JP2007284762A - Method for forming tin-plated film, and semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a mono-component tin-plated film free from lead while preventing a whisker from forming on the film and simultaneously keeping the adhesiveness to a base material. <P>SOLUTION: This film-forming method comprises the steps of: forming a tin-plated film on a material to be plated; and then forming a silver film on the tin-plated film with an electroless or electrolytic plating method by using a silver-plating bath containing an organic acid, as a post-treatment step. The method thereby controls an orientation of crystals in the tin-plated film so that the orientations of the (211) face, the (220) face, and the (101) face precede that of the (321) face. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for forming a unitary tin plating film applied to an electronic component such as an outer lead, and a semiconductor device having an outer lead on which a unitary tin plating film is formed.

  Semiconductor elements such as IC and LSI fixed on the lead frame are connected to the lead frame by wire bonding and then sealed with a mold resin. In order to connect this semiconductor element to a printed circuit board or the like, the outer leads exposed outside the mold resin are connected to the printed circuit board or the like using solder or the like. For this reason, it is necessary to apply plating to the outer lead.

  The plating applied to the outer lead requires characteristics such as whisker resistance, solder wettability, adhesion, bendability, and heat resistance. Conventionally, tin-lead alloy plating containing 5% by mass to 20% by mass of lead has been widely used because it satisfies all of these characteristics.

  However, in recent years, since the influence of lead on the environment has been pointed out, switching to plating not containing lead, that is, lead-free plating, has rapidly progressed as an environmental measure. Currently, binary plating such as tin-bismuth, tin-silver, tin-copper, etc. is the mainstream for plating on the outer leads.

  However, compared with tin-lead alloy plating, the shift to single-unit tin plating is desired because of its high cost, stability of the plating bath, difficulty in composition control, and the like.

  Tin plating is widely used in the fields of electronic industry parts and light electrical industry as a film for improving solderability or a film for connectors. A process flow of tin plating according to the prior art is shown in FIG.

  However, in the obtained tin plating film, tin whiskers that grow depending on the environment after plating are generated, which becomes a problem. Such a tin whisker is a narrow pitch IC with a narrow lead interval and causes a short between leads. For this reason, it has been difficult to develop tin plating on narrow pitch ICs from the viewpoint of reliability required for electronic components. For this reason, the countermeasure which suppresses a tin whisker is desired.

  Conventionally, in the tin plating film and tin alloy plating film formed on the outer lead of the semiconductor device, the following method has been performed in order to prevent the occurrence of tin whiskers.

  Japanese Patent Application Laid-Open No. 2000-174191 describes a method of changing the composition in a film. In this method, the tin alloy plating film is formed so as to have a concentration gradient in which the content of the alloy component changes in the film thickness direction of the plating. However, even if such a method is effective for binary tin alloy plating, it cannot be applied to single tin plating.

  Japanese Patent No. 3513709 describes a method of forming a base metal thin film between a material and a plating film. In this method, a base metal thin film selected from the group consisting of platinum, palladium, silver, bismuth, titanium, zirconium, aluminum, and antimony having a thickness of 0.005 μm to 5.0 μm is formed on a material, and then tin or tin An alloy plating film is formed. However, by interposing a metal thin film between the material and the plating film, there is a high possibility that the adhesion of the plating film is lowered.

  Japanese Patent Application Laid-Open No. 2005-109373 describes a method of forming a plating layer of tin or a tin alloy after forming a base layer formed of a nickel layer and a nickel-phosphorus alloy on a material. However, it becomes multilayer plating and the process becomes complicated.

  Japanese Patent Application Laid-Open No. 2004-156094 describes a method of performing surface treatment after plating. In this method, the plating surface of tin and tin alloy is treated with a surface treatment agent containing a benzimidazole compound and / or a salt thereof. However, assembly manufacturers that handle the resulting products often determine that treatment with an organic coating on the plating surface is undesirable, limiting their application.

  Further, Japanese Patent No. 3314754 proposes improvement of solder wettability by preferential orientation of a specific surface for a tin-silver alloy film from the viewpoint of improving film characteristics by controlling crystal orientation. However, the object of this method is only a tin-silver alloy film, and its purpose is also to improve solder wettability.

