http://dx.doi.org/10.1595/205651317X695271
Johnson Matthey Technol. Rev., 2017, 61, (2), 156–164
JOHNSON MATTHEY
TECHNOLOGY REVIEW
www.technology.matthey.com
Lowering the Silver Content in Automotive
Conductive Pastes
Experiments in thrifting with selected base metals, base metal alloys and
silver-coated base metals
By Edwin S. Raj* and Jonathan Booth
Johnson Matthey Technology Centre, Blount’s Court,
Sonning Common, Reading, RG4 9NH, UK
*Email: [email protected]
The high conductivity of silver and its oxidation
resistance make it the metal of choice for automotive
applications in defogging and other areas. There is
scope to reduce the cost by reducing the content of
silver, a costly metal. This article reports the results
from testing formulations with reduced silver content.
A range of silver-coated and base metal fillers were
tested however none of these resulted in performance
to match the commercially available silver automotive
pastes.
Introduction
Automotive conductive pastes are formulations
consisting of an active metal component, a glass frit
and an organic medium; the latter two facilitating the
adhesion and application on to the target substrate,
which is usually glass. Applications for these pastes
include print-based in-car defogging systems, antennae
and alarm circuits. The term ‘frit’ refers to ceramic
mixtures that have been melted to form a glass and
then crushed into a fine powder. Modern glass frits used
in the silver pastes are lead-free to meet environmental
regulations and are often predominantly bismuth-rich
156
to effect good flow properties (1). Screen printing
is the preferred mode of application for automotive
pastes being cost-effective and scalable for industrial
production.
Silver is the material of choice for automotive
conductive paste due to its high electrical conductivity
and oxidation resistance. In order to meet the industry
defogger standards, the conductive tracks must
operate with a 12 V line and the tracks need to have
a resistivity of 6 mW sq–1 at a thickness of 10 μm after
being fired (2), which can only be met by using a noble
metal such as silver. Typically, the silver loading in the
paste is in the range of 60–88 wt% depending on the
intended function. Table I summarises the automotive
silver pastes produced by Johnson Matthey (3).
Although silver has excellent conductivity and stability,
it is expensive. It is estimated that as much as 40% of
the cost of the automotive silver paste is that of the
silver content and between 2–3.4 g of silver is used per
car for defogging purposes (2).
Any potential replacement for silver in automotive
pastes should meet requirements of conductivity,
solderability, adhesion, chemical resistance, optimal
stability of resistivity over temperature and overall
compatibility with automotive glass technology. Thrifting
of silver with fillers is not uncommon in silver pastes
in applications such as photovoltaic current collectors
where fillers such as metals, silver coated metals and
alloys have been attempted (4–6). However, stringent
requirements for automotive silver pastes such as
needing to pass environmental tests makes the choice
© 2017 Johnson Matthey
Johnson Matthey Technol. Rev., 2017, 61, (2)
http://dx.doi.org/10.1595/205651317X695271
Table I Silver Pastes for Automotive Glass Applications Available from Johnson Matthey
Product
Ag content, %
Resistance, mΩ sq–1
Firing range, ºC
AG7500-60
60
±8.1
600–720
AG7500-80
80
±2.8
600–720
AG7500-85
85
±2.1
600–720
AG7500-88
88
±1.8
600–720
of fillers rather limited. More than a decade ago,
when the automotive industry considered the option
of increasing the voltage in vehicles from 12 V to
24–42 V, some work was carried out to develop resistive
silver pastes (to maintain the same power across the
heating window) which involved thrifting silver pastes
with base metals and their alloys (7). However, the
industry abandoned the switch to high-voltage sources
and no progress has been made in the area of thrifted
automotive silver pastes since. Furthermore, in the
past 10 years, the price of silver has decreased from
its peak price of US$48.5 per troy ounce in April 2011
to US$17.9 per troy ounce in February 2017 (Figure 1)
which has been a disincentive to the thrifting approach.
