Materials Transactions, Vol. 48, No. 3 (2007) pp. 594 to 599
#2007 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Pb-Free Silver Conductive Paste with High Reliability
Kazutaka Nakayama1; * , Atsushi Nagai1 and Noritaka Iida2
1
2
Development & Engineering Headquarters, Noritake, Co., Limited., Nagoya 451-8501, Japan
Technical Department, Noritake Kizai Co., Ltd., Nishikamo-gun, Aichi 470-0293, Japan
We have been engaged in developmental activities aimed at improving the properties of Ag conductive paste which can be used for various
kinds of electronic components including circuit boards and both chip and high frequency components. By applying the uniform ceramic coating
technology to Ag powder, we produced a variety of options to achieving the goal of lead elimination. The properties, namely solder leaching,
adhesives strength (aging, heat cycles, strength in high heat and humidity) and migration, have successfully been improved to a level which is the
same or higher than those of Ag/Pd (80/20) and Ag/Pt (99/1), which are currently used for conductive paste material. In this study, we
succeeded in eliminating not only lead, but also frit, which, as a result, paves the way for the application of Ag conductive paste to highfrequency components, LTCC (Low Temperature Co-fired Ceramic) terminations and surface electrodes.
[doi:10.2320/matertrans.48.594]
(Received October 11, 2006; Accepted January 9, 2007; Published February 25, 2007)
Keywords: coating, powder, lead-free, silver paste, solder
1.
Introduction
With continual advancements in technology, electronics
such as cellular phones and personal computers are becoming
smaller, thinner and lighter, and consequently the demand for
rapid development of the inner electronic components
requires further downsizing, lower costs and higher frequency.1–4) In addition, as the demand for lead elimination has
increased due to environmental concerns, this task, as well as
the improvement of the properties of various electrode
materials from solder to plating materials, has become an
urgent matter.5) From the beginning, we focused on multilayered electronic components, and studied the possibility of
the co-firing of ceramic and electrode. One critical challenge
we faced during the study was the structural defect caused by
a mismatch between ceramic and electrode shrinkage. Since
controlling this defect requires the integration sintering of
ceramic and electrode, we examined various ways to improve
the heat resistance of the metal powder as a potential
countermeasure. First, we tried to increase the heat resistance
by enhancing the crystallinity of the metal powder.6) As a
result, since the film of the internal electrodes of the multilayered ceramic capacitor is as thin as 1–2 mm, the structural
defect could be controlled by adding fine ceramic powders
which improves crystallinity and controls firing shrinkage.
However, the same method could not be used to control the
structural defect of such thick films as those of highfrequency components and heaters (5 mm or thicker), and we
concluded that the heat resistance needed to be further
improved. Therefore, we examined various uniform coating
methods to the metal powder.7,8) As a result, the size of the
particles in oxide and hydroxide sols were too large (50 nm
range) and therefore the processing of defect-free coatings
were difficult so far. Hence, the application of metal chelates
that were able to produce very small and uniform metallic
particles by controlling the heat-treatment conditions (atmosphere etc.) was considered. As a result, the thermal stability
of the electrode was improved considerably and the possi*Corresponding
author, E-mail: knakayama@n.noritake.co.jp
bility of its application in co-fired multi-layered external
electrode in addition to its application in internal electrode
has been enhanced. Conventionally, the inorganic materials
of the external electrodes and terminations of electronic
components have been composed of such adhesive ingredients as various kinds of metal powder and glass frit
containing lead.9) For electrodes that undergo direct soldering, Ag/Pd (e.g. 80/20) or Ag/Pt (e.g. 99/1) rather than Ag
conductor has been used to avoid solder leaching.9) However,
Ag/Pd and Ag/Pt are no longer able to meet the escalating
demands for higher frequency and lower costs. The present
authors paid attention to giving characteristics (solder
leaching, adhesive strength and migration10)) equivalent to
Ag/Pd (e.g. 80/20) and Ag/Pt (e.g. 99/1) by making the best
use of the characteristic of single Ag (low resistance etc.) and
did various examinations with this purpose. As targets of
development, the achievement of adhesive strength characteristics without the addition of glass frits for binding the
coating to the substrate and the direct of application of the
new material as a solder were considered. It was expected
that the possibility of elimination of lead would be achieved
as a result. The bonding between the ceramic in the ceramic
coating and the oxide binder was expected to improve the
adhesive strength characteristics as well as the solder
leaching characteristics when used as solder. Results are
reported here.
