Technical Report
Thick Film Resistors
Failure Mechanisms and Reliability
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Thick film resistors, failure mechanisms and reliability
The electronics industry has developed at an impressive pace over the past 25 years. New
technologies and progress in almost every field have helped products to include more and more
components while constantly reducing the amount of space they take up.
There is constant pressure on component manufacturers to develop smaller, more reliable
components. Among these are chip components such as capacitors and resistors which are used in
vast numbers in almost all electronic products.
They are sturdy components that rarely fails because of errors introduced in their manufacturing
processes; they are also expected to work in environments in which high temperature and moisture
occur, as well as aggressive gasses in some instances, but these environments may change their
properties. When they fail, it often has fatal consequences for the function of the equipment in
which they are being used.
The purpose of this article is to describe why and how thick film resistors in particular fail and how
to choose the correct type of resistor for an application, and to describe a number of testing options
which can be used to verify that the end product is compliant with the specified requirements.
Description of a thick film resistor
Thick film resistors became popular in the 1970s, and the vast majority of SMD resistors used
nowadays are of this type. A serigraphic process is used in the fabrication of these resistors,
whereby a resistance paste is
printed onto a ceramic substrate.
PROTECTIVE COVER
RESISTOR ELEMENT
The ohmic resistance of the paste
printed on, is not particularly
CERAMIC SUBSTRATE
accurate, so subsequent laser
trimming is necessary to achieve
SOLDER TERMINAL
the desired value.
CONSTRUCTION OF A THICK FILM
The first thick film resistors made
RESISTOR
had a tolerance of 5%, but since
then this has been improved to 2% and 1%. The temperature coefficient for thick film resistors is
high, typically 200-250 ppm/K.
The resistance paste discussed above comprises metal oxides and glass-based fillers, which together
with a solvent forms a paste which can be printed Serigraphically. After burning off the solvents
and the subsequent hardening processes, the resistor element has a sponge like structure, with more
or less random irregularities. If the resistor element is not sealed fully, moisture and gases could
penetrate it and change the properties of the component. Laser trimming, as mentioned above, may
also cause micro cracks in the resistor element and have the same consequences.
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Manufacturer’s testing of thick film resistors
Of course, there are differences in the quality of the cheapest resistors, such as those used in
consumer electronics, and the most expensive resistors used in e.g. medical equipment, where
extremely high reliability is a requirement.
As a rule, data sheets for thick film resistors describe which tests have been carried out in order to
support data concerning the reliability of components. These international standardized tests include
temperature cycling, the effects of temperature/moisture, thermal shock, HAST (Highly Accelerated
Stress Test) and HALT (Highly Accelerated Life Testing).
It may be expected that the manufacturer, on the basis of a random sample plan, will implement the
test programme on a rolling basis in order to ensure that a constant high level of quality will be
maintained in production.
MIL-STD-202, “Test Methods, Standard Electronic Component Parts”, is one of these standards. It
is divided into four sections: an instruction forms the first section, the second describes
environmental tests, the third describes mechanical tests, and the fourth describes electrical tests.
Compliance with the requirements in this standard is not required solely for manufacturers of
components for military applications. They are also applied in a civilian context together with
equivalent international standards such as IEC 60115-1, IEC 60068 and JIS C 5201.
Failure mechanisms in thick film resistors
There are three ways in which any resistor, including a thick film resistor can fail: there may be a
change in resistance compared with the original value, it may be disconnected, or it may be shortcircuited. Below is a description of the causes and effects of each failure mechanism, with
references to further information on this.
Resistance changes
Predictable changes in resistance values, which the designer has to take into account, occur as a
function of electrical conditions and the environment in which the resistor is used. The
manufacturer describes these conditions, such as changes as a function of the ambient temperature
and pulse load, in the data sheet for the component.
However, the environment in which the resistor is used can bring about more drastic, less
predictable changes in the resistance value.
Electrochemical migration in damp and aggressive surroundings is a growing problem,1 with fewer
and fewer components, small insulation distances, high component density and contamination of the
PCB and components.
For example, the two terminals on a chip resistor may act as anode and cathode if they are
connected by a (thin) layer of water. In this instance, dissolved metal ions from the positive
1
Daniel Minzari, Morten S. Jellesen, Per Møller, Pia Wahlberg and Rajan Ambat: Electrochemical Migration on
Electronic Chip Resistors in Chloride Environments. IEEE Transactions on Device and Materials Reliability, Vol. 9,
No. 3, September 2009
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electrode (anode) will migrate towards the oppositely charged electrode (cathode) and be deposited
here. This process, which is also termed electrolytic migration, is similar to what happens during
galvanic metal plating, apart from the manner in which the metal is deposited; in the form of
dendrites here. The growth of dendrites from the cathode to the anode creates an electrical
connection in parallel with the resistor body between the terminals of the component. The
concentration of salt in the water film and the tension difference between the terminals are of
significance to the growth of dendrites.
