Tuesday, June 13, 2006

"CHIPS: Decoupling capacitors return to simpler times"

AVX Corp. has made what it claims is a revolutionary step backward with its invention of a patented new land-grid array (LGA) architecture for decoupling capacitors. Decoupling capacitors provide on-demand power reserves that can keep the edges sharp on the quickly changing signals from high-speed microprocessors, graphics coprocessors, digital signal pro- cessors and field-programmable gate arrays. Unfortunately, as clock frequencies have climbed, capacitor designers have quelled the parasitic inductance that can limit switching speed by resorting to hard-to-use multilayer architectures that require terminals that exit from the side. AVX has lowered parasitic inductance by returning to the simpler design of old-school capacitors, which feed directly into the printed-circuit board from the bottom. Most capacitors are just parallel metal plates separated by an insulator, and hence have essentially no inductance. In working capacitors, however, parasitic inductance is a fact of life. When current flows, the flux creates a magnetic field that loops around the internal electrodes of the capacitor and its external termination, the power planes, vias, mounting pads and solder fillets of the printed-circuit board. AVX's LGA capacitors solve the parasitic inductance problem through a reorientation of the internal electrodes. Instead of a horizontal orientation, in which the electrodes are parallel to the substrate, the electrodes are given a vertical orientation. The LGA architecture enables multilayer capacitors to gang together their multiple electrodes internally, with only two terminals exiting the capacitor from the bottom of the package. This allows printed-circuit-board makers to use a single two-terminal component with bottom exits, rather than using multiple small capacitors, each of which might have up to eight terminals exiting from the sides.