Method of electroplating printed circuit board using magnetic field having periodic directionality

Abstract

Disclosed is a method of electroplating a printed circuit board using a magnetic field having periodic directionality, in which the printed circuit board is electroplated using a magnetic field that is generated substantially perpendicular to a current direction and that has periodic directionality between a forward wave and a reverse wave. According to the method of the present invention, since the magnetic field having the periodic directionality is used, the aging of the plating solution may be prevented and the ionic concentration thereof may be made uniform even without the use of an external liquid flow device, and as well, the substrate may be uniformly plated.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of electroplating a printed circuit board (PCB) using a magnetic field having periodic directionality. More particularly, the present invention relates to a method of electroplating a PCB, capable of realizing uniform plating while preventing the aging of a plating solution without the need for an additional external liquid flow device, using a magnetic field which is generated substantially perpendicular to the current direction and which has periodic directionality between a forward wave and a reverse wave.

2. Description of the Related Art

Generally, PCBs for use in the operation of electronic products are produced through numerous fabrication processes comprising complicated process steps. Among the fabrication processes, a plating process, that is, a metallization process, plays an important role in the application of current and voltage to electronic devices. Moreover, according to the recent trend toward the fabrication of light, slim, short and small PCBs, since the sizes of via holes used in the connection of metal wires and electronic devices are further decreased, a more uniform plating process is required.

In order to form a circuit or plate a via hole in the PCB plating process, a technique of forming a seed layer using an electroless chemical plating process and then forming a metal layer through electroplating is typically employed. Such a technique is advantageous because it is relatively inexpensive and also because a uniform plating layer may be formed in the case where the distance between the metal wire and the insulating layer is 60 μm or less and the via hole has a large diameter of 100 μm or more.

In this regard, FIG. 1 schematically illustrates the plating device used in the method of electroplating the PCB, according to a conventional technique.

As shown in FIG. 1, in a plating bath 1 filled with an electrolyte solution 3, voltage is applied from a power source 6 to an anode 2 for transferring a plating ion and a substrate 4 (cathode) on which a wire is formed, according to Faraday's law, such that current may flow to conduct a plating process .

However, with the recent requirements for miniaturization and high functionality of the electronic products, the size of the PCB should be drastically decreased in order to increase the mounting densities of electronic devices. Under these circumstances, conventional plating techniques are incapable of fill-plating a via hole having a high aspect ratio or of realizing a uniform plating layer for the formation of a metal wire having a fine pattern. To solve such problems, general electroplating methods using magnetic fields have been proposed.

In light of this, although Japanese Patent Laid-open Publication Nos. Hei. 7-169714 and 5-78885 disclose an electroplating method using a magnetic field, aging of the plating solution and non-uniform ionic concentration thereof are caused by the application of a magnetic field having a fixed flow direction, undesirably resulting in non-uniform plating.

FIG. 2 illustrates the plating device used in the electroplating method using a magnetic field according to the conventional technique. In the conventional electroplating method using a magnetic field, when voltage is applied using a power source 6, the plating ion is detached from the anode 2 and then deposited on the substrate 4 through the electrolyte solution 3 in the plating bath 1. In this case, when a magnetic field is applied perpendicular to a current direction using a magnetic field generator 5 (electromagnets or permanent magnets), the flow of the plating ion is activated by Lorentz force. In FIG. 2, if the magnetic field generator is not supplied, the device of FIG. 2 is equal to the plating device shown in FIG. 1.

However, in the case of the magnetic field application electroplating method using the plating device of FIG. 2, because the direction of the generated magnetic field is fixed, unidirectional Lorentz force in the plating device results. Accordingly, the fixed flow direction of the magnetic field may negatively affect the ionic concentration gradient (FIG. 3).

Ultimately, there is an urgent need for a method of electroplating a PCB, which can control the flow direction and density of the plating ion present in the plating solution without the use of an additional external liquid flow device to retard the aging of the plating solution and to uniformly plate the substrate.

SUMMARY OF THE INVENTION

Leading to the present invention, intensive and thorough research on electroplating methods using a magnetic field, carried out by the present inventors, aiming to overcome the problems encountered in the prior art, resulted in the finding that the direction of the magnetic field is periodically changed to control the flow direction and density of the plating ion, thereby increasing flow of the electrolyte solution and decreasing the variation in plating, leading to the formation of a uniform plating layer.

Accordingly, one aspect of the present invention is to provide a method for electroplating on a PCB using a magnetic field having periodic directionality, which can prevent aging of a plating solution and can make the ionic concentration of the plating solution uniform without the use of an additional external liquid flow device.

