KR101685578B1

KR101685578B1 - Method for electroless palladium plating

Info

Publication number: KR101685578B1
Application number: KR1020160018979A
Authority: KR
Inventors: 전성욱; 정보묵; 박재승; 박유미
Original assignee: 와이엠티 주식회사
Priority date: 2016-02-18
Filing date: 2016-02-18
Publication date: 2016-12-12

Abstract

Provided is a method of electroless palladium plating, comprising: a step of providing an electroless palladium plating liquid; a step of injecting an inert gas into the electroless palladium plating liquid; a step of immersing a substrate in the electroless palladium plating liquid; and a step of electroless plating the substrate with the plating liquid.

Description

[0001] METHOD FOR ELECTROLESS PALLADIUM PLATING [0002]

The present invention relates to an electroless palladium plating method and an electronic component using the electroless palladium plating method. More particularly, the present invention relates to a method for uniformly maintaining the electroless palladium plating rate and an electronic component using the electroless palladium plating method.

Electroless plating refers to plating in which electrons emitted at this time are combined with metal ions in the solution and are reduced to metal on the plating object as the reducing agent chemical is oxidized by the plating which does not conduct electricity. The feature of electroless plating is that when the plating liquid is circulated, a uniform thickness can be obtained, and the plating film has less pinhole and corrosion resistance than the electroplated film. Even in the hardness, the plating film containing phosphorus (P) and boron (B) can be heat treated at 400 ° C to increase the Vickers hardness to 1,000 or more and exhibit hardness values comparable to hard chrome. Also, the change in magnetism of the plated film, the solderability, and the conductivity characteristics are superior to the electroplating.

Depending on the composition of the plating solution used for the electroless plating, the plating rate, the characteristics of the plating film and the like may be changed, and the degree of reaction accumulation by the chemical reaction during plating may vary. This is a very important factor because it affects not only the quality of the plating product but also the efficiency of the work.

Electroless plating solutions of noble metals are widely used in cutting-edge electronic industries such as electronic parts and substrates due to the nature of the coating. In the conventional electronic parts and substrates, the so-called ENIG is the mainstream in which electroless nickel plating is performed on copper wirings and gold plating is performed thereon. Since ENIG precipitates gold on the nickel film by substitution, the gold film has a lot of pin-holes, and corrosion of the nickel film due to substitution is a problem. As a result, the connection failure during solder connection or wire bonding It happens frequently. In order to compensate for the drawback, there is ENIGAG in which a thicker gold plating is formed on the substituted gold, and it is disclosed in Japanese Patent Laid-Open No. 1997-008438. However, if the thickness of the gold layer is increased, it leads to an increase in cost. Since the pinhole is not completely removed, the improvement of the connection reliability is insufficient as compared with the ENIG.

2. Description of the Related Art In recent years, there has been a growing demand for high-reliability electronic parts at low cost and miniaturization of wiring. Palladium, which can replace expensive gold as a plated film, has been attracting attention. There is also ENEPIG in which electrocatalytic electroless palladium is formed on an electroless nickel film and gold plating is performed thereon. Such contents are disclosed in Japanese Patent Laid-Open No. 1997-008438.

Most of the conventional palladium plating solutions have a problem in that the deposition rate is rapidly decreased or the thickness of the plating layer is varied as time passes during the continuous plating operation. There is a problem that the same phenomenon occurs even when the fresh solution is moistened and left for a long time without performing the plating operation.

Therefore, efforts are needed to minimize the occurrence of deposition rate and thickness variations during long-term use and long-term storage of the palladium plating solution.

A problem to be solved by the present invention is to provide a plating method for minimizing occurrence of deposition rate and thickness variation of palladium metal during long-term use and long-term storage of the palladium plating solution.

According to an aspect of the present invention, there is provided a method of forming a palladium plating solution, comprising: providing an electroless palladium plating solution; Introducing an inert gas into the electroless palladium plating solution; Immersing the substrate in the electroless palladium plating solution; And electroless plating the substrate using the plating liquid.

In one embodiment, the inert gas may be at least one selected from helium, neon, argon, and nitrogen.

In one embodiment, the feed flow rate of the inert gas may be from 0.000001 to 50 L / min.

In one embodiment, the reduction in electroless plating rate may be less than 5% compared to immediately after bathing on a 3 MTO basis.