However, there is still scope to further decrease the
cost per kilogram of paste if a suitable replacement
for some or all of the silver in automotive paste is
found. This article provides an overview of the effects
of reducing the silver content in 80% automotive silver
paste using fillers which includes selected base metals,
silver-coated base metals and base metal alloys.
Experimental
Silver pastes and thrifted silver pastes were fabricated
with 80% metal loading by triple roll milling appropriate
quantities of metal component, glass frit, additives
and organic medium to achieve a viscosity suitable for
screen printing. For the sake of comparison, the same
bismuth(III) oxide (Bi2O3)-based glass frit was used in
the present study the loading of which was typically
2–5%. Printed glass tiles were fired in a pre-heated
fast-fire roller kiln, set to the target temperature, in
air. Electrical resistivity of the tracks was measured
by the Van der Pauw four-point probe method (8) on
meander circuits. Colour measurements were carried
out in a Datacolor spectrophotometer after calibrating
using standard black trap and white tile. Temperature-
45
40
US Dollars, $
35
30
25
20
15
10
5
0
2005
2000
2005
2010
2010
2015
2015
Fig. 1. Variation of silver price (US$ per troy ounce) between years 2000–2017 (© Kitco Metals Inc)
157
© 2017 Johnson Matthey
Johnson Matthey Technol. Rev., 2017, 61, (2)
http://dx.doi.org/10.1595/205651317X695271
programmed oxidation (TPO) studies were carried out
in an Altamira AMI-200 instrument by heating ~200 mg
of the powder to 600ºC in 10% O2/He gas.
Thrifting with Base Metals
A simple approach to thrifting silver is to replace
with cheaper base metals. The closest conductivity
match for silver (6.30 × 107 S m–1 at 20ºC) is copper
(5.96 × 107 S m–1 at 20ºC) (9). However, copper suffers
from poor oxidation resistance, with onset of oxidation
around 140ºC, making it an unlikely candidate for firing
applications which involve temperatures in excess of
600ºC. Other metals with a conductivity match close
to silver are aluminium (3.50 × 107 S m–1 at 20ºC) and
zinc (1.69 × 107 S m–1 at 20ºC). Zinc oxidises on firing
to form zinc oxide (ZnO) whereas aluminium tends to
either oxidise or alloy with silver (10), depending on the
loading; all of which significantly increases the resistivity
of the silver track. Although nickel is one of many
carcinogenic metals known to be an environmental
and occupational pollutant which requires extreme
care during handling (11), it is a promising metal filler
with conductivity of 1.43 × 107 S m–1 at 20ºC and
resists oxidation on firing. In order to test the effect of
adding nickel filler to the silver paste, screen printable
pastes were prepared by triple-roll mixing a mixture of
appropriate quantities of silver, nickel (average particle
size 10 µm, 8.4–32.4 wt% loading), glass frit, additives
and organic medium. The paste was screen printed
Resistivity, ohm cm
1.2 × 10–5
8.4 wt% Ni
Pure Ag
16.3 wt% Ni
onto float glass substrates, dried around 100ºC and
subsequently fired in a roller kiln.
Figure 2 shows the variation of resistivity of 80%
nickel-thrifted silver tracks fired at temperatures
between 620–680ºC in a fast-fire roller kiln. For
comparison, resistivities of 80% Ag tracks fired at
the same temperature are also plotted alongside.
The resistivity of a pure silver track decreases with
increasing firing temperature. For example, the
resistivity of a silver track fired at 620ºC was 2.6 µΩ cm
which dropped to 1.9 µΩ cm after firing at 680ºC, which
is attributed to sintering of silver particles. As the nickel
concentration in a silver track is increased, the resistivity
of the track increases. The variation in resistivity (ΔR)
with firing temperature between 620–680ºC is between
3.8–13.7%, in the studied composition range. ΔR
increases initially with increasing nickel concentration
and then drops to lower values.