Another aspect of the present invention is to provide a method of electroplating a PCB using a magnetic field having periodic directionality, which can form a uniform plating layer on the PCB.

According to a preferred embodiment, the present invention provides a method of electroplating a PCB using a magnetic field having periodic directionality, comprising (a) bringing an electrolyte solution into contact with the surface of a PCB; and (b) simultaneously applying a magnetic field to the electrolyte solution and applying voltage between the PCB and the electrolyte solution, wherein the magnetic field is generated substantially perpendicular to the current direction and has periodic directionality between a forward wave and a reverse wave.

As such, the magnetic field may be supplied using permanent magnets, and the magnetic field having the periodic directionality is preferably supplied through the rotation of a pair of permanent magnets.

In addition, the magnetic field may be supplied using electromagnets. According to a first preferred embodiment of the present invention, the magnetic field having the periodic directionality may be supplied through change in a current waveform using a pair of electromagnets and a pulse modulator/demodulator connected thereto. According to a second preferred embodiment of the present invention, the magnetic field having the periodic directionality may be supplied through change in a current waveform using a pair of electromagnets and a reverse pulse rectifier connected thereto.

In a preferred embodiment of the present invention, the electrolyte solution may include a copper ion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the plating device used in a method of electroplating a PCB according to a conventional technique;

FIG. 2 is a schematic view showing the plating device used in a method of electroplating a PCB according to another conventional technique;

FIG. 3 is a view showing the direction of Lorentz force with respect to the directions of current and magnetic field in the plating device of FIG. 2;

FIG. 4 is a schematic view showing the plating device used in a method of electroplating a PCB, according to a first embodiment of the present invention;

FIG. 5 is a schematic view showing the plating device used in a method of electroplating a PCB, according to a second embodiment of the present invention;

FIG. 6 is a schematic view showing the plating device used in a method of electroplating a PCB, according to a third embodiment of the present invention;

FIGS. 7A to 7D are schematic views showing the current waveforms resulting from the electroplating method using the plating devices of FIGS. 4 and 5;

FIGS. 8A and 8B are views showing the direction of Lorentz force, varying depending on the direction of a magnetic field, in the plating method of the present invention;

FIGS. 9A to 9C are electron micrographs (×500) showing the cross-sections of the plated substrates in the example and comparative examples of the present invention; and

FIGS. 10A to 10C are SEM photographs (×30,000) showing the surfaces of the plated substrates in the example and comparative examples of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a detailed description will be given of the present invention, with reference to the appended drawings.

According to the present invention, a method which can be used to form a uniform plating layer on a PCB using a magnetic field is provided. To this end, the method of the present invention is characterized in that a magnetic field is applied to a plating solution so as to control the flow direction and density of the ion in the plating solution. When the magnetic field is applied to the electrolyte solution, an electric field and a magnetic field induce large convection flow through Lorentz force, and thus small MHD (Magneto Hydrodynamics) flow is generated near the cathode on which the plating process is conducted so as to make the plating process uniform. Also, the applied magnetic field functions to enlarge the mean free path of the ion near the plating surface so as to increase the quantity of charge. Thereby, the current amount is increased, leading to an increased plating rate.

Further, the method of the present invention is characterized in that the direction of the magnetic field is periodically changed, thus increasing the flow of the electrolyte solution and decreasing the variation in plating. Accordingly, through the method of the present invention, excellent plating may be conducted on the PCB.

FIGS. 4 to 6 illustrate the plating devices used in the method of electroplating the PCB, according to the preferred embodiments of the present invention.

As shown in FIG. 4, a plating bath 101 is filled with an electrolyte solution. 103 containing a plating metal ion, preferably a copper ion, and a PCB 104 (cathode), on which a circuit is formed, is supported by a support member and thus is in contact with the electrolyte solution 103.

While a magnetic field is applied to the electrolyte solution 103 using a magnetic field generator 105a, that is, electromagnets 105a, voltage is applied between the substrate 104 and the electrolyte solution 103 using a power source 107 such that current flows. During such a procedure, ions in the plating solution are detached from the anode 102 and then deposited on the substrate 104 through the electrolyte solution 103, thus forming a plating layer.

In such a case, the magnetic field is applied substantially perpendicular to the current direction using the electromagnets 105a, and therefore the flow of the plating ion is activated by Lorentz force. Simultaneously, using a current pulse modulator/demodulator 106a connected between the electromagnets 105a, the phase of current may be varied over time, as shown in FIGS. 7A to 7D. Accordingly, the direction of the magnetic field is periodically changed between the forward wave and the reverse wave, thereby varying the direction of Lorentz force.

