JPH0697631A - Electroless plating method and equipment - Google Patents

Abstract

PURPOSE:To form an uniform plating layer in a through hole, and prevent copper and dust from adhering to a printed board, by keeping the ratio of the outer circulation flow rate of solution circulating in an electroless plating vessel to the inner circulation flow rate and the flow velocity to be in specified values, and using air bubble flow together with the plating solution flow. CONSTITUTION:An overflow dam 2 for outer circulation and a lower overflow dam 3 for inner circulation are installed in an electroless plating tank 1. By a circulating pump 4, the plating solution exceeding the overflow dam 2 is made to pass a filter 6, and to circulate from a plating solution supply port 8 of an outer circulation return channel 7 to the bottom part of the plating tank 1. In this case, the inner circulation amount is set as 5-10% of the outer circulation amount of the plating solution. A plurality of aeration pipes 10 for generating an air bubble flow composed of many fine air bubbles by using compressed air are installed at upper positions of the plating solution supply port 8. Thereby an uniform plating layer can be formed in a through hole.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroless plating method and apparatus for plating a printed circuit board,
The present invention relates to an electroless plating method and apparatus most suitable for through-hole plating in which small holes (through holes) formed between circuits on the upper and lower surfaces of a printed circuit board are electrically connected by copper plating.

[0002]

2. Description of the Related Art Generally, the electroless plating method (chemical plating method) involves immersing an object to be plated in a plating solution containing components such as a metal salt, a reducing agent, a pH adjusting agent, a complexing agent and a stabilizer. The metal salt ions are deposited on the object to be plated by the action of the reducing agent.

Compared with the electroplating method, this electroless plating method has more uniform deposition, and therefore has fewer defects and has a higher hardness,
Since it is excellent in corrosion resistance, wear resistance, magnetic characteristics, solderability, and the like, and has the advantage that a rectifier or the like is unnecessary, it has come to be used in many fields in recent years. For example, in the field of electronic device manufacturing, it is used as a base material for gold plating on a transparent conductive film (ITO film, indium oxide film, tin oxide film, etc.) formed on a glass substrate or a ceramic substrate. There is.

By the way, the electroless copper plating method (chemical copper plating method) is also used as an undercoat for plastics, but the electroless copper plating method can form a predetermined plating layer on the inner surface of a small hole. It has become an integral part of printed circuit board manufacturing. If a printed wiring board is required to have a higher density in the future, the board thickness / hole diameter ratio, that is, the aspect ratio, is more than twice the aspect ratio of the conventional printed circuit board, regardless of whether it is used in multiple layers or a small diameter. Becomes

Therefore, when the conventional electroless copper plating method is used, the throwing power of the plating in the through holes is extremely deteriorated, and it is difficult to form a uniform plating layer in the through holes. Become.

Therefore, when the electroless copper plating method is used, an invention aimed at forming a uniform plating layer in the through hole is disclosed in JP-A-59-161895 and JP-A-63-163. 83282, JP-A-63-3
There are those disclosed in JP-A-12983, JP-A-1-263278 and the like.

[0007]

However, in the invention disclosed in the above-mentioned Japanese Patent Laid-Open No. 59-161895, the electroless copper plating solution having a constant temperature and concentration is flown in parallel to the substrate in the plating tank. As a result, the flow rate of the plating solution is made uniform over the entire surface of the substrate, whereby the temperature and concentration distribution of the plating solution is averaged over the entire surface of the substrate, and the plating bath in the through hole is formed to have a uniform thickness.

The invention disclosed in Japanese Unexamined Patent Publication No. 63-83282 is a copper which prevents sodium sulfate from accumulating in a chemical copper plating solution by the addition of copper oxide and extends the life of the plating solution. This is a smooth copper ion replenishment method that establishes an ion replenishment method and also prevents abnormal copper deposition on the tank wall and decomposition of the liquid.

Further, the invention disclosed in Japanese Patent Laid-Open No. 63-312983 is an electroless copper plating method for an additive method printed wiring board having a fine circuit, which contains oxygen having a bubble diameter of 0.5 mm or less in a plating solution. Disperses the gas to keep the dissolved oxygen concentration in the electroless copper plating tank locally uniform and constant, and suppresses the stop of plating reaction and abnormal deposition of plating when plating fine and dense wiring patterns. Is what you are trying to do.

In the invention disclosed in Japanese Patent Laid-Open No. 1-263278, the decomposition reaction of the conventional plating solution progresses little by little during the plating operation or the storage, so that 1-2
Precipitation of copper on the plating tank wall, etc. may become remarkable after a week, so to prevent these, self-decomposition reaction is suppressed, stability is improved, and it can be used for a long time without trouble and is excellent. It is intended to provide an electroless copper plating solution capable of forming a plating film.

