JPH04224682A - Electroless copper plating method, oxygen supplying device used to this method and electroless copper plating apparatus used to this method - Google Patents

Abstract

PURPOSE:To stabilize plating solution and to prevent lowering of precipitating velocity of a copper by increasing soluble oxygen in the plating solution up to the specific value according to increase of sp. gr. of the electroless copper plating solution. CONSTITUTION:During using the electroless copper plating solution containing >10g/l copper surface as Cu ion source, >4.5ml/l formalin as reducing agent for Cu ion, >60g/l ethylenediamine-tetraacetic acid as complexing agent and material refining agent and non-cyanogen series stabilizer and having >12.4 pH, lowering of plating capacity of the plating solution is detected with increase of this sp. gr. The plating solution in a plating vessel 9 is supplied into a vessel providing a static mixer element 5 by a pump 10, and oxygen from an oxygen cylinder 8 is supplied from an introducing hole 2 to increase oxygen concn. in the plating solution up to within the range of 6-20ppm. The copper having excellent quality is stably plated with the electroless method without lowering the precipitating velocity of copper.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroless copper plating method for forming a conductor by plating copper on the surface of a substrate for a printed wiring board and the inner surface of a through-hole.

0002

BACKGROUND OF THE INVENTION Conventional electroless copper plating mainly contains copper salts, complexing agents, reducing agents, pH adjusters, stabilizers and the like. In general, the copper salt is copper sulfate, the complexing agent is ethylenediaminetetraacetic acid (EDTA), the reducing agent is formalin, the pH adjuster is caustic soda, and the stabilizer is 2
, 2' dipyridyl, etc. are used. Furthermore, when performing plating using such an electroless copper plating solution, the plating solution is stabilized by drawing piping into the plating tank from the outside and supplying air to perform air agitation.

[0003] In such an electroless copper plating method,
When plating is performed continuously over a long period of time, the following problems occur. For example, copper sulfate, ethylenediaminetetraacetic acid (EDTA), formalin, caustic soda, and 2,2' dipyridyl are used as a plating tank with a volume of 1,000 to 8,000 liters. When continuous operation is performed at a plating deposition rate of 2.0 to 2.5 μm/hr with a quality pipe installed and air agitation,
Over time, as the plating reaction progresses, the amount of sodium sulfate and sodium formate is approximately 0.5 mol/l and approximately 0.1 mol/l, respectively.
1 is produced, which impedes the stability of the plating solution and causes copper deposits to occur between circuit conductors. In addition, physical properties of copper, such as copper elongation rate,
It is smaller than ordinary rolled copper foil, and when used in printed wiring boards, circuits are easily broken due to stress caused by heat from soldering. In order to solve these problems, we have developed techniques to replace the plating solution, techniques to use stabilizers and additives to improve the physical properties of copper, techniques to remove harmful by-products, techniques that do not generate harmful by-products, and techniques to use oxygen. Various technologies have been developed, including those that use plating solutions to stabilize them. Each conventional example will be explained below.

(Conventional example of replacing the plating solution) In order to suppress such harmful by-products, a method of replacing the plating solution by adding a large amount of water or the main component of the plating solution and allowing it to overflow. Also, manage the usage time of the plating solution,
A method is known in which the liquid is replaced with freshly prepared bath liquid every hour.

(Conventional Examples of Using Stabilizers, etc.) In addition, sodium cyanide, vanadium pentoxide, etc. are added as stabilizers, carboxylic acids and germanium are added as copper physical property improvers, or interfacial Efforts have been made to add an activator and to use copper mothate as a copper salt.

(Conventional example that does not remove or generate harmful by-products) As disclosed in Japanese Patent Publication No. 57-43630, a part of the electroless copper plating solution is taken out,
It is known to distill the removed solution, make the concentration of the by-product higher than its solubility, precipitate it, remove the precipitate, and return the remaining concentrated solution to the original electroless copper plating solution.

