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DESIGN AND PROCESS CONSIDERATIONS FOR THICK FILM SURGE
RESISTORS TO INCREASE RELIABILITY
Dennis Raesner
CTS Corporation
Resistor Networks SBU
406 Parr Road
Berne, IN 46711
Phone: 219-589-3111 Fax: 219-589-3243
Peter Bokalo, J. Michael Robertson
EMCA-REMEX Products
Ablestik Electronic Materials and Adhesives
A subsidiary of National Starch and Chemical Company
160 Commerce Drive
Montgomeryville, PA 18936
Phone: 215-855-1000 Fax: 215-855-8202
ABSTRACT
Recent improvements in materials and
processing techniques have helped make thick film
a preferred technology to produce surge protection
circuits. However, size constraints coupled with
lower resistance requirements have made it difficult
to design circuits that meet both physical and
electrical requirements. This paper presents design
and process techniques that increase the reliability
of designs while improving the "manufactureability" of thick film surge resistors. These
techniques are useful when designing surge circuits
for both performance and high yields in production.
Samples were built to demonstrate both acceptable
and unacceptable designs. Studies will demonstrate
that proper design and processing methods can be
used to ensure conformance to product
requirements.
no inductive influences on the circuit. Recent
material and process developments have made thick
film an even more preferred alterative, since very
low ohm resistors that meet customer specifications
can be built using this technology. However, in
order to achieve the optimum performance of the
resistors, design and process methods need to be
carefully examined in order to ensure long term
reliable products.
Commercial thick film surge devices can
range in resistance value between 5 and 400 ohms.
These devices are constructed using palladium
silver formulations that range in value from 0.1
Ω/o to 1.5 Ω/o. The characteristics of these
pastes include temperature coefficients of resistance
(TCR) less than ±100 ppm/ºC and power handling
that is associated with a metallic material. The
market trends for these devices are for lower
resistor values and decreased real estate per device.
INTRODUCTION
Manufacturers of telecommunications
equipment have used various technologies to
protect devices from voltage peaks or "surges".
Typically, primary surge protection is provided by
carbon blocks, gas tubes, and high power solid state
devices. Secondary surge protection is provided by
crowbar devices that are in series with surge
resistors. These surge resistors utilize both thick
film and wirewound technology. Until recently,
very low ohm value resistors (< 50 Ω) utilized
wirewound technology, while higher ohm values
made use of thick film technology. The wirewound
resistors have better power handling capability for
the designated footprint, but are more costly and
have some technical drawbacks due to an inductive
influence on the circuit.
There are several reasons why thick film is
becoming the preferred technology for lower ohm
devices. These reasons include cost, improved
material performance, less real estate required, and
The resistor circuits are designed to
conform to a published specification, namely the
Bellcore spec #TA NWT 001089. It defines the
power surge requirements for both AC and DC
power surges. Designs for AC requirements have
been previously reported 1. This work concentrates
on designs for DC surge conformance. The
Bellcore spec defines two different DC pulses that
are used to test circuits for the ability to withstand
lightning or voltage surges. These two pulses are
1000V peak, 10 µsec rise, 1000 µsec decay to half
peak voltage (10/1000 µsec); and 2500V peak, 2
µsec rise, 10 µsec decay to half peak voltage (2/10
µsec). The devices must withstand 100 pulses of
the 10/1000µsec at 1000V and 20 pulses of the
2/10µsec at 2500V. Because of the high voltages
involved, power handling of the materials is
important, particularly on lower ohm and smaller
devices. (see table 1 for common resistor values
and power handling)
POWER REQUIREMENTS
FOR MOST COMMON CIRCUIT VALUES
Circuit Resistance in Ohms
Power For
1000V Applied (KW)
Power For
2500V Applied (KW)
390
2.6
16
200
5.0
31
100
10
63
50
20
125
20
50
313
10
100
625
179
1125
5.6
Table 1
Fig. 1
Fig. 2
This paper will describe a team approach
by a materials supplier and a component
manufacturer that focuses on end product
conformance, utilizing the material supplier's input
on material properties and resistor design and the
component manufacturer's input on process control
and implementation. The results are an increase in
final circuit yields, and final product reliability.
applications of voltage on a fixed design until
failure of each material occurred. A failure was
defined as a spark or open of the circuit. A change
in resistance of the failed circuit was observed to be
greater than 20%. The failure voltage was noted,
and another part was used to verify the failure
voltage for one pulse. After these voltages were
verified, the voltage was decreased in increments of
10% until a resistor survived the required pulses for
the specification (20 X 2/10µs, 100 X 10/1000µs).
The power was then calculated (P= V2/R) and
normalized to watts per mm2 of resistor material.
The results of these experiments allowed power
handling curves to be generated for the 4500 series
of materials (see fig. 1 and fig. 2).
