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A degradation model for metallized polyester capacitors
Microelectron. Reliab., Vol. 27, No. 1, pp. 145-163, 1987. Printed in Great Britain.
0026-2714/8753.00 + .00 © 1987 Pergamon Journals Ltd.
A DEGRADATION MODEL FOR METALLIZED POLYESTER CAPACITORS G. SPECCHIULLI Telettra S.p.A, Reliability and Quality Department Via Trento 30, 20059 Vimercate, Italy (Received for publication 30 September 1986)
ABSTRACT
T i l l now i t was usual to think that the physical degradation of the end-spray contacts of metallized polyester capacitors, accelerated by exposure to high temperature , only implied an increase of the equivalent series resistance. This paper reports that together with this resistance increase, a contribution due to a parasitic capacitance must be taken into account to describe this degradation mechanism correctly. A degradation model is presented that enables f a i r l y good fitting of experimental measurements of capacitance, dissipation factor and impedance vs. frequency of capacitors submitted to long-term tests, that exhibit an electrical behaviour deviating from the one described by the usual equivalent series circuit. The paper illustrates a technological analysis method that enables the consistence of the proposed model to be confirmed and that can currently be used to make quick estimations of the reliability performance of components by different manufacturers and to carry out periodic quality control. I t was also seen that, in well manufactured capacitors, the described degradation mechanism is rather slow, with respect to the high temperature exposure time, so that the r e l i a b i l i t y level of these components is not affected.
I - INTRODUCTION
In these last years metallized polyester capacitors (M.P.C.) with l e a d spacing of 5 mm, have become a good alternative to high dielectric constant ceramic capacitors, mainly for their good electrical behaviour and low cost, but also because of the serious r e l i a b i l i t y problems involved in the ceramic capacitors themselves: that is, their low voltage failures. Many studies have been done on ceramic capacitors to understand their low voltage failures and today the failure physics of these capacitors is generally well understood: the main failure mode is short circuit due to metal migration between opposite electrodes through voids, cracks and/or high porosity into the ceramic dielectric, accelerated by humidity and temperature [1]-[7]. Even though the comprehension of this failure mechanism led to plenty of improvement in the r e l i a b i l i t y level of ceramic capacitors, neverthelessthe risk of getting a bad lot may s t i l l be high, mainly for low cost ceramic capacitors with the highest dielectric constants. This risk may be too high for manufacturers of equipment requiring long useful l i f e with high r e l i a b i l i t y level as is the case with telecom equipment. So in order to use ceramic capacitors in telecom applications i t may become necessary to introduce some MR 27: l-J
145
146
G SP~('(:HIUL~] screening procedure [ 8 ] - [ 1 3 ] to reduce the Freak population and/or to c e r t i f y the l o t s , thus g i v i n g up the benefit of low cost of these capacitors. On the other hand, even though only a few studies have been performed on the r e l i a b i l i t y of M.P.C. with small dimensions [ 1 4 ] , [ 1 5 ] , i t appears that these capacitors generally e x h i b i t a good r e l i a b i l i t y level. In a previous work [14], i t was demonstrated , with long-term l i f e tests, that the main f a i l u r e mechanism of these components is a progressive degradation of the contact between the end-spray and the m e t a l l i z a t i o n of the electrodes, i n v o l v i n g , as correlated f a i l u r e modes, a gradual change of the frequency response of the capacitor parameters. This f a i l u r e mechanism is a wear-out process, that could a f f e c t the r e l i a b i l i t y during the useful l i f e , i f the degradation rate is not s u f f i c i e n t l y slow. So f o r a user the most important step in q u a l i f y i n g H.P.C. manufacturers, is to evaluate the q u a l i t y of the contact between spray layers and electrodes, and to do some p e r i o d i c a l check on the maintenance of t h i s q u a l i t y . In t h i s paper a degradation model is presented that j u s t i f i e s the observed f a i l u r e modes of M.P.C. submitted to long-term t e s t s ; a technological c h a r a c t e r i z a t i o n procedure is described that can serve as a very useful and rapid tool to estimate 'a p r i o r i ' the r e l i a b i l i t y performance of these components and to choose the most r e l i a b l e manufacturers.
2- EQUIVALENTCIRCUIT The equivalent series c i r c u i t of a metallized capacitor, according to Holmberg [15], is sketched in Fig.1.
RI
Rc
,vvvw
O
Fig.l
VvW
Re
Re
Co
,vVvV
VvW
I
Ls ,, o
Equivalent series c i r c u i t of an unaged capacitor.
The symbols in the i l l u s t r a t i o n have the following meaning: -R z
leads
resistance
including
end-spray
resistance
-R c
contact resistance
-R e
electrodes resistance
-Rd
resistance due to i n t r i n s i c d i e l e c t r i c losses
-Co
geometric capacitance
-L s
inductive contribution electrodes.
between end-spray and electrodes
due to
leads, end-spray and
Let us deal f i r s t with the contact resistance. For a capacitor for which the contact between the end-spray and the electrodes is perfectly continuous, the current in the electrodes w i l l only flow orthogonally to the winding direction. But in real capacitors the contact is Far from being
Metallized polyester capacitors
147
perfect and, as a consequence of t h i s imperfection, there w i l l also be a longitudinal current flow. The larger t h i s imperfection , the higher the longitudinal current flowing in the electrodes .This current flow increases the capacitor losses that in the equivalent seri~s c i r c u i t can be interpreted as an increase of the contact resistance. Thus i t is clear that Re depends on winding technique and end-spraying operation efficiency. The sum of the resistive and d i e l e c t r i c losses is generally known as the equivalent series resistance (ESR) of the capacitor. With respect to Fig. I, the ESR can be written as: tg 6~
Roeq
= (R/
+ R c + Re) + R d = R s + ~
(1) o) CO
in which the r e s i s t i v e and d i e l e c t r i c losses have been subdivided, tg6 d is the i n t r i n s i c dissipation factor and ~=2nf is the pulsation, while Rs is clearly defined by the expression itself. I t is worth noting that Rz is frequency dependent (increases with frequency) and i t s value can be estimated with the formulae generally used to take into account the contribution due to the skin effect (see for example Snelling [16]). However i t s influence on the total ESR is generally negligible up to I MHz, after which, i t has to be taken into account. Rr and Rc can be considered constant with respect to the frequency, while Rd, due to ( I ) , decreases with increasing frequency (neglecting, as a f i r s t approximation, the frequency dependence of the i n t r i n s i c dissipation factor). The apparent or equivalent capacitance is generally defined by: Co
Co
Coeq :
:
(2) I - (f/fo) 2
1 - o2LsCo
where fo is the self-resonant frequency of the capacitor. So the impedance of the capacitor can be written as: I Zo = Roeq - j
(3)
Cot q and the dissipation factor: mCoRs + tg6 d
tg 60 = ~ Coeq Roeq =
(4) I - 2 LsCo
Fig.2 reports the plot corresponding to eq.(2), in terms of (Cocq - C,)/Co x100 for a I uF capacitor.
