Printed Circuit Board Power Bus Decoupling Questions

5. Should decoupling capacitors be located near the Vcc pins or near the ground pins of an active device?

As discussed above, if the board doesn't have power planes or if the power and ground planes are closely spaced, it doesn't matter where the capacitor is located as long as the connection inductance is as low as possible. However, for boards where the power and ground planes are not closely spaced (e.g. >0.5 mm), it is important to maximize the mutual inductance between the decoupling capacitor connection to the planes and the active device connection to the planes. This requires the capacitor to be located near the pin (Vcc or Gnd) that connects to the most distant plane. For example, if a 4-layer board has a Vcc plane on layer 2 and a ground plane on layer 3 and all components are mounted above layer 1, then the ground pad of the capacitor should be as close as possible to the ground pad of the active device. If all components on this board were below layer 4, then it would have been important to locate the two Vcc pads close to each other.

References

[1] T. H. Hubing, J. L. Drewniak, T. P. Van Doren, and D. Hockanson, "Power Bus Decoupling on Multilayer Printed Circuit Boards," IEEE Trans. on Electromagnetic Compatibility, vol. 37, no. 2, May 1995, pp. 155-166.

[2] T. H. Hubing, T. P. Van Doren, F. Sha, J. L. Drewniak, and M. Wilhelm, "An Experimental Investigation of 4-Layer Printed Circuit Board Decoupling," Proceedings of the 1995 IEEE International Symposium on Electromagnetic Compatibility, Atlanta, GA, August 1995, pp. 308?312.

[3] J. Fan, J. Drewniak, J. Knighten, N. Smith, A. Orlandi, T. Van Doren, T. Hubing and R. DuBroff, "Quantifying SMT Decoupling Capacitor Placement in DC Power-Bus Design for Multilayer PCBs," IEEE Trans. on Electromagnetic Compatibility, vol. EMC-43, no. 4, November 2001, pp. 588-599.

[4] J. Fan, W. Cui, J. Drewniak, T. Van Doren, J. Knighten, "Estimating the Noise Mitigation Effect of Local Decoupling in Printed Circuit Boards," IEEE Trans. on Advanced Packaging, vol. 25, no. 2, May 2002, pp. 154-165.