Method for selecting and placing bypass capacitors on multi-layer printed circuit boards
A method places bypass capacitors for controlling the signal integrity of a signal routed between devices on a multi-layer printed circuit board by analysis of the routing of the signal and the signal characteristics. In general, when an interconnecting signal between devices is routed adjacent to an impedance control plane that does not serve as the circuit supply voltage plane or the reference voltage plane for all devices interconnected by the signal trace, the method selects locations for addition of bypass capacitors based upon the routing of the interconnecting signal trace to improve signal integrity.
1. A method of determining locations for placing bypass capacitors for controlling the signal integrity of a signal routed from a first device to a second device via an interconnecting signal trace on a multi-layer printed circuit board, the printed circuit board including at least a first layer, a second layer, a third layer, and a fourth layer, the method comprising:
analyzing the routing of the interconnecting signal trace; and
determining a location for a bypass capacitor based upon routing of the interconnecting signal trace in the third layer adjacent to the fourth layer, where the first layer powers the first and second devices, the second layer serves as a voltage reference plane for the first and second devices, the third layer serves as a routing layer for the interconnecting signal trace, and the fourth layer serves as an impedance control plane for the interconnecting signal trace but not powering nor serving as the voltage reference plane for the first and second devices, the location of the bypass capacitor decided in accordance with frequency and waveform characteristics of the interconnecting signal and required signal integrity.
2. The method of claim 1 , wherein determining a location for a bypass capacitor comprises:
determining a location near a source point of the signal at the first device between the second layer and the fourth layer.
3. The method of claim 1 , wherein determining a location for a bypass capacitor comprises:
determining a location near a termination point of the signal at the second device between the second layer and the fourth layer.
4. The method of claim 1 , wherein determining a location for a bypass capacitor comprises:
determining a location near a source point of the signal at the first device between the second layer and the fourth layer; and
determining a location near a termination point of the signal at the second device between the second layer and the fourth layer.
5. The method of claim 1 , wherein the printed circuit board further includes a fifth layer and a sixth layer, the fifth layer also serving as a routing layer for the interconnecting signal trace, and the sixth layer also serving as an impedance control plane for the interconnecting signal trace, and determining a location for a bypass capacitor comprises:
determining a location near a routing via of the signal between the fourth layer and the sixth layer.
6. The method of claim 1 , wherein the printed circuit board further includes a fifth layer and a sixth layer, the fifth layer also serving as a routing layer for the interconnecting signal trace, and the sixth layer also serving as an impedance control plane for the interconnecting signal trace and determining a location for a bypass capacitor comprises:
determining a location near a source point of the signal at the first device between the second layer and the fourth layer;
determining a location near a termination point of the signal at the second device between the second layer and the fourth layer; and
determining a location near a routing via of the signal between the fourth layer and the sixth layer.
7. The method of claim 6 , wherein:
the first layer is coupled to a first supply voltage;
the second layer is coupled to a first ground voltage;
the fourth layer is coupled to a second supply voltage; and
the sixth layer is coupled to a second ground voltage.
8. The method of claim 1 , wherein the fourth layer powers a third device.
9. A printed circuit board with a first device and a second device mounted on the printed circuit board for routing a signal from the first device to the second device via an interconnecting signal trace, the multi-layer printed circuit board comprising:
a first layer coupled to and powering the first device and second device;
a second layer coupled to and serving as a voltage reference plane for the first device and second device;
a third layer serving as a routing layer for the interconnecting signal trace;
a fourth layer serving as an impedance control plane for the interconnecting signal trace but not powering nor serving as a voltage reference plane for the first device and the second device; and
at least a bypass capacitor coupled between the second and the fourth layer.
10. The printed circuit board of claim 9 , wherein the bypass capacitor comprises:
a bypass capacitor coupled near a source point of the signal at the first device between the second layer and the fourth layer.