3- EXPERIMENT
We tested about 500 pieces of MPC,from three manufacturers, with lead spacing of 5 mm, subdivided in four capacitance values (I/0.47/0.1/0.01 uF). The tests ran for more then 30 Khrs at I00"C in three electrical stress conditions: without voltage , with 2 Vdc and with rated voltage applied.
148
G. SPECCHIULL1
•
•
•
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• •L_~J EIR
100
Z o u U
_J LIJ (:3
-100
I~JF -200
1o
lOOk
1M
IOM
FREQUENCY [Hz}
Fig.2
Percentage of apparent capacitance change with respect to the I0 KHz capacitance as a function of the frequency for an unaged luF capacitor. The crosses are experimental points, while the continuous l i n e were calculated from eq.(2).
Before and during the l i f e tests capacitance, dissipation factor (D.F.) and insulation resistance (IR) of a l l capacitors were periodically measured in accordance with IEC Publ. 384-2 [17]. Moreover IR measurements of capacitors in low voltage and storage tests were performed at 2 V, to avoid possible self-healing effects. Capacitance and D . F . measurements w e r e carried out automatically @ 10 KHz with an LCR meter GR 1688 controlled by an HP 9825 desk-top computer. All the data were s t a t i s t i c a l l y treated for each group of capacitors (manufacturer/capacitance value/type of test) and graphs of C, AC/C and D.F. vs. test time were obtained. After 25 Khrs of test an e x t e n d e d frequency characterization was performed on all capacitor groups in the IOKHz - IOMHz range with C, D.F.and IZJ measurements, to find any deviation from a normal behaviour, and to see which parameters were more sensitive for depicting the degradations suffered. The measurements were done automatically with an impedance analyzer HP 4192A controlled by a desk-top computer. The results on capacitance and IR deviations with time are not reported here because they l i e outside the purpose of this paper. They are reported elsewere [14].
4- DEGRADATIONMECHANISM The analysis of the data from these tests enabled us to ascertain that the main f a i l u r e mechanism of these capacitors is a progressive degradation of the contacts between the narrow tips of the Al-electrodes and the f i r s t spray layer and/or between the two spray layers themselves (the f i r s t spray layer is generally of zinc in order to have a good contact with the Al-electrodes, while the second one, sprayed over the zinc, is a more or less complex t i n alloy to f a c i l i t a t e soldering with the leads). The degradation was physically interpreted as a gradual separation at the boundaries (that could also be a growth of some insulating compounds) between these d i f f e r e n t metals as a consequence of the long temperature exposition of the specimens.
Metallized polyester capacitors
149
E l e c t r i c a l l y such a separation implies an increase of the contact resistance, both by a ' t r u e ' increase of resistance at the contact points where the separation occurs, and by an increase due to a ceduction of the contact areas that causes, as explained before, an increase in the longitudinal component of the current. Associated with these separations must also be taken into account a parasitic capacitance whose value w i l l be very high when the separation e n t i t y is very limited, so that i t does not affect the equivalent capacitance; as the thickness of the separation grows, the associated parasitic capacitance can become comparable with the nominal capacitance of the capacitor or decidedly smaller , so affecting the total capacitance value as measured between the external leads. This influence on the total capacitance w i l l clearly be frequency related and w i l l also affect the D.F and impedance vs. frequency response. I f we look at the degradation evolution with time, at f i r s t we shall observe a progressive increase of the contact resistance, l a t e r also a progressive capacitance decrease; eventually a complete separation could occur leading to an open c i r c u i t as f a i l u r e mode.
5- EQUIVALENTCIRCUIT FOR DEGRADEDCAPACITORS
The f a i l u r e mechanism just explained can be described by assuming the equivalent c i r c u i t shown in Fig.3,where: -R~
takes into resistance
account the
increment
-r~
loss resistance of the parassitic capacitance
-Cc
the contact or parasitic capacitance
while the meaning and values of the unchanged.
Cc
other
of
the
contact
parameters
remain
rs
Rs
Rd
Co
Ls o
0
Rc Fig.3
Equivalent c i r c u i t of a degraded capacitor.
The total impedance is again given by the formula: 1 Z t = Rte q
(5)
- j ~Ctcq
where the t o t a l equivalent series resistance becomes: 1 Rte q
= Roeq + R~ ~
1
+ Rp 1 + A
(6) 1 + 1/A
150
(J. SPEC('itlUL[ 1
and the total apparent capacitance: 1
1
l
(°)CoRe)2
+ -Co Cc
Ct~q
2 - ~ Ls
(7)
i + A
where: R!
Rp
(8)
R~+ r~
A : [eCc.(R~ + r , ) ] 2
(9)
The total dissipation factor then becomes: tg61 = ~ Ct<.q RI,.q
(i0)
In Fig.4 a set of graphs are reported in which the experimental points are compared with the theoretical plots obtained from eq.(7) in terms of (Ctro - Co)/Co x]O0, for five i uF capacitors after 25 Khrs @ I00°C with different degradation levels; the parasitic parameters being obtained from a b e s t - f i t t i n g non-linear regression program, and where Co is the 10 KHz capacitance. The agreement between experimental points and theoretical plots is quite good, taking into account that in the model the parameters were assumed constant with respect to the frequency and to the measuring current. Fig.5 is a comparison between IZI and tgO~ vs. freq. plots for an unaged capacitor and the most degraded capacitor of Fig.4. The continuous lines were obtained with the previously developed formulae. In table I, as an example, the parasitic parameters found for the five capacitors of Fig.4 are reported. I t is of interest to note that a measurement of C. and tg6~ @ 10 or 100 KHz, may be not sufflcent to defln~ the degradation level completely.
loo
-~-
100 °0
o
. . . . .
