FAILURE MECHANISMS THAT CAUSE HIGHELECTRICAL...

Electrocomponent Science and Technology, 1981, Vol. 8, pp. 175-1800305-3091/81/0804-0175 $06.50/0

(C) 1981 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

FAILURE MECHANISMS THAT CAUSE HIGH ELECTRICALLEAKAGE IN MULTILAYER CERAMIC CAPACITORS

HIROFUMI IKEO, SANPEI SAKAMOTO, KEN SATO’, HISAKAZU NISHIURA’, andKATSUHIRO OHNO$

Kita-Itami Works, Mitsubishi Electric Corporation, ltami, Japan 664?Central Research Laboratory, Mitsubishi Electric Corporation, Amagasaki, Japan 661

Manufacturing Development Laboratory, Mitsubishi Electric Corporation, Amagasaki, Japan 661

The causes of failure mechanisms of multilayer ceramic capacitors that result in zero resistance (shorts) or lowresistance (high leakage) behaviour in the capacitors when the capacitors are subjected to low voltages have beenstudied. It has been found that failures resulted from the electrochemical reaction of electrode materials in themicroscopic open pores which pass across a ceramic layer between inner electrodes and are filled with watercontaining C1- ions. The mechanism found in this investigation has been applied to enable screening processes to beevolved for manufactured capacitors to eliminate the possibility of failure and hence to improve the reliability ofcircuits using multilayer ceramic capacitors.

1. INTRODUCTION

There are three types of typical failure modes inmultilayer ceramic capacitors; electrical shorts or lowresistance, open circuits failure, and finally capacitancevalue being out-of-tolerance. The most common of thethree is the electrical short circuit or low resistancefailure mode.

It has been reported by T. F. Brennan that in manycases multilayer ceramic capacitors failed insulationresistance testing at less than 1/10th of their ratedvoltage. Voids (small or large), microcracks,delaminations, impurities in the ceramic, migration ofexternal electrode materials (e.g. silver), andcontamination on the ceramic surface have all beenconsidered as the causes of poor insulationresistance. 2,3 However, failure due to low resistance atlow voltage stress has not been examined.

In this paper, we describe the following: finding theconditions for insulation resistance failures; clarifyingthe materials responsible for the leakage bymicroanalysis; confirming a mechanism byelectrochemical measurements; and proposing methodsto reject failed capacitors during manufacture, in orderto ensure high-reliability capacitor manufacturing.The capacitors which we studied consisted of multiple

layers of Pd metal electrodes with the inner electrodesseparated by ceramic dielectric material (mainlybarium titanate).

175

2. ELECTRICAL BEHAVIOUR OF FAILEDCAPACITORS

Typical properties of leaky capacitors which failed inthe reliability test were as follows:

1) Voltage-current characteristic at low voltages wasohmic as shown in Figure 1.

2.0

R=10.6 K ohms

._ 2 2 K ohms- R=I./7 K ohms

0 2 3 4 5 6 7 8

Applied voltage V

FIGURE Voltage-current characteristic of failed capacitor.

176 H. IKEO et al.

2) Most of the leaky capacitors recovered as thevoltage was increased beyond several volts (theychange discontinuously, with irreversible change, to thehigher resistance level) as shown in Figure 1.

3) Temperature coefficients of resistance in theohmic region were positive (about + 10-3/C).

4) Failed capacitors frequently recovered theirinsulation resistance at high temperature (above+200C).Most of the leakage failures took place at relatively

low dc voltages and tended to depend on the specificbatch of capacitors examined.

lOO tet t v

dgradltlon of irub.tlen rbtance

0 2 3 4 5Test time (hour)

FIGURE 3 Abrupt degradation of insulation resistance atV, at +85C and in 85% RH.