[0007] Furthermore, Japanese Patent Publication No. 58-5983 describes a method for producing a solution of a complexing agent and copper ions that does not contain such harmful by-products, in which copper is immersed as an anode in a solution containing a complexing agent. However, a method is disclosed in which a potential is applied between the anode and another cathode to dissolve the copper that is the anode to form a compound with a complexing agent.

Furthermore, there is a method of recovering only copper ions and complexing agents using a copper precipitation device or a complexing agent recovery device and reusing them, a method of removing sulfate ions and formate ions using an ion exchange membrane, and A known method is to reduce the generation of harmful by-products by supplying the copper source of the plating solution in the form of copper hydroxide or cupric oxide.

(Conventional example of stabilization using oxygen) Furthermore, Japanese Patent Laid-Open No. 63-3220 and Japanese Patent Laid-Open No. 63-49
As disclosed in Publication No. 50, oxygen gas is bubbled into the plating tank or injected from external piping to make the dissolved oxygen concentration in the plating solution higher than the saturation value of air.
There are known methods for extending the life of aged plating solutions.

0010

[Problem to be Solved by the Invention] With the development of electronic equipment, the characteristics required of wiring boards have of course increased.
Customer delivery times, prices, etc. must also be shortened. In response to such customer requests,
There has arisen a need to provide faster and lower cost electroless plating on the surface of insulating substrates and in through-holes, and at lower manufacturing costs. Therefore, increasing the deposition rate of plating, lowering the price of materials used for manufacturing, and reducing costs related to manufacturing equipment have become important technical issues.

By the way, in the above-mentioned conventional example in which the plating solution is replaced, a large amount of plating waste solution is generated and processing equipment is required, so that it is difficult to realize it economically.

[0012] In addition, in the conventional example using stabilizers, etc., sodium cyanide used as a stabilizer in the plating solution is a powerful chemical, requires careful handling, and requires processing equipment. Since vanadium is expensive, it is difficult to put it into practical use. In addition, in the case of copper physical property improvers, carboxylic acid converts to formic acid during the plating reaction, reducing the stability of the plating solution, germanium is expensive, and surfactants accumulate harmful byproducts. However, its solubility decreases and it loses its effectiveness, making it difficult to put it into practical use. It is difficult to use copper mothate because it is less effective in suppressing harmful by-products and the stability of the plating solution is low.

Furthermore, in (conventional example in which harmful by-products are not removed or generated), Japanese Patent Publication No. 57-4363
As disclosed in Publication No. 0, a part of the electroless copper plating solution is taken out, the taken out solution is distilled, the concentration of the by-product is made higher than its solubility and precipitated, and the precipitate is removed. Although it is possible to remove sodium sulfate by returning the remaining concentrated solution to the original electroless copper plating solution,
Since sodium formate cannot be removed, the effect of increasing the stability of the plating solution is small.

As disclosed in Japanese Patent Publication No. 58-5983, copper as an anode is immersed in a solution containing a complexing agent, and a potential is applied between it and another cathode to dissolve the copper as an anode. In the method of making a compound with a complexing agent, the equipment is expensive, and the operation is complicated and the workability is low.

[0015] In the method of recovering only copper ions and complexing agent using a copper precipitation device or complexing agent recovery device and reusing them, or in the method of removing sulfate ions and formate ions with an ion exchange membrane, the equipment is expensive. At the same time, it is difficult to realize this because it requires maintenance and management of equipment such as ion exchange membranes.

[0016] In the method of reducing the generation of harmful by-products by supplying the copper source of the plating solution in the form of copper hydroxide or cupric oxide, the material is expensive and has low solubility. , it is not practical to use it as a copper source for plating solutions.

[0017] In these conventional techniques, the plating deposition rate is usually 2 μm/hr and at most 3.2 μm/hr.
r, and in terms of the physical properties of copper, the elongation rate is about 1 to 5%. In addition, even if the elongation rate of copper is improved by heat treatment,
It is about 10%. For this reason, electroless plated copper was thought to be hard and brittle. Therefore, in order to increase the plating deposition rate using conventional technology, we conducted an experiment by increasing the concentration of copper sulfate, aiming at a rate of about 4 μm/hr. This caused a phenomenon in which the circuit conductors were completely short-circuited. The harmful by-products of this process, sodium formate, were 0.04 mol/l and sodium sulfate was 0.2 mol/l, and at the conventional plating deposition rate, the content was not such that copper precipitation would occur. .

