METHOD AND APPARATUS FOR ELECTROPLATING A METAL ONTO A SUBSTRATE

Abstract

Method for electroplating a metal onto a flat substrate P. Surfaces are electrically polarized for metal deposition by feeding thereto at least one first and second forward-reverse pulse current sequences. The first forward-reverse pulse current sequence includes a first forward pulse generating a first cathodic current during a first forward pulse duration t.sub.f1 and having a first forward pulse peak current i.sub.f1, and a first reverse pulse generating a first anodic current during a first reverse pulse duration t.sub.r1 and having a first reverse pulse peak current i.sub.r1, the second forward-reverse pulse current sequence including a second forward pulse generating a second cathodic current during a second forward pulse duration t.sub.f2 and having a second forward pulse peak current i.sub.f2, and a second reverse pulse generating a second anodic current during a second reverse pulse duration t.sub.r2, the second reverse pulse having a second reverse pulse peak current i.sub.r2.

Claims

1. A method for electroplating a metal onto a substrate (P), wherein said substrate (P) is a flat substrate having opposing first and second substrate surfaces (P.sub.1, P.sub.2), said method comprising: (a) providing said substrate (P), an electroplating apparatus (100, 200) comprising at least one counter electrode (120, 130; 220, 230), and an electroplating liquid (L); (b) bringing each of said substrate (P) with said opposing first and second substrate surfaces (P.sub.1, P.sub.2) and said at least one counter electrode (120, 130; 220, 230) into contact with said electroplating liquid (L); (c) electrically polarizing said first and second substrate surfaces (P.sub.1, P.sub.2) of said substrate (P) to effect metal deposition onto said first and second substrate surfaces (P.sub.1, P.sub.2) by feeding at least one first forward-reverse pulse current sequence each one being composed of successive first forward-reverse pulse periods to said first substrate surface (P.sub.1) and at least one second forward-reverse pulse current sequence each one being composed of successive second forward-reverse pulse periods to said second substrate surface (P.sub.2); (d) each one of said at least one first forward-reverse pulse current sequence at least comprising, in each one of first consecutive forward-reverse pulse periods, a first forward pulse generating a first cathodic current during a first forward pulse duration (t.sub.f1) at said first substrate surface (P.sub.1), said first forward pulse having a first forward pulse peak current (i.sub.f1), and a first reverse pulse generating a first anodic current during a first reverse pulse duration (t.sub.r1) at said first substrate surface (P.sub.1), said first reverse pulse having a first reverse pulse peak current (i.sub.r1); and each one of said at least one second forward-reverse pulse current sequence at least comprising, in each one of consecutive second forward-reverse pulse periods, a second forward pulse generating a second cathodic current during a second forward pulse duration (t.sub.f2) at the second substrate surface (P.sub.2), said second forward pulse having a second forward pulse peak current (i.sub.f2), and a second reverse pulse generating a second anodic current during a second reverse pulse duration (t.sub.r2) at the second substrate surface, said second reverse pulse having a second reverse pulse peak current (i.sub.r2); (e) wherein said first and second forward pulses are further superposed with a respective first or second superposing cathodic pulse having a respective first or second superposing cathodic pulse duration (t.sub.c1, t.sub.c2) which is shorter than said respective first or second forward pulse duration (t.sub.f1, t.sub.f2); and wherein a phase shift φ.sub.r between said first reverse pulse of said at least one first forward-reverse current sequence and said second superposing cathodic pulse of said at least one second forward-reverse current sequence is set to 0°±30°.

2. The method for electroplating a metal onto a substrate (P) according to claim 1, wherein said first and second forward-reverse pulse current sequences are offset to each other by a phase shift (φ.sub.s) of about 180°.

3. The method for electroplating a metal onto a substrate (P) according to any one of claims 1 and 2, wherein the durations (t.sub.r1, t.sub.r2) of said first and second reverse pulses equal the respective durations (t.sub.c1, t.sub.c2) of said first and second superposing cathodic pulses.

4. The method for electroplating a metal onto a substrate (P) according to any one of the preceding claims, wherein said first reverse pulse and said second superposing cathodic pulse are applied simultaneously and wherein said second reverse pulse and said first superposing cathodic pulse are applied simultaneously.

5. The method for electroplating a metal onto a substrate (P) according to any one of the preceding claims, wherein the method further comprises, subsequent to performing said at least one first and second forward-reverse pulse current sequences in accordance with method steps (d) and (e) in a first method section period, applying at least one further first and second forward-reverse pulse current sequences, each one comprising a plurality of consecutive first or second forward-reverse pulse periods, respectively, wherein each one of said consecutive first and second forward-reverse pulse periods comprises a respective first or second forward pulse generating a cathodic current during a respective first or second forward pulse duration (t.sub.f1, t.sub.f2) at the respective first or second substrate surface (P.sub.1, P.sub.2), said respective first or second forward pulse having a respective first or second forward pulse peak current (i.sub.f1, i.sub.f2), and a respective first or second reverse pulse generating a respective first or second anodic current during a respective first or second reverse pulse duration (t.sub.r1, t.sub.r2) at the respective first or second substrate surface (P.sub.1, P.sub.2), said first and second reverse pulses having a respective first or second reverse pulse peak current (i.sub.r1, i.sub.r2), without superposing said respective first or second forward pulses with a respective first or second superposing cathodic pulse, in a second method section period.