MATERIAL CHARACTERIZATION AND
DESIGN METHODS
An important part of the development of
the EMCA-REMEX 4500 series surge resistor
materials
was the characterization of the power handling for
both pulse shapes. Experiments were performed on
each resistor formulation that involved sequential
ENERGY REQUIRED FOR EACH PULSE SHAPE
Circuit
Resistance in Ohms
10/1000 µsec
@ 1000V
2/10 µsec
@ 2500V
390
1.3 (J)
0.1 (J)
200
2.5
0.2
100
5
0.3
50
10
0.6
20
25
1.6
10
50
3.1
5.6
90
5.6
Table 2
Since there are two different pulse shapes,
it is important to note that each power handling
curve should be treated independently, since the
power handling for each material differs greatly
depending on the pulse shape. A relationship can be
established to normalize these two power handling
curves into a single curve. If the power is defined,
and the pulse shape is known, the energy dissipated
is defined as:
fig. 2). Since 2/10 power handling has already been
defined in figure 2, this curve can be used to design
for 2/10 performance. When used in conjunction
with the curve for 10/1000 power handling (fig. 1),
designs can be generated that will be robust for
both pulse shapes.
E= ∫ P ⋅ dt
where:
E=Energy (J, Joules)
P= Power (W)= V2/R
t= time (sec) or waveshape
Table 2 shows energy dissipation as a
function of circuit resistor value for both pulse
shapes. In theory, it appears that the power
handling (or in this case energy dissipation) need
only be considered for the 10/1000 pulse, since the
energy dissipated is so much greater than those of
the 2/10 pulse. This assumption was made early in
the development process of the 4500 series.
However, after finding failures during 2/10 pulsing,
it became clear that the consideration of only the
energy parameter is questionable for designing
reliable circuits. It appears there is another
parameter that is contributing to 2/10 type failures.
Some possible explanations are voltage breakdown
of the nonmetallic portion of the resistors, or arcing
between microscopic flaws in the fired film.
While it is still not clear what causes the
2/10 failures, steps can be taken to consider circuit
design for 2/10 performance. This involves taking a
step back from the energy calculations and reconsidering the power handling curves (fig. 1 and
Fig. 3
Using these power handling curves, actual designs
can be derived for specific circuit values, based on
mm2 of resistor material. The benefit of designing
surge resistor circuits using this method is the
optimization of real estate for performance. It
allows the use of the least amount of space for a
particular performance requirement. Designs were
then derived for several circuit values. These
designs showed a consistent trend: the lower ohm
(higher metal content) materials were better across
the board for all circuit values. These designs are
shown in fig. 3.
While all of these designs are valid in
theory, some of the designs are not realistic for the
manufacturing environment. Surge resistors are
very sensitive to defects, both voids and
contamination.
A defect of 0.01mm (0.0004")
diameter could account for a large portion of the
fine line, and will undoubtedly cause a surge
failure. The yields would be extremely poor.
Therefore, design rules were generated to account
for high volume manufacturing concerns. These
design rules were based on experience during the
development process. The minimum recommended
resistor width tracks is 0.625 mm (0.025") and the
minimum recommended space between resistor
tracks is 0.3mm (0.012") for reliable high volume
production. Using these design rules, actual circuit
designs were derived (fig. 4) for the same circuit
values as fig. 3. These designs represent minimum
area requirements for reliable circuit designs.
After these design parameters were
established, it was necessary to "prove" the method.
Circuits were designed using the proposed design
method. The parts were surged using both Bellcore
pulse requirements. These results are shown in
Table 3. There were no observed failures after the
2/10 µsec@ 2500V or 10/1000 µsec@ 1000V pulse
tests. In addition, to further validate the design
method proposed, other circuits were designed that
violated the design method. The results showed
poor performance in the surge tests, as expected.
These results are shown in Table 4.
Fig. 4
In addition to line width and spacing, other
design factors need to be considered. For optimum
surge performance reported earlier, the EMCAREMEX 4500 series was processed through a
conventional 850ºC, 60 minute profile. The
resistors were also overglazed with a high
temperature (850ºC, 60 minute profile) glaze,
EMCA-REMEX 7518D.
Once the material characterization was
completed and the design rules developed, they
were implemented in a production environment
along with sophisticated process controls to acheive
optimum yield and reliability of surge protection
circuits.
Circuit
Resistance
(Ω
Ω)
Area
(mm2)
Paste Value
(Ω
Ω /¨
¨)
Wave Shape
(µ
µ sec)
Voltage
(V)
No. of Pulses
%dR
5.4
220
0.125
2/10
2500
20
-0.074
5.4
220
0.125
10/1000
1000
100
+0.13
5.8
220
0.125
2/10
2500
20
-0.017
5.8
220
0.125
10/1000
1000
100
+0.12
Table 3
Circuit
Resistance
(Ω
Ω)
Area
(mm2)
Paste Value
(Ω
Ω /¨
¨)
Wave Shape
(µ
µ sec)
Voltage
(V)
No. of Pulses
%dR
5.6
56
0.1
2/10
2500
20
Failed
5.6
56
0.1
10/1000
1000
100
Failed
10
100
0.1
2/10
2500
20
Failed
10
100
0.1
10/1000
1000
100
+0.25
20
80
0.25
2/10
2500
20
Failed
20
80
0.25
10/100
1000
100
Failed
Table 4
PRODUCTION OF CIRCUITS
To successfully incorporate these materials
into a production environment, a number of
vehicles were used to insure product robustness.
