate. Its upper limit is not critical, but not more than 5µm is desirable since the adherence of the plated layer is liable to decrease when its thickness is beyond 5µm. Indium is costly and therefore it will be sufficient to have a thickness of not more than 1.0µm in practical use under consideration of its cost . The thickness of the tin or tin alloy plated layer is 0.1 to 1.0µm, preferably 0.5 to 0.7µm. If the thickness is less than 0.1µm, the solderability is impaired. A thickness in excess of 1.0µm is not particularly objectionable, but excessively thick plating is economically unwarranted and has a danger that the tin or tin alloy melts causing a short circuit when a bonding process is subsequently conducted. An electroplating bath for indium includes a sulfuric acid bath, fluoborate bath, sulfamic acid bath, methanesulfonic acid bath and others. An electroless plating bath for indium is either an alkaline or acidic bath. While the plating bath and plating procedure is not particularly limited in the present invention, it is recommended to use an acidic bath for a composite substrate using a resin easily corrosive to an alkali such as a film carrier. Typical indium electroplating and electroless plating baths are as follows: Ex. 1 : Indium electroplating bath Indium sulfate 10 - 25 g/ℓ Sodium sulfate 0 - 10 g/ℓ Temperature room temperature pH 2 - 2.7 Cathode current density 2 - 4 A/dm² Ex. 2 : Alkaline indium electroless plating bath Alkaline bath containing a sodium borohydride as a reducing agent Ex. 3 : Acidic indium electroless plating bath Hydrochloric acid 3 % Thiourea 100 g/ℓ Indium chloride 20 g/ℓ Temperature 60°C Ex. 4 : Acidic indium electroless plating bath sulfuric acid 1.5 % Thiourea 75 g/ℓ Indium sulfate 10 g/ℓ Temperature 60°C Among them, the electroless indium plating baths as shown in Ex. 3 and Ex. 4 are acidic and overcome the problems of conventional alkaline baths; they are excellent in the stability of the plating bath and are also high in deposition rate. They are suitable in an application where indium is plated on a copper surface of a film carrier having as a base a polyimide film which is weak to an alkali. Below the acidic indium electroless plating bath will be explained in more detail: Acidic indium electroless plating bath This plating bath is an acidic, electroless indium plating bath characterized by including an indium salt and thiourea or a derivative thereof. Unlike a conventionally used alkaline bath containing a sodium borohydride as a reducing agent, this has unique characteristics in that it is excellent in the stability of the plating bath, is high in deposition rate and is applicable to any parts including resin parts or the like which are poorly resistant to an alkali. The indium salts for use in this plating bath may be any compound of indium with an inorganic or organic acid which is in the form of a solution at a pH of 3.0 or less and include e.g. In₂(SO₄)₃, In(NO₃)₃, InCl₃, indium sulfamate or indium methanesulfonate. The concentration of the indium salt in the plating bath ranges from 1 to 50 g/ℓ , preferably from 10 to 20 g/ℓ . If the concentration is less than 1 g/ℓ , the deposition of indium does not take place, whereas at a concentration of more than 50 g/ℓ the deposition rate comes down although the plating itself is possible. Useful derivatives of thiourea for use in this plating bath include 2,4-dithiobiuret and thiosemicarbazide. The concentration of thiourea or its derivative in the plating bath is in the range of 50 to 200 g/ℓ , preferably 60 to 100 g/ℓ . With a concentration of less than 50 g/ℓ indium deposition does not occur, while on the other hand a higher concentration above 200 g/ℓ is not desirable because it forms white floating ( insoluble ) matters. This indium electroless plating bath has a pH of 3.0 or less, preferably 1.0 or less. When the pH exceeds 3.0, indium is transformed into a hydroxide, forming a precipitate. In view of this and also taking the deposition rate into consideration, a pH of 1.0 or less is preferred. The temperature of this plating bath is not critically limited, but from the standpoint of deposition rate it is preferable to adopt a temperature of 50 to 70 °C . This indium electroless plating bath performs an plating operation based on a replacement type reaction with the use of no reducing agent. The bath is so stable that it basically does not require the addition of a stabilizer or complexing agent. However, where necessary, for example, for the purpose of stabilizing indium, copper and other metal ions in the plating bath and of ensuring the uniform deposition, one or two more compounds such as mentioned below may be used: Examples of such compounds are aminocarboxylic acids such as ethylenediaminetetraacetic acid ( EDTA ) and derivatives thereof, nitrilotriacetic acid ( NTA ) and its derivatives etc., polycarboxylic acids such as tartaric acid and its derivatives, citric acid and its derivatives etc., organic compounds such as gelatine and surfactants such as polyalkylen glycols. This indium electroless plating bath is used for plating indium on a metal less noble than indium as well as copper and copper alloys and is applicable to a glass, plastic ceramic or the like by precoating its surface with copper or by preliminarily covering its surface with a copper foil etc. with any known technique. The functions of the plating operation with the use of this indium electroless plating bath will now be explained in connection with the case where copper or a copper alloy is the substrate: It will be generally thought that deposition of indium