6. The method for electroplating a metal onto a substrate (P) according to claim 5, wherein, in said second method section period, said first and second forward-reverse pulse current sequences are offset to each other by a phase shift (φ.sub.s) of about 180°.

7. The method for electroplating a metal onto a substrate (P) according to any one of the preceding claims, wherein none of said at least one first and second forward-reverse pulse current sequences, either in one of said first and second method section periods or in both method section periods, comprise any method section period wherein current applied to said substrate (P) is set to zero.

8. The method for electroplating a metal onto a substrate (P) according to any one of the preceding claims, wherein said metal is copper.

9. An apparatus (100, 200) for electroplating a metal onto a flat substrate (P) having opposing first and second substrate surfaces (P.sub.1, P.sub.2), said apparatus (100, 200) comprising: (a) means for holding (140) the substrate (P); (b) at least one counter electrode (120, 130; 220, 230); (c) means for accommodating (110, 210) an electroplating liquid (L); (d) means for electrically polarizing (150, 160; 250, 260) said substrate (P) to effect metal deposition onto said first and second substrate surfaces (P.sub.1, P.sub.2); wherein said means for electrically polarizing (150, 160; 250, 260) said first and second substrate surfaces (P.sub.1, P.sub.2) of said substrate (P) is designed to feed at least one first forward-reverse pulse current sequence each one being composed of successive first forward-reverse pulse periods to said first substrate surface (P.sub.1) and at least one second forward-reverse pulse current sequence each one being composed of successive second forward-reverse pulse periods to said second substrate surface (P.sub.2), wherein each one of said first forward-reverse pulse current sequence at least comprises, in each one of consecutive first forward-reverse pulse periods, a first forward pulse generating a first cathodic current during a first forward pulse duration (t.sub.f1) at the first substrate surface (P.sub.1), said first forward pulse having a first forward pulse peak current (i.sub.f1), and a first reverse pulse generating a first anodic current during a first reverse pulse duration (t.sub.r1) at the first substrate surface (P.sub.1), said first reverse pulse having a first reverse pulse peak current (i.sub.r1); and each one of said second forward-reverse pulse current sequence at least comprising, in each one of consecutive second forward-reverse pulse periods, a second forward pulse generating a second cathodic current during a second forward pulse duration (t.sub.f2) at the second substrate surface (P.sub.2), said second forward pulse having a second forward pulse peak current (i.sub.f2), and a second reverse pulse generating a second anodic current during a second reverse pulse duration (t.sub.r2) at the second substrate surface (P.sub.2), said second reverse pulse having a second reverse pulse peak current (i.sub.r2); and wherein said first and second forward pulses are further superposed with a respective first or second superposing cathodic pulse having a respective first or second superposing cathodic pulse duration (t.sub.c1, t.sub.c2) which is shorter than said respective first or second forward pulse duration (t.sub.f1, t.sub.f2).

10. The apparatus (100, 200) for electroplating a metal onto a substrate (P) according to claim 9, wherein at least one first counter electrode (120, 220) is arranged opposite a first substrate surface (P.sub.1) and wherein at least one second counter electrode (130, 230) is arranged opposite a second substrate surface (P.sub.2) and wherein said means for electrically polarizing (150, 160; 250, 260) said substrate (P) are designed to feed a first forward-reverse pulse current sequence having, in each first forward-reverse pulse period, a first forward pulse, a first reverse pulse, and a first superposing cathodic pulse, to a first substrate surface (P.sub.1) and a second forward-reverse pulse current sequence having, in each second forward-reverse pulse period, a second forward pulse, a second reverse pulse, and a second superposing cathodic pulse, to a second substrate surface (P.sub.2), wherein said first and second forward-reverse pulse current sequences are offset to each other by a phase shift (φ.sub.s) of about 180°.

