Portland, Ore. -- 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.
"Our new land-grid array decoupling capacitors offer higher-frequency performance in a simpler package that lowers assembly costs, with terminals that exit from the bottom," said Craig Hunter, strategic-marketing manager at AVX (Myrtle Beach, S.C.). The decoupling capacitors offer "as low an inductance as harder-to-use components with multiterminals that exit from the side," Hunter said.
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.
To date, two strategies have been used to reduce parasitic inductance. First, the capacitor is made in the shape of a rectangle instead of a square. The shorter distance between the terminals exiting the side of the capacitor along the narrow dimension makes the loop smaller. The long dimension of the rectangle, called the span, is perpendicular to the direction of current flow and thus does not contribute to the part's inductance.
The second strategy has been to reduce loop area further by using smaller plates in multiple layers. Not only is the distance between terminals smaller with this approach, but because the current path of the lowest loop has the least resistance, it also pins the inductance for the whole part on the area of the bottom layer loop. AVX offers high-performance decoupling capacitors that combine both strategies.
Unfortunately, as processor frequencies have continued to rise, more layers have had to be added to achieve higher capacitance at low parasitic inductance levels. And these multilayered capacitors have had to use a separate set of terminals for each layer. This approach has resulted in side-exiting, multiterminal capacitors, increasing the complexity of their fabrication as well as the mounting and assembly of printed-circuit boards.
To meet the performance needs of gigahertz applications, some chip makers have gone even further by using multiple small capacitors connected together, each with as many as eight terminals. As a result, both fabrication and assembly costs have in- creased in proportion to increases in clock frequency.
"In our land-grid array capacitors," said Mark Obuszewski, AVX's business manager of signal integrity, "maximum capacitance does not have to be traded off against lower inductance, because unlike conventional capacitors, the inductance of our devices is actually lower for larger case sizes. The current-loop area is set by the design of our internal electrodes--not its external dimensions."
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.
The LGA capacitors have "less than 50 percent of the inductance" of eight-terminal components, and offer very similar performance, Hunter said, "but you also get lower fabrication and assembly costs" because fewer parts are used.
"We already have two customers using our land-grid array capacitors inside their high-speed FPGAs," said Hunter. "We are in volume production now, and our sister company Kyocera will also manufacture them as a second source."
Because the LGA capacitors are available in the same kinds of packages already in use today, printed-circuit-board makers can use their current pick-and-place machines. The LGA capacitors are characterized at up to 9 GHz and will be available in 0204 through 1206 case sizes with thicknesses ranging from 0.5 to 1.5 mm. n