An object of the present invention is to provide an electroless copper plating method that does not cause abnormal precipitation such as copper precipitation, has an excellent plating deposition rate, and has excellent physical properties of the deposited copper.

[0019]

[Means for Solving the Problems] The present inventors conducted an experiment to confirm the relationship between the plating precipitation rate and the copper fall-off phenomenon, and as a result of intensive study of the results, the copper fall-off phenomenon when the deposition rate is high is as follows: It is not only affected by harmful by-products, but is also caused by the intense oxygen consumption in the plating solution during high-speed precipitation, the decrease in dissolved oxygen in the plating solution, and the formulation composition that tends to cause a decrease in dissolved oxygen. I got the knowledge that it can be done. We also found that the specific gravity of the plating solution is useful as an indicator for detecting the composition and harmful byproducts.

[0020] Based on these findings, the present inventors
The present invention has been made possible by establishing a new oxygen supply method and a technique for making the amount of oxygen supplied correspond to the specific gravity of the plating solution.

[0021] The electroless copper plating method of the present invention uses an electroless copper plating solution containing a copper salt, a complexing agent, a reducing agent, a pH adjuster, a physical property improver, a non-cyanide stabilizer, etc. The dissolved oxygen concentration in the plating solution is increased by 6 as the specific gravity of the solution increases.
It is characterized in that plating is performed while increasing the amount in the range of ~20 ppm.

[0022] It is preferable that the concentration of ethylenediaminetetraacetic acid as the complexing agent be 60 g/l or more, since this has the effect of suppressing the decrease in copper elongation and the generation of voids during long-term continuous plating.

[0023] When the concentration of copper sulfate as a copper salt in the electroless copper plating solution is set to 10 g/l or more, the concentration of formalin as a reducing agent to 4.5 ml/l or more, and the pH to 12.4 or more, the plating This is preferable because passivation of the copper surface due to excess oxygen, which tends to occur initially, can be suppressed.

Next, as shown in FIG. 1, the apparatus for supplying oxygen to the electroless copper plating solution of the present invention includes an oxygen supply tank having a plating solution inlet, a plating solution outlet, and an oxygen supply port. , a vibrating static mixer element, and the static mixer element is housed in the oxygen supply tank.

Furthermore, as shown in FIG. 2, the device for supplying oxygen to the electroless copper plating solution of the present invention has a partition wall therein for separating gas and liquid, and the discharge port of the line mixer is connected to the device for supplying oxygen to the electroless copper plating solution. A line mixer equipped with a connected oxygen supply tank, a vibrating static mixer element, a plating solution inlet, an oxygen supply port, and an ejector that pressurizes and sends out a mixed state of the plating solution and oxygen. The static mixer element is housed in the oxygen supply tank.

In such an apparatus, providing means for controlling the amount of oxygen supplied between the oxygen supply port and the oxygen cylinder connected to the supply port means that the amount of oxygen supplied from the plating tank and the plating tank is a means for taking out a portion of the plating solution;
It has a means for supplying oxygen to a portion of the removed plating solution in an oxygen supply tank, a means for monitoring the specific gravity of the plating solution in the plating tank, and a means for keeping a record of the usage time of the plating solution. It is preferable that the device has means for controlling the amount of oxygen supplied based on the usage time of the solution and the specific gravity of the plating solution, and the plating work can be automatically operated.

[0027]

[Function] Based on the findings obtained through extensive research by the inventors, the following phenomena are believed to occur in the plating solution to increase the plating deposition rate.

(1) The reaction of the main components of the electroless plating solution is CuSO4+2HCHO+4NaOH→Cu0+2
HCOONa+Na2SO4+2H2O+H2
As a result of the reaction, metallic copper is precipitated, sodium formate and sodium sulfate are produced as by-products, and water and hydrogen are generated. therefore,
The specific gravity of the plating solution is determined by the concentrations of the main components and byproducts.