11. The apparatus (100, 200) for electroplating a metal onto a substrate (P) according to any one of claims 9 and 10, wherein said means for electrically polarizing (150, 160; 250, 260) said substrate (P) are further designed to set said durations (t.sub.r1, t.sub.r2) of said first and second reverse pulses equal to said durations (t.sub.c1, t.sub.c2) of said first and second superposing cathodic pulses and to apply said first reverse pulse and said second superposing cathodic pulse simultaneously and to apply said second reverse pulse and said first superposing cathodic pulse simultaneously.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0097] FIG. 1 shows an apparatus of the invention in a first embodiment in a schematic perspective view;

[0098] FIG. 2 shows an apparatus of the invention in a second embodiment in a schematic perspective view;

[0099] FIG. 3 shows a forward-reverse pulse current sequence according to the present invention being applied to one surface of a flat substrate;

[0100] FIG. 4 shows forward-reverse pulse current sequences in a first embodiment of the invention, a first one of these forward-reverse pulse current sequences being applied to a first side of a flat substrate and a second one of these forward-reverse pulse current sequences being applied to a second side of the flat substrate;

[0101] FIG. 5 shows forward-reverse pulse current sequences in a second embodiment of the invention, each one being applied to one of the sides of the flat substrate;

[0102] FIG. 6 shows forward-reverse pulse current sequences having no superposing cathodic pulse;

[0103] FIG. 7 shows a forward-reverse pulse current sequence in a third embodiment of the invention;

[0104] FIG. 8 shows a forward-reverse pulse current sequence having no superposing cathodic pulse, but having a pulse break;

[0105] FIG. 9 shows photographs of coppered through holes obtained with a forward-reverse pulse current sequence having no superposing cathodic pulse, but a pulse break;

[0106] FIG. 10 shows photographs of coppered through holes obtained with a forward-reverse pulse current sequence having no superposing cathodic pulse and no pulse break;

[0107] FIG. 11 shows photographs of coppered through holes obtained with a forward-reverse pulse current sequence having a superposing cathodic pulse;

[0108] FIG. 12 shows diagrams of copper surface thickness variations at different conditions;

[0109] FIG. 13 shows a diagram indicating the dependency of the plated copper thickness between the surface of the board and surface of the through holes, relating to the plated surface copper thickness [%], from the ratio of the active surface area on the surface of the board to the surface area on the through hole walls;

[0110] FIG. 14 shows a diagram indicating copper thickness on the board surface in a hole region and outside the hole region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0111] Elements having the same function are designated with the same reference signs in the figures.

[0112] The apparatus of the invention may be of a vertical type of treatment apparatus 100 (FIG. 1) or a horizontal (conveyorized) type of apparatus 200 (FIG. 2).

[0113] In the vertical type of apparatus 100 (FIG. 1) the substrate P, a printed circuit board for example, which has a first surface (side) P.sub.1 and a second surface (side) P.sub.2, is vertically immersed into the treatment liquid L contained in a container 110. The board is provided with through holes and/or blind holes. The substrate is placed between two counter electrodes 120, 130 (anodes) which are also oriented in a vertical direction and which are arranged facing each other: a first counter electrode 120 facing the first surface P.sub.1 of the board and a second counter electrode 130 facing the second surface P.sub.2 of the board. Both, the board and the counter electrodes are immersed into the treatment liquid. The board is held by a holding means 140 like a frame or a claw. The counter electrodes may for example be made from expanded metal like from expanded titanium which is surface coated with a noble metal. The treatment liquid may be a copper electroplating liquid like a sulfuric acid electroplating liquid containing copper sulfate, sulfuric acid, sodium chloride, and organic additives in water. In addition, the apparatus may contain a heating, nozzles for injecting air into the liquid, nozzles for injecting treatment liquid into the container, stirring means, filtering means, and the like (not shown). Each one of the counter electrodes and the board are electrically connected to a respective current source like a rectifier. A first counter electrode 120 and the board are connected to a first rectifier 150 (represented by its electrical contacts) and a second counter electrode 130 and the board are connected to a second rectifier 160 (represented by its electrical contacts). The current sources independently apply pulse currents to the counter electrodes and respective surfaces P.sub.1, P.sub.2 of the board. Each one of the pulse currents has a defined pulse shape and frequency.

[0114] The horizontal type apparatus 200 (FIG. 2) also comprises a container 210 holding the treatment liquid. Two rows of counter electrodes 220, 230 (anodes) are arranged one after the other in the conveyance direction in the container. A space is formed between the rows wherein a board P having two surfaces (sides) P.sub.1, P.sub.2 and being provided with through holes and/or blind holes, is conveyed through the container on a horizontal conveying path. The board is conveyed using rollers 240. The rollers transport the board in a horizontal direction (arrow H) through the container. The container is preferably flooded with the treatment liquid L so that the counter electrodes and the board are completely immersed in the treatment liquid. In this case too, each one of the counter electrodes and the board are electrically connected to a respective current source like a rectifier (well-known in the art). The first counter electrodes 220 and the board are connected to a first rectifier 250 (represented by its electrical contacts) and the second counter electrodes 230 and the board are connected to a second rectifier 260 (represented by its electrical contacts). The current sources independently apply pulse currents to the counter electrodes and surfaces P.sub.1, P.sub.2 of the board. Each one of the pulse currents has a defined pulse shape and frequency.