Below, respective waveforms of FIGS. 7A to 7D are specifically described.

The case of FIG. 7A is suitable for use in plating a substrate that has a via hole having a large diameter of 100 μm or more and requires large liquid flow due to the simple phase change of current waveform. In the case of FIG. 7B, since a pause time of current is present, the thickness of a hydraulic boundary layer may be adjusted through the control of such a pause time, thus realizing the plating of a substrate having a complicated pattern. In the case of FIG. 7C, the small flow of the magnetic field may be generated due to the fine change in current and thus the throwing power of the plating may be increased with respect to a substrate having both a via hole and a pattern. Therefore, such a waveform is suitable for plating a substrate having a pattern and a via hole. Since the case of FIG. 7D helps greatly increase the small throwing power through application of the same fine waveforms having a short period, it is suitable for use in plating a substrate having a small via hole having a diameter of 50 μm.

Hence, as shown in FIGS. 8A and 8B, the direction of Lorentz force is periodically changed, such that no concentration gradient of the electrolyte ion is induced.

In contrast, in the case of the plating device used in the conventional electroplating method as in FIG. 1, if external flow is not provided, during the plating, the concentration of the plating ion capable of being deposited near the substrate (cathode) becomes 0. At this point, the current applied to the substrate is referred to as a threshold current density. In this way, when the current reaches a threshold value, the plating process is not further conducted and the current density is drastically decreased. To alleviate the problems, although an external flow device, for example, an air blower or an E-ductor, may be used, it may negatively affect the formation of a uniform plating layer. Thus, the control of such flow is regarded as important in the fabrication of a fine metal wire substrate, such as a PCB.

According to the present invention, using the current pulse modulator/demodulator 106a as shown in FIG. 4, the current direction may be changed as in FIGS. 7A to 7D, such that the N pole and S pole of the electromagnet are alternately changed as in FIGS. 8A and 8B. Thereby, periodic change in Lorentz force is caused which therefore contributes to making the concentration gradient of the plating solution uniform. Further, MHD convection flow is induced near the substrate to realize the formation of a uniform plating layer without the use of an additional device, such as an air blower or an E-ductor, functioning to negatively affect the uniform plating process.

In the case of the magnetic field applied in the present invention, the period and interval of the magnetic field are depicted in FIGS. 7A to 7D. The magnetic field increases in proportion to the magnitude of current inducing the magnetic field depending on the law of Lorentz force. The magnetic field is proportional to the magnitude of current as represented by the following equation: $\Delta B=\frac{{\mu }_0}{4\pi }\frac{I\Delta \stackrel{\rightharpoonup }{l}\times \stackrel{\rightharpoonup }{r}}{r^2}$
wherein ^B is the magnitude of a magnetic field, ^{{right arrow over (l)}} is the extension distance of a magnetic field, ^I is a current, ^r is the radius of a conductor for inducing a magnetic field, ^{{right arrow over (Y)}} is a unit vector in a current flow direction, μ₀ is a dielectric constant related to magnetism, and ^πis a ratio of a circumference of a circle to its diameter. The magnetic field thus obtained functions to transfer the force in a direction perpendicular to the charge direction through the law of Lorentz force as represented by the following equation:
{right arrow over (F)}=q{right arrow over (ν)}×{right arrow over (B)}
wherein ^{{right arrow over (F)}} is the direction of force, ^q is the charge, and ^{{right arrow over (ν)}} is the directional vector of the charge. Thus, the extent of liquid flow in the plating bath may be adjusted by controlling the magnitude of current inducing the magnetic field. In the present invention, the magnitude of the magnetic field suitable for plating the substrate preferably falls in the range of 0.1 T to 10 T. If the magnitude of the magnetic field is beyond the above range, there is no effect thereof, or an effect impeding the movement of the ion may result.

The plating time and the current density are proportional to the plating thickness according to Faraday's law. $d=\frac{M_{w}I\Delta t}{nFAD}$

wherein ^d is the plating thickness, ⁿ is the number of charges, ^F is the Faraday constant, ^A is the area of the plating surface, ^D is the metal density, ^Mw is the atomic weight of metal, ^I is the current density, and ^Δt is the plating time. That is, when the plating time and the current density are increased, the plating thickness may be increased. In the present invention, a method of increasing the current density for plating within the same time using a magnetic field to improve the plating rate is proposed.

FIG. 5 illustrates the plating device using a reverse pulse rectifier 106b, in which the pulse modulator/demodulator 106 and the power source 107 of FIG. 4 are combined. Even with the use of such a device, the current waveforms of FIGS. 7A to 7D may also be formed, and thus the direction of the magnetic field of FIGS. 8A and 8B is periodically changed, leading to uniform plating.