on copper or a copper alloy by a replacement reaction is theoretically impossible because of their ionization tendency. However, according to the principle of this plating bath, the use of thiourea or its derivative shifts the dissolution potential of copper toward the less-noble direction rendering it less noble than the deposition potential of indium. As the result, the progress of the replacement reactions may be effected as represented by the following formulas: Also, because the complex formation constant of thiourea and Cu²⁺ is high as indicated as follows and therefore thiourea forms a stable complex with the Cu²⁺ that has dissolved out by a replacement reaction with indium, the reverse reaction of the formula (3) can be conveniently inhibited. In addition to these actions, thiourea plays the role of a dissolution promotor for copper as indicated by the formula (3). Consequently, this plating bath has an advantage of remarkably high plating ( deposition ) rates about 4 to 10 times those of conventional alkaline plating baths. Returning back to the explanations of the present invention, a tin or tin alloy plating is formed on the indium plated layer thus produced. Various kinds of tin electroplating or electroless plating baths have been known and the present invention is not limited to specific plating baths, but may employ the following plating baths for example: Ex. 5 : Tin electroplating bath Tin sulfate 40 g/ℓ Sulfuric acid 100 g/ℓ Brightener AC ( commercially saled product ) Temperature 20°C Cathode current density 3 A/dm² Ex. 6 : Tin electroless plating bath " Tinposit LT-34 " ( manufactured by Shipley Co. ) Liquid temperature 50 °C Indium, with a low melting point ( 156°C ), easily forms an alloy with tin or a tin alloy. Where necessary, therefore, it is further effective for preventing the whisker generation to heat treat the plated layer at a temperature low enough not to produce oxides on a tin or tin alloy surface, namely 50 to 150°C . As described, it has been proofed that the platings of a substrate first with indium and then with tin or a tin alloy prevent the formation of whiskers. As to the reasons why the generation of whiskers can be prevented according to the present invention, it is considered that presumably the indium undercoat of the tin or tin alloy plating has changed to a material which does not induce the whisker generation and that the internal stress of the tin or tin alloy plated layer will be relaxed in view of the fact that indium and tin or tin alloy has diffused to each other to partially form an alloy. For the purpose of the present invention, the tin alloys to be used include Sn-Pb or other alloys which do not undergo any reduction in corrosion resistance or solderability. EXAMPLES The present invention is further illustrated by the following examples: Example 1 The indium electroplating bath of Ex. 1 was used. A 0.4 mm thick copper sheet was made a cathode and an indium sheet was used as an anode. Current was passed across the cathode and anode for one minute to form an indium plated film of 0.05 µm thickness. Next, using the tin electroplating bath of Ex. 5 with the use of a tin sheet as anode, current was passed for 3 minutes. A 0.65 µm thick tin plated film was formed on the indium undercoat. The specimen so obtained was allowed to stand at 40 °C and 95% RH for one month. No whisker growth was obserbed. Example 2 In the indium electroless plating bath of Ex. 3 was immersed a piece of 0.4 mm copper sheet for 5 minutes to form an indium plated film 0.5 µm thick. This indium-undercoated sheet was immersed in the electroless plating bath of Ex. 6 to form a tin plated film of 0.65 µm thickness. The specimen thus obtained was allowed to stand at 40 °C and 95% RH for one month. No whisker growth was obserbed. Example 3 A copper foil 35µm in thickness was laminated onto an ordinary 35 µm wide film of a polyimide as a base and etched to produce a film carrier having microcircuits with a lead portion as shown in Fig. 1. This film carrier was immersed in the indium electroless plating bath of Ex. 3 for 5 minutes to form a 0.5µm thick indium plated film on the lead portion. The film carrier was then immersed for 5 minutes in the above-mentioned electroless tin plating bath " Tinposit LT-34 " of Shipley Co. to form a 0.5 µm tin plated film over the indium plated film. The film carrier sample so obtained was kept at 40°C and 95% RH for one month. No whisker growth was observed. Comparative Example 1 Using the tin plating baths of Ex. 5 and Ex. 6, 0.65µm thick plated films were formed two copper sheets each having a thickness of 0.4 mm. After standing the sheets at 40 °C and 95% RH for one week, it was shown that whiskers were generated in a number of more than 100 per square millimeter. Comparative Example 2 A film carrier was made in the same way as described in Example 3 except that the indium plating was omitted. After standing the film carrier at 40 °C and 95% RH for one week, it was shown that whiskers were generated in a number of more than 100 per square millimeter. Advantages of the invention As explained above, according to the present invention, the generation of whiskers in tin or a tin alloy plated articles can be easily prevented and hence shorting or other whisker related troubles can be precluded while maintaining not only good appearance of the tin or tin alloy plated layer, but also good corrosion resistance and solderability. In particular, this invention can furnish tin or tin alloy coated articles including electro and electronic parts having beautiful appearance and high reliability.

