JPH0375376A - Circulating device for plating solution in electroless plating bath - Google Patents

Abstract

PURPOSE:To uniformize the temp. and concn. distribution of a plating soln. and to uniformize the thickness of a plating layer by joining the first and second plating soln. circulating lines respectively connected to the bottom and rear discharge ports of the plating bath to a supply pipe connected to the plating bath on its front plate side. CONSTITUTION:Many printed boards 3 are arranged orthogonally to the front plate and rear plate of the plating bath 1 at regular intervals, and plural rear discharge ports 7 and bottom discharge ports 6 are opened on the rear plate and the bottom plate close to the rear plate. The first and second plating soln. circulating lines A and B respectively joined to the discharge ports 6 and 7 are connected to the suction sides of the main circulating pump 8 and heat- exchange pump 16, and the delivery sides are connected to the common supply pipe 13 and joined. The supply pipe 13 is connected to the plating bath 1 on the front plate side of the board 3. The pumps 8 and 16 are driven to circulate the plating soln., and the plating soln. flows smoothly in parallel with the surface of the board 3.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an apparatus for circulating a plating solution in a plating bath when performing electroless plating on through holes of a printed circuit board.

(Prior Art) A printed circuit board with double-sided wiring provides conduction between both sides of the board by attaching a plating layer to the inner surface of a hole drilled in the board to form a through hole.

When forming a through hole, in the traditional electroplating method, a thin conductive layer is first applied to the inner surface of the hole by electroless plating, and then a plated layer is deposited thereon by electroplating. In comparison, the electroless plating method finishes a predetermined plating layer on the inner surface of the hole using only electroless plating, so it has the advantage that the plating layer can be formed uniformly and the process can be shortened.

(Problems to be Solved by the Invention) However, in the case of electroless plating, the thickness of the plating layer varies significantly depending on the temperature and concentration of the plating solution, which makes it difficult to handle.

However, in the past, the plating solution tended to stagnate in a part of the plating tank, causing unevenness in the temperature and concentration of the solution there, which was one of the reasons why the thickness of the plating layer was not uniform.

The present invention improves this by allowing the plating solution to flow parallel to the substrate in the electroless plating tank without stagnation, thereby controlling the temperature and concentration distribution of the plating solution in the tank. The purpose is to average the thickness and thereby form the plating layer of the through hole to have a uniform thickness.

(Means for Solving the Problems) In the present invention, a printed circuit board is installed between the front plate and the back plate of the plating tank in a direction perpendicular thereto, and the back board is installed on the back plate and the bottom plate of the plating tank near the back plate, respectively. Open the outlet and bottom outlet.

The bottom outlet is located at one end of the first plating solution circulation line.
The rear discharge ports are each connected to one end of the second plating solution circulation line, and the other ends of these first and second plating solution circulation lines are connected to a common supply pipe, and this supply pipe is connected to the plating tank. Connect closer to the front board than the printed circuit board.

(Function) The plating solution in the plating tank exits from the tank through the back outlet and the bottom outlet, passes through the first and second plating solution circulation lines, and returns to the plating tank via the supply pipe.

During this circulation process, the plating solution is heated and replenished, but in the present invention, the supply pipe is connected closer to the front plate of the plating tank than the printed circuit board, and the supply pipe is connected to the rear outlet and the bottom outlet near it. Since the supply pipe is located on the opposite side of the printed circuit board, the plating solution discharged from the supply pipe crosses the plating tank from the front side to the back side.

Since the printed circuit board in the plating tank is installed along a direction perpendicular to the front plate and the rear plate, the plating solution flows parallel to the surface of the printed circuit board.

Therefore, the flow of the plating solution in the plating bath is smooth and does not stagnate, so that the temperature and concentration distribution of the plating solution on the surface of the printed circuit board are averaged, and the thickness of the plating layer in the through hole is made uniform.