FIG. 6 is a view showing the plating device for periodic application of the magnetic field through the rotation of permanent magnets 105b. The means for rotating the permanent magnets 105b is not particularly limited so long as it is known in the art. For example, while a fan belt 109 connected to a rotation motor 108 is moved, the permanent magnets 105b provided to both ends thereof are rotated, but the present invention is not limited thereto. Thereby, the periodic change in the direction of the magnetic field is caused as in FIGS. 8A and 8B, consequently forming a uniform plating layer.

For reference, the substrate actually used in the plating process has a size of about 40.5 cm (width)×51 cm (length). Thus, the plating bath should have a volume capable of accommodating one to four substrates. Such a substrate may have a via hole of 50˜120 μm and a pattern having a size of 100 μm.

In the present invention, since the periodically changeable magnetic field is applied, the trajectory of the metal ion is made more precise through the MHD convection flow than in a conventional unidirectional magnetic plating device. Hence, even though no external liquid flow device, such as an air blower or an E-ductor, is additionally provided, a via hole having an aspect ratio about 1˜2 times higher than conventional via holes may be plated, or a fine pattern having a via hole diameter of about 20˜60 μm may be uniformly plated.

Conventionally, upon continuous application of the unidirectional magnetic field, in the case of using the electromagnets, the aging of the plating solution rapidly progresses, or the ionic concentration of the plating solution may be undesirably inclined in only one direction by unidirectional Lorentz force. However, according to the present invention, the current waveforms may be changed using the pulse modulator/demodulator or the reverse pulse rectifier, or various magnetic waveforms may be formed through the rotation of the permanent magnets, thus preventing the aging of the plating solution and making the ionic concentration in the plating solution uniform.

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

EXAMPLE 1

In an electroplating device of FIG. 4, a copper plating solution, comprising 50 g/l sulfuric acid, 200 g/l copper sulfate, 50 ppm chlorine, and various additives, that is, 100 mg of a carrier, 2.5 mg of a brightener, and 7.5 mg of a leveller, was used as an electrolyte solution, and conditions shown in Table 1 below were applied, thus forming an electroplating layer on a PCB.

To this end, as the PCB, a 6 cm (width)×6 cm (length) sized test sheet having a via hole having a diameter of 50 μm and a pattern of 40 μm was used. The test sheet was sonicated for 5 min to remove organic material from the substrate and then washed with 0.1 N sulfuric acid for 30 sec and subsequently with distilled water to increase affinity with the plating solution, before copper electroplating. As such, the above pretreatment procedure was the same as a plating pretreatment process typically known in the art.

COMPARATIVE EXAMPLE 1

An electroplating layer was formed on the PCB in the same manner as in Example 1, with the exception that the electroplating process was conducted using the conventional electroplating device of FIG. 1 without the application of a magnetic field. The electroplating conditions are given in Table 1 below.

COMPARATIVE EXAMPLE 2

An electroplating layer was formed on the PCB in the same manner as in Example 1, with the exception that the electroplating process was conducted using the conventional electroplating device of FIG. 2 through the application of a unidirectional magnetic field. The electroplating conditions are given in Table 1 below.

TABLE 1
Conditions	Ex. 1	C. Ex. 1	C. Ex. 2
Sulfuric Acid	50	g/l	50	g/l	50	g/l
Copper Sulfate	200	g/l	200	g/l	200	g/l
Chlorine	50	ppm	50	ppm	50	ppm
Additive A(Carrier)	100	mg	100	mg	100	mg
Additive B(Brightener)	2.5	mg	2.5	mg	2.5	mg
Additive C(Leveller)	7.5	mg	7.5	mg	7.5	mg
Air	—	—	—
Volume of Plating Bath	500	ml	500	ml	500	ml
Current Density	1	ASD	1	ASD	1	ASD
Plating Time	60	min	60	min	60	min
Max. Plating Time	100	min	60	min	80	min
Magnetic Field	Periodic Reverse	—	Unidirection
	Direction (0.3 T)		(0.3 T)

The SEM photographs of the cross-sections and surfaces of the PCB samples having the electroplating layer formed under the conditions shown in Table 1 are shown in FIGS. 9A to 9C and FIGS. 10A to 10C.

