Discussion: Many heat sinks have tines that protrude perpendicularly from a base plate such as the one shown in Figure 1. Today's clock frequencies are so high as to be able to excite the tines as quarter wave monopoles resulting in something like an antenna farm on top of the IC, potentially radiating a clock harmonic or other source resulting in an emissions problem.

One way to measure the resonant frequency of physical structures in general and heat sink tines in particular is shown in the June 2006 Technical Tidbit, Measuring Structural Resonances on this site. Basically, one would hold a very small shielded loop at the base of one of the heat sink tines to couple energy into the tine. By measuring the reflected signal from the small shielded loop, the resonant frequency of a tine can be measured as the frequency where the reflected signal experiences a dip as the tine absorbs energy from the loop. The lowest frequency this would occur at is the frequency at which the tine is 1/4 wavelength. The method is described in detail in the June 2006 article referenced above. Figure 2 shows the method being applied to a PCB test board mounted over a copper clad board. The object in that case is to measure the resonant frequency of the LC tuned circuit composed of the capacitor formed by the PCB and copper clad board, and the inductance of the short wire between them.