11. The printed circuit board of claim 9 , wherein the bypass capacitor comprises:
a bypass capacitor coupled near a termination point of the signal at the second device between the second layer and the fourth layer.
12. The printed circuit board of claim 9 , wherein the bypass capacitor comprises:
a first bypass capacitor coupled near a source point of the signal at the first device between the second layer and the fourth layer; and
a second bypass capacitor coupled near a termination point of the signal at the second device between the second layer and the fourth layer.
13. The printed circuit board of claim 9 , wherein the printed circuit board further includes a fifth layer and a sixth layer, the fifth layer also serving as a routing layer for the interconnecting signal trace, and the sixth layer also serving as an impedance control plane for the interconnecting signal trace, and the printed circuit board further comprises:
a bypass capacitor coupled near a routing via between the fourth layer and the sixth layer.
14. The printed circuit board of claim 9 , wherein the printed circuit board further includes a fifth layer and a sixth layer, the fifth layer also serving as a routing layer for the interconnecting signal trace, and the sixth layer also serving as an impedance control plane for the interconnecting signal trace, and the bypass capacitor comprises:
a first bypass capacitor coupled near a source point of the signal at the first device between the second layer and the fourth layer;
a second bypass capacitor coupled near a termination point of the signal at the second device between the second layer and the fourth layer; and
the printed circuit board further comprises a third bypass capacitor coupled near a signal routing via between the fourth layer and the sixth layer.
15. The printed circuit board of claim 14 , wherein:
the first layer is coupled to a first supply voltage;
the second layer is coupled to a first ground voltage;
the fourth layer is coupled to a second supply voltage; and
the sixth layer is coupled to a second ground voltage.
16. A computer program product stored on a computer readable medium and adapted to perform a method of determining locations for placing bypass capacitors for controlling the signal integrity of a signal routed from a first device to a second device via an interconnecting signal trace on a multi-layer printed circuit board, the printed circuit board including at least a first layer, a second layer, a third layer, and a fourth layer, the first layer powering the first and second devices, the second layer serving as a voltage reference plane for the first and second devices, the third layer serving as a routing layer for the interconnecting signal trace, and the fourth layer serving as an impedance control plane for the interconnecting signal trace but not powering nor serving as the voltage reference plane for the first and second devices, the method comprising:
analyzing the routing of the interconnecting signal trace; and
determining a location for a bypass capacitor based upon routing of the interconnecting signal trace in the third layer adjacent to the fourth layer.
17. The computer program product of claim 16 , wherein determining a location for a bypass capacitor comprises:
determining a location near a source point of the signal at the first device between the second layer and the fourth layer.
18. The computer program product of claim 16 , wherein determining a location for a bypass capacitor comprises:
determining a location near a termination point of the signal at the second device between the second layer and the fourth layer.
19. The computer program product of claim 16 , wherein determining a location for a bypass capacitor comprises:
determining a location near a source point of the signal at the first device between the second layer and the fourth layer; and
determining a location near a termination point of the signal at the second device between the second layer and the fourth layer.
20. The computer program product of claim 16 , wherein the printed circuit board further includes a fifth layer and a sixth layer, the fifth layer also serving as a routing layer for the interconnecting signal trace, and the sixth layer also serving as an impedance control plane for the interconnecting signal trace, and determining a location for a bypass capacitor comprises:
determining a location near a routing via of the signal between the fourth layer and the sixth layer.
21. The computer program product of claim 16 , wherein the printed circuit board further includes a fifth layer and a sixth layer, the fifth layer also serving as a routing layer for the interconnecting signal trace, and the sixth layer also serving as an impedance control plane for the interconnecting signal trace and determining a location for a bypass capacitor comprises:
determining a location near a source point of the signal at the first device between the second layer and the fourth layer;
determining a location near a termination point of the signal at the second device between the second layer and the fourth layer; and
determining a location near a routing via of the signal between the fourth layer and the sixth layer.