In one embodiment, the concentration of oxygen or carbon dioxide in the plating solution may be lowered to less than 100 ppm during electroless plating by the introduction of the inert gas.

In one embodiment, the electroless plating may be performed by immersing the substrate in the plating solution under continuous or periodic exposure of the inert gas.

According to the present invention, there is an advantage that the plating solution is prevented from aging by blowing a certain amount of nitrogen into the palladium plating solution to keep the deposition rate upon electroless plating, and even when the plating is performed several times or more, there is almost no thickness variation of the plating layer.

1 is a process flow diagram illustrating an electroless palladium plating method according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

1 is a process flow diagram illustrating an electroless palladium plating method according to an embodiment of the present invention. Referring to FIG. 1, in step S1, an electroless palladium plating solution is provided. Palladium plating herein includes pure palladium and palladium alloys. The electroless palladium plating solution may include a palladium ion source, a reducing agent, a complexing agent, and a stabilizer.

Generally, the palladium ion source of the electroless palladium plating solution is not particularly limited as long as it is a water-soluble palladium compound, but it may be palladium chloride, palladium sulfate, palladium nitrate, ammonium chloride palladium chloride, ethylenediamine palladium chloride and the like. The palladium ion may have a concentration of 0.5 to 500 mmol / l, preferably 1 to 100 mmol / l.

The electroless palladium plating solution further comprises a reducing agent that makes the plating solution autocatalytic. Palladium ions are reduced to metallic palladium in the presence of a reducing agent. The electroless palladium plating solution is particularly suitable for depositing a pure palladium layer in the presence of formic acid, acetic acid, and derivatives or salts thereof as a reducing agent. Suitable derivatives of formic acid are, for example, esters of formic acid, e. G. Formic acid methyl ester, formic acid ethyl ester and formic acid propyl ester. The formic acid derivative may be formaldehyde. Other suitable derivatives of formic acid are, for example, substituted and unsubstituted amides such as formamide and N, N-dimethylformamide. Suitable counterions for salts of formic acid are, for example, selected from hydrogen, lithium, sodium, potassium and ammonium. Suitable reducing agents for the deposition of palladium-phosphorous alloys may be phosphorous acid, hypophosphorous acid, and derivatives or salts thereof. For example, sodium hypophosphite and potassium hypophosphite, which form an alloy that is, for example, palladium. Suitable reducing agents for the deposition of palladium boron alloys can be borane derivatives, such as amine borane compounds such as methylamine borane, dimethylamine borane, diethylamine borane, and the like. In the electroless palladium plating solution, the reducing agent may have a concentration of 10 to 1000 mmol / l.

The complexing agent is a compound that forms a stable ring structure by coordinating with a metal ion, and forms a complex with the metal ion in the solution, thereby reducing the concentration of the liberated metal ion. It is important to secure the stability of the metal ion to some extent by reducing the free metal ion in the role of such a complexing agent. However, use of a complexing agent that is too strong may cause a decrease in the speed, and if a weakly complexing agent is used, the stability of the solution may be lowered, which may cause the decomposition of the bath.

Materials which can be used as a complexing agent may be an organic acid having a carboxyl group or a salt thereof or an amine or a salt thereof selected from the group including primary amines, secondary amines and tertiary amines. For example, the complexing agent may be selected from the group consisting of malic acid, succinic acid, adipic acid, lactic acid, acetic acid, glycolic acid, citric acid, ethylenediamine, 1,3-diaminopropane, 1,2- -Diethylenetriamine, diethylenetriaminepentaacetic acid, nitroacetic acid, N- (2-hydroxyethyl) ethylenediamine, ethylene-diamine-N, N-diacetic acid, 2- ) Ethylamine, 1,2-diaminopropylamine, 1,3-diaminopropylamine, 3- (methylamino) propylamine, 3- (dimethylamino) propylamine, 3- Bis (3-aminopropyl) amine, 1,2-bis- (3-aminopropyl) alkylamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylene hexamine and salts thereof .

The kind and concentration of the complexing agent can be determined in consideration of the precipitation rate and good plating appearance. The concentration of the complexing agent is preferably 0.01 to 80 g / l, more preferably 0.1 to 8 g / l in the whole plating solution in order to secure economical efficiency and appropriate plating rate. In the electroless palladium plating solution used in one embodiment of the present invention, the molar ratio of the complexing agent containing no phosphorus to the palladium ion is in the range of 2: 1 to 50: 1.

