GB2116729A

GB2116729A - Electrical testing

Info

Publication number: GB2116729A
Application number: GB08217105A
Authority: GB
Inventors: Robert Carnegie Chittick
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-03-02
Filing date: 1982-06-11
Publication date: 1983-09-28
Also published as: GB2115937A; GB2116328B; GB2116328A

Abstract

Insulation faults in electrical components are detected by measurement of 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 insulation. <IMAGE>

Description

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GB 2 116 729 A

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SPECIFICATION Electrical testing

5 This invention relates to testing insulation faults such as may occur in electrical components and to techniques for improving the quality control of such components.

A major problem with the manufacture of electric-10 al components incorporating an insulating material is that of detecting insulation faults. Such faults may, at the time of manufacture, be only minor and thus difficult to detect but can subsequently be a cause of failure of the component. It would therefore be a 15 considerable advantage if an incipient insulation failure could be detected by non destructive test proceedure.

For example, Ceramic dielectric capacitors, and particularly multilayer ceramic capacitors are widely 20 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 25 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 30 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 35 with electrodes using an ink formed from the 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 40 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 45 has assumed great important. 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 manufac-50 tured 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 55 termination and can lead to subsequent failure in service. The mechanism of this failure is not fully understood, but it is thought to involve electrochemical dissolution and migration of the materials electode or end termination which then provides a 60 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 65 becomes manifest after the capacitor has been in use for an extended period. It is clearly desirable to reduce such long term failures to a minimum.

Further problems arise with components provided with insulation e.g. in the form of an encapsulating plastics material. Again it is desirable to detect flaws in this insulation to eliminate the risk of subsequent failure.

Previous attempts to detect insulation faults have generally involved some form of electrical destructive testing on a batch basis. Such tests cannot be employed to provide 100 percent screening of components. Furthermore since they generally involve the provision of relatively costly precautions.

A number of techniques have for example 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 term component as employed herein is understood to relate not only to capacitors but also to other electrical components and devices including inter alia integrated circuits, wire products and cables.

The object of the present invention is to minimise or to overcome the disadvantages of the prior art techniques by providing an insulating fault detection process that is inexpensive, reliable and nondestructive.

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

According to another aspect of the invention there is provided a process for testing for an insulation 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 an insulation fault.

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

We have found that treatment of an insulator with a mobile ionising solvent, for example a lower alkyl alcohol, provides an efficient non-destructive means for fault detection. The electrical leakage current of the insulator is measured prior to treatment with the liquid and is measured again after treatment. The two measurements are then compared. Alternatively

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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 faults in 5 the dielectric.

A variety of solvents may be used for this process. In general the solvent is of the type that produces an incease in leakage current when applied to a capacitor having a cracked dielectric.

10 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. It should be understood that this description is byway 15 of example only and that the techniques described herein are in no way limited to capacitor applications.

Referring to the drawing, capacitors 11 to be treated are mounted on a conveyor 12 and are 20 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 25 ionising solvent, to a drying station 16 whereby excess solvent is removed. Typically drying is effected in a current of air at ambient temperature. In some applications the capacitors may be preheated e.g. to 100°C priorto immersion to enhance penetra-30 tion by the solvent.

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 35 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 40 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 45 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 greaterthan the reference value are rejected.

50 In a typical test process a batch of 0.1 microfarad multilayer capacitors formed from an X7R dielectric were tested for electrical leakage at 10 volts. In each case the leakage current was less than or equal to 10-9 amps. The capacitors were then immersed in 55 methanol for 10 minutes, air dried and remeasured for leakage. The majority maintained a leakage of 10~9 amps but a fewshowed an increase in leakage to 5 x 10~9 amps or greater. These latter capacitors when subsequently sectioned and microscopically 60 examined were found to exhibit dielectric cracking.

A variety of liquids may be employed in the technique. Typically we employ methanol, but etha-nol, isopropyl alcohol, industrial methylated spirit or mixtures of any of these solvents may be used. Less 65 advantageously water containing a wetting agent can be employed, although in certain applications the use of water is undesirable. This list of solvents is given by way of example only and is not to be considered as limiting. Preferably the solvent should 70 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 some applications the efficiency of the solvent can be enhanced by the addition of a small quantity, e.g. 0.01 %, of an 75 ionogen or mixtures thereof. Materials of this nature, such as triethylamine, enhance the generation of ions in the solvent and hence increase the electrical conductivity.

It is preferred, although not essential, that the 80 ionogen is relatively volatile so that subsequent removal of the material from the component can be readily effected.