JP 2000-174191 A

Japanese Patent No. 3513709

JP 2005-109373 A

JP 2004-156094 A

Japanese Patent No. 3314754

  The object of the present invention is applicable to lead-free single-component tin plating, preventing whisker generation, preventing wettability from being reduced by heat treatment after plating, and maintaining adhesion to the material. Then, it is providing the formation method of a tin plating film | membrane with a simple process.

  The method for forming a tin plating film according to the present invention includes a post-treatment step of forming a silver film on the tin plating film using an organic acid silver plating bath after forming the tin plating film on the material to be plated. It is characterized by having.

  Specifically, after forming the tin plating film, a silver film is formed on the tin plating film, whereby the (211) plane and the (220) plane with respect to the (321) plane with respect to the crystal orientation of the tin plating film. And the orientation of the (101) plane is given priority.

  As a result, the ratio of (321) plane intensity / (211) plane intensity is less than 1.0, and the value of (321) plane intensity / (220) plane intensity is 1.0 as the ratio of diffraction intensity by X-ray diffraction measurement. And the crystal orientation of the tin plating film is controlled so that the value of (321) plane strength / (101) plane strength is less than 1.5.

  In addition, when the crystal orientation of a tin plating film can be controlled in this way, formation of a silver film is not restricted to the case where an organic acid silver plating bath is used. As the post-treatment process for forming the silver film, either electroless plating or electrolytic plating can be employed.

  A semiconductor device according to the present invention is characterized in that a tin plating film having a silver film on its surface is applied to an outer lead which is a material to be plated, using the method for forming a tin plating film.

  That is, a tin plating film is formed on the outer lead of the semiconductor device, and by forming a silver film on the tin plating film, the tin plating film has a ratio of diffraction intensity by X-ray diffraction measurement. 321) plane strength / (211) plane strength value less than 1.0, (321) plane strength / (220) plane strength value less than 1.0, and (321) plane strength / (101) plane strength The crystal orientation has a value of less than 1.5.

  In addition, it is preferable that the said silver film is formed so that the film thickness may be set to about 0.01-1.0 micrometer.

  The present invention is particularly applied to a material to be plated and an outer lead made of 42 alloy.

  According to the present invention, lead-free, in particular, the formation of a unitary tin plating film formed on an outer lead made of 42 alloy can be carried out without reducing the productivity under the conventional plating conditions, and Thus, it is possible to easily prevent the occurrence of tin whiskers in the tin plating film.

  As a result, a semiconductor device that does not deteriorate wettability and does not generate whiskers even after heat treatment can be provided at low cost.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a process flow showing a method for forming a tin plating film according to the present invention.

  In the process flow of this embodiment, two processes of a silver film formation process and a water washing process are added to the process flow of tin plating film formation according to the prior art shown in FIG. Can be implemented by a series of continuous processes. For this reason, productivity is not particularly reduced by the addition of the step. The structure of the plating apparatus may be a conventional sheet plating type or a rack plating type, and the structure of the plating apparatus is not particularly limited.

  First, a tin plating film is formed on the material to be plated. The tin plating process itself is the same as the conventional process. Conventional commercial chemicals can also be used for the tin plating solution. In general, an alkanesulfonic acid bath system is often used. For example, an MST400 bath (manufactured by Reybold Co.) can be used. Table 1 shows the liquid composition of the MST400 bath and the plating treatment conditions.

  The feature of the present invention is that the crystal orientation of the tin plating film is controlled by forming a silver film on the surface of the tin plating film after the tin plating film is formed.

  The step of forming the silver film can be performed by electroless plating, and only needs to be immersed in a plating bath containing silver, and does not particularly require electrolytic treatment. However, a silver film may be formed by electrolytic treatment.

  A plating bath for forming a silver film needs to contain silver and control the deposition rate of silver. As such a plating bath, for example, an organic acid silver plating bath can be used. Specifically, organic acid silver plating bath AG-110 (manufactured by Reybold Co.) can be mentioned. The organic acid silver plating bath AG-110 is composed of AG-111 (manufactured by Reybold) and AG-112 (manufactured by Reybold) and has the liquid composition shown in Table 1. Moreover, what is necessary is just to use the conditions shown in Table 1 for the plating process conditions for silver film formation. By adopting such a plating bath composition and plating treatment conditions, the immersion time becomes constant at the apparatus conveyance speed, and the state of the obtained silver film is kept constant only by liquid concentration and liquid temperature management. It is.