In the electroless palladium plating solution, metal palladium is a noble metal with a relatively high standard reduction potential (Pd / Pd ²⁺ : 0.987). Therefore, the tendency to exist as Pd ⁰ , which is a metal state rather than Pd ²⁺ , is very high. This causes instability of the solution and promotes decomposition of the plating solution. Therefore, a stabilizer is further included in the plating liquid to prevent the decomposition of such plating liquid. Examples of the type of stabilizer include, but are not limited to, bismuth or bismuth compounds. As the bismuth compound, for example, bismuth oxide, bismuth sulfate, bismuth sulfite, bismuth nitrate, bismuth chloride, bismuth acetate and the like can be used. The stabilizer may be contained in an amount of 0.1 to 1000 mg / L, preferably 1 to 100 mg / L in the whole plating solution. If the concentration of the stabilizer is less than 0.1 mg / L, the stability of the plating solution is deteriorated. If the concentration exceeds 1000 mg / L, the plating rate may be lowered.

In step S2, an inert gas is introduced into the electroless palladium plating solution. The inert gas may be helium, neon, argon, nitrogen or a combination thereof. Preferably, the inert gas is nitrogen in terms of availability and economy. When the inert gas is introduced, it is possible to prevent the plating reaction by-products from accumulating excessively in the plating solution and to prevent the gas such as oxygen and carbon dioxide gas from being dissolved in the plating solution. As a result, it is possible to maintain the physical properties of the plating solution when the continuous operation using the electroless palladium plating solution is performed, or even when the inert gas is continuously supplied for a long period of time, for example, several days to several tens of days, Or the thickness variation of the plating layer can be effectively mitigated. The flow rate of the inert gas to the plating solution may be 0.000001 to 50 L / min, preferably 0.001 to 10 L / min, more preferably 0.002 to 5 L / min with respect to 1 L of the solution volume. When the flow rate of the inert gas is less than the above range, the effect of the deposition rate maintenance and the thickness deviation is insufficient, and when the flow rate is larger than the above range, it may be economically disadvantageous.

The method of introducing the inert gas is not particularly limited. For example, the inert gas may be continuously blown into the plating liquid at the above-mentioned rate, or blown into the plating liquid at regular intervals. Considering that the working environment in the mass production line is not uniform, continuous charging is more advantageous for the plating solution speed and deviation, but considering the economical aspect, it is also possible to divide the plating solution periodically. Since the work is not done continuously on the production line, it is effective to continuously inject nitrogen even during work as well as during work.

In order to verify the effect of inert gas injection, three palladium solutions were prepared in each 1 L volume of beaker, and oxygen, carbon dioxide gas and air were introduced into each solution at a rate of 0.1 L / min for 4 hours, The plating rates were compared and observed. The plating rate of each solution before blowing each gas was 0.26 μm / 15 minutes, 0.05 μm / 15 minutes when oxygen was supplied, 0.09 μm / 15 minutes when carbon dioxide was added, and 0.11 μm / 15 minutes when air was supplied Minute, the plating rate was drastically reduced. In addition, it was confirmed that the plating rate was restored to 0.26 μm / 15 minutes when nitrogen, oxygen, carbon dioxide gas and air were blown into the solution and nitrogen was supplied at an amount of 0.1 L / min for 1 hour. That is, substitution of oxygen or carbon dioxide gas by the inert gas can prevent the lowering of the plating rate.

For example, when the inert gas is introduced, the concentration of oxygen or carbon dioxide in the plating solution during electroless plating is 100 ppm or less, preferably 50 ppm or less, more preferably 10 ppm or less, particularly preferably 5 ppm or less Or less. For example from 0.01 to 10 ppm. The plating rate can be kept constant in the concentration range.

In this experiment, it was found that the plating rate was lowered when the gas in the atmosphere was dissolved in the palladium solution. In order to prevent this, it was confirmed that blowing an inert gas such as nitrogen was effective.

Generally, when formic acid and its derivatives are used as a reducing agent in a plating process, carbon dioxide gas may be generated as a reaction by-product. Therefore, even in such a case, it can be expected that the plating rate will be lowered as the carbonaceous gas is forcibly injected as in the foregoing results.