Referring to FIGS. 9A to 9C, the plating layer of Example 1 (FIG. 9A) had a maximum thickness of 26 μm, and the plating layers of Comparative Examples 1 and 2 (FIGS. 9B and 9C) were 21 μm and 15 μm thick, respectively. In this case, the reason why the plating layer of Comparative Example 2 (FIG. 9C) was the thinnest despite the application of the magnetic field is believed to be because the concentration of the plating solution was non-uniform due to the application of the unidirectional magnetic field and because heat was emitted from the electromagnet and thus the temperature of the plating solution increased, thus facilitating the aging of the plating solution.

As shown in FIGS. 10A to 10C, the plating layer of Example 1 (FIG. 10A) had the finest texture, whereas the plating layers of Comparative Examples 1 and 2 had coarse textures, in which the texture of the plating layer of Comparative Example 2 (FIG. 10C) was coarser than that of the plating layer of Comparative Example 1 (FIG. 10B). These results could be confirmed with the naked eye by evaluating the brightness of the plating layer.

Moreover, when the current is applied to the electrolyte solution (plating solution) in the absence of the external liquid flow, the copper ion is deposited on the substrate (cathode) with time. After a certain time period, the concentration of the copper ion capable of being deposited near the substrate becomes 0. At this point, the current applied to the substrate is referred to as a threshold current. In this way, when the current reaches a threshold current, the plating process does not proceed any further. As is apparent from the results in Table 1, the plating process in Example 1 was seen to be conducted for a period of time increased by about 60% compared to the plating process of Comparative Example 1 without the application of the magnetic field, and by about 25% compared to the plating process of Comparative Example 2 through the application of the unidirectional magnetic field. As such, that the plating process is conducted for a longer period of time without the external liquid flow indicates the realization of a uniform plating process on the metal wire substrate having various thicknesses.

In this way, in the case where the PCB is electroplated using a magnetic field having periodic directionality, the plating rate may be increased. Further, by changing the magnetic waveform, the liquid flow is increased, therefore preventing the non-uniform copper ion that has occurred in a conventional electromagnetic plating process.

In the case where the predetermined electromagnetic field is applied according to the conventional technique, the plating solution may be easily aged due to the non-uniform solution concentration and the generation of heat. However, according to the method of the present invention, the liquid flow can be activated through various current waveforms or using permanent magnets. In addition, upon the use of the electromagnetic magnets, less heat is generated and thus the aging of the solution may be slowed. Therefore, a uniform plating process is realized, leading to the formation of the fine circuit of the substrate.

Although the preferred embodiments of the present invention, in regard to the PCB electroplating method using the magnetic field having the periodic directionality, have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention.

As described hereinbefore, the present invention provides a method of electroplating a PCB using a magnetic field having periodic directionality. According to the present invention, a periodically changeable magnetic field is applied, such that the trajectory of a metal ion is made more precise through MHD convection flow, compared to the use of a conventional unidirectional magnetic plating device. Thus, it is possible to realize a process of plating a via hole having a high aspect ratio or of uniformly plating a fine pattern, even without the use of an external liquid flow device.

In addition, although the use of a conventional plating device continuously applying the unidirectional magnetic field results in fast aging of the plating solution or in the ionic concentration of the plating solution inclined in only one direction through unidirectional Lorentz force, in the present invention, the aging of the plating solution can be prevented and the ionic concentration thereof can be made uniform thanks to the change of current waveforms using a pulse modulator/demodulator or a reverse pulse rectifier or thanks to the formation of various magnetic waveforms through the rotation of permanent magnets.

Such variations and modifications are intended to be within the scope of the present invention and the appended claims.

Claims

1. A method of electroplating a printed circuit board using a magnetic field having periodic directionality, comprising:

(a) bringing an electrolyte solution into contact with a surface of a printed circuit board; and

(b) simultaneously applying a magnetic field to the electrolyte solution and applying voltage between the printed circuit board and the electrolyte solution;

wherein the magnetic field is generated perpendicular to a current direction and has periodic directionality between a forward wave and a reverse wave.

2. The method as set forth in claim 1, wherein the magnetic field is supplied using permanent magnets.

3. The method as set forth in claim 1, wherein the magnetic field having the periodic directionality is supplied through rotation of a pair of permanent magnets.

4. The method as set forth in claim 1, wherein the magnetic field is supplied using electromagnets.

5. The method as set forth in claim 1, wherein the magnetic field having the periodic directionality is supplied through change in a current waveform using a pair of electromagnets and a pulse modulator/demodulator connected thereto.

6. The method as set forth in claim 1, wherein the magnetic field having the periodic directionality is supplied through change in a current waveform using a pair of electromagnets and a reverse pulse rectifier connected thereto.

7. The method as set forth in claim 1, wherein the electrolyte solution includes a copper ion.