1
.o
J
LU
-
-
100
20(
• 1Ok
lOOk
IM
® IOM
Frequency [Hz]
Fig.4
Percentage of apparent capacitance change with respect to the IO KHz capacitance as a function of the frequency for five ]uF capacitors with different degradation levels, after 25Khrs @ IO0°C. Theoretical plots compared with experimental measurements.
Metallized polyester capacitors
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IpF
151
.......
100 ° C
[~]i02
10
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Z .< 0
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10-1
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/ . . . .
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,,
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. . . .
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1ook 1M FREQUENCY [Hz]
10-2 IOM
.........
Fig.5
Comparison between impedance and D.F. vs. frequency plots of an unaged capacitor and the most degraded capacitor of Fig.4 (n'5). The continous lines are the theoretical plots to be compared with the experimental measurements.
Tab. I
Parasitic parameters of five degraded capacitors after 25 Khrs @I00°C. Rc'
Rp
Cc
Ls
[mohm]
[mohm]
[uF]
[nH]
1
169
50
6.77
13.57
2
134
50
7.36
6.0
3
308
150
0.51
5.33
4
340
140
0.17
4.5
5
1050
170
0.07
5.0
This clearly emerges from a look at equations (10),(6) and (7). In fact, at relatively low frequencies the total apparent capacitance is practically equal to Co, while the contribution of the parasitic parameters to Rteq is mainly due to the term in R~ becauseof the frequency dependenceof A. At higher frequencies the total apparent capacitance is a combination of Co and Co, while the total equivalent series resistance is mainly affected by Rp. So, i f a complete definition of the degradation levels is desired, i t is necessary to perform an extended frequency characterization of the failed specimens or, at least, to integrate a measure of the dissipation factor at low frequencies with a measure of the apparent capacitance near the self-resonant frequency of the capacitor. Fig. 6 (a), (b) and (c) show some graphs of the cumulative distribution of the per cent capacitance difference between IMHz and 10 KHz for I uF capacitors (A and B manuf.) and 0.1 uF (C manuf.), after 25 Khrs at the three performed stress
~D
G . SPECCHIULLI
152
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CUMULATIVE PERCENTAGE
Fig.6
Cumulative distribution of the capacitance change between ] MHz and ]0 KHz; continous l i n e : IO0"C/Vr; dashed l i n e : 100°C/2Vdc; dots and dashes:storage @ IO0°C.
Metallized polyester capacitors
153
conditions. As can be seen, these plots give a good idea of the degradation suffered by the d i f f e r e n t capacitor groups, showing up the contribution due to the parasitic capacitance. A q u a l i t a t i v e correlation was found between D . F . and [C(IMHz)-C(IOKHz)]/C(IOKHz) (see Table I I ) of degraded capacitors: that is, the higher the D.F., the higher the negative capacitance change. Thus to get an idea of the degradation e n t i t y , the usual measurement of the D.F. @ 10 or 100 KHz could be s u f f i c i e n t , but only as a f i r s t approach.
Tab. I I
Comparison among capacitance deviation between I MHz and 10 KHz, Dissipation Factor and self-resonance frequency for f i v e degraded capacitors. AC/CO
D.F.
fo
[%]
[10 .3 ]
[MHz]
I
65.5
21
1.7
2
lg.6
17
2.3
3
5.3
36
3.0
4
-12.9
38
4.1
5
-52.8
74
7.5
6- DEGRADATIONRATE From the analysis of the whole set of data i t became evident that the described f a i l u r e mechanism did not extend uniformly over all the specimens in test, but a few marked differences among manufacturers and also among d i f f e r e n t capacitance values were found. These results are summarized in Table I l l that reports the 10 KHz D.F. values after 25 Khrs of tests for the three applied conditions, in terms of minimum, mean and (mean + 2 sigma) D.F. values. Looking at t h i s table the following observations can be made: - capacitors of the same ratings by manufacturers A and e x h i b i t a similar behaviour during the tests;
B
- 0.01 and 0.1 uF by manufacturer C exhibit a very high D.F. increase compared to those of the other two manufactures; - except for the 0.47 uF, that remains p r a c t i c a l l y constant, the maximum D.F. d r i f t increases on increasing the nominal capacitance value; In addition two other facts were evident: - the D.F. increases l i n e a r l y with test time; - the voltage applied during the tests has no influence e n t i t y of the D.F. increase.
on the
These last results clearly emerge from Fig.7, in which a set of graphs of tgSt (@ 10 KHz) vs. test time for I uF capacitors by manufacturers A and B is reported. The three lines on each graph for each manufacturer show the minimum, mean and (mean + 2 sigma) D.F.values, respectively.
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T ME ( K h r s ) Fig.7
10 KHz Dissipation Factor vs. test capacitors @different stress levels.
time
of
luF
Moreover, comparing these plots with the corresponding ones of Fig. 6(a) and (b), a clear qualitative agreement can be found between D.F. d r i f t s and AC/C .
6- TECHNOLOGICAL ANALYSIS 6.1- Technological analysis procedure To understand these behaviour differences, that are clearly technological related, and bearing in mind that the c r i t i c a l point of this technology is located in the contact zones, a technological analysis method was developed. Cross-sections of several capacitors w e r e performed (without incapsulating the parts),in a plane orthogonal to the leads, up to half of the height of the capacitors. This was done by employing a commercial polisher with papers of different grade; the last polishing was done with a 1 um Alumina. A f i n a l step of about ]0 minutes in an oxygen plasma was performed to avoid overlapping effects due to the extreme softness of the polyester film. After that the parts were flashed with gold to enable SEM analysis. 6.2-Technological analysis results Fig.8(a) is a SEM micrograph of a cross section of a 0.47 uF capacitor. The f i r s t spray layer of zinc and the second one, that is the t i n alloy, are evident. Moreover i t is possible to see that the zinc spray penetrates deeply between plastic f o i l s , assuring a very tight contact with Al plates. Greater details are given in figures 8(b) and 8(c). As mentioned before, this capacitor rating suffered p r a c t i c a l l y no degradation during the tests. Fig.9(a) instead shows a cross-section of a I uF capacitor. As can be seen, zinc penetration is poor, so the contact is generally realised only between the inner spray surface and the tips of the electrodes, as can be better seen in Fig.9(b).