As a result, when an inert gas such as nitrogen is injected in the electroless plating process in accordance with an embodiment of the present invention, it is effective to maintain the plating rate.

A printed circuit board (PCB) with a copper circuit was used as a test substrate. The process conditions are shown in Table 1 below. SAC302, Caroat, CATA855, PEN855, ZEP100, and IR Gold 810 are product names of WYMITE.

Process Name Chemical name or solution composition Immersion time Temperature Degreasing SAC302 5 minutes 50 ℃ Pickle 95% sulfuric acid 100 ml / L 1 minute 25 ℃ Soft Etching 100 g / L Caroat + 95% sulfuric acid 10 ml / L 1 minute 30 ℃ Free dip 95% sulfuric acid 50 ml / L 1 minute 25 ℃ catalyst CATA855 2 minutes 30 ℃ Electroless nickel PEN855 25 minutes 82 ° C Palladium plating ZEP100 15 minutes 50 ℃ Electroless IR Gold 810 10 minutes 80 ℃

In step S3, the substrate is immersed in the electroless palladium plating solution. The substrate includes a printed circuit board, an IC substrate, a semiconductor wafer or other electronic components. The time for immersing the substrate in the electroless palladium plating solution varies depending on the thickness of the plating layer and the plating rate, but is usually 1 to 20 minutes. For example, if the plating is performed at 50 ° C. for 2 minutes, the thickness of the plating layer becomes 0.052 μm, and if the plating is performed for 17 minutes under the same conditions, the thickness of the plating layer becomes 0.29 μm.

In step S4, the substrate is plated using the plating liquid. In the case of electroless plating, it is common that the deposition rate increases with temperature. For example, the optimum plating temperature and pH can be appropriately determined according to the deposition rate, the characteristics of the precipitated structure, and the plating liquid management characteristics, and the preferable plating temperature for palladium plating may be 40 to 60 캜. If the temperature is lower than the above temperature range, the plating rate is lowered to 40 ° C., so that the deposition time is prolonged in order to obtain a desired plating thickness, resulting in poor working efficiency. On the other hand, since the electroless palladium plating solution is unstable at a pH value of less than 4, the pH value of the electroless palladium plating solution is in the range of 4 to 7. Preferably, the pH value of the plating solution is in the range of 4.5 to 6.5. NaOH, ammonia water, or a phosphate compound may be used as the pH adjusting agent.

Maintaining the characteristics of the plating solution for MTO (Metal Turn Over) which can determine the lifetime of the solution in the electroless plating solution is very important factor. In general, as the MTO increases, phenomena such as an increase in internal stress, an increase in P content due to a decrease in deposition rate (only in the case of Pd-P alloy plating), a decrease in corrosion resistance, and a decrease in fatigue resistance occur. This is because the accumulation of the reaction products generated as the plating reaction proceeds causes roughness of the plating film, degradation of gloss and liquid dispersion. However, as described above, when the inert gas is introduced, the aging is reduced, and the plating rate is not reduced even when the MTO is increased, and the uniform plating thickness can be maintained.

According to the electroless palladium plating method according to an embodiment of the present invention, the decrease in the electroless plating rate may be 5% or less, preferably 1% or less, immediately after bathing on the basis of 3 MTO. For example, a plating rate of 0.26 μm / 15 minutes immediately after drying can be uniformly maintained at 3 MTO or 10 MTO, 20 MTO.

The electroless palladium plating method of the present invention can be applied to an object to be plated such as a semiconductor bump, a package substrate on which a semiconductor is mounted, and an electronic part such as a printed wiring board. The electroless palladium plating method of the present invention can be suitably used, for example, for electroless palladium plating inserted between electroless nickel plating and electroless gold plating.

Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, the technical spirit of the present invention is not limited by the following Examples.

[Example]

The change of plating thickness was observed for each 1L of palladium plating solution by the process conditions, and the thickness after plating for 15 minutes was measured for each condition. In Comparative Example 1 and Comparative Example 2, the change in the thickness of the plating was observed while nitrogen was not blown at 50 ° C and room temperature, respectively. The substrate to be plated and other process conditions are as described in Table 1. [

In Examples 1 to 4 shown in Table 2, the substrate was immersed in a plating solution for 15 minutes after being allowed to stand at room temperature for 30 minutes under a nitrogen-blowing condition, and the variation in the plating thickness was observed. 8 shows the change in the thickness of the plating by immersing the substrate in the plating solution for 15 minutes after allowing the substrate to stand for a time at 50 DEG C under a nitrogen blowing condition. In Comparative Examples 1 and 2, plating was performed under the same conditions as in Examples 1 to 4 and 5 to 8, respectively, but no nitrogen was added.