  The crystal orientation of the tin plating film can be controlled by performing a post-treatment step of forming a silver film on the tin plating film. However, in order to suppress the generation of tin whiskers, it is important to prioritize the orientation of the (211) plane, the (220) plane, and the (101) plane with respect to the (321) plane with respect to the crystal orientation of the tin plating film. It is.

  The present inventor has intensively studied the crystal orientation of the tin plating film and the generation of tin whiskers using an organic acid silver plating bath, a mixed solution of sodium thiosulfate and silver nitrate, and a tin-silver plating solution. In the examination, the mixed solution of sodium thiosulfate and silver nitrate includes sodium thiosulfate 7.9 g / L + silver nitrate 4.2 g / L and sodium thiosulfate 0.79 g / L + silver nitrate 0.42 g / L. Prepared, and SA-silver (manufactured by Reybold Co.) was used as the tin-silver plating solution.

  Of these, the tin-silver plating solution cannot be used as a product as a defective appearance because the surface of the tin plating film turns black. On the other hand, whisker having the same or more number is generated in the case of using a mixed solution of sodium thiosulfate and silver nitrate as compared with the case where no silver film is formed. On the other hand, whisker generation is suppressed for those using an organic acid silver plating bath.

  Next, the inventor performed X-ray diffraction measurement for each sample, and investigated the relationship between the crystal orientation of the tin plating film and the whisker.

  According to the comparison of the ratio of the X-ray diffraction intensity and the number of whiskers generated, the value of (321) plane strength / (211) plane strength is less than 1.0, and the value of (321) plane strength / (220) plane strength is If all of the conditions of less than 1.0 and (321) plane strength / (101) plane strength satisfy less than 1.5, the occurrence of whiskers is suppressed, and if none of these conditions is met Does not have the effect of suppressing the occurrence of whiskers.

  The sum of the values of (321) plane strength / (211) plane strength, (321) plane strength / (220) plane strength, and (321) plane strength / (101) plane strength is as follows: When the correlation between the X-ray diffraction intensity ratio and the number of whisker occurrences was considered, the correlation coefficient was 0.678 and not a strong correlation, but it was 2% significant and had a positive correlation.

A × {(321) / (211)} + B × {(321) / (220)} + C × {(321) / (101)}
The coefficient of each term was determined as A = 1.0, B = 0.8, and C = 0.4 in consideration of the intensity of each diffraction surface.

  As described above, in the same tin plating conditions, whisker generation is large in the conventional tin plating film in which the crystal orientation of the tin plating film is not controlled and in the tin plating film according to the present invention in which the crystal orientation is controlled as described above. Different. In the former, many whiskers of 50 μm or more are confirmed, but in the latter, whiskers of 50 μm or more are not generated, and the number of whiskers of 50 μm or less is greatly reduced. In this way, it is possible to effectively prevent whisker generation by appropriately controlling the crystal orientation of the tin plating film.

  By the method for forming a tin plating film according to the present invention, a single-system tin plating film having a silver film formed on the surface of a material to be plated can be obtained. The structure of the plating film is arbitrary depending on the type and application of the material to be plated. In the case where the tin plating film is formed on the outer lead of the lead frame made of 42 alloy, the film thickness of the tin plating film is 5 μm to 20 μm. In that case, the film thickness of the silver film is preferably 0.01 μm to 1.0 μm. If it is 0.01 μm or less, the crystal orientation of the tin plating film cannot be controlled, and if it exceeds 1.0 μm, the wettability decreases.

  By applying the method for forming a tin plating film according to the present invention, the generation of tin whiskers of 50 μm or more is completely prevented in the unitary tin plating film. Moreover, generation | occurrence | production of the tin whisker of 50 micrometers or less (for example, about 20 micrometers) can also be suppressed significantly.

  The inventor further examined the relationship between whisker generation and heat treatment. The final step of the plating process includes a process of performing a heating (drying) process for drying. Such heat treatment is performed in order to prevent contamination on the plating surface. The conditions for the heat treatment vary depending on the structure of the apparatus, but are generally about 80 seconds to 150 ° C. and about 5 seconds to 5 minutes.