The preferred liquid is methanol since it has a higher conductivity than the other alcohols and a 85 suitably low viscosity. The relative sensitivies of the above mentioned liquids can be demonstrated by the following example. A multilayer ceramic chip capacitor had a reference value of insulation resistance equal to 1010ohms measured 10 seconds after 90 applying 10VDC. The device, remeasured after 15 minutes immersion in methanol followed by surface drying, had an insulation resistance of 108 ohms indicating the presence of a crack between two or more electrodes that had a path to the external 95 environment. The corresponding values of insulation resistance after immersion in ethanol and isopropyl alcohol were 5 x 10s ohms and 5 x 109 ohms respectively. Addition of an ionogen, e.g. triethylamine at a concentration of about 0.01%, can 100 significantly increase the conductivity of methanol thus enhancing the sensitivity of the technique. The mechanism of the effect is simply a shunting of electrodes connected by a crack or other flaw by the penetrant liquid that remains in the crack after the 105 surface liquid has evaporated. The rapid evaporation of this surface liquid is necessary to prevent masking of the effect by surface conductivity.

It has been found possible to differentiate between types of defect in multilayer ceramic chip capacitors 110 according to the relative behaviour of the post-immersion insulation resistance and the reference value. A simple crack between two opposing internal electrodes will have an effect as described in the above example. The insulation resistance recovers 115 to the reference value as the liquid evaporates from the crack. The time to recovery is dependent on the dimensions of the crack but is usually of the order of minutes.

If the defect bridging the electrodes is linked fine 120 porosity, as is often the case in the ZSU dielectric, the recovery time is very long and in extreme cases there may be no noticeable recovery after several hours.

Another type of defect, known as a knit line fault, 125 can be detected. This is in effect a non-lamination of successive dielectric layers and is seen as a crack extending from one end termination of the device to an internal electrode of the opposite polarity. Since the end termination material (usually silver) is 130 normally different from the internal electrode mate

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rial the result of the test has a polarity dependence. If, for example, the end termination (silver) is positive and the internal electrodes, say Palladium/ silver, are negative, silver migration can readily 5 occur in the presence of methanol along the knit line crack from the end termination and a silver dendrite can grow back from the internal electrode to the end termination. This results in an insulation resistance that decreases with time. If the polarity is reversed 10 this rapid migration does not occur and the test behaviour is the same as that for a simple crack between internal electrodes.

The presence of these defects in capacitors with the above described test behaviour has been con-15 firmed by destructive physical analysis. The relevance of this test with regard to multilayer ceramic capacitors is its use for example as a screening technique for potential low voltage failure. This failure mode is believed to be due to the electroche-20 mical dissolution and migration of electrode materials under an applied d.c. electric field in the presence of atmospheric moisture and various impurities. An essential feature for the occurrence of this failure mode would then be a flow connecting 25 two or more opposing electrodes and a path to the outside environment from this flaw. It is claimed that the above procedure can detect such a flaw.

The following example illustrates the invention:

A life test conducted in an environment at 85 deg C 30 and 100% RH (non-condensing) contained 40 chip capacitors 17 of which had failed the above screening technique. After 1400 hours, 8 of these had failed at an insulation resistance below their specification. None of those that had passed the screening failed 35 on life test.

The test can also be extended to encapsulated capacitors and other components. In the case of encapsulated devices a test failure is indicative of an encapsulant defect that allows the outside environ-40 ment access to an internal defect bridging two or more of the opposing electrodes that are connected to the circuit. The bridging defect need not be within the body of the device but can be across the surface along a path formed by lack of bonding between the 45 encapsulant and the surface of the device. This type of defect is particularly important in encapsulated multilayer ceramic capacitors with silver end terminations as silver migration can readily occur in a humid environment resulting in the growth of a 50 shorting silver dendrite between end terminations on the ceramic surface. If the circuit impedance is sufficiently low, the heat generated when the dendrite shorts out the end terminations may even cause the encapsulant to carbonise and catch fire. 55 This example demonstrates the feasibility of the techniques described herein forthe non destructive treating of ceramic capacitors prior to use.

Claims

CLAIMS 60

1. A process for testing for an insulation fault in an electrical component, the process including subjecting the component to a volatile mobile ionising solvent, measuring a parameter associated with the 65 electrical condition of the insulation, and comparing the parameter with a reference value to provide an indication of the presence of an insulation fault.

2. A process for testing for an insulation fault in a component, the process including subjecting the

70 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 an insulation fault.

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

4. A method as claimed in claim 1,2 or 3,

wherein said solvent contains an ionogen or mix-

80 tures thereof.

5. A method as claimed in claim 4, wherein the ionogen is triethylamine.

6. A method of testing an electrical component, which method is substantially as described herein

85 with reference to the accompanying drawings.

7. An apparatus for measuring and testing for insulation faults in components, the apparatus including means for applying a mobile ionising solvent to the component so as to penetrate any

90 discontinuity in the insulation, means for measuring a parameter associated with the electrical condition of the solvent treated component, and a comparator whereby the measured parameter is compared with a reference value corresponding to the condition of

95 an untreated component.

8. An electrical testing apparatus substantially as described herein with reference to the accompanying drawing.