Printed circuit board

1. Technical Field

The invention relates to printed circuitry, and more particularly to a printed circuit board compensating for capacitance characteristics of a via stub.

2. Description of Related Art

A printed circuit board (PCB), such as a multilayer PCB used with a high-level server main board, a motherboard, or a backplane has signal layers, ground layers and source layers. Effective via technology is thus important in the PCB design.

As shown in FIGS. 5A and 5B , a lateral view and cross section of a PCB 1 such as multilayer PCB, PCB 1 has a plurality of copper foil layers 11 , a first line layer 12 , a second line layer 13 , a pair of symmetrical vias 14 A, 14 B, an isolation layer 15 and a plurality of line layers 16 . Each copper foil layer 11 has a symmetrical pair of void holes 11 A, 11 B both of which are disposed between the first line layer 12 , the second line layer 13 and each line layer 16 . The isolation layer 15 is disposed between each copper foil layer 11 , the first line layer 12 , the second line layer 13 and each line layer 16 . The first line layer 12 is disposed on the second line layer 13 . The vias 14 A, 14 B are disposed through the first line layer 12 , the second line layer 13 , the avoiding holes 11 A, 11 B and the isolation layer 15 .

The first line layer 12 and the second line layer 13 have a pair of symmetrical first conductors 12 A, 12 B, symmetrical second conductors 13 A, 13 B, symmetrical first signal lines L 1 A, L 1 B, symmetrical second signal lines L 2 A, L 2 B. The first conductors 12 A, 12 B and the second conductors 13 A, 13 B, such as solder pads, are disposed around the vias 14 A, 14 B, respectively and correspond to the through holes 11 A, 11 B. The first signal lines L 1 A, L 1 B and the second signal lines L 2 A, L 2 B are coupled to the first conductors 12 A, 12 B and the second conductors 13 A, 13 B, respectively.

When the first signal lines L 1 A, L 1 B receive a pair of input signals S 1 , the input signals are transmitted through the first conductors 12 A, 12 B, the vias 14 A, 14 B, the second conductors 13 A, 13 B and output from the second signal lines L 2 A, L 2 B. The vias 14 A, 14 B below the second signal line layer 13 do not path the input signal S 1 , thus creating a via stub structure W, as shown in FIG. 5B , elevating capacitance and lowering impedance. thereby preventing the high frequency of the input signal S 1 from passing, slowing the time wave. As shown in FIG. 5C , a channel response oscillogram, a curve C 1 decays before the frequency reaches 9 GHz. Vias such as 14 A, 14 B, below the second signal line layer 13 and the second signal lines L 2 A, L 2 B act as branches, and the part of the input signal S 1 is input to the vias 14 A, 14 B, and reflected back therethrough to generate multiple reflection, adding the original input signal S 1 by the second signal lines L 2 A, L 2 B to bad effect, as shown in FIG. 5D and FIG. 5E . FIG. 5D is a serial signal eye diagram of the signal path including via stub structure W, causing input signal S 1 to experience severe jitter and resulting high noise. FIG. 5E shows a serial signal eye diagram of the signal path with no via stub structure W.

FIG. 6 shows, in an attempt to improve the described problem, implementation of a process, after copper-plating of the vias, in which a drill D creates holes from line layers 16 of the PCB 1 to the first line layer 12 to completely clear the via stub structure W and reduce its effect.

However, the added process complicates manufacture of the PCB 1 , and difficulty in ensuring precision of drill D positioning of the vias can result in decreased yield and increased costs.

What is needed, therefore, is a printed circuit board compensating for capacitance characteristics of a via stub requiring no added process.

FIG. 1 is a side view of a printed circuit board, in accordance with the first embodiment of the disclosure.

FIG. 2 is a cross section of the printed circuit board of FIG. 1 taken along line IV-IV′.

FIG. 3A is a side view of the printed circuit board of FIG. 1 showing widths of the curved first portions exceeding those of the straight second portions, in accordance with the disclosure.

