Ta Capacitors with Conductive Polymer Robust to Lead Free

Ta CAPACITORS WITH
CONDUCTIVE POLYMER ROBUST
TO LEAD FREE PROCESS
A B S T R A C T :
Tantalum capacitors with conductive polymer
cathodes have found a place in the market as a low
ESR component with reduced ignition. Conductive
polymer cathodes however, suffer from instability
during multiple high temperature thermal treatments
such as lead free soldering due to its limited self
healing ability and lower thermal and mechanical
strength compared with manganese dioxide. This
makes capacitors with conductive polymer cathode
more sensitive to thermo-mechanical stresses, which
appear during soldering and to negative influences of
storage condition after removal from the protective
packaging prior to soldering. This paper describes
methods of improving the robustness of such
capacitors with lead free processes without a negative
impact on ESR. Presented procedures enhance the
stability and reliability of Tantalum capacitors with
conductive polymer cathodes making them
comparable to capacitors containing a manganese
dioxide counter electrode system.
Z. Sita
AVX Czech Republic s.r.o.,
Dvorakova 328, 563 01 Lanskroun,
Czech Republic
Phone: +420 465 358 626
Fax: +420 465 358 229
E-mail: [email protected]
M. Biler
AVX Czech Republic s.r.o.,
Dvorakova 328, 563 01 Lanskroun,
Czech Republic
Phone: +420 465 358 626
Fax: +420 465 358 229
E-mail: [email protected]
TA CAPACITORS WITH CONDUCTIVE POLYMER
ROBUST TO LEAD FREE PROCESS
Z. Sita
AVX Czech Republic s.r.o., Dvorakova 328, 563 01
Lanskroun, Czech Republic
Phone: +420 465 358 626
Fax: +420 465 358 229
E-mail: [email protected]
Introduction
L425 Leadfree LF A
(°C)
Lead-free technologies are being increasingly adopted
across all of the industry. Both legislation and customer
demands are pushing the producers to replace lead in
their products and to provide compatibility of parts to
lead-free soldering processes. The robustness of
components to these new conditions is one of the key
focuses for AVX. All new product developments are
considered for both their environmental impact and
their stability under lead-free reflow conditions.
Specifically conductive polymer product, where AVX
has engaged in over 8 year’s development activity, is
taken with special care. The goal of this development
was to introduce onto the market a reliable product
which meets the lead-free requirements and maintains
the high reliability level of AVX components.
Lead Free Process
The reflow profile used in lead-free technology is driven
first by the solder melting point, which is typically 20°C
higher than for conventional SnPb solder. In more
complicated assembly systems, where dimensionally
large components are soldered next to small ones, the
temperature of soldering is designed so that the solder
at the larger component is properly heated. In such
cases the temperature of the small components can
easily reach or even exceed 260°C and it leads many
customers to excessive lead-free profile specifications.
A typical current customer lead-free profile specification
is described in Table 1 and Figure 1.
- peak temp. max
- time at 260°C
- time at >230°C:
- max heat up gradient:
M. Biler
AVX Czech Republic s.r.o., Dvorakova 328, 563 01
Lanskroun, Czech Republic
Phone: +420 465 358 626
Fax: +420 465 358 229
E-mail: [email protected]
255±5°C
10 sec.
60s
2.5°C /s
Table 1: Specification of lead-free reflow profile
250.0
200.0
150.0
100.0
50.0
0:30.0
50
1:00.0
1:30.0
100
2:00.0
150
2:30.0
200
3:00.0 (MMM.SS.T)
250cm
Figure 1: Lead-free profile
Lead-free processes are a threat for many component
technologies. Lead-free solder systems will require a
peak reflow temperature up to 260°C. Component
manufacturers have to review all design parameters
necessary to enable the product to cope with the higher
thermal stress resulting from these soldering processes.
Replacement of some materials and development of low
stress alternatives will be required. Components have
problems with cracking, material decomposition, popcorning, delaminating and other mechanisms caused by
high temperature or temperature gradients during
heating and cooling. Teams of experts around the world
are intensively working on new materials with high
temperature stability, high glass transition
temperatures, systems with minimal temperature
mismatches and adhesives with improved bonding.