[0115] In a first embodiment of the method of the present invention, the pulse shape of the pulse current applied to the board (or a flat substrate having any other shape than being board-shaped) is shown in FIG. 3. This diagram shows the current i over time t with cathodic current being above the zero current line (0) and anodic current being below the zero current line (0). The pulse current sequence shown represents one periodic cycle having a cycle time T.sub.p. A plurality of such cycles (forward-reverse pulse periods) follow each other. In this embodiment a forward pulse having a forward pulse peak current i.sub.f is applied during a forward pulse duration t.sub.f and a reverse pulse having a reverse pulse peak current i.sub.r is applied during a reverse pulse duration t.sub.r. Furthermore, during the forward pulse duration t.sub.f a superposing cathodic pulse having a superposing cathodic pulse duration t.sub.c is applied. This superposing cathodic pulse has a superposing cathodic pulse peak current i.sub.c which adds to the forward pulse peak current i.sub.f to yield an overall cathodic peak current i.sub.c+f. This pulse current sequence repeats permanently at a frequency f, so that the period T.sub.p=1/f.

[0116] The pulsed current applied to the substrate P is provided by rectifiers 150, 160, 250, 260 which are accordingly programmed to provide such pulse current sequence. This current sequence is applied to the substrate and counter electrodes 120, 130, 220, 230 being arranged opposite this substrate.

[0117] As a flat substrate like a board P is used, the two board's surfaces P.sub.1, P.sub.2 are independently loaded with forward-reverse pulse current sequences by applying a first forward-reverse pulse current sequence to a first counter electrode 120, 220 and a first surface P.sub.1 of the board and by applying a second forward-reverse pulse current sequence to a second counter electrode 130, 230 and a second surface P.sub.2 of the board. The first forward-reverse pulse current sequence applied to the first side of the board is shown in the upper graph of FIG. 4, whereas the second forward-reverse pulse current sequence applied to the second side of the board is shown in the lower graph of FIG. 4.

[0118] The first forward-reverse pulse current sequence comprises a first forward pulse having a first forward pulse duration t.sub.f1 and a first forward pulse peak current i.sub.f1 and a first reverse pulse having a first reverse pulse duration t.sub.r1 and a first reverse pulse peak current i.sub.r1. Furthermore, there is a first superposing cathodic pulse having a first superposing cathodic pulse duration t.sub.c1 and a first superposing cathodic pulse peak current i.sub.c1. The first superposing cathodic pulse peak current i.sub.c1 adds to the first forward pulse peak current i.sub.f1 to yield a first overall cathodic peak current i.sub.c+f1. The second forward-reverse pulse current sequence comprises a second forward pulse having a second forward pulse duration t.sub.f2 (not shown) and a second forward pulse peak current i.sub.f2 and a second reverse pulse having a second reverse pulse duration t.sub.r2 and a second reverse pulse peak current i.sub.r2. Furthermore, there is a second superposing cathodic pulse having a second superposing cathodic pulse duration t.sub.c2 and a second superposing cathodic pulse peak current i.sub.c2. The second superposing cathodic pulse peak current i.sub.c2 adds to the second forward pulse peak current i.sub.f2 to yield a second overall cathodic peak current i.sub.c+f2. Both pulse current sequences are offset by a phase shift φ.sub.s of 180°, such that the first reverse pulse is offset to the second reverse pulse by 180°. Furthermore, the first superposing cathodic pulse of the first pulse current sequence and the second reverse pulse of the second pulse current sequence are applied simultaneously and the second superposing cathodic pulse of the second pulse current sequence and the first reverse pulse of the first pulse current sequence are also applied simultaneously (φ.sub.r=0°), because the superposing cathodic pulse and the reverse pulse within the same forward-reverse pulse current sequence are offset relative to each other by an angular offset ξ.sub.c=180° and because t.sub.c1=t.sub.r2 and t.sub.c2=t.sub.r1. As will be shown hereinafter, this type of pulse current treatment is very advantageous for X- (bridge-) plating. If t.sub.c1 would not be equal to t.sub.r2 and t.sub.c2 would not be equal to t.sub.r1, the reverse and superposing cathodic pulses would not completely overlap.

[0119] In a further embodiment (FIG. 5), each one of both pulse current sequences comprises a forward pulse, a reverse pulse, and a superposing cathodic pulse. The angular offset ξ.sub.c between the superposing cathodic pulse and the reverse pulse in a pulse current sequence is 110°. The phase shift φ.sub.s between the first and the second forward-reverse pulse current sequences is less than 180°, 150° for example.

[0120] In yet a further embodiment (FIG. 6), each one of both forward-reverse pulse current sequences comprises a forward pulse and a reverse pulse, but no superposing cathodic pulse. These forward-reverse pulse current sequences may be applied in a second method section period, after, in a first method section period, the forward-reverse pulse current sequences having superposing cathodic pulses (FIGS. 4, 5) have been applied to provide X- (bridge-) plating of through holes, so that thereafter the through holes may be filled efficiently. In this case, the phase shift φ.sub.s between the reverse pulses of the two forward-reverse pulse current sequences is preferably 180°.