However, in the inventions disclosed in the above publications, no consideration has been given to the ratio of the external circulation flow rate to the internal circulation flow rate of the circulating plating solution and the flow velocity thereof. When the plating method is used, stirring of the plating solution is not uniform and 5 μm / h
Even if the deposition rate is about r, the film thickness is not uniform,
The flow of the liquid in the through-hole of the printed circuit board is poor, and there is a drawback that the thickness of the film in the through-hole is smaller than that on the surface of the substrate with a high aspect ratio, and the liquid is not completely circulated in the plating tank because the circulation of the plating liquid is not perfect. There is a problem in that a stagnant portion is generated, and copper powder, dust, and the like generated in the plating solution at the stagnant portion of the solution adheres to the printed circuit board, and the surface thereof becomes rough.

Therefore, the inventors of the present invention made an external circulation flow and an internal circulation flow so as to make the circulating plating solution in the electroless plating tank uniform, and conducted research by paying attention to the ratio and the flow rate. As a result, a uniform plating layer can be formed in the through holes by setting the flow rate ratio and its flow rate to specific values, and by using the bubble flow together, and further, copper powder and dust generated in the plating solution can be printed. It has been found that adhesion to the substrate can be prevented and the surface layer circuit can be made higher in density.

The present invention is based on the above knowledge, and maintains the ratio of the external circulation flow rate and the internal circulation flow rate of the circulating plating solution in the electroless plating tank and the flow rate thereof to a specific value, and uses the bubble flow together. By doing so, it is possible to form a uniform plating layer in the through holes, and to form a uniform plating layer without depositing a copper plating layer having a thickness larger than necessary on the substrate surface layer,
Further, it is an object of the present invention to provide an electroless plating method and apparatus capable of preventing copper powder, dust, etc. generated in a plating solution from adhering to a printed circuit board and enabling high density of surface layer circuits.

[0014]

In order to achieve the above-mentioned object, the present invention reduces the metal ions in the circulating plating solution by the action of a reducing agent and deposits them on the printed circuit board. In the electroless plating method of plating on the above, while supplying a bubble flow consisting of a large number of fine bubbles from below in the plating tank to the printed circuit boards suspended in an electroless plating tank and arranged in large numbers, The electroless plating method is characterized in that an internal circulation amount is adjusted to 5 to 10% with respect to an external circulation amount of the plating solution by an overflow weir provided in a plating tank.

Further, the present invention provides an electroless plating apparatus for plating on a printed board by reducing metal ions in a circulating plating solution by the action of a reducing agent to deposit on the printed board. A plurality of the aforesaid printed circuit boards are arranged side by side in a tank and suspended, and a plurality of aeration pipes for supplying a bubble flow composed of a large number of fine bubbles to the printed circuit board from below in the plating tank are returned to the plating tank from an external circulation channel. The electroless plating apparatus is characterized in that an overflow weir for internal circulation, which is provided above the plating solution supply port and lower than an overflow weir for external circulation of the plating tank, is provided.

[0016]

Next, the operation of the present invention will be described. An electroless plating tank containing a plating solution is provided with an overflow weir for internal circulation, which is lower than the overflow weir for external circulation, while generating a bubble flow composed of a large number of fine bubbles. A plurality of aeration pipes to be supplied are installed above the plating solution supply port where the plating solution is cleaned and returned from the external circulation flow path to the plating tank to perform aeration, and at the same time the plating solution in the tank is circulated by a pump device. On the other hand, a large number of printed circuit boards are arranged and suspended in the plating solution. Then, the pump device for circulating the plating solution is adjusted so that the amount of the plating solution flowing to the external circulation flow passage beyond the overflow weir for external circulation provided in the plating tank is exceeded by the overflow weir for internal circulation. The amount of the plating solution returning from the circulation channel to the plating tank is adjusted to 5 to 10%. As a result, copper powder and dust generated in the plating solution will not adhere to the printed circuit board,
Further, a uniform plating layer is formed in the through hole.

[0017]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a vertical sectional view of this embodiment, and FIG. 2 is a top view thereof. In this embodiment, an overflow weir 2 for external circulation is provided in an electroless plating tank 1 filled with a plating solution, and an overflow weir 3 for internal circulation, which is lower than the overflow weir 2 for external circulation, is provided inside the overflow weir 3 for external circulation. The plating solution that has flowed over the overflow weir 2 is introduced into the external circulation flow path 5 that communicates with the circulation pump 4, passes through the filter 6 by the circulation pump 4, and then returns to the plating tank 1 in the external circulation return flow path 7. It is circulated from the liquid supply port 8 to the bottom of the plating tank. In addition, the overflow weir 3 for internal circulation
The plating solution having flowed over the internal circulation overflow weir 3 flows into the internal circulation flow path 9 formed between the internal circulation flow channel 9 and the external circulation overflow weir 2.