Capacitor technologies are fighting with the lead-free
process requirements with varying results. Whereas
polymeric film capacitors, wet aluminum capacitors or
large high CV ceramic capacitors suffer from high
temperature stress and often do not meet the industry
needs, Tantalum and niobium oxide technology with
manganese dioxide cathodes has been successfully
developed to survive the lead-free process [1]. The new
technology with a conductive polymer electrode is more
sensitive, however, and suppliers of Tantalum capacitors
with conductive polymer cope with this task with
varying degrees of success.
Tantalum capacitors with conductive polymer are
required to withstand up to three exposures to the
reflow profile, which is a more stringent specification
than is often required of manganese dioxide capacitors.
A quick search in specifications of maximum reflow
temperature among five suppliers (Table 2) from end of
year 2004 shows that the recommended profiles are
lagging the industry need.
Supplier
Peak Temp
Time at peak
°C
sec
A
240
10
B
240
5
C
240
10
D
245
10
E
260
0
Reflow Stability of Commercial
Products
To prove the specifications and to search the real
situation on the market, Tantalum capacitors with
conductive polymer electrodes from five suppliers were
subjected to reflow stability testing.
It is well known within the industry that conductive
polymer capacitors are sensitive to humidity, which
strongly affects their stability on reflow. Producers of
parts with conductive electrodes mitigate this problem
by putting the capacitors in a vacuum sealed bag with
labels strongly restricting the usage time after removal
from the bag or necessity of drying if the minimum time
is exceeded. Therefore not only have multiple reflow
tests been chosen, but also accelerated humidity
followed by reflow to emphasize differences in
technologies.
a) Multiple reflow test
The capacitors were removed from their closed sealed
bags and within 30 min. subjected to 3x245°C reflow.
Second group was treated to a single 255°C reflow.
Electrical parameters were then measured after 24 hrs.
recovery. Results are summarized in Table 3.
C (µF) U (V) Reflow 1Ref Last Ref
B
D
100
10.0
3x245
0
0
B
Y
100
6.3
3x245
0
0
B
Y
100
6.3
1x255
0
0
C
D
100
10.0
3x245
0
C
D
100
10.0
1x255 1xSC
Table 3: Stability on multiple reflow
b) Humidity followed by multiple reflow test
This test was designed to show the sensitivity of
capacitors to humidity prior to reflow. Typically
capacitors with conductive polymer are classified in
JEDEC level 3, i.e. the floor life at factory ambient
below 30°C/60%RH is 168 hrs. The parts were removed
from the sealed bag, and within 30 min. measured, then
stored at accelerated conditions 90% humidity at 40°C
for 72 hours and then subjected to 3x reflow as in the
previous case. Electrical results were measured
between each step and are summarized in Table 4.
Supplier Case C (µF) U (V) Reflow Hum 1Ref
Table 2:
Maximum peak temperature recommended by suppliers
Supplier Case
All parts survived the 3x245°C reflow, but parts from
supplier C did not pass the 255°C reflow test.
0
1xSC
10
245°C
0
2Ref
3Ref
1%SC
2%SC
5%SC
5%SC
5%SC
4%SC
8%SC 10%SC
B
D
100
1%SC
B
D
100
10
255°C
0
B
Y
100
6.3
245°C
0
C
D
100
10
245°C
0
0
1%SC 10%SC
C
D
100
10
255°C
0
5%SC
5%SC
5%SC 10%SC
5%SC
C
D
150
6.3
245°C
0
5%SC
D
Y
150
6.3
245°C
0
0
0
2%LI
D
Y
150
6.3
255°C
0
0
0
4%SC
Table 4: Stability in humidity 72 hrs. 90%40°C
and multiple reflow
Short circuits appeared mostly after the first reflow.
Higher reflow temperatures had greater impact.
c) Accelerated pressure cooker and reflow
test
Accelerated humidity testing has been developed
internally in AVX to emphasize the effect of humidity
before reflow. The parts were subjected to lead-free
reflow according to Table 1. After reflow, the parts were
allowed to relax for 1 hour and subjected to pressure
cooker at 121°C for 4 hrs. Then, after 1 hour relaxation,
they were reflowed with the same profile again. Results
are summarized in Table 5.