  Regarding the tin plating film, regardless of whether or not the silver film is formed, the heat treatment increases the number of whiskers for unknown reasons. However, for the tin plating film obtained by the method for forming a tin plating film according to the present invention, the number of whiskers generated is 0 as long as the heat treatment is performed at a temperature of 80 ° C. to 150 ° C. for 5 sec to 10 sec. It can be. Further, even under other heating conditions, the number of whiskers generated can be remarkably suppressed in the tin plating film obtained by the method for forming a tin plating film according to the present invention.

  However, when the present invention is applied to an outer lead of a semiconductor device, from the viewpoint of the reliability of the obtained semiconductor device, the heat treatment conditions for the purpose of drying are set so as to be short at a low temperature as described above. It is preferable to do.

  At present, in order to control the crystal orientation of the tin plating film as described above, it is effective to use an organic acid silver plating bath. However, when the crystal orientation can be controlled as described above, The immersion liquid used for forming the film is not limited to the organic acid silver plating bath.

Example 1
A tin plating film and a silver film having a film thickness of about 10 μm were formed on the outer leads of a lead frame made of 42 alloy by the process flow shown in FIG. As for the tin plating conditions, a sheet plating apparatus was assumed, and the current density was set to 12 A / dm ² . The silver film was formed using AG-110 (manufactured by Reybold Co., Ltd.) with a silver concentration of 2.5 g / L, a liquid temperature of 30 ° C., and a treatment time of 10 sec.

  In the obtained silver film, a slight blackening was observed as compared with Conventional Examples 3 and 4 described later. However, this was not a problem level in terms of product appearance.

  The obtained sample was subjected to heat treatment at the temperatures and times shown in Table 2 for the purpose of diffusing silver from the silver film into the tin plating film.

  In addition, as an accelerated test for growing whiskers, 500 cycles were performed using a temperature cycle tester at a temperature of −50 ° C. to + 80 ° C. and 1 hour as one cycle. Thereafter, an area common to Conventional Example 1 and Comparative Examples 3 to 4 described later was set and observed with a 100 × metal microscope, and the number of needle whiskers of 20 μm or more was measured.

  On the other hand, for the obtained sample, by X-ray diffraction measurement using an X-ray diffractometer (manufactured by PANalytical, X'Pert-PRO), (321) surface strength, (211) surface strength, 220) surface strength and (101) surface strength are measured, and the ratio of (321) surface strength / (211) surface strength, (321) surface strength / (220) surface strength, 321) The value of plane strength / (101) plane strength was calculated.

  Moreover, the wettability measurement was implemented about the obtained sample. As pretreatment for wettability measurement, PCT treatment (105 ° C., 100% RH) was performed for 8 hours. The wettability was evaluated by measuring the zero-crossing time, which is the time when the force received from the solder bath is zero, after the test piece is immersed in the solder bath by the meniscograph method.

  The respective measurement results and evaluation results are shown in Table 2.

(Conventional example 1)
On a lead frame made of 42 alloy, tin plating with a film thickness of about 10 μm was applied by the process flow shown in FIG. As for the tin plating conditions, a sheet plating apparatus was assumed, and the current density was set to 12 A / dm ² . Further, heat treatment was performed at the temperature and time shown in Table 2.

  Thereafter, in the same manner as in Example 1, the obtained sample was subjected to an acceleration test for growing whiskers, and the number of needle-shaped whiskers having a size of 20 μm or more was measured. Further, the obtained sample was subjected to X-ray diffraction measurement and wettability evaluation. The respective measurement results and evaluation results are shown in Table 2.

(Comparative Example 1)
In the same manner as in Examples 1 to 3, except that SA-silver (manufactured by Reybold) 100 mL / L, which is a silver solution for tin-silver plating solution, was used as the solution for forming the silver coating, Formed. The surface of the obtained silver film turned black at the moment of immersion. Since the blackening becomes defective as a product appearance, subsequent tests and evaluations were not performed.

(Comparative Example 2)
A tin film and a silver film were formed in the same manner as in Examples 1 to 3, except that the liquid for forming the silver film was changed to 5 mL / L of SA-silver (manufactured by Reybold). The surface of the obtained silver film gradually turned black during immersion. Since the blackening becomes defective as a product appearance, subsequent tests and evaluations were not performed.