156
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Figure 8a
Figure 8b
Metallized polyester capacitors
157
Figure 8c Fig.8
SEM views of a 0.47uF capacitor head. (a) view of about h a l f capacitor head, (b) zinc penetration through Al electrodes, (c) t i g h t contacts between zinc spray and electrodes.
Moreover, on these capacitors we found defects l i k e those shown in Fig.9(c) and (d), that are cracks or separations near the contact zone. The 0.1 uF by manufacturer C was the one with the highest degradation levels. From the technological point of view we found some extended defects l i k e those shown in Fig.10(a), that is, a separation between the two spray layers running along h a l f capacitor head. Near the contact zone with the electrodes some pofosity is also v i s i b l e . For this rating too, zinc penetration is generally poor, as i l l u s t r a t e d in Fig.tO(b) and
IO(c). Thus the failure mechanism postulated finds a good support in these pictures.
at the
beginning
7- RELIABILITYCONSIDERATIONS
Above a l l i t is important to note that capacitors with an impedance vs. frequency plot l i k e that shown in Fig.5, are s t i l l usable as general purpose capacitors, while capacitors by manufacturer C have not been considered for any r e l i a b i l i t y consideration because of the unacceptableness of the obtained results. Therefore, having had neither catastrophic nor ' t r u e ' parametric f a i l u r e s , i t was impossible to v e r i f y any f a i l u r e d i s t r i b u t i o n . In an attempt to get some more information on t h i s point, an arbitrary D.F. f a i l u r e c r i t e r i o n of 0.03 was assumed. With t h i s assumption i t was v e r i f i e d that the times to f a i l u r e have a Weibull d i s t r i b u t i o n with shape parameter greater than one; that means, as expected, a wear-out process.
158
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Figure 9a
Figure 9b
Metallized polyester capacitors
Figure 9c
Figure 9d Fig.9
SEM views of a luF capacitor head. (a) view of about h a l f capacitor head, (b) contacts between zinc and electrodes, (c) voids and cracks near the contact zone, (d) deep crack in the contact zone.
159
160
( i . SPI ¢ tttlt',l.l I
Fiaure lOa
Figure IOb
Metallized polyester capacitors
Figure lOc Fig.t0
SEM views of a O.]uF capacitor. (a) large separation between zinc and tin alloy; cracks and voids in zinc are also visible, (b) contacts between zinc and electrodes, (c) detail of picture (b).
For example, for I uF capacitors by manufacturer B in the three test conditions, shape parameters between 1.2 and 6.5 were estimated. The scale and shape parameters found could not, of course, be used to get information about the failure rate. In conclusion, the only thing that can be said from the r e l i a b i l i t y point of view, is that i f the end-spray operation has been carefully performed, the related failure mechanism, having a quite low degradation rate, does not affect the r e l i a b i l i t y level of the capacitors.
8- CONCLUSIONS
The long term tests performed enabled us to ascertain that the main failure mechanism of metallized polyester capacitors is a temperature dependent degradation of the end-spray contacts which introduces, besides the already known ESR increase, a parasitic series capacitance, that decreases on increasing the degradation level. This parasitic capacitance, together with increasing ESR, causes a gradual change of the frequency response of degraded capacitors. In well manufactured capacitors, the degradation rate is quite low, so that i t does not affect their r e l i a b i l i t y level. A technological analysis method was developed that, together with a frequency characterization especially for capacitance, enables very rapid estimation of the r e l i a b i l i t y performance of these components and the more reliable manufacturers to be choosen. These tools can, of course, be used for periodic quality control. MR 2 7 : 1 - K
161
G. SPECCHIL'LLI
162
AKNOWLEDGEMENTS
I would like to thank R. Bottini for valuable assistance with the technological analysis and A. Scorzoni, from LAMEL, for SEM analysis.
REFERENCES
[1]
T.F. Brennan: "Ceramic capacitor insulation resistance failures accelerated by low voltage", I6th Annual Proc. on Rel. Physics, pp. 68-74, 1978.
[2]
T. Kobayashi et al.: "Reliability evaluation and failure analysis for multilayer ceramic capacitors", IEEE Trans. Components, Hybrids, and Manuf. Tech., Vol. CHMTI, pp. 316-324, Sep. 1978.
[3]
K. Sato et al.: "Mechanism of ceramic capacitor leakage failures due to low DC stress", 18th Annual Proc. on Rel. Physics, pp. 205-212, 1980.
[4]
S. Ropiak: "Low-voltage failure mechanism for ceramic capacitors", Int. Symp. for Testing and Failure Analysis, pp. 101-105, 1981.
[5]
E. Loh: "A model for low voltage i n s t a b i l i t y in ceramic capacitors", 2 0 t h Annual Proc. on Rel. Physics, pp. 144-151, 1982.
[6]
W.J. Minford: "Accelerated l i f e testing and r e l i a b i l i t y of high K multilayer ceramic capacitors", IEEE Trans. Components, Hybrids, and Manuf. Tech., Vol CHMT5, pp. 297-300, 1982.
[7]
A. Joshi, J. Cruzan: "An analysis of low voltage multilayer ceramic capacitor failures", 2nd Capacitor and Resistor Tech. Symp., pp. D3-I/D3-8, 1982.
[8]
K. Sato et al.: "A low voltage screening of ceramic capacitors from leakage failures", Int. Symp. for Testing and failure Analysis, pp. 225-229, 1980.
[9]
T. Murata et al.: "Low voltage failures of monolithic capacitors and their screening method", Int. Symp. for Testing and Failure Analysis, pp. 105-110, 1981.