Supplier Case
C (µF) U (V)
Reflow
PC+RFL
A
D
220
6.3
6%SC
13%SC
A
D
100
10
2%SC
2%SC
B
Y
100
6.3
8%SC
4%SC
D
B
47
6.3
18%LI
60%LI
E
D
100
10
12%SC
28%SC
E
Y
100
6.3
100%SC
100%SC
Table 5: Stability in
lead-free reflow – pressure cooker – reflow test
From all three tests it followed that Tantalum capacitors
with conductive polymer electrodes are very sensitive
to storage condition and lead-free processing. As a
consequence it was undertaken as a key challenge in the
development of AVX’s conductive polymer series to
produce a more robust polymer product.
To illustrate this point the stability of capacitors with
two different anode structure designs after the pressure
cooker/reflow test is shown in Figure 2.
Development of Lead Free Reflow
Stable Product
DCL STABILITY - POOR ANODE DESIGN
120
80
REFLOW
60
PC+REFLOW
LIMIT
40
20
0
1
7
13
19
25
31
37
43
49
55
61
67
Part No.
DCL STABILITY - GOOD ANODE DESIGN
120
100
DCL (µA)
PEDT has many advantages over other conductive
polymers, because of its simple processability, relatively
good mechanical strength and atmospheric stability.
PEDT is not a film creating polymer, however. Its
structure is brittle and depending on the way of
preparation is also often porous and non uniform.
External thermo-mechanical stresses during lead free
reflow, caused by temperature coefficient expansion
mismatches within the capacitor, can easily damage the
polymer layer, and result in short circuits or increasing
ESR up to open circuits. Key factors which have to be
managed, are:
DCL (µA)
100
A poly[3,4-ethylenedioxy)thiophene] (PEDT) [2]
conductive polymer has been chosen as the most
suitable material for development of lead free reflow
resistant Tantalum capacitors.
80
REFLOW
60
PC+REFLOW
LIMIT
40
20
0
1
8
15
22
29
36
43
50
57
64
71 78 85
92
99
106 113 120 127
Part No.
• Anode design
• Quality of dielectric
Figure 2: Effect of anode design (DCL Stability of Y case)
Lead Free Reflow – Pressure cooker and LF reflow test
• Adhesion of polymer to dielectric
b) Strong dielectrics
• Polymer uniformity
The solid electrolytic capacitor is designed to utilize a
self healing process. If a defect appears in the dielectric,
the resistivity is locally reduced and increased current
flows through the lower resistivity site. The solid
electrolyte must have the property that when
overheated, it changes its structure irreversibly to a
non-conductive structure to effectively block defects in
the conductive site.
• Stress barrier
• Low stress encapsulation
• In line 100% verification
a) Anode design
Conductive polymer layers can be created in-situ
directly in the anode by various methods. It can be
deposited by chemical oxidative polymerization from a
monomer, an oxidizing agent and a dopant, deposited in
one solution or sequentially, or by electro-deposition.
In all cases the polymer process must be tailored to the
anode design to obtain a highly conductive and
mechanically strong polymer layer. Polymer
impregnation and waste removal must be managed.
Anode design depends on the selection of tantalum
powders with the proper distribution of pores, which is
driven by particle size and agglomerate distribution,
pore size distribution within and between agglomerates,
and the presence of sinter retarding doping elements.
Sinter activity of powder and press density must be
taken into account when a sintering profile is set up.
Lubricating and adhesive binders together with modern
uniform pressing technique will also help to transfer the
desirable properties of the powder to a suitably
designed anode pellet.
PEDT conductive polymer does not have as positive an
effect on Tantalum pentoxide dielectrics, as manganese
dioxide. Therefore the dielectrics must be more robust
to external thermo-mechanical stresses and impurity
migration. It is well known that shell formation, which
creates a thick external dielectric, will increase the
dielectrics robustness. It was found that the thickness of
both internal dielectric, and external shell, are crucial
factors. Parameters of shell formation have been studied
from the point of view of the effect on dielectric
robustness. Figure 3 shows the improvement of
reflow/PC and reflow DCL stability with optimized shell
form conditions.