These included:
1.
Product design based upon the power
handling capabilities supplied by the
material supplier
2.
Further material characterization using
production product
3.
Process FMEA analysis
4.
Design for conformance using Design of
Experiment guidelines
5.
Process Control based on Cpk capability
As shown in Fig. 1 and Fig. 2, surge
resistor performance is defined using both the
10x1000 µsec, 1000 volt pulse and the 2x10 µsec,
2500 volt pulse. In the case of the 10x1000 µsec,
1000 volt requirement, product characterization
was accomplished by evaluation of joules per
square millimeter vs ∆R data. A scatter plot of this
data and the resulting regression line is shown in
Fig. 5.
Regression analysis of this data yielded a
low correlation coefficient (0.03) indicating very
little relationship between the joules per square inch
applied and the resultant change in resistance.
From this, it is implied that the data is random and
the material has not reached the region where
increasing the energy applied results in a
corresponding change in resistance. As shown in
Fig. 6, further testing using the 10x1000 µsec
waveform and extending the surge voltage through
1600 volts on a 40 ohm surge resistor established
the “knee” of the resistance change vs energy curve
at approximately 0.16 joules/mm2 . Allowing some
guardband to this data gave a design requirement of
0.11 joules/mm2 for the 10x1000 µsec design
requirement. The scatter plot in Fig. 5 would also
confirm the acceptability of this requirement with
an expected ∆R of 0.25% or less.
To insure conformance to the 2x10 µsec.,
2500 volt requirement, the characteristics shown in
Fig.2 were used to develop a design program for
determining acceptable resistor material and area
combinations for a given resistor value. An
example utilizing a 40 ohm resistor is shown in Fig
7.
The program compares the calculated
resistor areas for each resistor material against the
minimum area required to meet the 2500 volt
characteristics of that material. The designer is
presented with a series of resistor design options
which are in conformance with the 2500 volt
limitations of the selected resistor material.
Early in the production process, it was
determined to use a Potential Failure Mode and
Effects Analysis (Process FMEA) format as a
vehicle for fine tuning the manufacturing process.
As part of this work the resistor material screening
characteristics were identified as a potential cause
for yield loss during surge testing prior to shipment.
To optimize the screening process, a Design of
Experiments (DOE) was initiated which included
the resistive material viscosity as one of the
experimental factors. While a complete discussion
of these studies is beyond the scope of this paper, it
is significant to note that the resistive material
viscosity was identified as a major contributor to
yield loss. The viscosity was adjusted in accordance
with the DOE results, and retesting of the revised
viscosity material in the confirmation run verified
the improved performance. The revised material
was introduced into production and Fig. 8 and Fig.
9 show the corresponding improvement in
production yields:
relationship. It is also the result of applying a
systems approach to manufacturing which utilizes
the following steps:
1.
Characterization of the materials to the
critical
design
parameters
and
incorporation of this information into all
products using these materials.
2.
Optimization of the materials and
manufacturing processes through the use
of statistical analysis of experimental
results.
3.
The use of problem detection and
corrective action programs such as Process
Failure Modes and Effect Analysis and
Design Failure Modes and Effect Analysis.
4.
Statistical process controls to maintain
process integrity.
5.
Co-operative interaction between the
material supplier and the component
manufacturer.
The end result is a product which is both
predictable in production and reliable in use.
ACKNOWLEDGEMENTS:
Fig.8 and Fig. 9 show that scrap related to
screening was reduced in excess of 60% and scrap
related to surge failures at final test was reduced in
excess of 50% by matching the material viscosity to
the product application and screening equipment.
In addition to the results shown in Fig.8
and Fig. 9, work is in process to improve screening
characteristics further through screening rheology
improvements in addition to viscosity.
The final element in the successful
incorporation of these surge materials involved the
use of process control procedures which require
process capability as determined by set-up and
sample Cpk values at critical process operations
such as screen printing and laser trimming.
SUMMARY
This paper has shown the importance of
providing thick film materials which have been
designed and characterized to meet not only the
specific requirements of the end user but also are
user friendly to the component manufacturer who
purchases these materials. The ability to identify
and correct potential problems before they resulted
in end user dissatisfaction was the result of the
material supplier and component manufacturer
working together in an open and supportive
The authors would like to acknowledge J.
Durant, C. Matier, W. Sheppard, and C. Wyanske
for sample preparation and testing. The authors
would also like to acknowledge R. Shanks and G.
Stallard for their help in preparing the manuscript.
REFERENCES:
1. R. L. Reinhard, "CTS Surge Resistors in
Telecommunications", 15th Capacitor and Resistor
Technology Symposium, pp. 82-84, March 1995
2. O. W. Brown et. al., "Lightning Surge Behavior
of Thick Film Metallic Resistors", Proceedings of
International Symposium on Microelectronics", pp.
673 - 678, November 1993