3. REAPPEARANCE OF FAILURES

Reappearance of failure tests were performed by theprocedures as shown schematically in Figure 2.Capacitors were boiled in water for several hours.Afterwards the capacitors were dried, and insulationresistancce was measured at the rated voltages. Theboiled capacitors were classified according to theresistance level: below 10] ohms (rejected capacitors)and above 101 ohms (acceptable capacitors).Every capacitor was subjected to dc voltage stresses

from 0.25 V to 10 V in an atmosphere of 85% RH andat +85C in a thermo-hygrostat. Under voltage stresses,some of the capacitors failed at less than 104 ohms forapplied voltage between 0.5 V and 3 V as shown inFigures 3 and 4. Acceptable capacitors after boiling didnot fail at any voltage. The electrical properties offailed capacitors in this reappearance test were thesame as those of the failed capacitors in the reliabilitytest. This reappearance test method was evolved bytaking account of the electrical properties and theenvironment of the failed capacitor.

IBOIUNO TEST]II.R. TEST]

w 104 Mohms

0 o.TEST

1. Measurement of insulationresistance at the ratedvoltage

APPEARANCE’ 2. Test condition is at lowvoltage between 0.25vand 10v at+85C in85% RH

FIGURE 2 _Reappearance test flow chart.

1.0

"5 0.5E

0 o o0 2

0 actually tested voltage

o o o3 4 5 10

Test voltage (V)

FIGURE 4 Failure rate at dc voltage stresses from 0.25 V to10 V under high temperature and humidity

3.1 Weibull Distribution of Failure

Weibull distributions were plotted for the failedcapacitors in two cases. The first was for the failures inthe reappearancce test, and the second was for thefailures in the device reliability test. From Figure 5, theshape parameter rn of the Weibull distribution can beseen to be 0.3 to 0.4 in both cases. It is thereforepossible to detect failed capacitors because every failed

capacitor belongs to a decreasing failure rate group.

FAILURE MECHANISMS IN MULTILAYER CERAMIC CAPACITORS 177

001o.

re.appearance teS. m:O

...- m=O,3

,I

0,5 1.o 50 lOO

FIGURE 5 Weibull distribution analysis of failed capacitors.

3.2 Location of Leakage Path

The leakage path of the failed capacitor wasinvestigated by three steps.

First step: After confirming that there was noleakage on the ceramic surface, the individual innerelectrodes were isolated by removing one of the outerelectrodes. The leaky ceramic layer was determined bymeasuring the resistance between each isolatedelectrode and the other outer electrode as shownschematically in Figure 6.

Second step: The ceramic was planed down parallelto the inner electrode plate by sand paper or sand/airabrasion, until the dielectric layer between theelectrodes (1 and 1’ in Figure 6) appeared. One of theinner electrode plates across the dielectric layer wasthen exposed by using a narrow beam sand/air abrasionsystem and afterwards the outer electrode which hadbeen removed by the first step was reconnected usingsilver paste. Careful removal of the electrode plate a

little at a time, whilst watching the current across thedielecrtric, was performed until the observed resistancesuddenly became infinite. This location, just before theresistance became infinite, is the macroscopic highleakage portion of the dielectric (as shownschematically in Figure 7).

_1" II ndblasting device

por on"’" ’" ti

FIGURE7 Procedure to clarify macroscopically leakyportion of capacitor.

Third step: When the electron beam of aScanning-Electron Microscope (SEM) is allowed toimpinge on the insulator, a charging phenomenonoccurs. However the charging phenomenon does notoccur if the dielectic exhibits high leakage (see Figure8). It is thus possible to determine the exact position ofthe high leakage portion of the dielectric exposed instep 2.

terminal etec trode

miccapacitor element

15 Isolated inner electrodes (formerly,connected to leftward terminal

|t,.2e: Opposite electrodes (dotted line)

FIGURE 6 Procedure to clarify leakage layer (in this caseleakage layer is between 1’ and 2).

FIGURE 8 Charging image by SEM (insulation resistanceof sample is 80 ohms).

178 H. IKEO et al.

3.3 Structure of High Leakage Portion and theMaterials Involved

After charging, the ordinary secondary electron imageof the SEM was used to study the structure of the highleakage portions as shown in Figures 9 and 10. The

FIGURE 9 Secondary electron image of Figure 8.