(2) Along with the main reaction, 2Cu+HCHO+5OH-→Cu2O+HCOO
−+3H2O
A reaction as shown in formula 2 occurs, and the cuprous oxide produced at this time is Cu2O+2HCHO+2NaOH→2Cu0+H
2+2HCOONa+H2O
A reaction expressed by Formula 3 occurs in the plating solution, and metallic copper particles are generated in the plating solution. This is a phenomenon called copper fall, and these metallic copper particles adhere to the object to be plated, and in the case of wiring boards, conductor circuits are formed in unnecessary locations.

(3) Here, when oxygen is supplied by air stirring, the cuprous oxide produced by the above formula 2 is oxidized to form Cu2O+1/2O2+2H2O→2Cu2+4
OH-
Equation 4 is obtained, which suppresses the formation of metallic copper in the plating solution mentioned above, but if the amount of oxygen supplied is small, cuprous oxide acts as a catalyst, and the following happens: 2HCHO+O2+2NaOH→2HCOONa+
2H2O
The reaction shown in Formula 5 occurs, and a by-product of sodium formate is generated in addition to the main reaction, making the plating reaction unstable.

(4) In addition, oxygen is driven out of the plating solution by the hydrogen gas produced in the main reaction of plating that occurs in equation 1 above, which aggravates the phenomenon that occurs when the amount of oxygen supplied is reduced. let

(5) In the present invention, when the deposition rate of plating is increased, the instability of the plating solution is caused more by a decrease in the amount of oxygen supplied than by the accumulation of harmful by-products. This was done based on the knowledge that it was large. Based on this knowledge, the present invention has been developed to improve plating even when the plating deposition rate is increased by forcibly introducing oxygen and increasing the amount of oxygen supplied when plating is performed continuously. This ensures the stability of the liquid.

[0033]

[Example] Using an electroless copper plating solution with the composition shown below,
Electroless plating was performed using the apparatus shown in FIG. 1 while supplying oxygen. (Plating solution composition) Copper sulfate 10
g/l ethylenediaminetetraacetic acid 60g/l formalin (37% by weight) 5ml/l 2,2' dipyridyl 20mg/l polyethylene glycol 0.1g/l sodium hydroxide... Adjust the pH of the plating solution to 12.5 The plating conditions are: temperature 70℃, load 2dm2/l, plating tank volume 1.
000 l, and the substrate to be plated was a plating catalyst-containing epoxy resin insulating substrate ACL-141N (trade name, manufactured by Hitachi Chemical Co., Ltd.) on which a plating resist was formed except for the circuit areas. Each plating process is 9
Soaked for a period of time. At this time, a vibromixer manufactured by Reika Kogyo Co., Ltd. was used as the oxygen supply tank, and the vibration frequency was 15.
Hz. Dissolved oxygen was measured using a polarographic method manufactured by Orbis Fair Laboratories, Inc., and the plating solution was cooled to 45°C. Oxygen gas was injected from an oxygen gas cylinder into the above-mentioned oxygen supply tank at a pressure that was measured in advance so as to provide a predetermined amount of dissolved oxygen.

Example 1 During the first plating, the amount of dissolved oxygen was 6p for the initial 2 hours.
For the remaining 7 hours, the amount of dissolved oxygen was 7 ppm. As a result, no abnormality occurred in the plating, the deposition rate was as high as 4 μm/hr, and the elongation rate of copper was as high as 12%. The elongation rate of copper could be increased to 21% by heat treatment.

Example 2 Plating was performed 10 times, the first time was performed in the same manner as in Example 1, and the remaining plating was performed by increasing the amount of dissolved oxygen according to the increase in specific gravity, as shown in FIG. did. The relationship between dissolved oxygen concentration and specific gravity measured at this time is shown in FIG. As a result, in the second and subsequent platings, no abnormalities occurred in the plating, and the deposition rate did not decrease. In addition, the elongation rate of copper is high at 7-8%, and when heat treated it increases to 15-19%.
I was able to do it.