[0121] In a further method embodiment of the present invention a further (third) pulse is applied, in addition to the forward pulse, the reverse pulse, and the superposing cathodic pulse. This pulse current sequence is shown in FIG. 7. Furthermore in this case, the real pulse track exhibiting a finite time period for the rise from one current level to another current level is shown. Therefore each pulse has a rise time and a decay time, indicated as a slope, expressed in [A/s]. This slope may have a maximum value depending on electrical conditions of the apparatus setup. The respective rise and decay times (slope durations) of the reverse pulse is accordingly shown to be t.sub.sl. Taking the start time for the reverse pulse to be at 0 s, FIG. 7 further shows a couple of further parameters, i.e., the start time for the forward pulse t.sub.sf, the start time for the superposing cathodic pulse t.sub.sc, and the start time for the additional (third) pulse t.sub.sa.

EXAMPLE 1

[0122] In a setup using a horizontal conveyorized plating apparatus with a plating liquid flow of 15 m.sup.3/h such as shown in FIG. 2, copper deposition was performed to a printed circuit board having through holes. The board was held in the apparatus with clamps at one clamping edge thereof wherein the clamps also provided electrical contact to the two sides of the board. Each one of the two sides were electrically connected individually and powered from a respective rectifier independently with their own forward-reverse pulse current sequences. The rectifiers were driven by respective computer controlled devices to generate the forward-reverse pulse sequences. The copper plating bath was a sulfuric acid plating bath containing copper sulfate, sulfuric acid, sodium chloride and commonly used organic additives. The boards were provided with a thin copper layer all over the outer surface and the through hole walls. The through holes had a diameter of 0.2 mm and a length (thickness of the board) of 0.8 mm. 800 through holes were arranged in matrices (clusters) in an area of 20 mm by 20 mm with a pitch of 0.5 mm. A couple of these matrices were arranged on the board at various distances to the edge of the board.

[0123] Copper deposition was performed to effect X-plating, i.e., depositing copper in the through holes to generate a plug in the center thereof. Copper deposition was performed by applying a forward-reverse pulse current sequence to each one of the surfaces of the board, wherein the two pulse current sequences were phase shifted relative to each other by φ.sub.s=180°, i.e., the start time of the first reverse pulse was offset by 180° relative to the start time of the second reverse pulse. Furthermore, as the angular offset ξ.sub.c between the superposing cathodic pulses and the reverse pulses in the same first or second forward-reverse pulse current sequence was 180°, the start time of the first superposing cathodic pulse was at the same time as the start time of the second reverse pulse.

[0124] In a first experiment, deposition was performed with conventional forward-reverse pulse current sequences for both surfaces of the board, each one having, in each pulse sequence cycle (forward-reverse pulse period), one forward pulse, one reverse pulse, and one pulse break during which no current flows (Plating Condition 1). The first pulse break of the first forward-reverse pulse current sequence was applied at the same time as the second reverse pulse of the second forward-reverse pulse current sequence and vice versa. A diagram showing these pulse current sequences is shown in FIG. 8. The first pulse current sequence is shown in the upper diagram and the second pulse current sequence is shown in the lower diagram. The parameters for these pulse current sequences are given in Table 1.

[0125] In a second experiment, metal deposition was performed with other conventional forward-reverse pulse current sequences each one having, in each pulse sequence cycle (forward-reverse pulse period), one forward pulse and one reverse pulse, but no pulse break (Plating Condition 2). A diagram showing these pulse current sequences is shown in FIG. 6. The parameters for these pulse current sequences are given in Table 1.

[0126] In a third experiment according to the present invention, metal deposition was performed with forward-reverse pulse current sequences each one having, in each pulse sequence cycle (forward-reverse pulse period), one forward pulse, one reverse pulse, and one superposing cathodic pulse (Plating Condition 3). A diagram of such pulse current sequences is shown in FIG. 4. The parameters for these forward-reverse pulse current sequences are given in Table 1.

[0127] Results:

[0128] With the conventional forward-reverse pulse current sequence having a pulse break (first experiment, Plating Condition 1), marked differences of X-plating in the through holes were observed depending from the location of the through holes on the board: The through holes which were positioned nearest to the clamping edge of the board (Location 1: at 170 mm from the edge of the board opposing the clamping edge) were not yet plugged with copper in the center thereof while thickening of the copper layer in the center of the holes took place to some extent (FIG. 9a). The through holes located nearer to the edge of the board which was opposing to the clamping edge (Location 2: at 85 mm from the edge of the board opposing the clamping edge) experienced even less coppering so that only little thickening of the copper layer in the center of the holes took place (FIG. 9b). The through holes located in the vicinity of the edge of the board opposing to the clamping edge (Location 3: at 10 mm distance from the edge of the board opposing the clamping edge) did not show much coppering. A plug has not yet formed at all and thickening was almost not observed (FIG. 9c). Therefore, metal deposition is differing between the locations markedly.