[10]
G.J. Ewell, D . A . Demeo: "Electrical analysis of capacitors failing the 85"C/85%RH/1.5 VDC test", 2nd Capacitor and Resistor Tech. Symp., pp. El-I/El-12, 1982.
[11]
R.C. Chittick et al.: "Non-destructive screening for low voltage failure in multilayer ceramic capacitors", 3rd Capacitor and Resistor Tech. Symp., pp. 61-70, 1983.
[12]
R.C. Straessle, G . J . Ewell: "The 85°C-85% Relative Humidity-1.5 VDC bias test: can ceramic capacitor pass this new screen?", ibidem, pp. 70-82, 1983.
[13]
R.C. Chittick, E. Gray: "Improved moisture resistance of multilayer ceramic capacitor encapsulation by on-line screening", 4th Capacitor and Resistor Symp., pp.99-I05,1984.
[14]
G. Specchiulli: "Degradation of metallized capacitors submitted to log-term tests", 6th and Resistor Tech. Symp., pp. 100-108, 1986.
polyester Capacitor
Metallized polyester capacitors [15]
H.F. Holmberg: "Metallized polyester capacitors- Increase in ESR during tests", 5th Capacitor and Resistor Tech. Symp., pp. 35-40, 1985.
[]6]
E.C. Snelling: "Soft Ferrites", 1969.
[17]
IEC Publ. 384-2, "Fixed capacitors equipment", Part 2, I I e d . , 1982.
London ILIFFE Books LTD, for use in electronic
163
0026-2714/8753.00 + .00 © 1987 Pergamon Journals Ltd.
A DEGRADATION MODEL FOR METALLIZED POLYESTER CAPACITORS G. SPECCHIULLI Telettra S.p.A, Reliability and Quality Department Via Trento 30, 20059 Vimercate, Italy (Received for publication 30 September 1986)
ABSTRACT
T i l l now i t was usual to think that the physical degradation of the end-spray contacts of metallized polyester capacitors, accelerated by exposure to high temperature , only implied an increase of the equivalent series resistance. This paper reports that together with this resistance increase, a contribution due to a parasitic capacitance must be taken into account to describe this degradation mechanism correctly. A degradation model is presented that enables f a i r l y good fitting of experimental measurements of capacitance, dissipation factor and impedance vs. frequency of capacitors submitted to long-term tests, that exhibit an electrical behaviour deviating from the one described by the usual equivalent series circuit. The paper illustrates a technological analysis method that enables the consistence of the proposed model to be confirmed and that can currently be used to make quick estimations of the reliability performance of components by different manufacturers and to carry out periodic quality control. I t was also seen that, in well manufactured capacitors, the described degradation mechanism is rather slow, with respect to the high temperature exposure time, so that the r e l i a b i l i t y level of these components is not affected.
I - INTRODUCTION
In these last years metallized polyester capacitors (M.P.C.) with l e a d spacing of 5 mm, have become a good alternative to high dielectric constant ceramic capacitors, mainly for their good electrical behaviour and low cost, but also because of the serious r e l i a b i l i t y problems involved in the ceramic capacitors themselves: that is, their low voltage failures. Many studies have been done on ceramic capacitors to understand their low voltage failures and today the failure physics of these capacitors is generally well understood: the main failure mode is short circuit due to metal migration between opposite electrodes through voids, cracks and/or high porosity into the ceramic dielectric, accelerated by humidity and temperature [1]-[7]. Even though the comprehension of this failure mechanism led to plenty of improvement in the r e l i a b i l i t y level of ceramic capacitors, neverthelessthe risk of getting a bad lot may s t i l l be high, mainly for low cost ceramic capacitors with the highest dielectric constants. This risk may be too high for manufacturers of equipment requiring long useful l i f e with high r e l i a b i l i t y level as is the case with telecom equipment. So in order to use ceramic capacitors in telecom applications i t may become necessary to introduce some MR 27: l-J
145
146
G SP~('(:HIUL~] screening procedure [ 8 ] - [ 1 3 ] to reduce the Freak population and/or to c e r t i f y the l o t s , thus g i v i n g up the benefit of low cost of these capacitors. On the other hand, even though only a few studies have been performed on the r e l i a b i l i t y of M.P.C. with small dimensions [ 1 4 ] , [ 1 5 ] , i t appears that these capacitors generally e x h i b i t a good r e l i a b i l i t y level. In a previous work [14], i t was demonstrated , with long-term l i f e tests, that the main f a i l u r e mechanism of these components is a progressive degradation of the contact between the end-spray and the m e t a l l i z a t i o n of the electrodes, i n v o l v i n g , as correlated f a i l u r e modes, a gradual change of the frequency response of the capacitor parameters. This f a i l u r e mechanism is a wear-out process, that could a f f e c t the r e l i a b i l i t y during the useful l i f e , i f the degradation rate is not s u f f i c i e n t l y slow. So f o r a user the most important step in q u a l i f y i n g H.P.C. manufacturers, is to evaluate the q u a l i t y of the contact between spray layers and electrodes, and to do some p e r i o d i c a l check on the maintenance of t h i s q u a l i t y . In t h i s paper a degradation model is presented that j u s t i f i e s the observed f a i l u r e modes of M.P.C. submitted to long-term t e s t s ; a technological c h a r a c t e r i z a t i o n procedure is described that can serve as a very useful and rapid tool to estimate 'a p r i o r i ' the r e l i a b i l i t y performance of these components and to choose the most r e l i a b l e manufacturers.
2- EQUIVALENTCIRCUIT The equivalent series c i r c u i t of a metallized capacitor, according to Holmberg [15], is sketched in Fig.1.
RI
Rc
,vvvw
O
Fig.l
VvW
Re
Re
Co
,vVvV
VvW
I
Ls ,, o
Equivalent series c i r c u i t of an unaged capacitor.