On the other hand, a plurality of aeration pipes 10 for generating a bubble flow composed of a large number of fine bubbles by compressed air produced by a compressor (not shown) are provided at the bottom of the plating tank 1, and the plating solution is filled with the plating solution 1. The external circulation return passage 7 is provided at a position above the plating solution supply port 8. In this embodiment, there are three external circulation flow paths 5 and three external circulation return flow paths 7, respectively.

Next, a basic experiment carried out to confirm the effect of this embodiment will be described. The first experiment relates to the heights of two overflow weirs provided in the electroless plating tank 1 in order to make the flow of the plating solution in the electroless plating tank 1 uniform when an overflow weir for internal and external circulation is provided. This is an experiment investigating the mutual relationship. In this experiment, a transparent experimental tank was created and observed. FIG. 3 shows the state of the flow of the plating solution when the heights of the overflow weirs 2 and 3 for internal and external circulation are made equal, and the flow of the plating solution in the electroless plating tank 1 is good, but there are two liquid levels. A step is formed between the overflow weirs 2 and 3, and there is a slight problem in discharging copper powder, dust, etc. generated in the plating tank during electroless plating.

FIG. 4 shows the flow of the plating solution when the inner circulation overflow weir 3 is made higher than the outer circulation overflow weir 2, and in this case, the overflow weirs 2 and 3 having two liquid levels are provided. It has been found that there is a gap between them, and since a backflow is generated in the internal circulation flow path 9, the flow of the plating solution in the electroless plating tank 1 is stagnant or backflowed, which is not suitable for electroless plating.

FIG. 5 shows the flow of the plating solution when the inner circulation overflow weir 3 is lower than the outer circulation overflow weir 2. In this case, the liquid surface is formed flat and electroless plating is performed. It was found that the flow of the plating solution in the tank 1 was good and that the flow of the plating solution did not stagnate or flow backward. In this case, since the liquid surface is flat and no step can be formed between the two overflow weirs 2 and 3, the discharge of copper powder, dust, etc. generated in the plating tank during electroless plating was also good.

The second experiment is an experiment on the position of the aeration pipe provided in the electroless plating tank 1 when the overflow weir for internal / external circulation is provided, and is for the plating solution supply port 8 of the external circulation return passage 7. It is an experiment investigating the mutual relation regarding the position of the aeration pipe 10. FIG. 6 shows the flow of the plating solution when the aeration pipe 10 is provided lower than the plating solution supply port 8 of the return channel 7, and in this case, the plating solution supplied to the plating tank 1 is It was found that the flow of the plating solution in the tank 1 was not uniform because the rising force of the aeration from the aeration pipe 10 was too strong to reach the back of the plating tank 1.

FIG. 7 shows the situation of the flow of the plating solution when the plating solution supply port 8 of the return channel 7 and the aeration pipe 10 are provided at the same height, and in this case as well, the plating solution is supplied to the plating tank 1. The plating solution is aeration pipe 10
It was found that the flow of the plating solution in the tank 1 was not uniform because the rising force of the aeration from the above was too strong to reach the inside of the plating tank 1.

FIG. 8 shows the flow of the plating solution when the aeration pipe 10 is provided higher than the plating solution supply port 8 of the return channel 7, and in this case, the plating solution was supplied to the plating tank 1. It was found that the plating solution flows under the aeration pipe 10 and reaches the depth of the plating tank 1, so that the flow of the plating solution in the tank 1 rises uniformly.

Next, an experiment for clarifying the effect of the present invention will be described based on the above first and second basic experimental results. FIG. 9 shows an example of the present invention, and FIG. 10 shows a comparative example. The plating solution used in this experiment is KC-500 (Nichiko), the content of which is Cu
_{SO 4 · 5H 2 O 10g /} l, EDTA 28g / l,
Formalin 2 ml / l, PH 12.5, additive proper amount, and the plating solution temperature was set to 72 ° C.

In the present embodiment shown in FIG. 9, a large number of printed circuit boards 11 are suspended from above in a plating tank 1 and the overflow weir 3 for internal circulation is set lower than the overflow weir 2 for external circulation. 1 external overflow weir 2
5 to 10% of the amount of the plating solution that flows over the internal circulation flow path 9 beyond the internal circulation overflow weir 3 with respect to the amount of the plating solution that flows over the external circulation flow path 5
The height of the overflow weir 3 for inner circulation was adjusted to be 10 cm lower than the height of the overflow weir 2 for outer circulation. In addition, the amount of the plating solution returned from the external circulation channel 5 is set to 7
The aeration pipe 10 was set to a height of 50 liters with respect to the height of the plating solution supply port 8 of the return channel 7, and compressed air was sent by a compressor (not shown) to perform aeration.