2.
Experimental
Figure 1 shows the developed coating powder process.
Table 1 shows the test paste contents. We created samples
for evaluation by making patterns through screen-printing
various kinds of test paste. The substrate used for the
evaluation samples was made of alumina, and 40 mm by
30 mm by 0.63 mm in size. We used a #200 stainless steel
mesh screen as our form plate. The printing thickness of the
pattern was 20–25 mm and it was dried for 5–10 min at 110–
120 C. Firing was scheduled to be conducted in a belt
furnace at 850 C with 60 min of in-out and 10 min of topkeep. Firing thickness was to be 12–15 mm. For solder
Pb-Free Silver Conductive Paste with High Reliability
[ Conventional coating method ]
·Metal hydride
Metal powder
·Oxide sol
Sn-covered copper wire,
∅0.6 in diameter
[ Developed coating method ]
Metal powder
Metal chelate
compound solution
Coating
Coating and solvent drying
Drying
Heat treatment in inert gas
Coating powder
Coating powder
Solder
Fig. 2 Measurement sample figure.
Table 1 Test sample contents.
Percentage of
metal content
[mass%]
80 85
Ag/Pd
Ag/Pt Newly developed
(¼ 80=20) (¼ 99=1)
lead-free Ag
80 85
Additive to give Metallic oxide
adhesiveness
Plus glass frit
Viscosity [Pas]
80 85
Silver
Electrode
Al2O3
substrate
Fig. 1 Developed coating powder process.
Conventional
Ag
595
85
Metallic oxide
250
ization of 20 V direct current up to 1000 h after applying
overcoat glass to electrodes. For printing and firing line
shape, we conducted experiments with the printing shape of
L/S: 100/100 mm and with the firing condition of L/S: 50/
50 mm. We also conducted an SEM observation for the
surface of each electrode with the magnification of 1,000
times.
3.
Results and Discussion
Coating powder
wetting (230 C for 3 s) and solder leaching tests (first
condition: 260 C for 10 s, second condition: 280 C for 10 s),
tests were conducted by immersing the samples in the solder
(M705 (Sn-3.5%Ag-0.5%Cu), Senju Metal) vat under given
conditions after firing.
For an investigation of adhesives strength, we measured
pull-out strength under various conditions after conducting
the preliminary soldering of the samples for evaluation at
230 C for 3 s as shown in Fig. 2, and then Sn- covered copper
wire of 0:6’ was soldered on the pattern areas of 2 2 mm
by solder iron. For aging strength, heat-cycle strength and
heat/humidity resistance, deteriorations in strength were
measured up to 1000 h under 150 C, up to 1000 cycles under
40 C to 150 C, and up to 1000 h under 85 C/85%RH,
respectively. For migration measurement, we measured
insulation performance under 85 C/85%RH with the energ-
Uncoated
Figure 3 shows the photo images of FE-SEM, each of
which focuses on one particle from one coating type. As
shown clearly by these images, it was confirmed that the
newly developed coating powder could provide a more
desirable uniform coating condition than the conventional
coating powder. We could obtain uniform ceramic coating
because we have used coating solutions containing chelated
metals instead of the usual oxide or hydroxide sol. This
helped to produce a uniform coating made of very small
particles. Heat treatment of the film for removing the organic
content and for stabilizing the particles has been performed in
inert gas atmosphere rather than in air, so that the growth of
the particles would be inhibited, resulting in a very uniform
thin layer.