The symbols in the i l l u s t r a t i o n have the following meaning: -R z
leads
resistance
including
end-spray
resistance
-R c
contact resistance
-R e
electrodes resistance
-Rd
resistance due to i n t r i n s i c d i e l e c t r i c losses
-Co
geometric capacitance
-L s
inductive contribution electrodes.
between end-spray and electrodes
due to
leads, end-spray and
Let us deal f i r s t with the contact resistance. For a capacitor for which the contact between the end-spray and the electrodes is perfectly continuous, the current in the electrodes w i l l only flow orthogonally to the winding direction. But in real capacitors the contact is Far from being
Metallized polyester capacitors
147
perfect and, as a consequence of t h i s imperfection, there w i l l also be a longitudinal current flow. The larger t h i s imperfection , the higher the longitudinal current flowing in the electrodes .This current flow increases the capacitor losses that in the equivalent seri~s c i r c u i t can be interpreted as an increase of the contact resistance. Thus i t is clear that Re depends on winding technique and end-spraying operation efficiency. The sum of the resistive and d i e l e c t r i c losses is generally known as the equivalent series resistance (ESR) of the capacitor. With respect to Fig. I, the ESR can be written as: tg 6~
Roeq
= (R/
+ R c + Re) + R d = R s + ~
(1) o) CO
in which the r e s i s t i v e and d i e l e c t r i c losses have been subdivided, tg6 d is the i n t r i n s i c dissipation factor and ~=2nf is the pulsation, while Rs is clearly defined by the expression itself. I t is worth noting that Rz is frequency dependent (increases with frequency) and i t s value can be estimated with the formulae generally used to take into account the contribution due to the skin effect (see for example Snelling [16]). However i t s influence on the total ESR is generally negligible up to I MHz, after which, i t has to be taken into account. Rr and Rc can be considered constant with respect to the frequency, while Rd, due to ( I ) , decreases with increasing frequency (neglecting, as a f i r s t approximation, the frequency dependence of the i n t r i n s i c dissipation factor). The apparent or equivalent capacitance is generally defined by: Co
Co
Coeq :
:
(2) I - (f/fo) 2
1 - o2LsCo
where fo is the self-resonant frequency of the capacitor. So the impedance of the capacitor can be written as: I Zo = Roeq - j
(3)
Cot q and the dissipation factor: mCoRs + tg6 d
tg 60 = ~ Coeq Roeq =
(4) I - 2 LsCo
Fig.2 reports the plot corresponding to eq.(2), in terms of (Cocq - C,)/Co x100 for a I uF capacitor.
3- EXPERIMENT
We tested about 500 pieces of MPC,from three manufacturers, with lead spacing of 5 mm, subdivided in four capacitance values (I/0.47/0.1/0.01 uF). The tests ran for more then 30 Khrs at I00"C in three electrical stress conditions: without voltage , with 2 Vdc and with rated voltage applied.
148
G. SPECCHIULL1
•
•
•
i
• •L_~J EIR
100
Z o u U
_J LIJ (:3
-100
I~JF -200
1o
lOOk
1M
IOM
FREQUENCY [Hz}
Fig.2
Percentage of apparent capacitance change with respect to the I0 KHz capacitance as a function of the frequency for an unaged luF capacitor. The crosses are experimental points, while the continuous l i n e were calculated from eq.(2).
Before and during the l i f e tests capacitance, dissipation factor (D.F.) and insulation resistance (IR) of a l l capacitors were periodically measured in accordance with IEC Publ. 384-2 [17]. Moreover IR measurements of capacitors in low voltage and storage tests were performed at 2 V, to avoid possible self-healing effects. Capacitance and D . F . measurements w e r e carried out automatically @ 10 KHz with an LCR meter GR 1688 controlled by an HP 9825 desk-top computer. All the data were s t a t i s t i c a l l y treated for each group of capacitors (manufacturer/capacitance value/type of test) and graphs of C, AC/C and D.F. vs. test time were obtained. After 25 Khrs of test an e x t e n d e d frequency characterization was performed on all capacitor groups in the IOKHz - IOMHz range with C, D.F.and IZJ measurements, to find any deviation from a normal behaviour, and to see which parameters were more sensitive for depicting the degradations suffered. The measurements were done automatically with an impedance analyzer HP 4192A controlled by a desk-top computer. The results on capacitance and IR deviations with time are not reported here because they l i e outside the purpose of this paper. They are reported elsewere [14].
4- DEGRADATIONMECHANISM The analysis of the data from these tests enabled us to ascertain that the main f a i l u r e mechanism of these capacitors is a progressive degradation of the contacts between the narrow tips of the Al-electrodes and the f i r s t spray layer and/or between the two spray layers themselves (the f i r s t spray layer is generally of zinc in order to have a good contact with the Al-electrodes, while the second one, sprayed over the zinc, is a more or less complex t i n alloy to f a c i l i t a t e soldering with the leads). The degradation was physically interpreted as a gradual separation at the boundaries (that could also be a growth of some insulating compounds) between these d i f f e r e n t metals as a consequence of the long temperature exposition of the specimens.
Metallized polyester capacitors
149
E l e c t r i c a l l y such a separation implies an increase of the contact resistance, both by a ' t r u e ' increase of resistance at the contact points where the separation occurs, and by an increase due to a ceduction of the contact areas that causes, as explained before, an increase in the longitudinal component of the current. Associated with these separations must also be taken into account a parasitic capacitance whose value w i l l be very high when the separation e n t i t y is very limited, so that i t does not affect the equivalent capacitance; as the thickness of the separation grows, the associated parasitic capacitance can become comparable with the nominal capacitance of the capacitor or decidedly smaller , so affecting the total capacitance value as measured between the external leads. This influence on the total capacitance w i l l clearly be frequency related and w i l l also affect the D.F and impedance vs. frequency response. I f we look at the degradation evolution with time, at f i r s t we shall observe a progressive increase of the contact resistance, l a t e r also a progressive capacitance decrease; eventually a complete separation could occur leading to an open c i r c u i t as f a i l u r e mode.
5- EQUIVALENTCIRCUIT FOR DEGRADEDCAPACITORS
The f a i l u r e mechanism just explained can be described by assuming the equivalent c i r c u i t shown in Fig.3,where: -R~
takes into resistance
account the
increment
-r~
loss resistance of the parassitic capacitance
-Cc
the contact or parasitic capacitance
while the meaning and values of the unchanged.
Cc
other
of
the
contact
parameters
remain
rs
Rs
Rd
Co
Ls o
0
Rc Fig.3
Equivalent c i r c u i t of a degraded capacitor.
