ICEC2006
The Influence of Manufacturing Process, Metal Oxide Content,
and Additives on Switching Behavior of Ag/SnO2 in DC and AC
Relays (2)
Andreas Koffler,Peter Braumann, Bernd Kempf,
Umicore AG & Co. KG, Hanau , Germany
Summary
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New findings and conclusions from a multi-year program are
presented, which first part was published at the 22nd ICEC 2004.
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The current phase of work focused on investigating the influence
of three separate parameters on the switching behavior of DC and
AC relays: the grain size and dispersion of the components, the
metal oxide content, and the In2O3 content.
In automotive relays, materials with larger oxide particles
achieve equivalent or better results. In the case of general
purpose relays, the maximum life in regards to welding and
erosion is achieved with a large metal oxide content (12%).
Welding resistance and erosion behavior are counteracted less
effectively by reducing the amount of In2O3 and using larger
metal oxide particles.
It has been determined that in general, a very high degree of
oxide particle dispersion is only advantageous at low currents
like those are switched in general purpose relays.
Key words:
Silver tin oxide, relay applications, wet chemical precipitation
1. Introduction
The historical development of the use of Ag/SnO2 in
automotive relays and power relays has been previously
reported in a comprehensive survey of the literature /1/. In
switching experiments, the focus lay on comparing
Ag/SnO2 materials manufactured using the classic mixed
powder technology (PMT materials) with so-called NCF
materials where the composite powder is produced using
the wet chemical precipitation technology. The NCF
method was presented for the first time in /2/ and
explained in greater detail also in /1/.
The major advantages of the NCF technology compared
with the conventional powder blending technology can be
summarized as follows according to /1/:
- Extremely homogenous distribution of the
components
- High material deformability
Flexibility in the selection of the basic component
SnO2
Flexibility regarding additives, including the use of
In2O3
In /1/, numerous switching experiments were performed
on automotive relays and power line relays. The switching
performance of known material varieties was compared
with new materials based on the NCF technology, and a
few basic rules on the use of the materials within different
load ranges were presented.
Additional work concerning the overall topic addressed the
following questions:
- Under what conditions are extremely finely dispersed
Ag/SnO2 materials advantageous?
- What effect does varying the amount of SnO2 have on
the switching performance?
- What influence does the amount of In2O3 have?
The results will be discussed in the following paragraphs.
2. Contact Materials
Table 1 lists the Ag/SnO2 variants tested. Except for the
comparative material Ag/SnO2 80/12 VS1011, all are
based on wet chemical precipitation.
Designation
Degree of
dispersion
AgSnO2 88/12 NCF1 extremely
high
AgSnO2 90/10 NCF1 extremely
high
AgSnO2 92/8 NCF1
extremely
high
AgSnO2 88/12 NCF2 extremely
high
AgSnO2 88/12 NCF3 high
AgSnO2 88/12 VS1011 average
Table 1
In2O3 additive
average content
average content
average content
low content
average content
average content
Contact materials for switching tests
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processed into rivets. The chemical composition of the test
material VS1011 corresponds to that of Ag/SnO2 88/12
NCF3.
Fig. 1
Ag/SnO2 88/12 NCF 1
(NCF with extremely fine oxide particles)
If we use Ag/SnO2 88/12 NCF1 as the reference, the metal
oxide content was reduced from 12% to 8%. In the case of
variant NCF2, the amount of In2O3 additive was reduced
by 1/3 in comparison to NCF1. The variant NCF3
corresponds to NCF1 except for the somewhat coarser
structure (lesser degree of dispersion).
Fig. 3
Ag/SnO2 88/12 VS1011
(conventional mixed powder technology with
average metal oxide particles)
3. Automotive Relay Applications
The experiments done on automotive applications centered
on the influence of fineness of the metal oxide powder and
the effect of the method for producing the composite
powder on switching performance. For this reason, the
variants Ag/SnO2 88/12 NCF1, Ag/SnO2 88/12 NCF3 and
Ag/SnO2 88/12 VS1011 were included in the experiments.