FIGURE 11 Characteristic Pd L X-ray image.

the leakage path branched and penetrated more thanone dielectric layer. The concentration of C1- ionstogether with Pd metal was found to be about 0.1 wt%.A failed capacitor having relatively high insulationresistance contained smaller quantities of Pd than failedcapacitors having low insulation resistance.

FIGURE 10 Secondary electron image (magnification ofportion A in Figure 9).

existence of small voids in the high leakage portion wasthus confirmed. Subsequently element analysis in the

vicinity of voids was performed by an X-ray micro

analyzer (XMA). The material found to be responsiblefor the high leakage was Pd metal together with C1- inthe voids as shown in Figure 11. An agglomerate phasecontaining Ca was also found near the voids. After

bombarding the high leakage portion with Ar+ ions

step by step, it was also confirmed by SEM-XMA that

4. FAILURE MECHANISMS

On the basis of the experimental facts, the followingprocesses are suggested.

1) Formation of the microscopic leakage pathbetween inner electrodes during manufacturing orduring assembly processes.

2) Penetration into the microscopic leakage path,already contaminated with C1- ions, by water from theoutside of the capacitor.

3) Deposition of inner electrode material into themicroscopic leakage path based on an electrochemicalreaction of the inner electrode metal at low dc voltages.

5. CONFIRMATION OF FAILUREMECHANISMS

5.1 Penetration of Water into the MicroscopicLeakage Path

From micro-analysis of the failed capacitors, it has beenfound that there were deposits of Pd in the voids in themicroscopic path. In addition to this, whether waterpenetrates into the ceramic from the surface of the

FAILURE MECHANISMS IN MULTILAYER CERAMIC CAPACITORS 179

capacitors or not was examined by a penetrationmethod using a fluorescent dye solution. The traces ofthe fluorescent dye were confirmed in the interior ofthe ceramic layer. It was thus found that themicroscopic path was open to the atmosphere.

5.2 Pd Migration

In order to study Pd migration, the dissolution ofpalladium was measured as a function ofelectrochemical electrode potential in water and inaqueous solutions of Na2SO4, NaNO3, and NaC1. Therelationships between the applied voltage Va, thecurrent density J, the anode potential E and thecathode potential Ec, are shown in Figure 12 for thecase of aqueous NaC1 soltuion. To measure theminimum dissolution voltage for Pd, the currentO inFigure 12 is shown in Figure 13 on an enlarged scale.The beginning of the current increase appears around0.3 V. This anode potential is equivalent to the appliedvoltage of about 0.5 V given in Figure 12.As an attempt to observe Pd dissolution directly, two

sheets of filter paper containing 1N NaC1 solution weresandwiched between Pd metal plates, and a low dcvoltage was applied to the Pd plates. As expected, onecould observe directly the Pd migration from the colourchange (black) on the paper at the cathode. Pd migrationwas observed at 1.5 V, but applied voltages above 3 Vno longer gave Pd dissolution. Pd migration in theregion of 0.5 V to 3 V coincided with the voltage regionin which capacitors failed.

.oa:,_o,/, ol.’c o

o

lO-gL0.6 0.2 0 0.2 O.t. 0.6 O.S 1.0

Anode potential Ea(V SCE)

Enlargement of currentQ in Figure 12.FIGURE 13

5.3 Chlorine Ion EffectsIt is certain that C1- ions play an important part in thefailure of capacitors. Several capacitors were thereforeboiled in 1N NaC1 for several hours. The resultingcapacitors were classified into two groups: thoseshowing a resistance above 101 ohms on the ratedvoltage test and those showing resistance below 101ohms. After the capacitors were dried, life tests wereperformed at two voltages, 0.5 V and 1.5 V, in anatmosphere of 85% RH and at +85C. Results areshown in Figure 14 and can be classified into two

Mpplled volligelVi

FIGURE 12 Electrochemical reaction test in an aqueousNaCI solution.

1.0

00

m Pd

FIGURE 14 Dependence on test voltage of failure rate (fortwo cases: Pd inner electrode, Pd + x inner electrode).