[0129] With the conventional forward-reverse pulse current sequence having no pulse break (second experiment, Plating Condition 2), plug formation took place more distinctly at least in those holes which were at Location 1 and at Location 2 (FIGS. 10a, 10b). The holes located near the edge of the board remote from the clamping location (Location 3) showed marked thickening of the copper layer in the center of the holes, but coppering did not result in plug formation (FIG. 10c). Therefore, marked differences were still observed while plug formation was better than with the first experiment.

[0130] With the forward-reverse pulse current sequences having a superposing cathodic pulse according to the invention (third experiment, Plating Condition 3) almost no differences were observed with plug formation in the center of the holes irrespective of whether the holes were located at Location 1, Location 2, or Location 3 (FIG. 11a: Location 1; FIG. 11b: Location 2, FIG. 11c: Location 3).

EXAMPLE 2

[0131] Under the setup conditions of Example 1 (horizontal conveyorized plating line) with a plating liquid flow of 9 m.sup.3/h another experiment was performed showing superior results as regards uniformity of copper on the surface of a printed circuit board between through holes. A comparison was made between the copper thickness obtained between through holes arranged at a high density hole pitch (0.5 mm) and through holes arranged at a low density hole pitch (2.0 mm). Comparison was also made for different current conditions:

[0132] Plating Condition 1: DC plating (DC=direct current).

[0133] Plating Condition 2: forward-reverse pulse current sequences with pulse break (0 A/dm.sup.2), but without superposing cathodic pulse, corresponding to a pulse current sequence as is shown in FIG. 8.

[0134] Plating Condition 3: forward-reverse pulse current sequences with a superposing cathodic pulse, but without pulse break, corresponding to a pulse current sequence as shown in FIG. 4.

[0135] The board parameters were as follows: panel thickness: 0.8 mm; hole diameters 0.2 mm and 0.6 mm; hole pitch: 0.5 mm and 2.0 mm; block area (area of hole matrix): 20 mm by 20 mm.

[0136] DC current was set to 2 A/dm.sup.2 (Plating Condition 1). All other plating parameters are given in Table 2.

[0137] Results:

[0138] Copper thickness was measured on the surface of the board between the through holes and statistically evaluated. The values for those measuring positions where the hole pitch was small (pitch: 0.5 mm; high hole density) and where the hole pitch was large (pitch: 2.0 mm; low hole density) were determined separately. The results of these measurements are shown in FIG. 12:

[0139] FIG. 12a shows the results of copper surface thickness variation obtained with DC plating (2 A/dm.sup.2) at low and high hole density areas (“Low” and “High”, resp.), Plating Condition 1.

[0140] FIG. 12b shows the results of copper thickness variation obtained with forward-reverse pulse current sequences and with pulse break but without superposing cathodic pulse, Plating Condition 2. Again, results obtained at low and high hole density areas (“Low” and “High”, resp.) are shown.

[0141] FIG. 12c shows the results of copper thickness variation obtained with forward-reverse pulse current sequences, without pulse break, but with superposing cathodic pulse, Plating Condition 3. Again, results obtained at low and high hole density areas (“Low” and “High”, resp.) are shown.

[0142] Large relative variation in copper surface thickness was obtained with the pulse conditions using a forward-reverse pulse current sequence with pulse break and without superposing cathodic pulse (Plating Condition 2). Surface thickness variation is lower if a superposing cathodic pulse is used (Plating Condition 3). DC conditions are shown for comparison only (Plating Condition 1). DC conditions are not acceptable if an even metal thickness is to be achieved on the surface in high and low hole density areas.

[0143] In a further diagram (FIG. 13) a dependency of the plated surface copper thickness variation between the surface of the board (area without through holes, plain area) and the surface area including through holes, relating to the ratio of the active surface area between the surface of the board (area without through holes, plain area) to the real surface area in the through hole area region (board surface area plus surface area of the through hole walls) is shown for different hole diameters (0.2 mm: indicated by (1); 0.6 mm, indicated by (2)), different hole densities (hole pitches: 0.5 mm, indicated by (2); 2.0 mm, indicated by (1)), and for different plating conditions (Plating Condition 2 of Table 2, indicated by ‘x’; Plating Condition 3 of Table 2, indicated by ‘o’). The data given in Table 2 indicated by 1) and 2) correspond to substrates with hole diameters and hole densities indicated by (1) and (2), respectively. Accordingly, copper thickness as plated on the outer surface of the board in a region without through holes is compared to copper thickness as plated on the outer surface of the board in a region comprising through holes.

[0144] From the diagram it is apparent that a relatively small copper thickness difference between the plain area (without through holes) and the surface area where through holes were located was achieved if Plating Condition 3 was used. This effect was more pronounced if substrates with large holes and a large hole pitch were plated.

