Ceramic capacitor

A ceramic chip capacitor comprises electrically conductive layers extending alternately to two opposing edges of a rectangular ceramic body. The other two edges are provided with electrical terminations. The edges to which the conductive layers extend are partially covered with fusible metallic films which connect the layers to respective terminations. The fusible films isolate the capacitor from associated circuitry if the capacitor fails and effects an internal short circuit.

This invention relates to ceramic capacitors, such as are used for 
decoupling semiconductor integrated circuits from power supplies. Such 
capacitors are commonly used in the range 50 n F-1.mu. F. 
One type of ceramic capacitor is the rectangular leadless chip capacitor. 
One example of such a capacitor is described in British Pat. No. 
1,356,213. The capacitor is made up of a plurality of rectangular (or 
square layers), the two terminations are provided on two opposing edges of 
the capacitor, thereby leaving the other edges free from terminations. 
Each layer of electrically conductive material is so arranged as to extend 
to the edge on which is bonded its respective termination but not extend 
to the opposite edge. None of the layers of electrically conductive 
material extend to the other two edges, neither of which is provided with 
a termination. Thus, each layer of dielectric material is provided with a 
margin on two opposite edges which is not covered by electrically 
conductive material and, consequently, since the insulating and dielectric 
layers are generally made of identical or substantially identical 
materials, when the layers are bonded together, there is a substantially 
continuous margin at each of the two opposite edges of the capacitor 
formed by margins on each of the layers of dielectric material that are so 
firmly bonded together as to be essentially integral. 
When a multilayer ceramic capacitor fails under an overload it normally 
goes to a short circuit condition. Large currents may then pass, causing 
local damage or even fire in the circuitry associated with the capacitor, 
depending on the circumstances. A similar condition can occur in solid 
tantalum capacitors. In IEE Transactions on Components, Hybrids, and 
Manufacturing Technology, Vol. CHMT-3, No. 2, June 1980, at page 244 et 
seq. there is an article describing the incorporation of a miniature fuse 
module into a solid tantalum capacitor. The fuse module comprises a fine 
bimetallic wire housed within a cavity in the electrically and thermally 
insulative body surrounding the capacitor. The fuse wire is electrically 
in series with the capacitor. The bimetallic wire is composed of metals 
which at a predetermined current reach a temperature at which an 
exothermic effect destroys the wire. 
In yet another known construction disclosed in British patent application 
2013031 A a monolithic ceramic capacitor has two sets of mutually parallel 
interdigitated sets of electrodes extending to one end face and one side 
face respectively of a ceramic body, where they contact respective 
conductive terminal layers. The other end face has a third terminal layer 
that does not contact any electrodes. A wire fuse link is connected 
externally of the body between the third terminal layer and one of the 
other terminal layers. 
According to the present invention there is provided a ceramic capacitor 
comprising two or more parallel layers of electrically conductive material 
separated by dielectric material and enclosed in a body of ceramic 
material, the conductive layers extending alternatively to one only of two 
different edge or surface portions of the ceramic body, none of the layers 
extending to any other edge or surface portions of the body, two other 
edge or surface portions of the body being provided with respective 
electrically conductive terminations, each of said two edge or surface 
portions to which conductive layers extend being provided with an 
electrically conductive fusible film coating making electrical connection 
with all of the layers extending to that portion, said coatings each 
extending over a part of the body surface to make electrical connection to 
a respective one only of the terminations.

The capacitor illustrated comprises layers 1a, 1b, 1c of electrically 
conductive material, e.g. palladium, gold or alloys thereof, separated by 
and enclosed in ceramic material, alternate layers extending to opposite 
edges 2a, 2b of the rectangular ceramic body 2. None of the layers extends 
to either of the other two edges 2c, 2d. Edges 2c and 2d are provided with 
metallic terminations 3c, 3d which are conveniently silver. Edges 2a and 
2b are provided with thin films 3a, 3b which make electrical contact with 
terminations 3c and 3d respectively. Each of the films 3a, 3b only 
partially covers its respective edge surface such that it makes electrical 
connection with all the conductive layers extending to that edge but does 
not make contact with the other termination. Typically the films 3a, 3b 
are aluminium deposited by vacuum deposition. The melting point of the 
film must exceed soldering temperatures the capacitor would normally 
experience. In use any overload which causes the capacitor to fail 
internally to a short circuit will result in the film failing to an open 
circuit condition, thus preventing a prolonged excessive current passing 
through the capacitor. This would also provide a visual indication that 
the capacitor had failed. 
An alternative to a single metal film deposited as described above is the 
use of a multilayer film the components of which would react at an 
elevated temperature to produce a highly resistive film and cause the 
capacitor to become effectively open circuit. For example the film may be 
a multilayer film of aluminium, gold and silicon.