Not all metals play equally as active a part in electrolytic migration, but copper, silver, tin and lead which may occur in the material used for the resistor terminals - may cause migration.
A film of water may appear on the surface of the resistor in a damp utilisation environment. The
conditions for migration are in place with a voltage difference and salt residues from the production
process (manual handling, soldering processes, etc.) or the ambient environment. Eventually, this
will result in a leakage current, falling resistance value and then short-circuit.
Residues from what is known as no-clean flux, which is widely used in the industry, may leave salt
which can cause migration - see 2 - at soldering temperatures which are too low, caused by
physically large adjacent components which absorb heat, for example.
Short-circuit
Another form of silver migration may be caused by irregularities arising in the printing process of
the resistor material close to the solder terminals of the component. These irregularities concentrate
electrical fields around the resistor material, and this may cause silver from the terminals to migrate
into it.3
This failure type may occur when the resistor is placed in surroundings with a high ambient
humidity and temperature, but it has also been observed in “normal” surroundings, in which a major
voltage difference occurs over the resistor. Symptoms and consequences are the same as those
discussed above.
Disconnection
The connection between the resistor material and the solder terminals may be interrupted due to
silver sulphide. Silver sulphide may be formed in surroundings containing sulphur, hydrogen
sulphide H2S reacting with the silver in the resistor terminals. Silver sulphide builds up more in
2
Kirsten Stentoft Hansen, Danfos Drives A/S, Morten S. Jellesen PhD, Technical University of Denmark, Per Møller,
Technical University of Denmark, Peter Jacob Schwencke Westermannn, Technical University of Denmark, Rajan
Ambat, Technical University of Denmark: Effect of Flux Residues on Corrosion of Electronics, IEEE 2009
3
Anshul Shrivastava, Ahmed Amin, Bhanu Sood, Michael Azarian, Michael Pecht, CALCE University of Maryland:
Thick Film Resistor Failures. ISTFA 2008, 34th International Symposium for Testing and Failure Analysis. Portland,
OR, Nov. 2-6, 2008
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volume compared to pure silver, and over time this results in an disconnection between the
terminal and the resistor body4,5.
TIN/LEAD
SILVER SULPHIDE
NICKEL LAYER
SILVER LAYER
Aluminia substrate
photo: Hytek
Fig. 1 Interruption in a thick film resistor caused by silver sulphide
Sulphur build-up is caused by - for example - the resistor production processes, the manufacturing
processes for PCBAs, sulphur in the operating and storage environment, and vulcanised rubber
parts used in the environment near to the resistor. Several manufacturers/suppliers offer sulphurresistant thick film resistors.
Select the correct thick film resistor for your application.
The thick film resistors on a PCB are probably the cheapest components overall, even though there
are often many of them. But if one or more of them fails because the designer has not selected the
appropriate quality, this may have consequences of the same magnitude as the failure of a very
expensive BGA, for example.
4
Michael Reid, Jeff Punch, Claire Ryan, John Franey, Gustav E. Derkits Jr. William D. Reents, Jr. and Luis F. Gafias:
The Corrosion of Electronic Resistors. IEEE transactions on components and packaging materials, Vol. 30, No. 4,
December 2007.
5
Steve C Axtell, Vishay Intertechnology 2002: Failure of Thick Film Resistors in Sulphur-containing Environments
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Choosing the right quality for a thick film resistor (and other components) for a product starts by
describing its use, and the task is completed only when the end product has been proven to meet the
specified requirements.
JSTD-001 defines three classes of electronic products:
Group 1: Simple electronic products. Includes products which are suitable for use where the primary
requirement is the function of the finished product.
Group 2: Reliable electronic products. Includes products where continued function and extended durability are
required, and where constant operation is desirable but not critical. Typically, the operating environment will
not be able to cause failures.
Group 3: Electronic products high reliability. Includes products where continued function or performance is an
unconditional requirement. Stoppages cannot be tolerated, and the equipment must be capable of functioning
in unusually harsh environments where required, such as in respirators or other critical systems.
The environment for use must be described, the key parameters being temperature, moisture
(maximum, minimum and variation (cycling)), aggressive gases (see, for example, ISA
71.04), mechanical effects and EMC requirements.
The requirements in terms of function and reliability must be described and the component
quality selected on the basis of this.
