Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
1997-12-09
2002-04-02
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S321200, C361S303000
Reexamination Certificate
active
06366443
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally concerns “parallel plate” or “single layer” capacitors as are typically made by metallizing two faces of a thin sheet of ceramic so as to make a ceramic capacitor.
The present invention further concerns high-capacity ceramic capacitors having (i) internal conductive planes connected by (ii) multiple redundant (preferably radiation patterned) vias to (iii) improved, alignment-insensitive and anti-wicking, surface pads to which electrical connection may be made.
2. Description of the Prior Art
2.1 State of the Art in Capacitor Technology Circa 1997, and Industry Impetus for Certain Improvements of All Types
As reported by free-lance writer Hailey Lynne McKeefry in the article “Capacitor Technology Marches Ahead” appearing in the February, 1994, issue of Electronic Buyer's News, capacitors were then, and are now, a mature component in a mature market. Nonetheless to this maturity, U.S. manufacturers are striving mightily to improve both tantalum and ceramic capacitors. “The fact that everything you buy performs better and costs less than what you bought last year is what keeps this industry going,” said Terry Weaver, president and chief operating officer at Kemet Electronics Corp., Greenville, S.C. In this sentiment the inventors of the present invention concur.
As with just about any electronic component, the motto of capacitor purchasers and manufacturers alike is “smaller is better.” Capacitor manufacturers are working toward cases with smaller footprints and lower profiles. For example, the Sprague division of Vishay Electronic Components of North America and Asia developed a molded surface-mount tantalum capacitor with a rating of 6.8 microfarads in an R-size case. In September, 1997, Myrtle Beach, S.C.-based AVX Corp. announced a low-profile V-size case for its tantalum surface-mount capacitors. The TPS-V series is designed to provide capacitance/voltage (C/V) ratings higher than 3,500 and improved power dissipation. The V case measures less than 7 mm×6 mm, with a height of just 3.45 mm.
In the marketplace for ceramic capacitors (to which the present invention pertains), the 0805 case size remains popular although there is a significant move to 0603s, and even increased interest in 0402s. “Designers are leap-frogging the 0805 packages in favor of the 0603s in some designs,” said Kevin Rafferty, marketing manager for ceramic capacitors at Philips Electronic Components, Jupiter, Fla. Rafferty predicts that 0805s—with a 30% share of ceramic volume—will continue to take the biggest share of the market, while 0603s will claim 20% and 0402s 5%. The remaining 35% will be divided among products in 1206 and larger packages. In 1996, 0603 packages accounted for only 5% of the market share. “The 0805s are stable, while 1206 packages are losing share,” he said.
Part of what is holding the 0402s back is their diminutive size. “They are just too hard to handle,” Kemet's Weaver said. “A number of our customers can't place them at all.” To combat these complaints, some companies such as Murata Electronics North America Inc. are developing ways of handling the components. The manufacturer has created a small plastic case with a flip top that can hold 50,000 to 80,000 pieces. This box is slipped into a slot on the placement equipment. In comparison, a typical tape-and-reel holds about 5,000 parts. Although Murata developed the bulk cassette technology, others now offer it. For example, Kemet will ship the 0603 and 0402 sizes of its capacitors that use the X7R and C0G dielectrics in cassettes can hold 15,000 units of 0603 chips and 50,000 units of 0402 chips.
Another emerging solution is the use of capacitor arrays, in which several components are encased in a single package. The present invention will be seen to well support capacitor arrays. With these arrays OEMs are able to avoid the placement issues surrounding very small chips. In addition, they save on-board real estate and component placement costs. A standard array contains two to four chips. However, use of these packages is still limited to specialized niche markets. One of the biggest drawbacks of this packaging strategy is that all of the capacitors in the array have heretofore been required to be of same value. The present invention will be seen to readily overcome this limitation.
The fact that the Electronic Industries Association has not settled on a standard case size has also inhibited sales of arrayed capacitors. Heretofore the case size was strongly, even inflexibly, linked to the capacitance value. Therefore standardization of case sizes was, to some extent, a standardization of the capacitance values with which, for a particular packaging technology, circuit designers could work. The present invention will be seen to sever this relationship, and present a greater opportunity than heretofore of producing a ceramic capacitor of any desired value (within limits) inside a standard case, or a reduced number of standard cases.
A similar trend is occurring in the tantalum capacitor market. “We are seeing users of each case size [A, B, C, and D] going to the next-smallest case size to save real estate, while getting the same values,” said Willing S. King, marketing manager for tantalum products at AVX. “In turn, we are using less material and passing that savings on to the customer.” There is about a 10% price difference between case sizes, he estimated.
Computer makers primarily use C and D case sizes, while telecommunications and cellular manufacturers, the two biggest markets for tantalum capacitors, prefer the A and B sizes. A standard A case measures about 1.6 mm, while a B case is 2 mm high. Despite some similarities, the ceramic- and tantalum-capacitor businesses are, in fact, very different. In the ceramic marketplace, some of the primary focuses include advancing dielectric technologies and the advent of low-inductance products. Tantalum manufacturers, on the other hand, are concentrating on lowering equivalent-series-resistance (ESR) ratings and increasing C/V ratings.
Two popular dielectrics are the Y5V and X7R. Y5V is a general-purpose dielectric without the tighter tolerances offered by the X7R. It offers an operating temperature range of (approximately) −30 degrees C to +85 degrees C, and is favored in cost-sensitive consumer applications. The X7R, which is more temperature-stable, can handle from −55 degrees C to +125 degrees C. However, the dielectric coefficient K of the Y5V dielectric is approximately 15,000; the K of the X7R dielectric only about 4000. Accordingly, the advantage that a high-dielectric-constant dielectric offers for realizing much higher capacitance in a single chip comes at the expense of temperature stability. There has therefore heretofore been a trade-off: improved dielectric performance with resultantly increased capacitance (for the same form factor) versus enhanced temperature sensitivity. The present invention will again be seen to substantially obviate this concern, and to permit the production of ceramic capacitors of desirably high capacitance by use of only the dielectrics having lower dielectric coefficients.
OEMS have commenced to replace the tantalum chips with the ceramic parts in some instances. “Higher capacitance values and cost effectiveness make surface-mount ceramic capacitors an increasingly popular alternative to tantalum caps,” said Philips' Rafferty. “Other advantages of ceramic chips include higher breakdown voltages, lower ESR, higher insulation resistance, and better pulse response for frequencies greater than 100 kHz. This means that the capacitance values can be 50% less than the equivalent tantalum caps.
Low inductance is also in demand for the PC market, particularly as faster and faster microprocessors arc designed into PCs. “There has been lots of interest in low-inductance products since the speed of processors is increasing and voltages for processors are dropping to 2.2V” Vishay's Gormally said. “Using higher
Devoe Alan D.
Devoe Daniel
Devoe Lambert
Dinkins Anthony
Thomas Eric
Wood Herron & Evans L.L.P.
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