3.1 Switching parameters
Fig. 2
Ag/SnO2 88/12 NCF 3
(NCF with fine metal oxide particles)
Fig. 1 shows the structure of Ag/SnO2 88/12 NCF1. This
structure is characteristic of all NCF materials in which
extremely fine metal oxide powder is used. By using less
fine powder as in Ag/SnO2 88/12 NCF3 (Fig. 2), the
structure is somewhat coarser in comparison to NCF1. The
mechanical characteristics of NCF3 are more suitable for
use in forming rivets as noted in /1/.
In regard to component distribution, the material Ag/SnO2
88/12 VS1011 (Fig. 3) produced by powder blending has
weaknesses in the form of oxide agglomerates. They only
slightly affect the contact and switching performance;
however, they can be starting points for cracks when
Model switch, NO, contact force 150 cN
a) H4-lamp loads
1.5 s ON/ 3.5 s OFF, 50 000 ops.
4 H4 lamps: Ipeak = 170 A, Id = 22 A, We = 100 mWs,
Wa =38 mWs
6 H4 lamps: Ipeak = 210 A, Id = 33 A, We = 130 mWs,
Wa = 85 mWs
b) Ohmic inductive loads
1 s ON/ 5 s OFF, 50 000 ops.
40 A, 0.42 mH: We = 12 mWs, Wa = 550 mWs
50 A, 0.42 mH: We = 24 mWs, Wa = 800 mWS
The indicated values for the energy converted during the
switch-on peak and in the switch-off arc We and Wa are
typical averages calculated according to /3/.
The failure criterion in all experiments was the first
instance of failing to open within 2 s.
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3.2 Welding behavior
Fig. 4 shows the number of operations at which welding
first occurred. The results reveal generally large spreads
that are also typical for relays not modified as model
switches (series relays).
The picture is not uniform under ohmic-inductive loads of
40 A and 50 A. However, we can generally conclude that
the extremely finely dispersed NCF1 does not offer any
advantages.
Fig. 6
Fig. 4
Welding resistance under a lamp load and
ohmic inductive load
Under a lamp load of 4 H4, model switch 1 tends to weld
somewhat earlier. In this case, Ag/SnO2 88/12 NCF3 has a
longer life than the extremely finely dispersed NCF1. The
less homogenous comparative material VS1011 does much
more poorly than the other two. In model switch 2 where
both NCF1 and NCF3 reached the conclusion of the
experiment of 50,000 switches, VS1011 failed after 20,000
operations.
Fig. 5
Relative erosion under ohmic-inductive loads
3.3 Erosion and material migration
Under a lamp load (Fig. 5), the erosion was very slight in
each case. The material migration was largely over a broad
area and was not the result of failure. There was no
significant difference between the tested materials.
At a 40 A ohmic-inductive load (Fig. 6), we again saw
only small differences, with a slight tendency toward
greater erosion in the somewhat larger variants NCF3 and
VS1011. We saw a clearer picture after increasing the
current to 50 A: The extremely finely dispersed NCF1
revealed much less erosion and material migration.
Relative erosion under H4 lamp loads
As was expected, welding occurred after fewer operations
under a 6 H4 lamp load. However, no clear tendencies can
be concluded regarding the behavior of the different
variants.
Fig. 7
Contact resistance
(99.5% values)
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3.4 Contact resistance
The contact resistance was measured from the decrease in
voltage shortly before the contacts opened every time
switching occurs. Fig. 7 presents the 99.5% values. As
already known from /4/, the contact resistance under lamp
load is clearly lower than under an ohmic-inductive load,
with the three variants showing the same behavior.
The weak inductive load produces resistances that are
twice as high. There was no significant difference between
the variants at 40 A and 50 A loads.