Apart from conductivity, addition of fillers will also
have an impact on properties such as acid resistance
and reverse colour. The acid resistance test involves
evaluating the chemical resistance of silver tracks to
withstand acid attack. This was carried out in 0.1 N
sulfuric acid at 80ºC followed by a tape test; the latter
involving testing the adhesion of the tracks after the acid
test using sticky tape. Acid resistances of silver pastes
supplied by paste manufacturers vary significantly but
most pastes pass a 2–4 h acid test. Acid resistance
tests were carried out on silver tracks thrifted with
8 wt% nickel filler fired at 660ºC. The resistances of
24.2 wt% Ni
32.4 wt% Ni
8.0 × 10–6
4.0 × 10–6
0.0
620
640
660
Firing temperature, ºC
680
Fig. 2. Variation of resistivity of 80% silver paste and silver paste thrifted with nickel filler (various loadings) with firing
temperature
158
© 2017 Johnson Matthey
Johnson Matthey Technol. Rev., 2017, 61, (2)
http://dx.doi.org/10.1595/205651317X695271
the thrifted silver tracks were measured before and
after the acid test as well as after tape tests and are
summarised in Table II.
Acid resistance of nickel-thrifted silver tracks was
poor with all tracks falling apart after the tape test. This
is expected in the light of the fact that base metals
vigorously react with acid which opens up channels for
the acid to further penetrate into the track and attack
the glass frit which binds the metal particles to the glass
substrate, ultimately leading to delamination of the track
during the tape test. Due to poor acid resistance, base
metals are unlikely candidates for fillers in automotive
silver pastes.
conductivity and metal loading and may also offer
better acid resistance due to the silver shell protecting
the base metal core.
Investigations on the use of silver-coated copper
powders as fillers for automotive silver pastes suggest
that these Ag@Cu powders suffer the same fate as
their pure copper counterparts (12). TPO studies
carried out on two silver-coated copper powders with
different silver loadings (10 wt% and 28 wt% silver) in
10% O2/He ambient pointed that both samples oxidise
on heating, irrespective of the silver loading, with two
distinct oxidation steps involved (Figure 3). Increased
silver loading on copper just increases the oxidation
onset temperature. The onset of oxidation occurs
between 200–300ºC, which can be assigned to the
formation of copper(I) oxide (Cu2O) which on further
heating oxidises to copper(II) oxide (CuO) between
375–425ºC. The silver coating on copper delays the
oxidation on heating by about 100ºC however does
not prevent it. This can be explained on the basis of
Thrifting with Silver Coated Fillers
A straightforward strategy to reduce the silver loading
in silver paste is to use silver coated fillers such as
silver coated copper and silver coated nickel, which
in principle should meet the fine balance between
Table II Resistance of Silver Tracks Containing 8 wt% Ni Filler Before and After Acid Test and also After
Tape Test
Acid test duration, hour
Resistance, ohm
Before acid test
After acid test
After tape test
0
6.8 × 10–2
–
6.8 × 10–2
1
6.3 × 10–2
6.2 × 10–2
Damaged
2
6.8 × 10–2
6.8 × 10–2
Damaged
4
6.3 × 10–2
6.1 × 10–2
Damaged
Thermal conductivity detector signal
5
0
–5
–10
Ag@Cu (10% Ag)
–15
Ag@Cu (28% Ag)
–20
–25
0
100
200
300
400
Temperature, ºC
500
600
700
Fig. 3. TPO profiles of 10Ag@Cu and 28Ag@Cu powders in 10% O2/He
159
© 2017 Johnson Matthey
Johnson Matthey Technol. Rev., 2017, 61, (2)
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non-uniform coverage of silver on copper and also
due to the presence of pin holes in the silver shell
which allows the penetration of oxygen, eventually
resulting in the oxidation of copper (13). Oxidation of
silver-coated copper powders at temperatures above
200ºC makes them unsuitable for firing applications
such as automotive pastes.