Figure 4 shows the measured shrinkage curve of the paste
of each powder used for Fig. 3. These results clearly show
that the shrinkage behavior of the powder to which uniform
coating had been applied was close to that of ceramic, and it
Conventional coating
Fig. 3 FE-SEM of each particle.
Newly developed coating
596
K. Nakayama, A. Nagai and N. Iida
5
Newly
developed
powder
Shrinkage, %
0
-5
Ceramic
-10
Conventional
powder
-15
-20
-25
0
250
500
750
1000
Temperature, T / °C
Fig. 4 Shrinkage curves of ceramic and each electrode.
was confirmed that the uniform-coated powder had better
heat resistance.
Figure 5 shows the actual example of the solder leaching
of each test paste type. It is clear that the newly developed
lead-free Ag conductor is superior to the conventional Ag
conductor in terms of solder leaching. It was also confirmed
Wetting
(at 230°C for 3 s)
that the new lead-free conductor is equal to or slightly
superior to the electrodes of Ag/Pd and Ag/Pt. The solder
leaching behavior of the newly developed lead-free Ag
electrode was excellent compared to the conventional Ag
electrodes. It is believed that the ceramic contained in the
ceramic coated Ag coating material helped to suppress the
migration of silver to the solder material.
Figure 6 shows deterioration with age in strength at 150 C.
The newly developed lead-free Ag conductor had minimum
deterioration in strength, compared with Ag/Pt. Furthermore,
it demonstrated the best performance in terms of deterioration in strength at 40 C to 150 C as shown in Fig. 7 and
in terms of strength performance at 85 C/85%RH as shown
in Fig. 8, compared with Ag/Pt, as in the case of deterioration with age in strength at 150 C as shown in Fig. 6. First
of all, it is clear that the inclusion of ceramic coating in the
Ag powder helped to improve the properties of the powder in
comparison to the usual Ag powder. Further consideration
was done based on following.
Figure 9 shows the differences in strength performance
due to differences in metallic oxide additives. It is clear that
the additives of Bi2 O3 or Bi2 O3 /CuO to bond the layer to the
alumina substrate has helped to improve the adhesive
Solder
leaching
Solder
leaching
(at 260°C for 10 s)
(at 280°C for 10 s)
Newly
developed
lead-free
Ag
Ag/Pd
Ag/Pt
Fig. 5 Wetting and leaching test for lead-free solder.
20
Newly developed Ag
15
Ag / Pt
10
5
0
0
500
1000
1500
Aging time, t1 / h
Fig. 6
Pull-out Strength, τ p / N·mm-2
Pull-out Strength, τ p / N·mm-2
Pb-Free Silver Conductive Paste with High Reliability
Bi 2O3 : CuO = 2 : 1
50
30
20
CuO
0
0
100
200
Endurance time, t 3 / h
Newly developed lead-free Ag
Changes in strength with age due to differences in additives.
10
5
Ag / Pt
0
Shrinkage (%)
Pull-out Strength, τ p / N·mm-2
Bi 2O3
10
Fig. 9
0
500
1000
Fig. 7
-5
-10
-15
Newly developed Ag
-20
Ag/Pd(80/20)
Ag/Pt(99/1)
-25
1500
Number of cycles
Conventional Ag
-30
0
200
Deterioration in strength at 40 C to 150 C.
400
600
800
1000
Temperature, T / °C
Fig. 10 Shrinkage behavior of each electrode due to firing.
20
15
10
Newly developed lead-free Ag
5
Ag / Pt
0
0
500
1000
1500
Exposure time, t2 / h
Fig. 8 Strength performance 85 C-85%RH.
strength further. Although conventional Ag or Ag/Pt also
contain these bonding materials, it is believed that the use of
the newly developed ceramic coated Ag as well as the
elimination of the glass frits helped to improve the adhesive
strength of the layer to the substrate as well as helped to
control the deterioration of strength in the present case.