4. General Purpose Relay Applications
Fig. 9
At least three relays were tested, and the results are shown
in the Weibull diagram. The details of the switching
conditions are given in the legends of the figures. The
failure probability was plotted in reference to the target
life.
4.1 Influence of the metal oxide content on the welding
tendency and erosion
Only NCF1 variants were investigated. The metal oxide
content was 8%, 10% and 12%. The failures arose from
contact welding at an inrush peak of 51 A (Fig. 8). The
overtravel was sufficient in each case.
Weibull distribution of the failure rate by
erosion
General purpose relay, NO, 230 V, 5 A ohmic, 1
s ON/ 1 s OFF
Under a 5 A ohmic load, the failures were generated by
insufficient contact from insufficient overtravel due to
erosion. The results portrayed in Fig. 9 reflect the erosion
behavior of the material variants investigated.
There was no clear difference between NCF1 with 8% and
10% metal oxide. All initial failures were observed at
approximately 90% of the target life. At 12% metal oxide,
the lives were largely stable or lie above the target values.
When the metal oxide content was 8%, the relays clearly
tended to weld before reaching their target life. Increasing
the metal oxide content to 10% yielded a substantial
improvement. At a metal oxide content of 12%, the life
was consistently above the required target.
Fig. 10
Fig. 8
Weibull distribution of the failure rate from
welding
General purpose relay, NO, 230 V, Ipeak= 51 A, Id=
3 A, 5 s ON/ 5 s OFF
Weibull distribution of the failure rate from
welding
General purpose relay, NO, 230 V, Ipeak= 51 A,
Id= 3 A, 5 s ON/ 5 s OFF
4.2 Influence of the amount of In2O3 and particle size on
the welding tendency and erosion
The results in Fig. 10 and 11 were determined in the same
test series at a high inrush current of 51 A and at 5 A
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ohmic load. The results from Ag/SnO2 88/12 NCF1 from
the prior section were used as a benchmark. These are
contrasted with the results from AgSnO2 88/12 NCF2
(=1/3 of the In2O3 content) and Ag/SnO2 88/12 NCF3 (=
somewhat coarser).
The results which are partially based on a very narrow
statistical foundation need to be interpreted in light of the
authors’ experience which confirms the conclusions.
In particular, the following relationships can be derived
from the results:
5.1 Automotive relays
Ag/SnO2 88/12 NCF1, NCF3 and VS1011 were compared.
Fig. 11
Weibull distribution of the failure rate from
erosion
General purpose relay, NO, 230 V, 5 A ohmic,
1 s ON/ 1 s OFF
Under the given conditions, both the reduction of the In2O3
content and use of a less-fine metal oxide substantially
increased the tendency for failures with welding before the
target life was reached (Fig. 10).
Under a 5 A load (Fig.11) as well, the use of the variants
AgSnO2 88/12 NCF2 and NCF3 markedly shortened the
life (due to greater erosion) in comparison with AgSnO2
88/12 NCF1.
5. Discussion of the results
In comparing the different Ag/SnO2 variants, numerous
switching conditions were used: DC, AC, high make
current, ohmic and ohmic-inductive loads within different
current ranges. In summary, we can say that none of the
variants characterized by the production method of the
composite powder, degree of component dispersion, or
composition is significantly superior in comparison to the
other in terms of switching. This conclusion agrees with
the experiences of the authors that are far more wideranging than the results described in this article. All of the
investigated variants, whether produced by the powder
blending technology or the NCF method, are based on
cutting-edge technology and the results of many years of
material development. The performance level of all of the
utilized Ag/SnO2 variants is hence very high.
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The welding resistance of finely dispersed materials
produced using the NCF method (NCF1, NCF3) tends
to be somewhat better for automotive relays under low
lamp loads (4 H4 / Fig. 4). However, this difference
becomes less significant as the switch-on current
increases (6 H4 / Fig. 4).
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Under an ohmic-inductive load, all the three variants
investigated have the same level of welding resistance
(Fig. 4).