Silver-coated nickel (Ag@Ni) powders are also used
extensively as filler material in inks and are claimed to
have conductivity and chemical stability approaching
pure silver (14). Hence, silver-coated nickel powder
(40% Ag by weight) was tested as a filler for 80% silver
paste. X-Ray diffraction (XRD) of 680ºC fired silver
prints containing Ag@Ni filler showed no evidence
of nickel oxidation. Figure 4 shows the variation of
resistivity as a function of firing temperature and
Resistivity, ohm cm
8 × 10–6
Figure 5 shows the variation of resistivity as function of
Ag@Ni loading in the silver paste. For comparison, the
resistivity of 80% silver paste is also plotted alongside.
Resistivity of pure silver paste decreases with
increasing firing temperature. Resistivity of silver tracks
containing Ag@Ni also show similar behaviour, albeit
linked to Ag@Ni loading (Figure 4). For example, at
8% Ag@Ni loading, the resistivity of a track fired at
620ºC was 2.9 µΩ cm which dropped to 2.2 µΩ cm
after firing at 680ºC. However, at 32% Ag@Ni
loading, the change of resistivity with increased firing
temperature was almost negligible, remaining around
5 µΩ cm between 620 – 680ºC. The resistivity of the
Ag@Ni containing silver paste increases with
increasing Ag@Ni loading, which is more pronounced
at higher firing temperatures. Although the resistivity
8 wt% Ag@Ni
16 wt% Ag@Ni
32 wt% Ag@Ni
Pure Ag
24 wt% Ag@Ni
4 × 10–6
0
620
640
660
Firing temperature, ºC
680
Fig. 4. Variation of resistivity of 80% silver paste and silver paste thrifted with silver coated nickel, Ag@Ni, filler (various
loadings) with firing temperature
8 × 10–6
Resistivity, ohm cm
620ºC
640ºC
660ºC
680ºC
4 × 10–6
0
8 wt%
Ag@Ni
16 wt%
Ag@Ni
24 wt%
Ag@Ni
32 wt%
Ag@Ni
Pure Ag
Fig. 5. Variation of resistivity of thrifted 80% silver paste with Ag@Ni loading
160
© 2017 Johnson Matthey
Johnson Matthey Technol. Rev., 2017, 61, (2)
http://dx.doi.org/10.1595/205651317X695271
increases with increasing Ag@Ni loading, the latter
promotes stable resistivity values over the typical
firing range which is desired in automotive windscreen
manufacturing.
Compatibility of thrifted silver pastes with automotive
black enamel is critical as they are overprinted onto an
enamel layer prior to firing and shaping of windscreen
glass. Black enamels are made by pasting a mixture
of black pigment and low melting glass (usually in the
ratio of 1:4) and applied on to windscreens by either
screen printing or inkjet printing techniques. Black
pigments are usually derived from either copper
chromite spinel, chrome iron nickel spinel or iron cobalt
chromite spinel (15). The main purpose of the enamel
layer is to protect the glue that holds the windscreen
in place from degradation by ultraviolet (UV) light and
also to hide the electrical connections. For enamel
compatibility studies, thrifted silver pastes were printed
onto the pre-printed and dried black enamel and fastfired at 600ºC, 620ºC, 640ºC, 660ºC and 680ºC.
The difference between two colour samples is often
expressed as ΔE, which is the difference between
the L*, a* and b* values of the reference and sample.
Therefore, ΔE displays the difference between two
samples as a single value for both colour and lightness.
A detailed explanation of coordinates from Commission
Internationale de l’Eclairage (CIE) Lab Colour model
L*, a* and b* are explained elsewhere (16). Reverse
colour values L*, a* and b* were measured on black
enamel and black enamel under the silver print from
which ΔE values were calculated using the formula
(Equation (i)):
ΔE =
(L2 – L1)2 + (a2 – a1)2 + (b2 – b1)2
3
(i)
Table III shows the results of compatibility testing of
8 wt% Ag@Ni-containing silver track on a hiding black
enamel 1T3015 manufactured by Johnson Matthey.