EXAMPLE 3

[0145] Under the setup conditions of Example 1 (horizontal conveyorized plating line) with a plating liquid flow of 9 m.sup.3/h, another experiment was performed showing superior results as regards uniformity of copper on the surface of a printed circuit board in an area where through holes were located (thickness measured between through holes) and outside this area, i.e., in regions where no through holes were located.

[0146] The board parameters were as follows: panel thickness: 1.5 mm; hole diameters 0.4 mm and 0.6 mm; hole pitch: 0.2 mm, 0.4 mm, and 0.8 mm; block area (area of hole matrix): 20 mm by 20 mm.

[0147] Comparison is also made for different current conditions (the pulse consecutions and frequencies of all forward-reverse currents herein below were set to be identical):

[0148] Plating Condition 1: forward-reverse pulse current sequences on both sides of the board, each sequence without pulse break and without superposing cathodic pulse, the first reverse pulse on one side of the board being offset to the second reverse pulse on the other side thereof by φ.sub.s=187°.

[0149] Plating Condition 2: forward-reverse pulse current sequences on both sides of the board, each sequence comprising a superposing cathodic pulse, but no pulse break, the first superposing cathodic pulse on one side of the board being offset to the second reverse pulse on the other side thereof and vice versa by φ.sub.r=7°. The phase shift between the first and second forward-reverse pulse current sequences was set to φ.sub.s=187°. The angular offsets between the reverse pulses and the superposing cathodic pulses within the first forward-reverse pulse current sequence and within the second forward-reverse pulse current sequence, respectively, were set to be ξ.sub.c=180° in each case.

[0150] The parameters for the forward-reverse pulse current sequences are given in Table 3.

[0151] Results:

[0152] Copper thickness was measured on the surface of the board between the through holes on the one hand and in regions outside this area, i.e., in regions where no through holes were located. The data retrieved were statistically evaluated. The values for those measuring positions where through holes were present on the one hand and where no through holes were present on the other hand were determined separately. The results of these measurements are shown in FIG. 14:

[0153] Irrespective of the application of superposing cathodic pulses or not in the forward-reverse pulse current sequences, copper thickness in the area where through holes are present increases when through hole pitch increases. There is no remarkable effect on copper thickness due to the through hole diameter.

[0154] A remarkable increase of copper thickness was achieved when the forward-reverse pulse current sequences were applied which additionally included a superposing cathodic pulse as compared to when no such pulse was additionally included into the sequence. This result clearly shows that the advantageous effect of providing such superposing cathodic pulse in the forward-reverse pulse current sequence is not only effective if the phase shift φ.sub.s is 180°, but also when it is substantially higher, like 187° in this case. It has to be noted that this favourable effect is achieved by setting φ.sub.r to be greater than 0°, i.e., 7° in this case.

TABLE-US-00001 TABLE 1 Pulse Parameters for Example 1 Plating Plating Plating Condition 1: Condition 2: Condition 3. Forward- Forward- Forward- reverse reverse reverse pulse current pulse current pulse current sequence, sequence, sequence, with pulse no pulse no pulse break, no break, no break, with superposing superposing superposing cathodic cathodic cathodic pulse pulse pulse Forward current 3.89 3.68 2.78 density I.sub.f [A/dm.sup.2] Forward pulse 76 76 76 duration t.sub.f [ms] Reverse current 40 40 40 density I.sub.r [A/dm.sup.2] Reverse pulse 4 4 4 duration t.sub.r [ms] Overall cathodic peak ./. ./. 20 current density I.sub.c+f [A/dm.sup.2] Superposing ./. ./. 4 cathodic pulse duration t.sub.c [ms] Superposing ./. ./. 36 cathodic pulse duration start [ms] Pulse period T.sub.p [ms] 80 80 80 Start pulse 36 ./. ./. break [ms] Pulse break 4 ./. ./. duration t.sub.b Effective (= average) 1.5 1.5 1.5 current density I.sub.av [A/dm.sup.2] Phase shift φ.sub.s 180° 180° 180° between current sequences of both sides Plating time [min] 30 30 30 Phase shift φ.sub.r ./. ./.   0° between reverse pulse and superposing cathodic pulse of different current sequences

