Electricity: electrical systems and devices – Electrolytic systems or devices – Solid electrolytic capacitor
Reexamination Certificate
1999-04-01
2001-04-03
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Solid electrolytic capacitor
C361S529000, C361S540000, C029S025030
Reexamination Certificate
active
06212065
ABSTRACT:
BACKGROUND OF THE INVENTION
Solid tantalum capacitors use as anodes porous pellets which are compressed from fine tantalum and niobium powder. The pellets are first compressed and then they are sintered in a vacuum. The porous structure of the anode provides a high surface area and thereby high specific charge (CV) of the finished capacitors (where CV is a product of capacitance and rated voltage). Generally tantalum pellets contain a lead wire made of the same metal as the compressed powder. The lead wire is attached to the pellet either by welding after the pellet has been sintered, or by embedding in the pellet before the pellet is pressed and sintered.
A major trend of modern tantalum capacitors is reducing of the efficient radius of the powder particles so as to increase the pellet surface and thereby improve the capacitor CV per unit of weight or volume. Usages of very fine tantalum powder requires changing of the pelleting conditions. As the efficient particle radius becomes small the pellet density after pressing (green density) as well as the sintering temperature must be reduced to limit pellet shrinkage during sintering. Reducing of the green density and the sintering temperature causes weakening of the junction between the lead and the pellet for both embedded type pellets and welded type pellets. This causes capacitor rejects at manufacturing and failures at testing. The weakening of the junction between the lead and the pellet is one of the major limitations for usage of very fine tantalum with efficient particle radius of about one micron.
FIGS. 1 and 2
show a prior art capacitor
10
having a lead wire
14
welded to a tantalum pellet
12
.
During the welding process the lead wire
14
is pressed against the upper surface of the pellet
12
and heated by current pulse. For low density pellets sintered at low temperature this combination causes cracks of the pellet around welding zone in the area indicated by the numeral
18
. The resulting weld
16
may be weakened as a result of the fractured area
18
. This is particularly true for tantalum and niobium powder with a particle radius of about one micron or less.
Therefore, a primary object of the invention is the provision of an improved capacitor pellet and lead assembly and method for making same. A further object of the present invention is the provision of an improved capacitor having a lead wire welded to a pellet of low density and small particle size.
A further object of the present invention is the provision of a capacitor having a low density pellet of small particle size which exhibits good AC characteristics.
A further object of the present invention is the provision of a method for welding the lead wire to the pellet which minimizes the fracturing of the pellet during the welding operation.
A further object of the present invention is the provision of a capacitor and method for making same which is efficient in operation, durable in use, and includes high performance characteristics for small particle pellets.
SUMMARY OF THE INVENTION
The foregoing objects may be achieved by a capacitor comprising a porous pellet having compressed conductive particles therein. The pellet includes a lead receiving external surface. The conductive particles at the lead receiving surface are fused together to create a fused layer on the external surface of the pellet. A lead wire has one of its ends welded to the fused layer of the lead receiving surface.
Different methods may be used for fusing the lead receiving surface on the top of the pellet. The preferred method is the use of a laser beam which can be scanned back and forth to create a fused or partially melted area on the surface of the pellet.
The lead wire during the welding operation is pressed against the fused area on the surface of the pellet. The fused surface is harder then the remainder of the pellet, and therefore resists fracturing during the welding process. When the welding is complete, the lead wire and the fused area fuse together without any perceptible fractures at the junction or the weld joint.
REFERENCES:
patent: 4203194 (1980-05-01), McGrath
patent: 4479168 (1984-10-01), Green, Jr.
patent: 4520430 (1985-05-01), Long et al.
patent: 4791532 (1988-12-01), Gouvernelle et al.
patent: 5608601 (1997-03-01), Kuriyama
17thCapacitor And Resistor Technology Symposium Physical Principles Of The Solid Tantalum Capacitors By Dr. Yuri Pozdeev, Mar. 24-27, 1997.
17thCapacitor and Resistor Technology Symposium. CARTS '97. Mar. 24-27, 1997. Sponsored by The Components Technology Institute, Inc.
Akselrod Semion
Levi Boris
Pozdeev-Freeman Yuri L.
Dinkins Anthony
Vishay Sprague, Inc.
Zarley McKee Thomte Voorhees & Sease
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