On the other hand, in the comparative example shown in FIG. 10, a conventional plating tank is used, and a large number of printed circuit boards 11 are suspended from above and installed in the plating tank 1 as in the case of the present embodiment. The liquid was put in, discharged from the left side of the figure, and circulated.
Further, the aeration pipe 10 was installed near the bottom of the plating tank, and compressed air was sent by a compressor (not shown) to perform aeration. Table 1 shows the experimental results of the present example and the comparative example.

[0028]

[Table 1]

The throwing power in Table 1 above is the ratio of the thickness a of the plating film formed on the surface of the printed circuit board to the thickness b of the plating film in the through hole, that is, throwing power = b / a. Since it is shown by x100, the higher the value, the better. Further, the difference in the film thickness on the surface is the difference between the maximum value and the minimum value of the plating film formed on the surface of the printed circuit board. Therefore, the lower the difference, the more uniform the plating layer is formed. In addition, in the comparative example, in addition to the rise of the liquid due to the circulation of the plating liquid, the amount of winding of the plating liquid by air is quite large. The rolled plating liquid loses its place of travel and moves toward the bottom of the tank, forming a liquid pool. It was observed.

[0030]

As described above, according to the present invention, an electroless plating tank containing a plating solution is provided with an internal circulation overflow weir lower than the external circulation overflow weir,
A plurality of aeration pipes that supply a bubble flow consisting of a large number of fine bubbles are installed above the plating solution supply port where the plating solution is cleaned and returns from the external circulation flow path to the plating tank, over the overflow weir for external circulation. The amount of the plating solution flowing from the internal circulation channel into the plating tank beyond the internal circulation overflow weir is adjusted to 5 to 10% of the amount of the plating solution flowing to the external circulation channel. ,
Since the flow of the plating solution in the plating tank is a constant flow rate anywhere in the tank, a uniform plating layer can be formed in the through holes, and a copper plating layer with an excessive thickness is not deposited on the substrate surface layer and is uniform. It is possible to form a plating layer and prevent copper powder, dust, etc. generated in the plating solution from floating with air and being caught by the flow filter in the external circulation channel and adhering to the printed circuit board.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional explanatory view of the present embodiment.

FIG. 2 is a plan view of the present embodiment.

FIG. 3 is a vertical sectional view showing a first experimental state.

FIG. 4 is a vertical sectional view showing a first experimental state.

FIG. 5 is a vertical sectional view showing a first experimental state.

FIG. 6 is a vertical cross-sectional view showing a second experimental state.

FIG. 7 is a vertical sectional view showing a second experimental state.

FIG. 8 is a vertical sectional view showing a second experimental state.

FIG. 9 is a vertical cross-sectional view showing an experimental state of this example.

FIG. 10 is a vertical sectional view showing an experimental state of a comparative example.

[Explanation of symbols]

 1 Electroless plating tank 2 Overflow weir for external circulation 3 Overflow weir for internal circulation 4 Circulation pump 5 External circulation flow path 6 Filter 7 External circulation return flow path 8 Plating solution supply port 9 Internal circulation flow path 10 Aeration pipe 11 Printed circuit board

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ritsuji Toba No. 1 Horiyamashita, Hadano City, Kanagawa Prefecture Hiritsu Manufacturing Co., Ltd. Kanagawa Plant

Claims (2)

[Claims] 1. An electroless plating method in which metal ions in a circulating plating solution are reduced by the action of a reducing agent to be deposited on a printed circuit board to thereby plate the printed circuit board in an electroless plating bath. While supplying a bubble flow composed of a large number of fine bubbles from below in the plating tank to the printed circuit board that is suspended and arranged side by side, the overflow weir provided in the plating tank allows the plating solution to circulate externally. An electroless plating method characterized by adjusting an internal circulation amount to 5 to 10%. 2. An electroless plating apparatus for performing plating on a printed board by reducing metal ions in a circulating plating solution by the action of a reducing agent and depositing on the printed board, in an electroless plating tank. A plurality of aeration pipes are arranged side by side and suspended, and a plurality of aeration pipes that supply a bubble flow composed of a large number of fine bubbles to the printed circuit board from below in the plating tank are returned from the external circulation flow path to the plating tank. An electroless plating apparatus comprising an overflow weir for internal circulation, which is provided at a position above the mouth and is lower than an overflow weir for external circulation of the plating tank.