ELECTROLYTIC CORROSION OF MSTALLIC RESISTORS UNDER HUMIDITY

Dr. Friedrich Futschik

Industrial Components and laterials Division N.V.Philios'Gloeilamnenfabrieken

Eindhoven - Lath&lands

Abstract Eetallic resistors are encapsulated mainly

for two reasons. Firstly to avoid mechaniaal damage, secondly to provide protection against moisture.

If moisture penetration is not prevented and if at the same time ions are present capable of moving through or along the insu- lating materials, a resistance ohange will ooour with time, whenever the resistor is sub- jected to a high relative humidity and a d.c. load.

Under these conditions there will be an eleotrolytic current flowing through or along the insulating materials, which will oause damage to the resistance element at the anodic side of the resistor.

it is only a matter of time before water particles will penetrate through the encapsu- lating material. If there is enough water,parts of the insulating materials (i.e. ceramic material, upon which the resistance element is deposited or the encapsulating material) may act as electrical conductors, especially when mobile ions originating from impurities are present. Under a d.c. load there will be an electrolytical current flowing along the surface of the ceramic material or along or through the surface of the encapsulating material. As a result matter will be transported causing the metallio element at the anodio side to disappear. Open circuiting will occur, sometimes within a few hours if the oonditions are severe.

Typioal quantities expressing the behaviour are: the resistance value of the insulation resistance of the covering material under various humidity conditions and the oalorio resistanoe of the whole component.

Caloulated are formulae, which ae put to the test and whioh show that:

the eleotrolytio current flowing through or along the insulating materials as a funotion of applied load has a maximum value;

the load at whioh this maximum ocours depends on the resistor oonstruction and the materials used;

Electrochemical corrosion of resistors has stimulated everybody, manufacturersas well as customers, into a search for test methods providing information on corrosion resistance. There are a lot of tests each made for the purpose of speoifying oonditions as severe as possible. It is always a combination of eortain temperatures, high humidity and a small d.c. load, which is to produce these severe oondi- tions. The load has to be kept small,otherwise the self generated heat will raise the resistor to a temperature at whioh the water particles are removed and as a result of this the eleo- trolytio oonductivity of the insulating parts will cease to exist.

the moisture resistance of a metallic resistor has to be tested at several loads, otherwise it will be Impossible to draw con- olusions. Key words: Resistor, Moisture, Ions, Insulation Resistance, Electrolytic Current, Maximum.

Introduotion

Often there will be temperature-oycling oombined with intermittent loads. Temperature oyoling will aause some sort of "water breathing" (alternate oondensation and evaporation of water)and intermittent loads permit higher voltages with the self generated heat not eroeeding that obtained with a continuous but smaller load.

Wire-wound and metal film resistors, both belong to the best and most stable we know today. They develop only thermal noise, which means 0.1

c V/V or lower.Temperature ooefficient

oan be ma e almost zero (i.e. within a few parts per million per 'C). Yet these resistors require a better enoapsulation, beoause to some extent ions to step

%O-molecules always oause metal- out of the orystal struoture and

to become mobile.

From the foregoing it will be clear that any resistor not hermetically sealed may be caused to fail under humidity conditions.

If a resistor is not hermetioally sealed

Qualitative approach A particular test method always gives a parti- cular specifioation of the conditions under whioh the resistor haa to be tested. In thia oonneotion mention should be made of IEC- publication 115 which speoifies for the long

Moisture Tests

Behaviour under various loads

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term exposure during the damp heat test one temperature (4O'C) and for one half of the lot 5 V d.c.

However, when the moisture penetration ie not prevented by the encapsulation and ions are present capable of moving through or along the insulating materials, low d-c. voltages will oause small electrolytic currants to flow through or along these moderately conducting "insulating" materials. Hence, a very small amount of matter will be transported per unit of time, resulting in a small resistance change per unit of time. Increasing the volts@ will accelerate the electrolytic processes, which in turn will accelerate the changes of the resistance element.