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There are no substantial differences in regard to
erosion behavior and material migration under a lamp
load (Fig. 5).
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Under an ohmic-inductive load of 40 A (Fig. 6),
NCF1 has advantages in regard to erosion. At a higher
current of 50 A, this tendency reverses and is much
more significant. The tendency toward material
migration is also larger at higher loads for NCF1 than
for NCF3 or VS 1011 (Fig. 6).
In summary and again with reference to the authors’ wider
experience, we can conclude that extremely finely
dispersed Ag/SnO2 materials only have advantages over
coarsely dispersed materials at low currents.
For automobile relays, the variant Ag/SnO2 88/12 NCF3
hence represents an optimum compromise in regard to
switching performance and processability.
5.2. General purpose relays
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The use of Ag/SnO2 with a greater amount of metal
oxides in general purpose relays can alleviate
problems associated with high switch-on peaks. The
results shown in Fig. 8 confirm this known
relationship.
When general purpose relays fail under an ohmic load
due to the erosion of the contact, Ag/SnO2 containing
12% metal oxide is also advantageous (Fig. 9).
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In general purpose relays, optimum welding resistance
is achieved at high inrush peaks with extremely finely
dispersed materials such as Ag/SnO2 88/12 NCF1. A
slightly coarser metal oxide as in NCF3 can have a
negative effect. The same holds true when the In2O3
content is reduced to 1/3 as in Ag/SnO2 88/12 NCF2
(Fig. 10).
However, the results do not mean that variants NCF3 and
NCF2 are unsuitable for general purpose relays. Their
advantages (easier and hence more economical
processability of NCF3, and less of the expensive In2O3 in
NCF2) must be weighed against the switching advantages
of NCF1. This is particularly true when considered in light
of the fact that the somewhat higher welding resistance of
NCF1 is only manifested at the threshold of welding. The
advantages of improved processability can be critical,
especially when an effective processing method such as
direct riveting is desired.
With regard to erosion behavior, the relationship of the
three materials investigated, Ag/SnO2 88/12 NCF1, NCF2
and NCF3 is similar to welding: The somewhat coarser
structure and lower In2O3 content reduces the life by
approximately 20% in comparison to the results with
NCF1 (Fig. 11). In regard to the use of the materials, the
above arguments also apply.
References
We again refer to the comprehensive compilation of
literature on the “Use of Ag/SnO2 in Automobile Relays
and Power Relays“ in /1/.
/1/
/2/
/3/
/4/
Braumann, P., Koffler, A.: The Influence of
Manufacturing Process, Metal Oxide Content, and
Additives on the Switching Behaviour of Ag/SnO2 in
Relays.
50th Holm Conference on Electrical Contacts and
22nd International Conference on Electrical Contacts,
Seattle, USA, 2004, p. 90-97
Heringhaus, F. et al.: On the Improvement of
Dispersion in Ag-SnO2-Based Contact Materials.
Proc. 20th ICEC, Stockholm, 2002, pp. 199 – 204
Braumann, P., Koffler, A.,: The Importance of
Characterizing the Make and Break Operations to
Allow Effective Contact Material Development.
19th International Conf. on Electric Contact
Phenomena, Nürnberg, Sep. 1998, p. 325 - 333
Braumann, P.: Prüfung der elektrischen Lebensdauer
von Kfz-Relais [Testing the electrical Life of
Automobile Relays]. VDE Technical Report 55,
Berlin-Offenbach: VDE-Verlag, 1999, (15th
Conference, Albert Keil Contact Seminar, Karlsruhe
1999) , p. 49 – 59
Andreas Koffler received the Dipl.-Ing.
degree in 1992 in electrical engineering from
the University of Applied Science GiessenFriedberg, Germany. In the same year he
joined Umicore AG & Co. KG, Business Unit
Technical Materials, Hanau, Germany.
He is responsible for sales, applied
technology, and the contact testing lab.