Comparative Example 1 The composition of the plating solution and other conditions were the same, the supply of oxygen was stopped, and air was stirred by bubbling air from the bottom of the plating tank. As a result, the deposition rate of plating was the same, but
Copper rain occurred, and the circuits were short-circuited due to the abnormally deposited copper.

[0037]

Effects of the Invention As described above, by forcibly supplying oxygen in accordance with the increase in specific gravity of the plating solution, the plating method and oxygen supply device therefor exhibit the following effects. And a plating apparatus for carrying out this method can be provided.

[0038] No special additives or expensive chemicals are required to stabilize the plating solution.

[0039] The plating deposition rate can be made higher than before.

Even though the plating deposition rate is increased, copper precipitation does not occur.

[0041] Since no additives are required, there is no need to treat the waste liquid, making it safe and economical.

[0042] It is economical because no equipment or treatment is required to remove harmful by-products.

[0043] To stabilize the plating solution, it is sufficient to measure the specific gravity and control the amount of oxygen supplied based on the result, and the equipment is simple and its management and maintenance are simple.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an oxygen supply device used in an embodiment of the present invention.

FIG. 2 is a schematic diagram showing the entire electroless copper plating apparatus used in another embodiment of the present invention.

FIG. 3 is a diagram showing the relationship between the specific gravity of a plating solution and the dissolved oxygen concentration of supplied oxygen, for explaining an embodiment of the present invention.

FIG. 4 is a diagram showing the relationship between the dissolved oxygen concentration detected in the plating solution and specific gravity as a result of oxygen supply according to an embodiment of the present invention.

[Explanation of symbols]

1. oxygen supply tank
2. Oxygen supply port 3. Plating solution inlet
4. Plating solution outlet 5. Static mixer element 6. Ejector

Claims (7)

[Claims] [Claim 1] Copper salt, complexing agent, reducing agent, pH adjuster,
Using an electroless copper plating solution containing a physical property improver and a non-cyanide stabilizer, plating is performed while increasing the dissolved oxygen concentration in the plating solution in the range of 6 to 20 ppm according to the increase in the specific gravity of the plating solution. An electroless copper plating method that is characterized by: 2. The electroless copper plating method according to claim 1, wherein the concentration of ethylenediaminetetraacetic acid as the complexing agent is 60 g/l or more. 3. The concentration of copper sulfate as a copper salt in the electroless copper plating solution is 10 g/l or more, the concentration of formalin as a reducing agent is 4.5 ml/l or more, and the pH is 12.4 or more. The electroless copper plating method according to claim 1 or 2, characterized in that: 4. A sealed oxygen supply tank having a plating solution inlet, a plating solution outlet, and an oxygen supply port, and a vibrating static mixer element, the static mixer element being connected to the oxygen supply tank. A device that supplies oxygen to an electroless copper plating solution that is housed in a tank. 5. An oxygen supply tank having a partition wall for separating gas and liquid therein and connected to a discharge port of a line mixer, a vibrating static mixer element, and an inlet for a plating solution. and a line mixer equipped with an oxygen supply port and an ejector that pressurizes and sends out a mixed state of plating solution and oxygen, and the static mixer element is housed in the oxygen supply tank. A device that supplies oxygen to electroless copper plating solution. 6. The electroless device according to claim 4, further comprising means for controlling the amount of oxygen supplied between the oxygen supply port and an oxygen cylinder connected to the supply port. A device that supplies oxygen to the copper plating solution. 7. A plating tank, a means for taking out a part of the plating solution from the plating tank, a means for supplying oxygen to the taken out part of the plating solution in an oxygen supply tank, and a means for supplying oxygen to the part of the plating solution in the plating tank. The device has a means for monitoring specific gravity, a means for keeping records of the usage time of the plating solution, and a means for controlling the amount of oxygen to be supplied based on the usage time of the plating solution and the specific gravity of the plating solution. Electrolytic copper plating equipment.