Figure 10 compares the shrinkage curves of different
electrodes. The shrinkage curve of the newly developed Ag
electrode shifted to the high temperature side compared with
the conventional Ag electrode. This should be due to the
presence of the ceramic coated Ag powder, and shows that
sintering is delayed in the new powder. It is thought that the
improvement of strength by the reaction with the alumina
substrate has improved in the newly developed material
because it is easy for the Bi2 O3 and CuO, which gives
Log (Insulation Resistance, R / Ω )
Pull-out Strength, τ p / N·mm -2
Bi 2O3 : CuO = 4 : 1
40
Deterioration in strength due to aging at 150 C.
18
16
14
12
10
8
6
4
2
0
597
16
15
14
13
12
Newly developed lead-free Ag
Ag/Pd(=80/20)
11
10
9
0
500
1000
1500
Endurance Time, t4 / h
Fig. 11 Changes in insulation resistance at 85 C-85%RH with 20 V
impression.
adhesion with the alumina substrate to Ag, to infiltrate the
glass free interface between Ag and the alumina substrate.
Although Bi2 O3 gave good results even when it is present as
single, CuO failed to show any effect when present alone.
This should relate to the melting point and the sintering
temperature of the respective oxide, and it is thought that the
mixture system brought the synergy effect of the CuO-Bi2 O3
compound.
Figure 11 shows the measurement result of migration
performance. As this figure shows, the newly developed leadfree Ag conductor is considered to have presented no
598
K. Nakayama, A. Nagai and N. Iida
Newly developed
lead-free Ag
Ag / Pt
L / S = 100 / 100
dried film
L / S = 50 / 50
fired film
Fig. 12 Lines and density after printing/firing.
Conventional Ag
Newly developed
lead-free Ag
Ag / Pt
Ag / Pd
Fig. 13 SEM images of electrode surfaces (fired at 850 C).
problem in migration under the given conditions, compared
with Ag/Pd. No difference in sheet resistance was observed
between the newly developed lead-free Ag conductor and
conventional Ag.
Figure 12 shows lines after the printing and firing of each
electrode paste. The newly developed lead-free Ag realized
sharpness in print lines. It was confirmed that it demonstrated
a better performance both in 50 mm line shape and density
Pb-Free Silver Conductive Paste with High Reliability
than Ag/Pt. The dense structure of the electrode layer was
able to be confirmed from this figure. This should have an
influence on the strength characteristics, and therefore the
dense structure of the electrode layer is thought to be a major
factor.
Figure 13 shows the SEM images of electrode surfaces
fired at 850 C. It was confirmed that Ag/Pd and Ag/Pt had
quite a few pores on the surface while the conventional Ag
had an additive appearing on the surface. In contrast, the
newly developed Ag was smooth with few pores on the
surface. The formation of defect-less, dense and uniform film
as in Fig. 13 together with, the excellent solder leaching
properties of the newly developed material in comparison to
the usual materials as in Fig. 5, indicate that the following 2
points could be important. Firstly, the ceramic contained in
the newly developed ceramic coated Ag, could have helped
to reduce the migration of silver to the solder. Secondly, the
uniformity of the films (as in Fig. 13) would have helped to
reduce the migration of Ag to the solder.
4.
Summary
The combination of ceramic-coated Ag powder with the
metallic oxide additive allowed superior adhesiveness for
alumina boards. Furthermore, by providing excellent solder
leaching characteristics, Ag with characteristics comparable
to usual Ag/Pt or Ag/Pd have been developed. In this way,
the simultaneous elimination of glass frits as well as lead
could be achieved.
599
The Ag conductor developed this time demonstrated a
performance capable of substituting Ag/Pd and Ag/Pt and is
considered able to be applied to not only the electrodes of
circuit boards and various other electronic components, but
also to the terminations and surface electrodes of such highfrequency components as LTCC.
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