The reverse colour data suggest that the Ag@Ni
thrifted silver paste is compatible with 1T3015 enamel.
Both ΔL and ΔE values are lowest around 660ºC firing,
which is similar to that observed for the pure silver
paste. Although there is no general agreement on an
ideal value; ΔE value of 2.3 has been proposed (17) as
just noticeable difference (‘JND’), nevertheless, ΔE of
0.5 or less is preferred by windscreen manufacturers.
Acid resistance tests were carried out on 660ºC fired,
8% Ag@Ni loaded silver tracks. Figure 6 shows the
state of 8% Ag@Ni loaded silver tracks after 1 h, 2 h
and 4 h acid test, respectively.
The resistance of the tracks was measured before
and after acid tests as well as after tape tests
(Table IV). Acid attack was evident in all tracks but 8 wt%
Ag@Ni thrifted silver track passed the 1 h acid and tape
test. This is an improvement compared to pure nickel
fillers where delamination was observed in all samples
exposed to acid.
Table III L*, a* and b* Coordinates of 8 wt% Ag@Ni Containing Silver Track Printed over 1T3015
Hiding Enamel
Firing temperature, ºC
Sample
600
Black enamel
600
Black enamel under silver
620
Black enamel
620
Black enamel under silver
640
a*
b*
∆E
9.2
0.33
0.62
5.9
19.4
0.09
0.12
6.4
0.49
0.58
15.1
0.19
0.39
Black enamel
5.4
0.61
0.7
640
Black enamel under silver
8.6
0.33
0.49
660
Black enamel
5.5
0.63
0.59
660
Black enamel under silver
5.8
0.53
0.6
680
Black enamel
6.3
0.58
0.29
680
Black enamel under silver
5.4
0.61
0.63
161
L*
5.0
1.8
0.2
0.5
© 2017 Johnson Matthey
Johnson Matthey Technol. Rev., 2017, 61, (2)
http://dx.doi.org/10.1595/205651317X695271
(a)
(b)
a small amount of brass alloy had been oxidised
during fast-firing at 680ºC. Typical resistivity of an
80% silver paste thrifted with 32 wt% brass filler and
fired at 680ºC was 12 µW cm. However, the variation
in resistivity ΔR with increasing firing temperature
(620–680ºC) and increasing filler loading was >15%
which could be related to the oxidation of brass filler,
as evidenced from XRD. Base metal alloys that suffer
from oxidation during firing will negatively impact the
stability and compatibility of the silver paste with the
other automotive glass components.
Even alloys that do not exhibit oxidation behaviour may
still interact with either silver or glass components of
the paste during firing, leading to undesired properties.
XRD of silver tracks containing stainless steel (type
430-L) filler did not show evidence of oxidation on firing.
Resistivity of 80% silver paste thrifted with 8–32 wt%
stainless steel filler and fired at 680ºC varied between
4–16 µW cm. ΔR with increasing firing temperature
(620–680ºC) was >15%. However, migration of silver
into glass, commonly known as silver bleeding, was
observed in fired silver tracks containing 430-L filler.
Figure 8 shows the cross-sectional electron probe
(c)
1 cm
Fig. 6. Silver tracks containing 8% Ag@Ni on glass after
acid test: (a) 1 h; (b) 2 h and (c) 4 h
Thrifting with Base Metal Alloys
Suitable cost-effective base metal alloys can also
be used as fillers for silver paste as long as they are
stable at high temperatures and have conductivity in
the target range. Alloy powders including brass and
stainless steel were tested in silver pastes. The XRD
pattern of silver track containing 32 wt% brass filler
(70:30 Cu:Zn), fired at 680ºC is shown in Figure 7.