DCL STABILITY - BEFORE OPTIMIZATION OF SHELL FORM
ESR STABILITY - WITHOUT PRECOAT
120
120
100
80
REFLOW
PC+REFLOW
LIMIT
60
ESR (mohm)
DCL (µA)
100
40
80
REFLOW
60
PC+REFLOW
LIMIT
40
20
20
0
1
9
0
17
25
33
41
49
57
65
73
81
89
97
1
9
17
25
33
Part No.
41
49
57
65
73
81
89
97
Part No.
DCL STABILITY - AFTER OPTIMIZATION OF SHELL FORM
ESR STABILITY - WITH PRECOAT
120
120
100
REFLOW
80
PC+REFLOW
LIMIT
60
40
ESR (mohm)
DCL (µA)
100
80
REFLOW
60
PC+REFLOW
LIMIT
40
20
20
0
1
9
17
25
33
41
49
57
65
73
81
89
97
0
Part No.
Figure 3: Optimization of shell form
Reflow – Pressure cooker and lead-free reflow test
DCL stability of B case
1
9
17
25
33
41
49
57
65
73
81
89
97
Part No.
Figure 4: Effect of precoat on dielectrics
Reflow – Pressure cooker and lead-free reflow test
ESR stability of B case
c) Adhesion of polymer to dielectrics
Conductive polymer itself has a poor adhesion to the
dielectric. Peeling off and delaminating, caused by the
outgassing of humidity, leads to instability of
capacitance, dissipation factor or ESR. It is believed
that the adhesion of polymer can be improved by the
protective coating between dielectrics and polymer.
Adhesives based on electron donor organic layers,
preferably epoxysilane, deposited on the dielectric [3],
or added to the polymer solution [4], have been
proposed.
A new family of a protective coating containing an
insulative, resinous material selected from the group
consisting combinations of polyurethanes, polystyrenes,
or esters of unsaturated or saturated fatty acids has
been chosen [5]. It has been discovered that such a
protective coating can inhibit the short-circuiting of the
resulting capacitor and also reduce the ESR of the
capacitor and improve the ESR stability.
The stability of the interface has been improved, as
shown in following graphs. It results in a highly
continuous and dense conductive polymer with good
stability of ESR, DF and C and good yields.
d) Polymer uniformity
The mechanical properties of the polymer depend
largely on the structure of the polymer network and the
interaction of the network and solvents. The polymer
network’s toughness is determined by the crosslink
structure. PEDT forms a linear or simply branched
network, which has low mechanical strength. It is
furthermore impacted by the entrapped solvent.
The technology of polymer deposition had to be
optimized to allow a high level of crosslinking and to
effectively remove salts and solvents from the
polymerized material. The improved process gave
uniform and homogenous layers which improved reflow
stability of ESR (Figure 5).
ESR STABILITY - NON UNIFORM POLYMER
120
ESR (mohm)
100
80
REFLOW
60
PC+REFLOW
LIMIT
40
20
0
1
9
17
25
33
41
49
57
65
73
81
89
97
Part No.
Figure 6: Thick outer coatings causing delaminating
(A – anode, P – polymer, C – carbon, Ag – silver)
ESR STABILITY - UNIFORM POLYMER
120
f) Low stress encapsulation
ESR (mohm)
100
80
REFLOW
60
PC+REFLOW
LIMIT
40
20
0
1
9
17
25
33
41
49
57
65
73
81
89
97
Part No.
Figure 5: Effect of PEDT uniformity
Reflow – Pressure cooker and lead-free reflow test
ESR stability of B case
Encapsulation material were designed to reduce the
stresses coming from thermal shocks, and to create an
effective barrier to humidity penetration.
The resistance against thermal shocks was reduced by
low thermal expansion and high glass transition
temperature of the moulding compound. Reduction of
TCE decreased internal mechanical stresses caused by
the thermal expansion mismatch by 33% - see Table 6.
Encapsulant
Old
New
Expansion 25-260°C (%)
0.84
0.55
e) Stress barrier
The reduction of stress in the polymer requires soft and
stress absorbing external layers. This can be achieved
by flexible binders contained in both materials, and by
thick layers. There are physical barriers, however. Too
thick a layer causes delaminating on interfaces due to
thermal coefficient mismatch of included materials (see
Figure 6). Flexible coatings with good adhesion, high
internal strength allowing maximum thickness had to be
developed.
