BGA is a common component on PCB, such as CPU, North Bridge, South Bridge, AGP chip, card bus chip, etc In short, 80% of high-frequency signals and special signals will be pulled out of this type of package. Therefore, how to deal with the routing of BGA package has a great influence on important signals.

Generally, small parts around BGA can be divided into several categories according to their importance

1. by pass.

2. RC circuit of clock terminal.

3. Damping (in the form of series resistance and row group; for example, memory bus signal)

4. EMI RC circuit (in the form of dampin, C, pull height; for example, USB signal).

5. Other special circuits (special circuits added according to different chips; for example, the temperature sensing circuit of CPU).

6. Small power supply circuit group below 40mil (in the form of C, l, R; this kind of circuit often appears in the AGP chip or chip with AGP function, and different power groups are separated by R and L).

7. pull low R、C.

8. General small circuit group (in the form of R, C, Q, u; no wiring requirements).

9. pull height R、RP.

The circuits of items 1-6 are usually the focus of placement. They will be arranged as close as possible to BGA and need special treatment. The seventh circuit is less important, but it will also be arranged closer to BGA. 8 and Item 9, which are general circuits, belong to the signal that can be connected.

Relative to the importance priority of small parts near BGA, the requirements on routing are as follows:

1. When the same side of chip is the same as chip, the chip pin is directly connected to by pass, and then pulled out by pass to connect via to plane; when the side is different from chip, it can share the same via with VCC and GND pin of BGA, and the line length should not exceed 100mil

2. The RC circuit of clock terminal has the requirements of line width, line distance, line length or package GND; the wiring shall be as short and smooth as possible, and shall not cross the VCC separation line as far as possible.

3. Damping = > there are requirements for line width, line distance, line length and grouping routing; the routing should be as short and smooth as possible, with groups of lines running in groups, and other signals should not be mixed.

4. EMI RC circuit = has the requirements of line width, line distance, parallel wiring, package GND, etc.; it can be completed according to customer requirements.

5. Other special circuits have the requirements of line width, package GND or wiring clearance, etc. according to customer requirements.

6. Small power supply circuit group below 40mil = wire width and other requirements; try to finish with surface layer, reserve the inner layer space completely for the signal line, and try to avoid the power signal passing through the layer above and below in BGA area, causing unnecessary interference.

7. Pull low R, C =, no special requirements; smooth running.

8. There are no special requirements for general small circuit group.

9. Pull height R, RP =, no special requirements; smooth running.

In order to more clearly explain the processing of BGA part routing, a series of icons are shown as follows:

How to deal with part routing of BGA chip on PCB

A. BGA is divided by a cross from the center, and via is marked in the direction of upper left, lower left, upper right and lower right respectively; the cross can be adjusted asymmetrically due to the need of routing.

B. Clock signal has the requirements of line width and line distance. When the R and C circuits of clock signal are the same as chip, please try to deal with them in the way shown above.

C. USB signal at both ends of R and C should be wired in parallel.

D. If you can’t receive by pass, please lower the signal from the nearby plane. E. BGA assembly, pull out the outer three circles, and keep the original set line width and line spacing; via can adjust the routing sequence in the part entity and the forbidden zone of 3mm placement, and if the routing has no layer requirements, it can be extended without restriction. The inner ring is pulled inward or via is in the middle of pin and pin. In addition, the four corners of BGA should be pulled out with surface layer as far as possible to reduce the number of via in corners.

F. The signal of BGA module should be pulled out as far as possible in the radiation mode; the internal rotation should be avoided.

How to deal with part routing of BGA chip on PCB

F_ 2 is the placement and routing of BGA back by pass.

By pass as close as possible to the power supply pin. F_ 3 is the condition caused by via of BGA area in VCC layer, and the conduction state of thermal VCC signal in VCC layer.

Anti GND signal is separated in VCC layer.

Because BGA signal has regular lead wire and via, the power supply has sufficient conduction.

F_ 4 is the condition caused by via of BGA area in GND layer, and the conduction state of thermal GND signal in GND layer.

The isolation state of anti VCC signal in GND layer.

Because the BGA signal has regular lead wire and via, the grounding conduction is sufficient.

Analysis of BGA chip layout and routing design method

F_ 5 is the placement and routing proposal map of BGA area

The above BGA routing suggestions are as follows:

1. Regular leads are beneficial to the processing of special signals, so that except for the surface layer, the rest of the routing layers can be completed with the required line width and distance.

2. The VCC and GND inside BGA will have better continuity.

3. The cross dividing line in BGA center can be used; when there is more than one internal power supply in BGA and it is not easy to cut VCC layer, it can be processed in the wiring layer (40 ~ 80mil) to the power supply end. Or BGA’s own clock, or other signals with large line width and line spacing.

4. Good BGA routing and placement can minimize the interference of BGA signal.

Source: Internet

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