FIG. 3B is a side view of the printed circuit board showing the curved first portions of the first signal line and the second signal line disposed around the first via and the second via, in accordance with the second embodiment of the disclosure.

FIG. 4 shows an oscillogram of a channel response of the printed circuit board in accordance with the disclosure.

FIG. 5A is a side view of a commonly used printed circuit board.

FIG. 5B is a cross-section of the printed circuit board of FIG. 5A .

FIG. 5C shows an oscillogram of a channel response of the printed circuit board of FIG. 5A .

FIG. 5D is a serial signal eye diagram of the printed circuit board of FIG. 5A , showing a via stub structure.

FIG. 5E is a serial signal eye diagram of the printed circuit board of FIG. 5A , showing no via stub structure.

FIG. 6 is a schematic view of the printed circuit board of FIG. 5A , showing the via stub structure drilled.

Referring to FIG. 1 and FIG. 2 , a lateral view and a cross section of a printed circuit board (PCB) 2 of the first embodiment are shown. The PCB 2 includes a plurality of first layout layers 21 , second layout layers 22 , copper foil layers 23 and isolation layers 26 , a first via 24 , a second via 25 , a pair of symmetrical first conducting portions 27 A, 27 B, and a pair of symmetrical second conducting portions 28 A, 28 B.

In this embodiment, the first layout layers 21 are disposed above the second layout layers 22 . One of the copper foil layers 23 in the upper portion of the PCB 2 is disposed between two of the first and the second layout layers 21 , 22 . The isolation layers 26 are disposed to separate the first layout layers 21 , the second layout layers 22 and the copper foil layers 23 .

Each copper foil layer 23 includes an through hole 231 which is located in alignment. Shape of the through hole 231 may be any shape, and is irregular shown in this embodiment as seen in FIG. 1 , FIG. 3A and in the second embodiment as seen in FIG. 3B .

The first conducting portions 27 A, 27 B are disposed on one of the first layout layers 21 , and the second conducting portions 28 A, 28 B are disposed on one of the second layout layers 22 . The first conducting portion 27 A and the second conducting portion 28 A are disposed around and coupled to the first via 24 , and the second conducting portion 27 B and the second conducting portion 28 B are disposed around and coupled to the second via 25 . The first conducting portions 27 A, 27 B and the second conducting portion 28 A, 28 B may be solder pads.

In this embodiment, the first layout layer 21 , which has the first conducting portions 27 A, 27 B, includes a first signal line LA and a second signal line LB, which are symmetrical and disposed between the first via 24 and the second via 25 . The first signal line LA includes a curved first portion LA 1 and a straight second portion LA 2 integrally connected together. The second signal line LB includes a curved first portion LB 1 and a straight second portion LB 2 , both integrally connected.

The curved first portions LA 1 , LB 1 are formed such that shadows thereof are cast within the area of corresponding through holes 231 of the copper foil layer 23 , and coupled to the first via 24 and the second via 25 by the first conducting portions 27 A, 27 B.

In this embodiment, the curved first portions LA 1 , LB 1 of the first signal line LA and the second signal line LB are disposed generally between the first via 24 and the second via 25 , as shown in FIG. 1 and FIG. 3A , to cooperatively generate spiral inductance characteristics between the first layout layer 21 and the second layout layer 22 with respect to via stub portions S of third layout layers 29 which are around the first via 24 and the second via 25 , thereby compensating the capacitance characteristics of the via stub portions S. In the second embodiment, the curved first portions LA 1 , LB 1 of the first signal line LA and the second signal line LB are disposed around the first via 24 and the second via 25 , as shown in FIG. 3B . Shapes of the curved first portions LA 1 and the curved first portions LB 1 may be any shape, and are generally J-shaped, as shown in FIG. 1 and FIG. 3A , generally C-shaped, as shown in FIG. 3B , or generally curved.

The widths of the curved first portions LA 1 , LB 1 aren't limited, they may be bigger than the widths of the straight second portions LA 2 , 11 LB 2 of the first signal line LA and the second signal line LB, but shadows of the curve portions LA 1 , LB 1 should not exceed the areas of the corresponding through holes 231 , as shown in FIG. 3A .