Major phases are silver and brass alloy along with
minor amounts of CuO and ZnO, indicating that
Table IV Resistance of Silver Tracks Containing 8 wt% Ag@Ni Filler Before and After Acid Test and also
After Tape Test
Resistance, ohm
Acid test duration, hour
Before acid test
After acid test
After tape test
–
4.63 × 10–2
4.89 × 10–2
4.74 × 10–2
4.75 × 10–2
2
5.69 × 10–2
5.53 × 10–2
Damaged
4
4.51 × 10–2
4.56 × 10–2
Damaged
0
4.63 × 10
1
–2
25000
PDF 04-001-2617 Ag
PDF 04-004-8062 Cu0.76Zn0.24
PDF 01-089-7102 ZnO
PDF 00-048-1548 CuO
Counts
20000
15000
10000
5000
0
40
50
60
70
80
2θ, degree
90
100
110
120
Fig. 7. XRD pattern of silver track thrifted with 32 wt% brass filler, fired at 680ºC
162
© 2017 Johnson Matthey
Johnson Matthey Technol. Rev., 2017, 61, (2)
http://dx.doi.org/10.1595/205651317X695271
Ag level
Cr level
849
416
566
277
283
138
50 µm
Ag
Fe level
802
687
573
458
343
229
114
0
0
0
Average: 70
Average: 85
Cr
Si level 917
555
1132
50 µm
Element mix
1414
Average: 213
Si
50 µm
SL level 2040
CrAgFe 1.18
AgFe 1.20
CrFe 10.51
CrAg 0.02
Fe 0.26
Ag 17.43
Cr 0.01
1060
707
353
1785
1530
1275
1020
765
510
255
0
0
Average: 148
Fe
50 µm
CrAgFe
Average: 278
50 µm
SL
50 µm
Fig. 8. EPMA cross-sectional element map of silver track thrifted with 24 wt% stainless steel filler, fired at 640ºC
microanalysis (EPMA) image of silver track containing
24 wt% stainless steel powder fired at 640ºC.
Both silver and stainless steel particles are
homogeneously distributed in the track. The silver
track is also less dense even after 640ºC firing which
could be due to poor packing of silver and stainless
steel particles; d90 of the latter being 25 µm. Glass
frit distribution as seen from the silicon map is high
at the interface between the glass substrate and the
silver track which is expected due to glass melting and
flowing under gravity. Overlap element maps point
out that there is no evidence of detectable interaction
between silver and stainless steel. On the other hand,
silver bleeding into the glass substrate is evident.
Silver bleeding occurs when silver ions are exchanged
for sodium ions in glass (18), probably promoted by
interaction with the filler. Stainless steel thrifted prints
also showed poor acid resistance.
163
Summary
Attempts were made to reduce the silver content of
the automotive silver paste by thrifting with selected
base metals, base metal alloys and silver-coated
base metals. Inevitably, the conductivity decreases
with increasing filler loading which can be fine-tuned
through the choice of fillers depending on the
requirement. However, most fillers tested in this study
show poor acid resistance which limits their use in
automotive applications. The choice of the glass frit
plays a significant part in improving the acid resistance
which requires further research to fine tune the glass frit
composition to the type of filler used, if the concept of
thrifted silver pastes were to be a commercial success.
Further considerations need to be made on the use of
base metal fillers especially when the benefits of using
them does not outweigh the cost.
© 2017 Johnson Matthey
http://dx.doi.org/10.1595/205651317X695271
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The Authors
Edwin S. Raj is a Principal Scientist in the Catalyst & Materials Group at Johnson Matthey Technology
Centre, Sonning Common, UK. His current research activities are focused on conductive pastes
for electronic, automotive and photovoltaic applications as well as investigating pigments for glass
enamels.
Jonathan Booth is a Research Manager at Johnson Matthey Technology Centre. He has 29 years’
experience in applied materials research. He specialises in glass enamels and high-temperature
conductive pastes for both automotive and photovoltaic applications. Jonathan has a particular
interest in the relationship between glass structure and its macro properties.
164
© 2017 Johnson Matthey