BGA is a common component on PCB, such as CPU, North Bridge, South Bridge, AGP chip, card bus chip, etc In short, 80% of the high frequency signals and special signals will be pulled out from this type of package. Therefore, how to deal with the routing of BGA package will have a great impact on important signals.
Generally, small parts around BGA can be divided into several categories according to their importance
1. by pass.
2. Clock terminal RC circuit.
3. Damping (in the form of series resistance and row group; for example, memory bus signal)
4. EMI RC circuit (in the form of Damin, C, pull height; for example, USB signal).
5. Other special circuits (special circuits added according to different chips, such as temperature sensing circuit of CPU).
6. Small power supply circuit group below 40mil (in the form of C, l, R, etc.; this kind of circuit often appears near AGP chip or chip with AGP function, and different power supply 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, etc.; no wiring requirements).
9. pull height R、RP.
The circuits of items 1-6 are usually the focus of placement, and they will be arranged as close to BGA as possible, which needs special treatment. The importance of the seventh circuit is next, but it is also close to BGA. 8 and 9, which are general circuits, and belong to the signals that can be connected.
Relative to the importance priority of small parts near BGA, the requirements on routing are as follows:
1. By pass = > when it is on the same side as chip, it is directly connected from chip pin to by pass, and then via is pulled out from by pass and connected to plane; when it is on different sides from chip, it can share the same via with VCC and GND pin of BGA, and the line length should not exceed 100mil
2. Clock terminal RC circuit has the requirements of line width, line distance, line length or package GND, etc.; the routing should be as short and smooth as possible, and should not cross the VCC separation line as far as possible.
3. Damping = > has the requirements of line width, line distance, line length and group routing; the routing should be as short and smooth as possible, and one group of routing should not be mixed with other signals.
4. EMI RC circuit has the requirements of line width, line distance, parallel routing, package GND, etc.; it is completed according to the customer’s requirements.
5. Other special circuits need line width, package GND or routing clearance, etc.; according to customer requirements.
6. Small power supply circuit group below 40mil = > Cable width and other requirements; try to finish with the surface layer, reserve the inner layer space completely for the use of signal line, and try to avoid unnecessary interference caused by power signal passing through the upper and lower layers of BGA area.
7. Pull low R, C = > no special requirements; smooth routing.
8. General small circuit group = > no special requirements; smooth routing.
9. Pull height R, RP =, no special requirements; smooth routing.
In order to more clearly explain the processing of BGA parts routing, a series of icons will be used as follows:
A. BGA is divided by a cross from the center, and via is played in the directions of upper left, lower left, upper right and lower right respectively; the cross can be adjusted asymmetrically according to the routing needs.
B. The clock signal has the requirements of line width and line distance. When its R and C circuits are on the same side as chip, please deal with the above diagram as far as possible.
C. USB signal is at both ends of R and C, please complete parallel wiring.
D. By pass: try to connect chip pin to by pass and then enter the plane. If you can’t receive by pass, please lower the signal of the plane. E. BGA assembly nearby, pull out the outer three circles, and maintain the original set line width and line spacing; via can adjust the routing sequence in the part entity and 3mm placement forbidden area, if there is no level requirement for routing, it can be extended without restriction. Pull the inner ring inward or place via between 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 the corner.
F. The signals of BGA components should be pulled out radially as far as possible to avoid internal rotation.
F_ 2. BGA back by pass placement and routing.
Try to be close to the power pin. F_ 3 is the condition caused by via in BGA area in VCC layer, the conduction state of thermal VCC signal in VCC layer.
The separation state of anti GND signal in VCC layer.
The BGA signal has regular lead and via, which makes the power supply more sufficient.
F_ 4 is the condition caused by via in BGA area in GND layer, the conduction state of thermal GND signal in GND layer.
The separation state of anti VCC signal in GND layer.
Because the BGA signal has regular lead and via, the grounding conduction is sufficient.
Analysis of BGA chip layout and wiring design method
F_ 5 is the layout and routing proposal of BGA area
The functions of the BGA routing suggestions are as follows:
1. Regular leads are beneficial to the processing of special signals, so that all the routing layers except the surface layer can be completed with the required line width and line distance.
2. The VCC and GND in BGA have better conductivity.
3. The cross dividing line of BGA center can be used to cut the VCC layer when there is more than one power supply in BGA and it is not easy to cut the VCC layer, it can be processed in the routing layer (40 ~ 80mil) to the power supply end. Or the clock of BGA itself, or other signals with larger line width and line distance are routed in a forward direction.
4. Good BGA routing and placement can minimize the interference of BGA signal.