(Comparative Example 3)
Tin plating and silver film formation were performed in the same manner as in Examples 1 to 3, except that the liquid for forming the silver film was changed to sodium thiosulfate 7.9 g / L + silver nitrate 4.2 g / L.

The obtained silver film had no color tone change as compared with Conventional Examples 1 to 3.

  Thereafter, in the same manner as in Example 1, the obtained sample was subjected to an acceleration test for growing whiskers, and the number of needle-shaped whiskers having a size of 20 μm or more was measured. Further, the obtained sample was subjected to X-ray diffraction measurement and wettability evaluation. The respective measurement results and evaluation results are shown in Table 2.

(Comparative Example 4)
Tin plating and silver film formation were performed in the same manner as in Examples 1 to 3, except that the liquid for forming the silver film was changed to 0.79 g / L of sodium thiosulfate and 0.42 g / L of silver nitrate.

  The obtained silver film had no color tone change as compared with Conventional Examples 1 to 3.

  Thereafter, in the same manner as in Example 1, the obtained sample was subjected to an acceleration test for growing whiskers, and the number of needle-shaped whiskers having a size of 20 μm or more was measured. Further, the obtained sample was subjected to X-ray diffraction measurement and wettability evaluation. The respective measurement results and evaluation results are shown in Table 2.

  As shown in Table 2, the silver film was formed with AG-110, which is an organic acid silver plating bath, as compared with the tin plating film obtained in Conventional Example 1 in which the treatment for forming the silver film was not performed. 1 shows that the occurrence of tin whiskers is greatly reduced.

  However, in Comparative Examples 3 and 4 immersed in a mixed solution of sodium thiosulfate and silver nitrate, tin whisker is generated at the same level as conventional example 1 or more than that, and there is no effect. It can be seen that the occurrence is induced.

  In the present invention, when the heat treatment is performed after the formation of the silver film, at a temperature of 150 ° C., if the heat treatment is performed for about 10 seconds, the generation of tin whiskers can be completely suppressed to zero. Therefore, when the present invention is applied to the manufacture of a semiconductor device, it is preferable to control the heat treatment conditions during the heat treatment for drying.

  In addition, the tin plating film according to the present invention has a zero-crossing time of about 0.6 sec like the tin plating film that does not form a silver film, and the formation of the silver film does not affect the wettability. I understand.

It is the process flow using the whisker prevention method of this invention. It is the process flow of tin plating by a prior art. It is a graph which shows the relationship between the sum of X-ray diffraction intensity ratio and the number of whisker generation.

Claims (9)

  A method for forming a tin plating film comprising a post-treatment step of forming a silver film on the tin plating film using an organic acid silver plating bath after forming the tin plating film on the material to be plated .   After forming a tin plating film on the material to be plated, by forming a silver film on the tin plating film, the (211) plane and (220) plane with respect to the (321) plane with respect to the crystal orientation of the tin plating film And a method of forming a tin plating film, wherein priority is given to the orientation of the (101) plane.   After forming a tin plating film on the material to be plated, by forming a silver film on the tin plating film, the ratio of (321) plane strength / (211) plane strength in terms of the diffraction intensity ratio by X-ray diffraction measurement Is less than 1.0, (321) surface strength / (220) surface strength is less than 1.0, and (321) surface strength / (101) surface strength is less than 1.5. And a method for forming a tin plating film, wherein the crystal orientation of the tin plating film is controlled.   The method for forming a tin plating film according to any one of claims 1 to 3, wherein the post-treatment step of forming the silver film is performed by electroless plating or electrolytic plating.   The method for forming a tin plating film according to claim 1, wherein the material to be plated is 42 alloy.   A semiconductor device having an outer lead on which a tin plating film is formed, and a silver film using an organic acid silver plating bath formed on the tin plating film.   An outer lead having a tin plating film is formed, a silver film is formed on the tin plating film, and the tin plating film has a (321) plane in the ratio of diffraction intensity by X-ray diffraction measurement. Strength / (211) plane strength value is less than 1.0, (321) plane strength / (220) plane strength value is less than 1.0, and (321) plane strength / (101) plane strength value is A semiconductor device having a crystal orientation of less than 1.5.   8. The semiconductor device according to claim 6, wherein the film thickness of the silver film is 0.01 to 1.0 [mu] m.   The semiconductor device according to claim 6, wherein the outer lead is 42 alloy.

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