The total impedance is again given by the formula: 1 Z t = Rte q
(5)
- j ~Ctcq
where the t o t a l equivalent series resistance becomes: 1 Rte q
= Roeq + R~ ~
1
+ Rp 1 + A
(6) 1 + 1/A
150
(J. SPEC('itlUL[ 1
and the total apparent capacitance: 1
1
l
(°)CoRe)2
+ -Co Cc
Ct~q
2 - ~ Ls
(7)
i + A
where: R!
Rp
(8)
R~+ r~
A : [eCc.(R~ + r , ) ] 2
(9)
The total dissipation factor then becomes: tg61 = ~ Ct<.q RI,.q
(i0)
In Fig.4 a set of graphs are reported in which the experimental points are compared with the theoretical plots obtained from eq.(7) in terms of (Ctro - Co)/Co x]O0, for five i uF capacitors after 25 Khrs @ I00°C with different degradation levels; the parasitic parameters being obtained from a b e s t - f i t t i n g non-linear regression program, and where Co is the 10 KHz capacitance. The agreement between experimental points and theoretical plots is quite good, taking into account that in the model the parameters were assumed constant with respect to the frequency and to the measuring current. Fig.5 is a comparison between IZI and tgO~ vs. freq. plots for an unaged capacitor and the most degraded capacitor of Fig.4. The continuous lines were obtained with the previously developed formulae. In table I, as an example, the parasitic parameters found for the five capacitors of Fig.4 are reported. I t is of interest to note that a measurement of C. and tg6~ @ 10 or 100 KHz, may be not sufflcent to defln~ the degradation level completely.
loo
-~-
100 °0
o
. . . . .
1
.o
J
LU
-
-
100
20(
• 1Ok
lOOk
IM
® IOM
Frequency [Hz]
Fig.4
Percentage of apparent capacitance change with respect to the IO KHz capacitance as a function of the frequency for five ]uF capacitors with different degradation levels, after 25Khrs @ IO0°C. Theoretical plots compared with experimental measurements.
Metallized polyester capacitors
102
IpF
151
.......
100 ° C
[~]i02
10
10
c O
Lh L~ ~>
LLJ L-}
1
Z .< 0
Z
W (3_
~>
10-1
10-1 tg~o (NEw) ~0-2 10k •
,
,
,
/ . . . .
' ~ / ,
,
,
,,
,
. . . .
,
1ook 1M FREQUENCY [Hz]
10-2 IOM
.........
Fig.5
Comparison between impedance and D.F. vs. frequency plots of an unaged capacitor and the most degraded capacitor of Fig.4 (n'5). The continous lines are the theoretical plots to be compared with the experimental measurements.
Tab. I
Parasitic parameters of five degraded capacitors after 25 Khrs @I00°C. Rc'
Rp
Cc
Ls
[mohm]
[mohm]
[uF]
[nH]
1
169
50
6.77
13.57
2
134
50
7.36
6.0
3
308
150
0.51
5.33
4
340
140
0.17
4.5
5
1050
170
0.07
5.0
This clearly emerges from a look at equations (10),(6) and (7). In fact, at relatively low frequencies the total apparent capacitance is practically equal to Co, while the contribution of the parasitic parameters to Rteq is mainly due to the term in R~ becauseof the frequency dependenceof A. At higher frequencies the total apparent capacitance is a combination of Co and Co, while the total equivalent series resistance is mainly affected by Rp. So, i f a complete definition of the degradation levels is desired, i t is necessary to perform an extended frequency characterization of the failed specimens or, at least, to integrate a measure of the dissipation factor at low frequencies with a measure of the apparent capacitance near the self-resonant frequency of the capacitor. Fig. 6 (a), (b) and (c) show some graphs of the cumulative distribution of the per cent capacitance difference between IMHz and 10 KHz for I uF capacitors (A and B manuf.) and 0.1 uF (C manuf.), after 25 Khrs at the three performed stress
~D
G . SPECCHIULLI
152
i
r
I
r
1
I
i
i
I
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i
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i
i
i
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ltuF
-60 -40 -20
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x
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manuf.A
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u.,
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40 LF
manuf. B
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',",
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CUMULATIVE PERCENTAGE
I
i
r
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I
l!
I
r
I
i
r
iI
!
a"a -.~;-~.. 0.1
-60
pF
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0 20 40
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10
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I
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]
50
i
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90
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98
CUMULATIVE PERCENTAGE
Fig.6
Cumulative distribution of the capacitance change between ] MHz and ]0 KHz; continous l i n e : IO0"C/Vr; dashed l i n e : 100°C/2Vdc; dots and dashes:storage @ IO0°C.
Metallized polyester capacitors
153
conditions. As can be seen, these plots give a good idea of the degradation suffered by the d i f f e r e n t capacitor groups, showing up the contribution due to the parasitic capacitance. A q u a l i t a t i v e correlation was found between D . F . and [C(IMHz)-C(IOKHz)]/C(IOKHz) (see Table I I ) of degraded capacitors: that is, the higher the D.F., the higher the negative capacitance change. Thus to get an idea of the degradation e n t i t y , the usual measurement of the D.F. @ 10 or 100 KHz could be s u f f i c i e n t , but only as a f i r s t approach.
Tab. I I
Comparison among capacitance deviation between I MHz and 10 KHz, Dissipation Factor and self-resonance frequency for f i v e degraded capacitors. AC/CO
D.F.
fo
[%]
[10 .3 ]
[MHz]
I
65.5
21
1.7
2
lg.6
17
2.3
3
5.3
36
3.0
4
-12.9
38
4.1
5
-52.8
74
7.5
6- DEGRADATIONRATE From the analysis of the whole set of data i t became evident that the described f a i l u r e mechanism did not extend uniformly over all the specimens in test, but a few marked differences among manufacturers and also among d i f f e r e n t capacitance values were found. These results are summarized in Table I l l that reports the 10 KHz D.F. values after 25 Khrs of tests for the three applied conditions, in terms of minimum, mean and (mean + 2 sigma) D.F. values. Looking at t h i s table the following observations can be made: - capacitors of the same ratings by manufacturers A and e x h i b i t a similar behaviour during the tests;
B
- 0.01 and 0.1 uF by manufacturer C exhibit a very high D.F. increase compared to those of the other two manufactures; - except for the 0.47 uF, that remains p r a c t i c a l l y constant, the maximum D.F. d r i f t increases on increasing the nominal capacitance value; In addition two other facts were evident: - the D.F. increases l i n e a r l y with test time; - the voltage applied during the tests has no influence e n t i t y of the D.F. increase.
on the
These last results clearly emerge from Fig.7, in which a set of graphs of tgSt (@ 10 KHz) vs. test time for I uF capacitors by manufacturers A and B is reported. The three lines on each graph for each manufacturer show the minimum, mean and (mean + 2 sigma) D.F.values, respectively.