(Example) An example of the present invention will be described with reference to the drawings. Reference numeral 1 is an electroless steel plating tank with a rectangular plane. A substrate storage basket 2 is suspended from the open top surface of the tank, and the substrates inside the basket are 3 are installed in parallel in the short side direction of the plating bath 1. On the board 3,
Catalyst fine particles are adhesively applied in advance, and a resist is printed on the top surface according to the wiring pattern.

Then, the bottom plate 5 is gently inclined toward the back plate 4 that constitutes one long side of the plating tank 1, and a plurality of bottom outlet ports 6 are opened in the deepest part of the bottom plate 5 near the back plate 4. The bottom outlet 6 is located above the bottom outlet 6.
The same number of rear discharge ports 7 are opened.

Next, the plurality of bottom discharge ports 6 are combined and piped to the suction side of one main circulation pump 8, and the discharge side thereof is piped to the inlet of the mixing chamber 10 via the filter 9.

Reference numeral 11 denotes an automatic liquid replenisher, whose discharge pipe 11a is connected to a mixing chamber 10, and the outlet of the mixing chamber 10 is connected to a supply pipe 13 via a confluence pipe 12.

14 is a front plate 15 that constitutes the other long side of the plating tank l.
This perforated plate 14 is installed in the tank parallel to the perforated plate 14.
The open end of the supply pipe 13 is installed between the front plate 15 and the front plate 15.

In addition, the plurality of back discharge ports 7 are combined and piped to the suction side of one heat exchange system pump 16, and the discharge side is piped to the supply pipe 13 from a known heat exchanger 17 via the above-mentioned merging pipe 12. do. The heat exchanger 17 has a structure in which a large number of plastic tubes 17a extending from the inflow side to the outflow side are housed in an outer cylinder 17b, and the fluid inside the tubes 17a can be heated by injecting steam or hot water into the outer cylinder 17b. , or cool the fluid in tube 17a by injecting cold water.

Here, the path from the bottom outlet 6 to the supply tube 13 via the pump 8, filter 9, mixing chamber 10 and merging pipe 12 is the first plating liquid circulation line A, and from the back outlet 7 to the pump 16, the heat exchanger 17 and the confluence pipe 12 to the supply pipe 13 is the second plating liquid circulation line B.
It is.

Reference numeral 18 denotes a number of air diffuser pipes arranged at equal intervals along the short side of the bottom of the plating tank 1. These pipes are connected to an air pump (not shown), and are used to diffuse countless air bubbles into the tank through a large number of small holes drilled in the pipe wall. to suppress excessive liquid decomposition of the electroless copper plating solution, which will be described later.

19 is an injection pipe for newly injecting plating solution into plating tank 1;
Connect to a liquid tank (not shown).

Then, a specified amount of a low-temperature, inert electroless copper plating solution is injected into the plating bath l through the injection pipe 19. The electroless plating solution is a mixture of sulfuric acid steel and hydric soda, mixed with formalin, ethylenediaminetetraacid, and an additive solution dissolved in pure water, and is activated at a high temperature exceeding a specified temperature. It decomposes into a liquid and remains inactive at lower temperatures. When plating with nickel, an electroless copper plating solution may be used instead of electroless steel plating.

After the liquid is injected, the pumps 8 and 16 are driven, and the plating liquid in the tank is discharged from the bottom outlet 6 and the back outlet 7.
Plating solution circulation line A and second plating solution circulation line B
A common supply pipe 13 for these plating solution circulation lines
Then, return it to the plating bath L.

The liquid in the supply pipe 13 flows out in a dispersed manner from the entire surface of the perforated plate 14 while its flow rate is moderated by the perforated plate 14, and is discharged from the outlet 6.7 on the opposite side to the supply pipe 13.

Repeat this.

Here, since the discharge port opens not only at the bottom surface of the plating tank but also at the back surface, the plating solution flowing out from the porous plate 14 flows in the direction of the discharge port 7 on the back surface and does not concentrate at the bottom surface discharge port 6.

Therefore, the plating solution flows horizontally across the entire plating bath in parallel to the printed circuit board, and the plating solution does not stagnate in one place.