O.STest voltage (V)

180 H. IKEO et al.

groups: those associated with capacitors havingelectrodes of pure Pd metal and those associated withcapacitors having electrodes in which the Pd iscontaminated by a small amount of another unknownmetal represented by x in Figure 14.The fact that the failure rate of the Pd electrodes

contaminated with small amounts of another metal ismuch higher at 1.5 V than 0.5 V is interpreted asshowing that the voltage migration depends on theelectrode material. Furthermore measurements of thetime taken to arrive at failure gave figures of betweenone minute to 45 minutes for capacitors boiled in NaC1solution and 10 minutes to 10 hours for those boiled inwater.

It can be seen that boiling in NaC1 solution has an

accelerating effect on the failure of insulation resistancein the capacitors.

6. SCREENING METHODS FOR CAPACITORS

It has been suggested that in a capacitor, leakage pathsmay be formed by the electrochemical reaction of Pd(the inner electrode material), provided that thereexists small voids of microcracks passing through thedielectric, into which water containing C1- ions canpenetrate. These leakage paths result in low voltagefailures. It was thus possible to establish screeningmethods for capacitors as follows:

1) An electrical test using a voltage about ten timesthe rated voltage.

2) A boiling water or NaC1 solution test in which thecapacitors are immersed for several hours.

3) A low dc voltage life test under high temperatureand high humidity conditions (85C and around85% RH).

by C1- ions must also be eliminated because Pddissolution at low dc voltage is increased in thepresence of C1- ions.

8. CONCLUSIONS

The leakage mechanism causing failure in ceramiccapacitors has been studied using various techniques.The existence of small voids or microcracks passingthrough the dielectric, the presence of chlorinecontamination in water that has penetrated into thecapacitors through the microcracks and the subsequenteffect of chlorine on the dissolution of Pd of the innerelectrodes are all contributions to the leakagemechanism. Furthermore it has been shown that thedissolution of Pd can occur at low voltage stresses in thepresence of chlorine ions.As a result three screening tests have been suggested

for detecting possible failures in capacitors.With regard to improving the manufacturing of

ceramic capacitors, it is noted that it is important toeliminate C1- ion contamination, open pore structuresin the ceramic and any agglomeration phases in theceramic.

ACKNOWLEDGEMENTS

The authors wish to express their great appreciation to Messrs.Isoichi Kusajima, Tadashi Katashima and Yasuhiro Nakanishiof the Himeji Works for their helpful and stimulatingsuggestions. We would also like to thank Dr. KyoichiShibayama and Dr. Takeshi Koyama of the Kita-ItamiWorks, and Mr. Yozo NalCajima of the Central ResearchLaboratory for their continuous encouragement throughoutthis study. We are also indebted to Messrs. YoshihiroOgata and Hideaki Miyoshi of the Central ResearchLaboratory for microanalysis and for electro-chemicalmeasurements, respectively.

7. IMPROVEMENTS TO CERAMICCAPACITOR MANUFACTURING REFERENCES

It has been shown that the most important defectsleading to the degradation of insulation resistance aresmall voids and microcracks leading from the outside oft’he capacitor through the inner electrodes. The causesof these defects are very complicated. It is suggestedthat the appearance of agglomerate phases in theceramics, as found by XMA analysis must be avoided,since the difference in thermal expansion between thetwo phases may cause microcracking. Contamination

1. T. F. Brennan, "Ceramic Capacitor Insulation ResistanceFailures Accelerated by low Voltage", 16th AnnualProceedings of Reliability Physics Symposium, pp,69-74 (1978).

2. J. Piper, "Microstructure and Reliability of Ceramic ChipCapacitors", Proceedings International MicroelectronicsSymposium, pp. 4A2-1-4A2-10 (1972).

3. G. J. Ewell, and W. K. Jones, "Encapsulation, Sectioning,and Examination of Muitilayer Ceramic ChipCapacitors", Proceedings IEEE-EIA, ElectronicsComponents Conference, pp. 446-451 (May 1977).

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