Referring to FIG. 1 and FIG. 2 , the second layout layer 22 has a third signal line LC and a fourth signal line LD, disposed symmetrically. The third signal line LC has a curved first portion LC 1 and a straight second portion LC 2 integrally connected together. The fourth line LD has a curved first portion LD 1 and a straight second portion LD 2 integrally connected together.

The third signal line LC and the fourth signal line LD are disposed generally between the first via 24 and the second via 25 , and the curved first portions LC 1 , LD 1 of the third signal line LC and the fourth signal line LD are formed such that shadows of the curved first portions LC 1 , LD 1 are cast within an area of the corresponding through hole 231 of the copper foil layer 23 .

Curved first portions LC 1 , LD 1 of the third signal line LC and the fourth signal line LD are coupled to the first via 24 and the second via 25 by the second conducting portions 28 A, 28 B.

While curved first portions LC 1 , LD 1 may be of any generally curved shape, they are, here, generally J-shaped, as shown in FIG. 1 and FIGS. 3A and C-shaped, as shown in FIG. 3B . Curved first portions LC 1 , LD 1 can be disposed generally between the first via 24 and the second via 25 , as shown in FIG. 1 and FIG. 3A , or generally around the first via 24 and the second via 25 , as shown in FIG. 3B , to cooperatively generate spiral inductance characteristics between the first layout layer 21 and the second layout layer 22 with respect to via stub portions S of third layout layers 29 which are around the first via 24 and the second via 25 , thereby compensating the capacitance characteristics of the via stub portions S. Widths of the curved first portions LC 1 , LD 1 are not limited and can exceed those of the straight second portions LC 2 , LD 2 of the third signal line LC and the fourth signal line LD, but shadows of the curved first portions LC 1 , LD 1 need remain within the area of the corresponding through hole 231 of the copper foil layer 23 , as shown in FIG. 3A .

Referring to FIG. 1 , FIG. 3A and FIG. 3B , the setting direction V 1 of the curved first portions LA 1 , LB 1 of the first signal line LA and the second signal line LB is generally opposite to the setting direction V 2 of the curved first portions LC 1 , LD 1 of the third signal line LC and the fourth signal line LD, such that the shapes of the curved first portions LA 1 , LC 1 and the overall construction of the curved first portions LB 1 , LD 1 form a spiral and generate spiral inductance characteristics.

Referring to FIG. 1 , the first signal line LA, the second signal line LB, the third signal line LC and the fourth signal line LD transmit a pair of signals S 2 A, S 2 B. For example, the signals S 2 A, S 2 B can be transmitted via the first signal line LA and the second signal line LB and the vias 24 , 25 to be output from the third signal line LC and the fourth signal line LD. The signals S 2 A, S 2 B may be a pair of serial or differential signals.

Referring to FIG. 4 , an experimentally acquired oscillogram of a channel response of the PCB 2 and the conventional PCB 1 are shown. The yield of the curves C 2 , C 3 , C 4 of the PCB 2 is lower than that of C 1 of the conventional PCB 1 before the frequency reaches 9 GHz. Accordingly, spiral impedance characteristic generated by the curved first portions LA 1 , LB 1 , LC 1 , LD 1 of the first signal line LA, the second signal line LB, the third signal line LC, and the fourth signal line LD compensates for the capacitance characteristic of the via stub structure of PCB 2 .

In this embodiment, the curved first portions LA 1 , LB 1 , LC 1 , LD 1 , all of which form the spiral should not exceed the area of the corresponding through hole 231 of the copper layer 23 to compensate the capacitance characteristic of the PCB 2 , and the PCB 2 requires no added process. Along with decreased costs, the curved first portions LA 1 , LB 1 , LC 1 , LD 1 generate impedance characteristic by the changes in the circuit to compensate the capacitance characteristic of the via stub structure.

It is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.