The plating solution flowing through the second plating solution circulation line B is heated by steam or hot water in the heat exchanger 17, and when the plating solution in the tank exceeds a predetermined temperature, the solution decomposes due to catalytic action, and the palladium on the substrate 3 is heated. Metallic copper is deposited on the exposed portions, thereby forming through holes and wiring patterns.

Copper may precipitate around the catalyst separated from the substrate 3 and settle at the bottom of the plating tank 1, but such precipitates are adsorbed by the filter 9 through the bottom outlet 6 and the first plating solution circulation line A. and removed.

It takes a long time to finish the plating layer to a predetermined thickness (for example, 30 microns), but during that time, the plating solution that has evaporated or disappeared from the plating tank 1 is removed by the automatic solution replenisher 11.
The plating solution is replenished into the plating bath 1 via the first plating solution circulation line A via the mixing channel/<10.

The plating solution flowing through the first plating solution circulation line A is mixed with the high temperature plating solution from the heating line B in the confluence pipe 12, and then dispersed into the tank through the common supply pipe 13 through the perforated plate 14. The mixture is further mixed integrally with the perforated plate 14.

Then, this plating solution flows out from the plate surface of the perforated plate 14,
It flows parallel to the substrate and does not stagnate due to convection or vortices during the flow, and flows at a uniform flow rate throughout the tank. Therefore, the concentration and temperature of the plating solution over the entire surface of the substrate 3 are distributed evenly, so that the thickness of the plating layer is uniform in both the through holes close to the supply pipe 13 and the through holes far away from the supply pipe 13.

When plating is completed, the substrate storage basket 2 is pulled up and moved to the cleaning process. After moving, the pump 8.16 continues to be driven, and cooling water is injected into the heat exchanger 17 to lower the temperature of the plating solution, returning it to an inert state, and then draining the plating solution in the tank into the solution tank. , instead, add a cleaning solution to clean the plating tank l.

(Effects of the Invention) In short, in the present invention, when circulating the plating solution in the plating tank, the supply pipe for returning the plating solution to the plating tank is connected near the front plate of the plating tank, and on the other hand, the plating solution is transferred outside the tank. The plating liquid flows across the tank from the front plate side to the back plate side because the discharge ports for discharging the plating liquid are opened in the back plate and the bottom plate of the plating tank as a back discharge port and a bottom face discharge port, respectively.

Moreover, since the printed circuit board is installed in a direction perpendicular to these front and back plates, the plating solution flows parallel to the printed circuit board and is not obstructed by the board.

Therefore, the plating solution does not stagnate near the substrate 3 but flows smoothly, and the temperature distribution and concentration distribution of the plating solution over the entire surface of the substrate 3 are averaged.

Therefore, according to the present invention, it is possible to form a plated layer with a uniform thickness for all through holes located at any position within the tank.

[Brief explanation of drawings]

FIG. 1 is a flow diagram of the entire through-hole plating apparatus for printed circuit boards embodying the present invention, FIG. 2 is a plan view of its two-hole plating tank, and FIG. 3 is a cross-sectional view of FIG. 2. 1 is the plating tank, 3 is the printed circuit board, A is the first plating solution circulation line, B is the second plating solution circulation line, 6 is the bottom outlet, 7 is the back outlet, 8 is the main circulation pump, 13 is the supply pipe , 14 is a perforated plate, and 16 is a thermographic pump.

Claims (1)

[Claims] A large number of printed circuit boards are installed at intervals in a direction orthogonal to the front plate and the back plate of a plating bath for electroless plating, and each printed circuit board is installed on the bottom plate of the plating bath near the back plate and the back plate. A back outlet and a bottom outlet are opened, and the bottom outlet is connected to one end of the first plating solution circulation line, and the back outlet is connected to one end of the second plating solution circulation line. A plating solution circulation device for an electroless plating tank, in which the other ends of two plating solution circulation lines are piped to a common supply pipe to join together, and this supply pipe is connected closer to the front plate than the printed circuit board of the plating tank. .