Table 6. Comparison of relative expansion of old and
new type molding compound during lead-free reflow
High strength at high temperature and good adhesion
to lead frame reduces the chances of cracking the
encapsulant and opening the gate for humidity
penetration. Low water absorption of encapsulant can
further improve the humidity resistance.
WATER ABSORBTION @ 85°C/85%RH
0.45
Water absorbtion (%)
Carbon and graphite form the mechanical barrier
between the brittle polymer and the encapsulant. Both
materials consist of conductive filler, which are carbon,
graphite or silver flakes or particles, resin binder and
solvent.
0.4
0.35
0.3
0.25
Old
0.2
New
0.15
0.1
0.05
0
0
50
100
150
200
Time (hrs)
Figure 7: Moisture absorption of old and
new encapsulant at 85°C/85%RH
g) In Line 100% Verification
Case
Tantalum capacitors are 100% preconditioned during
their production at accelerated conditions to eliminate
potential failures. Robust polymer electrodes allowed
modification of the screening operations towards
temperatures as high as 260°C without losing reliability.
All capacitors see the high temperature already at
accelerated tests during its production, which
guarantees the reliability at end users when used under
recommended conditions.
Stability
An improved family of capacitors that are robust to
lead-free reflow has been achieved as a result of the
above mentioned technical developments. The new
series will survive lead-free reflow with peak
temperatures as high as 260°C for 10 sec. Humidity
stability was improved to such an extent that capacitors
survive the lead-free reflow post soaking in humidity,
simulated by 4 hrs. pressure cooker.
Figure 8 and Table 7 show typical stability of capacitors
during lead-free reflow/pressure cooker/reflow test.
DCL STABILITY
120
DCL (µA)
100
80
REFLOW
60
PC+REFLOW
LIMIT
40
20
C (µF)
U (V)
M0
Reflow
PC+RFL
A
10
10
0
0
0
B
100
6.3
0
0
0
C
47
10
0
0
0
W
150
6.3
0
0
0
Table 7: Stability in lead-free reflow – pressure cooker –
reflow test
Conclusions
The results demonstrated the ability of polymer
technology to fulfill lead-free process requirements.
Key factors influencing the stability at lead-free reflow
were identified and improvements adopted:
a) Anode quality - anode design has to be managed and
polymer tailored respectively.
b) Robust dielectric – Selective application of thicker
dielectric can reduce the possibility of damage.
c) Polymer adhesion and uniformity – addition of
special adhesive polymer improves the interface
between polymer and the dielectric. Uniform polymer
layers have greater toughness to lead-free process
stresses.
d) External materials – carbon, silver and encapsulant
have to create a low stress environment and have to
be designed has stress absorbing as possible.
e) In-line verification – parts have to be overstressed in
line to screen potential failures.
0
1
9
17
25
33
41
49
57
65
73
81
89
97
Literature
Part No.
[1] F. Priban et al, Green Environmentally Friendly
Technology for Tantalum and Niobium Oxide
Capacitors, CARTS Europe 2004
ESR STABILITY
120
[2] M. Biler et al, Poly(ethylenedioxy)-thiophene
Conductor Films, IUPAC 2004-MACRO, July 2004
ESR (mohm)
100
80
REFLOW
60
[3] Sakata et al, U.S. Patent No. 5,729,428
PC+REFLOW
LIMIT
40
20
0
1
16
31
46
61
76
91
106
121
136
151
166
181
196
[4] R. Hahn et al, U.S. Patent No. 6,072,694,
June 6, 2000
[5] J.A. Fife et al, U.S. Patent No. 6,674,635,
January 6, 2004
Part No.
Figure 8: Reflow – Pressure cooker and lead-free reflow test –
stability of B case after all improvements
NOTICE: Specifications are subject to change without notice. Contact your nearest AVX Sales Office for the latest specifications. All
statements, information and data given herein are believed to be accurate and reliable, but are presented without guarantee, warranty, or
responsibility of any kind, expressed or implied. Statements or suggestions concerning possible use of our products are made without
representation or warranty that any such use is free of patent infringement and are not recommendations to infringe any patent. The user
should not assume that all safety measures are indicated or that other measures may not be required. Specifications are typical and may not
apply to all applications.
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