GB2115937A

GB2115937A - Testing for dielectric faults

Info

Publication number: GB2115937A
Application number: GB08206030A
Authority: GB
Inventors: Robert Carnegie Chittick
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-03-02
Filing date: 1982-03-02
Publication date: 1983-09-14
Also published as: GB2116729A; GB2116328B; GB2116328A

Abstract

Ceramic capacitors are tested by measurement of their electrical leakage before and after immersion in a mobile ionising solvent, e.g. a lower alkyl alcohol. A significant increase in leakage is indicative of a fault, e.g. a crack in the dielectric. <IMAGE>

Description

SPECIFICATION Improvements in testing for dielectric faults This invention relates to testing dielectric faults such as may occur in ceramic capacitors and to techniques for improving the quality control of such capacitors.

Ceramic dielectric capacitors, and particularly multilayer ceramic capacitors are widely used in the electronics industry as they are relatively inexpensive and have a high capacitance/volume ratio. It is usual to employ a multilayer structure when fabricating ceramic capacitors, so that layers of ceramic are interleaved with layers of metal electrode in such a way that an interdigitated two-electrode component of high capacitance value is produced. Various methods are used to make the ceramic layers as thin 'leaves', usually formed from a mix of the finely powdered ceramic material and an organic binder solvent system. For example, in a typical conventional process, a ceramic/binder/solvent mixture is coated on to polyethylene strip, by a tape-drawing process.After drying, the ceramic/binder film is peeled off and then silk screen printed with electrodes using an ink formed from precious metal powders in an organic binder. A number of such 'leaves' are stacked and pressed together, diced, heated to remove the binder, then fired at a high temperature. End terminations and leads may be attached following normal practice and such processes as described above are well known in the art of multilayer ceramic capacitor manufacture. Following the present industry trend to decrease dielectric thickness the dielectric film integrity has assumed great importance. It is desirable to decrease the capacitor size for several reasons, mainly compatibility with micro-electronic trends and economy of materials.

A problem that has arisen with presently manufactured ceramic multilayer capacitors is that of occasional cracking of the dielectric, during the manufacturing process. This cracking provides an intrinsic breakdown path between the capacitor electrodes or between an electrode and the opposite plurality end termination and can lead to subsequent failure in service. The mechanism of this failure is not fully understood, but it is thought to involve silver migration from the precious metal electrodes or end termination which then provides a low resistive breakdown path. This migration is thought to occur in those cracked regions to which there is access to the atmosphere.

In most instances cracking of the dielectric cannot be detected by visual inspection and the defect only becomes manifest after the capacitor has been in use foray extended period. It is clearly desirable to reduce such long term failures to a minimum.

A number of techniques have been proposed for dielectric crack detection in ceramic capacitors. In one process acoustic emissions from the capacitors are monitored. These acoustic emissions are then computer processed to determine the good and bad capacitors. Such a process is however costly and somewhat uncertain.

the object of the present invention is to minimise or to overcome the disadvantages of the prior art techniques by providing a dielectric crack detection process that is both inexpensive and reliable.

According to one aspect of the invention there is provided a process for testing for a dielectric fault in a component, the process including subjecting the component to a mobile ionising solvent, measuring the electrical leakage current of the component, and comparing the leakage current with a reference value to provide an indication of the presence of a dielectric fault.

According to another aspect of the invention there is provided an apparatus for measuring and testing components, the apparatus including means for applying a mobile ionising solvent to the component so as to penetrate any discontinuity in the dielectric, means for measuring the leakage current of the solvent treated component, and a comparator whereby the leakage current is compared with a reference value corresponding to the leakage current of an untreated component.

We have found that treatment of a ceramic capacitor with a mobile ionising solvent, for example a lower alkyl alcohol, provides an efficient nondestructive means for crack detection. The electrical leakage current of each capacitor is measured prior to treatment with the liquid and is measured again after treatment. The two measurements are then compared. Alternatively a single measurement is made after solvent treatment and compared with a reference value. A significant increase in the leakage is indicative of the presence of one or more potentially active cracks in the dielectric.

A variety of solvents may be used for this process, but we prefer not to use water as this is often undesirable for electronic applications. In general the solvent is of the type that produces an increase in leakage current when applied to a capacitor having a cracked dielectric.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the single Figure is a schematic diagram of a capacitor measurement and test apparatus.

Referring to the drawing, capacitors 11 to be treated are mounted on a conveyor 12 and are carried through a first test station 13 where the electrical leakage of each capacitor 11 is measured and the result stored in a memory 14. The capacitors are then carried via a treatment station 15, where they are immersed in or sprayed with a mobile ionising solvent, to a drying station 16 whereby excess solvent is removed. Typically drying is effected in a current of air at ambient temperature.

The treated capacitors are carried from the drying station 16 to a second test station 17. The electrical leakage current of each capacitor is again measured and is compared, via comparator 18, with the measurement for that capacitor recalled from the memory 14. If the second leakage current measurement is significantly higher than the first, i.e. the measurement differ by a predetermined magnitude, then that capacitor is directed to a reject bin. In this way ceramic capacitors, and particularly multilayer ceramic capacitors, may be screened to remove those whose dielectric is imperfect.

In a further preferred embodiment the first test station 13 is dispensed with and the leakage current of each solvent treated capacitor is compared with a reference value corresponding to the leakage of an untreated good capacitor. Those capacitors whose leakage current is significantly greater than the reference value are rejected.

The test voltage applied to a capacitor to measure electrical leakage is dependent on the voltage rating of the capacitor. Typically this test voltage is in the range 5 to 15 volts.

Avariety of liquids may be employed in the technique. Typically we employ methanol, but ethanol, isopropyl alcohol, industrial methylated spirit or mixtures of any of these solvents may be used. This list of solvents is given by way of example only and is not to be considered as limiting. Preferably the solvent should be mobile, i.e. a low viscosity and surface tension to allow rapid penetration. The solvent should also be polar and of the ionising type.

In a typical test process a batch of 0.1 microfarad multilayer capacitors formed from an X7R dielectric were treated for electrical leakage at 10 volts. In each case the leakage was less than or equal to 10-9 amps. The capacitors were then immersed in methanol for 10 minutes, air dried and remeasured for leakage. The majority maintained a leakage of 10-9 amps but a few showed an increase in leakage to 5 x 10-9 amps or greater. These latter capacitors when subsequently sectioned and microscopically examined were found to exhibit dielectric cracking.

This example demonstrates the feasibility of the techniques described herein for the non destructive treating of ceramic capacitors prior to use.

Claims

1. A process for testing for a dielectric fault in a component, the process including subjecting the component to a mobile ionising solvent, measuring the electrical leakage current of the component, and comparing the leakage current with a reference value to provide an indication of the presence of a dielectric fault.

2. A method as claimed in claim 1,wherein the liquid is methanol, ethanol, isopropyl alcohol or mixtures thereof.

3. A method as claimed in claim 1 or 2, wherein the electrical leakage is measured at an applied voltage of 5 to 15 volts.

4. A method of testing a ceramic capacitor, which method is substantially as described herein with reference to the accompanying drawings.

5. An apparatus for measuring and testing components, the apparatus including means for applying a mobile ionising solvent to the component so as to penetrate any discontinuity in the dielectric, means for measuring the leakage current of the solvent treated component and a comparator whereby the leakage current is compared with a reference value corresponding to the leakage current of an untreated component.

6. A capacitor testing apparatus substantially as described herein with reference to the accompanying drawing,