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Metallized polyester capacitors
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10 KHz Dissipation Factor vs. test capacitors @different stress levels.
time
of
luF
Moreover, comparing these plots with the corresponding ones of Fig. 6(a) and (b), a clear qualitative agreement can be found between D.F. d r i f t s and AC/C .
6- TECHNOLOGICAL ANALYSIS 6.1- Technological analysis procedure To understand these behaviour differences, that are clearly technological related, and bearing in mind that the c r i t i c a l point of this technology is located in the contact zones, a technological analysis method was developed. Cross-sections of several capacitors w e r e performed (without incapsulating the parts),in a plane orthogonal to the leads, up to half of the height of the capacitors. This was done by employing a commercial polisher with papers of different grade; the last polishing was done with a 1 um Alumina. A f i n a l step of about ]0 minutes in an oxygen plasma was performed to avoid overlapping effects due to the extreme softness of the polyester film. After that the parts were flashed with gold to enable SEM analysis. 6.2-Technological analysis results Fig.8(a) is a SEM micrograph of a cross section of a 0.47 uF capacitor. The f i r s t spray layer of zinc and the second one, that is the t i n alloy, are evident. Moreover i t is possible to see that the zinc spray penetrates deeply between plastic f o i l s , assuring a very tight contact with Al plates. Greater details are given in figures 8(b) and 8(c). As mentioned before, this capacitor rating suffered p r a c t i c a l l y no degradation during the tests. Fig.9(a) instead shows a cross-section of a I uF capacitor. As can be seen, zinc penetration is poor, so the contact is generally realised only between the inner spray surface and the tips of the electrodes, as can be better seen in Fig.9(b).
156
(~
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Figure 8a
Figure 8b
Metallized polyester capacitors
157
Figure 8c Fig.8
SEM views of a 0.47uF capacitor head. (a) view of about h a l f capacitor head, (b) zinc penetration through Al electrodes, (c) t i g h t contacts between zinc spray and electrodes.
Moreover, on these capacitors we found defects l i k e those shown in Fig.9(c) and (d), that are cracks or separations near the contact zone. The 0.1 uF by manufacturer C was the one with the highest degradation levels. From the technological point of view we found some extended defects l i k e those shown in Fig.10(a), that is, a separation between the two spray layers running along h a l f capacitor head. Near the contact zone with the electrodes some pofosity is also v i s i b l e . For this rating too, zinc penetration is generally poor, as i l l u s t r a t e d in Fig.tO(b) and
IO(c). Thus the failure mechanism postulated finds a good support in these pictures.
at the
beginning
7- RELIABILITYCONSIDERATIONS
Above a l l i t is important to note that capacitors with an impedance vs. frequency plot l i k e that shown in Fig.5, are s t i l l usable as general purpose capacitors, while capacitors by manufacturer C have not been considered for any r e l i a b i l i t y consideration because of the unacceptableness of the obtained results. Therefore, having had neither catastrophic nor ' t r u e ' parametric f a i l u r e s , i t was impossible to v e r i f y any f a i l u r e d i s t r i b u t i o n . In an attempt to get some more information on t h i s point, an arbitrary D.F. f a i l u r e c r i t e r i o n of 0.03 was assumed. With t h i s assumption i t was v e r i f i e d that the times to f a i l u r e have a Weibull d i s t r i b u t i o n with shape parameter greater than one; that means, as expected, a wear-out process.
158
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Figure 9a
Figure 9b
Metallized polyester capacitors
Figure 9c
Figure 9d Fig.9
SEM views of a luF capacitor head. (a) view of about h a l f capacitor head, (b) contacts between zinc and electrodes, (c) voids and cracks near the contact zone, (d) deep crack in the contact zone.
159
160
( i . SPI ¢ tttlt',l.l I
Fiaure lOa
Figure IOb
Metallized polyester capacitors
Figure lOc Fig.t0
SEM views of a O.]uF capacitor. (a) large separation between zinc and tin alloy; cracks and voids in zinc are also visible, (b) contacts between zinc and electrodes, (c) detail of picture (b).
For example, for I uF capacitors by manufacturer B in the three test conditions, shape parameters between 1.2 and 6.5 were estimated. The scale and shape parameters found could not, of course, be used to get information about the failure rate. In conclusion, the only thing that can be said from the r e l i a b i l i t y point of view, is that i f the end-spray operation has been carefully performed, the related failure mechanism, having a quite low degradation rate, does not affect the r e l i a b i l i t y level of the capacitors.
8- CONCLUSIONS
The long term tests performed enabled us to ascertain that the main failure mechanism of metallized polyester capacitors is a temperature dependent degradation of the end-spray contacts which introduces, besides the already known ESR increase, a parasitic series capacitance, that decreases on increasing the degradation level. This parasitic capacitance, together with increasing ESR, causes a gradual change of the frequency response of degraded capacitors. In well manufactured capacitors, the degradation rate is quite low, so that i t does not affect their r e l i a b i l i t y level. A technological analysis method was developed that, together with a frequency characterization especially for capacitance, enables very rapid estimation of the r e l i a b i l i t y performance of these components and the more reliable manufacturers to be choosen. These tools can, of course, be used for periodic quality control. MR 2 7 : 1 - K
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G. SPECCHIL'LLI
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AKNOWLEDGEMENTS
I would like to thank R. Bottini for valuable assistance with the technological analysis and A. Scorzoni, from LAMEL, for SEM analysis.
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polyester Capacitor
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