But due to self generated heat the tempe- ratue of the resistor will rise as the applied voltage inoreases, the humidity in the resistcr will deorease.Thus,the aforo mentioned eleotro- lytic current also decreases and the behaviour of the resistance element becomes more stable.

Therefore there must be a particular load, at whioh there is a maximum PR per unit of time. Potential Fields

To obtain information on the eleotrolytio ourrent flowing through slightly conducting enoapsulating materials, the potential fields in the surroundings of a conducting cylinder have been investigated.

----__ &es of equal potential (&xperrin)

- fines of eiectr. current

Fig.1 Electric field around conducting cylinder

For this purpose the conducting cylinder was imitated by a silver painted reotangle on graphite-paper-much less oonductive than the silver (6A,:gpaper~ 2000 : l).Between the

_ -_ ends a constant potential differenoe was maintained and with the aid of a pin used as a probe the lines of constant potential have been determined. These lines show that the

electric ourrent flows mainly directly from one end to the other.

Fig.2 Eleotrolytic phenomena: uncoated wire-wound resistor immersedin water

This faot can also be demonstrated by a non-enoapsulated wire-wound resistor being in tap water. Applioation of a d.o. voltage will show that the electrolytio prooesses mainly ooour at both ends and scarcely along the central tarts of the body (fig.2).

On the left hand side Sn from the tinned cap and lead dissolves anodiaally and forms Sn(OH) , which is heavier than water. On the right P or the oathodio)side H2-gas is notioed going upwards. This means, that the current almost entirely flows from one end to the other.

If the resistanoe element is a spiralled metal film or a wire, wound around a ceramio core a oonfiguration is obtained at which also

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small currents flow from one turn to the adjacent one. This is why the photograph shows also a few small bubbles of hydrogen along the central parts of the resistor.

By reducing the dimensions of the non-or better, badly conduoting surroundings of the oonducting cylinder,

Fig.3 is obtained, showing the current Ii starting at the positive end, then going almost parallel to the boudaries of the oon- ducting cylinder and ending at the negative side of the oonduotor.

- - - - - - lines of equal potential (experim.)

- ihes of elects current

Fi6.3 Same as fig.1

Quantative Approach We shall now consider a current Ii through

or along the insulating parts of a resistor which causes the metallic resistance-element to disappear at the anodio side under a small d.c. load and a high humidity.

Responsible for the current Ii is the insulation resistance Ri according to

U Ii' q

where II is the voltage across the resistance element.

4---l-+ Fig.4 R and Ri in shunt connection

As shown in fig.4 the insulation resistance R. is taken to be in parallel with the risistance element R. Therefore the total current I t oaused by U :

(2) It - I + Ii

Furthermore it is assumed that Ri varies

under high humidity conditions with temperature according to

Ri = Rio.eaAT

c caps h heattbg element cer ceramic tube th thermocouple cem cement t tags

Fig.5 Test-specimen for Ri-measurements

tan>' is the value of the insulation resis-

, if the insulating material concerned is exposed to a pertain relative humidity in a chamber at To C. If only the temperatureoof the insulating material is raised by&T C the material will become drier, hence R will increase. The relation (3) was chec ed B experimentally on oement and on ceramic material.

A aeramio tube was used which had a heating element inside it, a thermocouple on its outside and two tags enabling the insulation resistance of the bare or cemented oeremio material to be measured (fig.5). This oon- struotion was placed in a ohamber with high relative humidity and a constant temperature. The results shown in fig. 6 were found on cemented ceramic material.

Temperature rise of a resistor may be expressed by

where W is a oonstant, depending on the con- struction of the resistor and the materials used. This constant may be called heat resis- tance, its dimensions being degrees per Watt. dT ia the temperature rise of the so-called

hot spot of the component.

Taking (l), (3) and (4) we get

e&w u2

(5) Ii - + . e - 77 * i0

(6)

This function has a maximum at

urn- AL- d-- 2- w

z9 f(U) l

which corresponds with a load

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Md

400~

3001

2m

1lW

Fig.6

Ri = R-/TAT); temper i of hum. - chamber:25”C t ;roO % rel. hum. ; maf end: cement on

i

1

Ri measured on cemented ceramic material

(7) Urn2 pm- y&q

Further, the current at this maximum is given by

(8) Iim = f(Um) ii + . io

According to AT = W.P, AT at which the maximum occurs is

(3) @Tjp ' =2* m

Conclusions 1. There is not much sense in testing re-

sistors at a high relative humidity with only one test voltage applied.

