PCB News - How to control PCB impedance and use signal integrity analysis tools

Many PCB engineers often draw computer motherboards and are very proficient with excellent tools such as Allegro. However, it is a pity that they rarely know how to control impedance and how to use tools for signal integrity analysis. For the IBIS model, I think the real PCB master should be a signal integrity expert, not just staying at the connection line, through the vias, it is easy to lay out a board, but it is very difficult to lay a good one.

Small information
After determining the number of power, ground and signal layers, the relative arrangement of them is a topic that every PCB engineer cannot avoid;

The general principle of layer arrangement:

The bottom of the component surface (the second layer) is the ground plane, which provides a device shielding layer and a reference plane for the top-level wiring;

All signal layers are as close as possible to the ground plane;

Try to avoid two signal layers directly adjacent; s

The main power supply is as close as possible to it correspondingly;

takes into account the symmetry of the laminated structure.
For the layer layout of the motherboard, it is difficult for the existing motherboard to control parallel long-distance wiring. For the board-level operating frequency above 50MHZ (refer to the situation below 50MHZ, appropriately relax it), it is recommended to arrange the principle:
The component surface and welding surface are a complete ground plane (shield);

no adjacent parallel wiring layers;

All signal layers are as close as possible to the ground plane;

The key signal is adjacent to the ground and does not cross the partition.
Note: When setting up specific PCB layers, the above principles should be flexibly mastered. Based on the understanding of the above principles, according to the actual requirements of the single board, such as: whether a key wiring layer, power supply, ground plane division is required, etc., Determine the arrangement of the layers, and don't just copy it or pick it up.
The following is a specific discussion on the arrangement of the veneer layers:
*Four-layer board, preferred plan 1, available plan 3
Scheme Number of power layers Number of ground layers Number of signal layers 1 2 3 4
1 1 1 2 S G P S
2 1 2 2 G S S P
3 1 1 2 S P G S
Scheme 1 The main selection layer setting scheme of the four-layer PCB of this scheme, there is a ground plane under the component surface, and the key signal is preferably arranged on the TOP layer; as for the layer thickness setting, the following suggestions are provided:
To meet the impedance control core board (GND to POWER) should not be too thick to reduce the distributed impedance of the power supply and ground plane; to ensure the decoupling effect of the power plane; in order to achieve a certain shielding effect, some people try to put the power supply and ground plane on the TOP, The BOTTOM layer, that is, the solution 2:
In order to achieve the desired shielding effect, this solution has at least the following defects:
The power supply and ground are too far apart, and the power plane impedance is large
The power supply and ground plane are extremely incomplete due to the influence of component pads, etc.
The signal impedance is not continuous due to the incomplete reference plane
In fact, due to the large number of surface mount devices, the power supply and ground of this solution can hardly be used as a complete reference plane when the devices are getting denser, and the expected shielding effect is difficult to achieve; the use of solution 2 is limited. But in individual boards, scheme 2 is the best layer setting scheme.
The following is a use case of Option 2;
Case (special case): During the design process, the following situations occurred:

A, the whole board has no power plane, only GND and PGND occupy one plane each;

B, the whole board is easy to wire, but as an interface filter board, the radiation of the wiring must be paid attention to;

C. The board has fewer SMD components, and most of them are plug-ins.

analyze:
1. Since the board has no power plane, the power plane impedance problem does not exist;

2. Due to the small number of SMD components (single-sided layout), if the surface layer is made as a plane layer and the inner layer is wired, the integrity of the reference plane is basically guaranteed, and the second layer can be laid with copper to ensure a small amount of top-level wiring reference plane;

3. As an interface filter board, the radiation of PCB wiring must be paid attention to. If the inner layer is wired, the surface layer is GND and PGND, the wiring is well shielded, and the radiation of the transmission line is controlled;
In view of the above reasons, when laying out the layers of this board, we decided to adopt option 2, namely: GND, S1, S2, and PGND. Because there are still a few short traces on the surface layer, and the bottom layer is a complete ground plane, we are in S1 The wiring layer is laid with copper to ensure the reference plane of the surface wiring. Among the five interface filter boards, based on the same analysis as above, the designer decided to adopt Option 2, which is also a classic layer setting.
Enumerating the above special cases is to tell everyone that we must understand the principle of layer arrangement, rather than copy it mechanically.
Scheme 3: This scheme is similar to scheme 1, and is suitable for the case where the main components are in the BOTTOM layout or the bottom wiring of key signals; in general, this scheme is restricted;
*Six-layer board: preferred option 3, available option 1, alternative options 2, 4 For six-layer boards, option 3 is preferred, wiring layer S2 is preferred, followed by S3 and S1. The main power supply and its corresponding ground are placed on the 4th and 5th layers. When the layer thickness is set, increase the spacing between S2-P and reduce the spacing between P-G2 (correspondingly reduce the spacing between G1-S2 layers), In order to reduce the impedance of the power plane, reduce the impact of the power supply on S2;
When the cost requirements are high, solution 1 can be used, preferably wiring layers S1, S2, followed by S3, S4.Compared with solution 1, solution 2 ensures that the power supply and the ground plane are adjacent to reduce the power supply impedance, but S1, S2, S3, S4 are all exposed, only S2 has a better reference plane;

For the occasions where the local and small signal requirements are higher, Option 4 is more suitable than Option 3. It can provide an excellent wiring layer S2.
*Eight-layer board: preferred plan 2, 3, available plan 1
In the case of a single power supply, solution 2 reduces adjacent wiring layers compared to solution 1, increases the main power supply adjacent to the corresponding ground, and ensures that all signal layers are adjacent to the ground plane. The cost is: sacrificing a wiring layer; for double In the case of power supply, solution 3 is recommended. Option 3 takes into account the advantages of no adjacent wiring layers, symmetrical laminate structure, and adjacent main power supply to ground, but S4 should reduce key wiring; Option 4: No adjacent wiring layers, layers The voltage structure is symmetrical, but the power plane impedance is high; 3-4, 5-6 should be appropriately increased, and the layer spacing between 2-3 and 6-7 should be reduced;
Option 5: Compared with Option 4, it ensures that the power and ground planes are adjacent; but S2 and S3 are adjacent, and S4 uses P2 as the reference plane; there are fewer key wiring at the bottom and the lines between S2 and S3

The above is an introduction to the points that PCB engineers need to pay attention to. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.