TABLE-US-00002 TABLE 2 Pulse Parameters for Example 2 Plating Plating Condition 2: Condition 3: Forward- Forward- reverse pulse reverse pulse current current sequence with sequence, no pulse break, no pulse break, with superposing superposing cathodic cathodic pulse pulse Forward current 1) 4.44 1) 3.33 density I.sub.f [A/dm.sup.2] 2) 7.74 2) 7.21 Forward pulse 1) 76 1) 76 duration t.sub.f [ms] 2) 78 2) 78 Reverse current 1) 40 1) 40 density I.sub.r [A/dm.sup.2] 2) 14 2) 14 Reverse pulse 1) 4 1) 4 duration t.sub.r [ms] 2) 2 2) 2 Overall cathodic peak 1) ./. 1) 20 current density I.sub.c+f 2) ./. 2) 20 [A/dm.sup.2] Superposing cathodic 1) ./. 1) 4 pulse duration t.sub.c [ms] 2) ./. 2) 2 Superposing 1) ./. 1) 36 cathodic 2) ./. 2) 38 pulse duration start [ms] Pulse period 1) 80 1) 80 T.sub.p [ms] 2) 80 2) 80 Start pulse 1) 36 1) ./. break [ms] 2) 38 2) ./. Pulse break 1) 4 1) ./. duration t.sub.b 2) 2 2) ./. Effective (= average) 1) 2 1) 2 current density I.sub.av 2) 7 2) 7 [A/dm.sup.2] Phase shift φ.sub.s between 1) 180° 1) 180° current sequences of 2) 180° 2) 180° both sides Phase shift φ.sub.r between ./. 0° reverse pulse and superposing cathodic pulse of different current sequences Plating time [min] 36 36

TABLE-US-00003 TABLE 3 Pulse Parameters for Example 3 Plating Condition 1: Plating Condition 2: Forward-reverse pulse Forward-reverse pulse current sequence, no current sequence, no pulse break, no pulse break, with superposing cathodic superposing cathodic pulse pulse Forward current density 7.8 7.3 I.sub.f [A/dm.sup.2] Forward pulse duration 76 73 t.sub.f [ms] Reverse current density 20 20 I.sub.r [A/dm.sup.2] Reverse pulse duration 3 3 t.sub.r [ms] Overall cathodic peak ./ 20 current density I.sub.c+f [A/dm.sup.2] Superposing cathodic ./. 3 pulse duration t.sub.c [ms] Superposing cathodic ./. 36.5 pulse duration start [ms] Pulse period T.sub.p [ms] 79 79 Effective (= average) 6 6 current density I.sub.av [A/dm.sup.2] Phase shift φ.sub.r between reverse pulse and superposing cathodic ./. 7° pulse of different current sequences Plating time [min] 26.7 26.7

LIST OF REFERENCE SIGNS

[0155] 100, 200 electroplating apparatus
110 means for accommodating an electroplating liquid
120, 220 first counter electrode
130, 230 second counter electrode
140 means for holding the substrate
150 first means for electrically polarizing the substrate, rectifier
160 second means for electrically polarizing the substrate, rectifier
210 means for accommodating an electroplating liquid
250 first means for electrically polarizing the substrate, rectifier
260 second means for electrically polarizing the substrate, rectifier
f frequency
H transport direction
i current
i.sub.a third pulse peak current
I.sub.a third pulse peak current density
i.sub.c superposing cathodic pulse peak current
I.sub.c superposing cathodic pulse peak current density
i.sub.c1 first superposing cathodic pulse peak current
i.sub.c2 second superposing cathodic pulse peak current
i.sub.c+f overall cathodic peak current
I.sub.c+f overall cathodic peak current density
i.sub.c+f1 first overall cathodic peak current
i.sub.c+f2 second overall cathodic peak current
i.sub.f forward pulse peak current
I.sub.f forward pulse peak current density
i.sub.f1 first forward pulse peak current
i.sub.f2 second forward pulse peak current
i.sub.r reverse pulse peak current
I.sub.r reverse pulse peak current density
i.sub.r1 first reverse pulse peak current
i.sub.r2 second reverse pulse peak current
L electroplating/treatment liquid
P (flat) substrate, board
P.sub.1 first substrate surface
P.sub.2 second substrate surface
t time
t.sub.a third pulse duration
t.sub.b pulse break duration
t.sub.c superposing cathodic pulse duration
t.sub.c1 first superposing cathodic pulse duration
t.sub.c2 second superposing cathodic pulse duration
t.sub.f forward pulse duration
t.sub.f1 first forward pulse duration
t.sub.f2 second forward pulse duration
T.sub.p cycle time
t.sub.r reverse pulse duration
t.sub.r1 first reverse pulse duration
t.sub.r2 second reverse pulse duration
t.sub.sa start time of the third pulse
t.sub.sb start time of the pulse break
t.sub.sc start time of the superposing cathodic pulse
t.sub.sf start time of the forward pulse
t.sub.sl slope duration
ξ.sub.a angular offset between the reverse pulse and the third pulse within the same forward-reverse pulse current sequence
ξ.sub.b angular offset between the reverse pulse and the pulse break within the same forward-reverse pulse current sequence
ξ.sub.c angular offset between the reverse pulse and the superposing cathodic pulse within the same forward-reverse pulse current sequence
φ.sub.r phase shift between reverse pulse of first forward-reverse pulse current sequence and superposing cathodic pulse of second forward reverse pulse current sequence
φ.sub.s phase shift between forward-reverse pulse current sequences (between start times of reverse pulses applied to the two opposing sides of the substrate)