2. AR (which will be directly proportional to I ) will show a maximum when measured at a cons ant t time as a function of P.

3. This maximum depends on o( and W, two constants, which are dependent only on the construction of the resistor (materials and dimensions). The maximum is independent of the resistance value R (if it does not influence heat transfer and therefore W).

4. If there are many mobile ions oontri- buting to Ii under humidity, Rio will be small and therefore I im will be high.

Fig.7 Ionic ourrent as a function of P, theoretically

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%

2

I

3%

5

4

3

2

Al?

AR

.Ol .I i la 100 mW Fig.8 Measurements of AR as a function of P

Me-f&, 365kO at lUU % relafive hwnid.fy

P I

1 IO 100 moo mW Fig.9 Same as fig.8

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Experimental data Plotting (5) by taking P ard1 as units,

we get fig.7. Fig.8 shows m!asure%&ts ofARts after 63 days made on metal film resistors which had been in a chamber at 98 - 10% rel. humidity.R was 150 kOhm, W was meagured using a thermocouple and found to be 152 C/W.

According to the graph P,lies between 0.8 and 1.6210-3 %, therefore&between 2.06 and 4.1 per degree.

Resistors with other dimensions, but of the same construction showed the results given in fig-g.

R was 365 kOhm, heat resistance W = 120°C/W. Taking P between 1.4 and 2.8x10-3 W, d must lie betwgen 1.5 and 3.0 per degree. These data are in good agreement and suggeet that AT at P will be something like 0.4 C (acoordi~g to (9)).

Shown in fig. 10 is the time to failure of wire-wound resistors measured as a function of P. These resistors were coated with cement which contained a lot of mobile ions (Rio at lOC$ rel. humidity was about 10 Yohm).

IEC test at 5 V for 10 days wouldn't have revealed anything, as can be seen from the graph (our tests were continued up to 10 days). Rut in this particular case the resistors were open-circuited within 65 minutes at 50 V d.c. !

h-

1

I -“r u2 404 &oft)”

RBsum6 bletallic resistors, when not hermetically

sealed, can always be brought to failure (open circuit or excessive nR) under high humidity conditions with a d.o. load. Especially when tested under constant conditions, i.e. at one temperature, one rel.hunidity and at a continuous load, time to failure depends on the load.

Depending only on the construction of the resistor and the materials used there exists a load, at which the time to failure is minimum. It is important to know this minimum for a given oonstruction.

It would be desirable to amend IEC publica- tion 68, where the long term exposure during the damp heat test is concerned.

Formula (8) shows how the time to failure can be influenced almost entirely by choosing enoapsulating materials with a high value Rio and a high d . This also implios there must be no impurities at the surfaces.

'if can be measured under high humidity on a res stor without a resistance element. As has been shown,d can be found by measuring W and determining Pm.

In spite of the fact that a higher W would improve moisture performance, moderate values should be chosen, because firstly a substantial improvement is not attainable and secondly AT should be kept as low as possible for good performance under nominal load (according to (4) rAT I W).

It is important to note, that humidity alone is not disastrous. Only a high humidity and a poor R failurHOs

will provide the conditions for early .

Acknowledgements The author wants to thank Dr.J.G.Bos and

Kr.J.Kunnen for the many discussions on the subject. Thanks are also due to Mr.K.A.de Gier who oontributed by measuring the potential fields and to Yr.J.Deliege and Kr.T.Yoormans who made all the measurements on the wire- wound and metal film resistors.

Reference Cl.M.ROFBAUER: "Die Feuchtigkeits- und Klimabesttindigkeit von SchichtwiderstZnden", Radio Mentor, & 1961, pp 400/401.

Fig.10 Time to failure at different loads

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