The numbers in Tables 2 and 3 are plating thicknesses in μm.

Comparative Example 1 Example 1 Example 2 Example 3 Example 4 Nitrogen input method none 0.001 L / min Continuous input 0.01 L / min continuous feed 0.001 L / min, 10 minutes every 3 hours 0.01 L / min, 10 minutes every 3 hours Neglect temperature 50 ℃ 50 ℃ 50 ℃ 50 ℃ 50 ℃ Immediately after bathing 0.26 0.26 0.26 0.26 0.26 Leave for 5 days 0.27 0.26 0.27 0.25 0.26 Leave for 10 days 0.17 0.26 0.25 0.24 0.24 Leave for 15 days 0.13 0.25 0.25 0.25 0.25 Leave for 20 days 0.08 0.25 0.24 0.24 0.25 Leave for 30 days 0.03 0.25 0.25 0.24 0.25

Comparative Example 2 Example 5 Example 6 Example 7 Example 8 Nitrogen input method none 0.001 L / min Continuous input 0.01 L / min continuous feed 0.001 L / min, 10 minutes every 3 hours 0.01 L / min, 10 minutes every 3 hours Neglect temperature RT RT RT RT RT Immediately after bathing 0.26 0.26 0.26 0.26 0.26 Leave for 5 days 0.21 0.24 0.24 0.24 0.23 Leave for 10 days 0.13 0.25 0.25 0.24 0.25 Leave for 15 days 0.05 0.24 0.25 0.25 0.24 Leave for 20 days 0.03 0.24 0.25 0.24 0.24 Leave for 30 days 0.02 0.24 0.25 0.23 0.24

Table 4 below shows the plating solution aging phenomenon with increasing MTO. Referring to Table 4, in Comparative Example 3, the change in the thickness of the plating solution after the aging of the plating solution under the condition of not blowing nitrogen was measured, and in Examples 9 to 12, the change in the plating thickness due to aging of the plating solution under the nitrogen- will be.

In Comparative Examples 1 to 3 in which nitrogen was not blown, the plating rate was continuously decreased as the plating time or the aging progressed, while the nitrogen-blown Examples 1 to 12 maintained a relatively uniform plating thickness.

Comparative Example 3 Example 9 Example 10 Example 11 Example 12 Nitrogen input method none 0.001 L / min Continuous input 0.01 L / min continuous feed 0.001 L / min, 10 minutes every 3 hours 0.01 L / min, 10 minutes every 3 hours 0 MTO 0.26 0.26 0.26 0.26 0.26 1 MTO 0.20 0.25 0.25 0.26 0.26 2 MTO 0.15 0.26 0.26 0.26 0.26 3 MTO 0.09 0.27 0.27 0.26 0.27

Claims

In the electroless palladium plating method by reduction of palladium ions,
Providing an electroless palladium plating solution comprising a palladium ion source, a reducing agent, a complexing agent, and a stabilizer;
Introducing an inert gas into the electroless palladium plating solution;
Immersing the substrate in the electroless palladium plating solution; And
And electroless plating the substrate using the plating solution,
The palladium ions are reduced by the electrons emitted by the oxidation reaction of the reducing agent, and the palladium metal is precipitated on the substrate. The reduction of the electroless plating rate is not more than 5% immediately after the bath treatment based on 3 MTO (Metal Turn Over) Electroless palladium plating method.

The method according to claim 1,
Wherein the inert gas is at least one selected from the group consisting of helium, neon, argon, and nitrogen.

The method according to claim 1,
Wherein the flow rate of the inert gas is 0.000001 to 50 L / min.

delete

The method according to claim 1,
Wherein the concentration of oxygen or carbonic acid gas in the plating solution is lowered to 100 ppm or less during electroless plating by the introduction of the inert gas.

The method according to any one of claims 1 to 3 and 5,
Wherein the electroless plating is performed by immersing the substrate in the plating solution under continuous or periodic exposure of the inert gas.