Claims: A tin or tin alloy plated article protected against the generation of tin whiskers, said article having a substrate selected from the group consisting of iron and iron based alloys as well as copper and copper based alloys, an indium plated layer on said substrate and a tin or tin alloy plated layer on said indium plated layer. A tin or tin alloy plated article protected against the generation of tin whiskers of the claim 1 wherein said substrate is a film carrier for mounting thereon electronic components such as semiconductor chips or the like. A tin or tin alloy plated article protected against the generation of tin whiskers anyone of the claims 1 to 2 wherein the thickness of said indium plated layer is not less than 0.01 µm. A tin or tin alloy plated article protected against the generation of tin whiskers anyone of the claims 1 to 2 wherein the thickness of said indium plated layer is not less than 0.05 µm. A tin or tin alloy plated article protected against the generation of tin whiskers anyone of the claims 1 to 4 wherein the thickness of said tin or tin alloy plated layer is in the range of 0.1 to 1.0 µm. A tin or tin alloy plated article protected against the generation of tin whiskers anyone of the claims 1 to 4 wherein the thickness of said tin or tin alloy plated layer is in the range of 0.5 to 0.7 µm. A process for producing a tin or tin alloy plated article protected against the generation of tin whiskers, said article having a substrate selected from the group consisting of iron and iron based alloys as well as copper and copper based alloys, characterized in that indium is plated on said substrate by electroplating or electroless plating and then a tin or a tin alloy is plated on said indium plated layer by electroplating or electroless plating. A process of the claim 7 wherein said substrate is a film carrier for mounting thereon electronic components such as semiconductor chips or the like. A process of the claim 8 wherein indium is plated on the substrate by electroless plating and then tin or a tin alloy is plated on said indium plated layer by electroless plating. A process of anyone of the claims 7 to 9 wherein the thickness of said indium plated layer is not less than 0.01 µm. A process of anyone of the claims 7 to 9 wherein the thickness of said indium plated layer is not less than 0.05 µm. A process of anyone of the claims 7 to 11 wherein the thickness of said tin or tin alloy plated layer is in the range of 0.1 to 1.0 µm. A process of anyone of the claims 7 to 11 wherein the thickness of said tin or tin alloy plated layer is in the range of 0.5 to 0.7 µm. A process of anyone of the claims 7 to 13 wherein after said tin or tin alloy plating, heat treatment is conducted at 50 to 150°C . A process of anyone of the claims 7 to 14 wherein said indium plating is conducted using an acidic, electroless plating bath including an indium salt and thiourea or a derivative thereof. A process of the claim 15 wherein said indium plating is conducted using an acidic, electroless plating bath including 1 to 50 g/ℓ of an indium salt and 50 to 200 g/ℓ of thiourea or a derivative thereof and having a pH of 3 or less. A process of the claim 16 wherein said indium plating is conducted using an acidic, electroless plating bath including 10 to 20 g/ℓ of an indium salt and 60 to 100 g/ℓ of thiourea or a derivative thereof and having a pH of 1 or less.

Inventors: SHIMAUCHI HIDENORI NIKKO METAL, SUZUKI KEIJIRO, SHIMAUCHI, HIDENORI NIKKO METAL PLATING CO.,LTD., SUZUKI, KEIJIRO
Applicants: NIPPON MINING CO, NIPPON MINING COMPANY LIMITED
Application year: 1989
Main classification: C25D5
IPC codes: H05K3, H01L23, C23C18, C25D5