Degrade the use of the resistor. For example, the maximum permitted power rating for a
thick film resistor in the data sheet is normally specified at 25°C. In a safe design, this rating
is reduced with regard to the intrinsic temperature of the component, which is dependent on
the current, voltage and ambient temperature, often degraded to 80% of the data sheet
parameters. Select the physically largest component possible and use it at the lowest
possible temperature and the lowest possible voltage.
Select the correct quality. There are many manufacturers, each of whom offers many variants of
their products at a wide range of prices. At the bottom of the scale are “general purpose” resistors;
these are inexpensive and best suited to consumer electronics, for example, which are not expected
to survive a particularly long time. At the top of the scale are resistors which are approved to JSTD001, class 3 products, where function is critical and a long service life is expected in harsh
environments. Between these two ends of the scale are resistors for special applications, such as
types with low (electrical) noise levels for acoustic purposes and microwave products, sulphurresistant types and types approved for use in the automotive industry and products.
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16
14
12
10
8
lowest
6
highest
4
2
0
general purpose
industrial
sulphur
resistent
high precision
The figure above, from one of the suppliers on the market in Denmark offering several different
makes, shows price factors in relation to the cheapest general purpose-type thick film resistor (set at
1, and based on 1 million units/order prices) and up to the most expensive, high precision type,
where the price is dependent in part on the tolerance and temperature coefficient required.
Ask the supplier/manufacturer what type of resistor is best suited to a product which has to work
flawlessly and offer a long service life in the specified environment; a reliable supplier should also
be able to demonstrate that the specifications in the data sheets are underpinned by decent test
results.
If the requirements for the resistor include a lower temperature coefficient or lesser tolerance, for
example, a thick film resistor may perhaps not be the best choice for the application; there are a
number of alternative technologies available, each with their own specific properties. See the figure
below.
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Source: Draloric/Beyschlag Resistors
Thick film resistors are vulnerable components, not least due to their physical dimensions – the
smallest available at present is 0.4x0.2 mm – and they have to be handled with care, so:
Before use, store thick film resistors in sealed packs at the lowest possible temperature and
not in environments containing sulphur and/or chlorine.
Avoid mechanical damage to thick film resistors during assembly, ensure that the pressure
of the placement tool is not too high.
Use the correct temperature profile in the soldering processes so that no defects occur due to
thermal overstressing of the components.
Implement design and handling measures which ensure that no contamination is left on the
finished PCBA which may later initiate the corrosion of components and/or PCBs. Known
sources of such contamination include soldering flux and manual handling.
If manual retrofitting and/or repair are necessary, use proven instructions and specially
trained operators.
If the end product is to be used in damp environments and/or in locations where airborne
contamination is present, protection with full or partial lacquering (conformal coating) can
be very effective at helping to improve its function and service life.
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Testing the end product
Verification of whether the end product meets the specified requirements in respect of function and
reliability by means of various test forms, among other things, some of which refer to the IEC/EN
60068-2 standards, is mentioned below:
During environmental testing, the end product is exposed to the following, for example:
CLIMATE TEST
Temperature
GAS/CORROSION
SO2 test
MECHANICAL TEST
Vibration
IEC/EN 60068-2-1 Cold
IEC/EN 60068-2-2 Dry Heat
IEC/EN 60068-2-14 temp. change
IEC/EN 60068-2-42
IEC/EN 60068-2-6 sinus
IEC/EN 60068-2-64 random
Moisture
H2S
Drop
IEC/EN 60068-2-3 damp heat steady
IEC/EN 60068-2-30 damp heat cyclic
IEC/EN 60068-2-43
IEC/EN 60068-2-32
Temperature/Moisture
Corrosion
Shock
IEC/EN 60068-2-38 comp. temp./hum.
cyclic
IEC/EN 60068-2-60
IEC/EN 60068-2-11
IEC/EN 60068-2-52
IEC/EN 60068--27
If failure of the end product would be critical – in a JSTD-001 class 3 product, for example – if the
end product is very expensive or if a large production run is to take place, HALT and HASS are
often used as professional testing tools during the development and production process.
The purpose of HALT is to identify, effectively and at an early stage, and correct any weaknesses in
the newly developed product.
HASS is used to indicate weak areas, such as component damage, caused by the production
processes.
The Highly Accelerated Life Test (HALT) is a process used during development of a
new product in order to validate the robustness of the structure. This method is well
suited to identification of potential weaknesses in electronic products.
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Design modifications, such as changes to more robust components and modifications
in mechanical structure, can then be introduced on the basis of this. Volume
production can then begin with a mature product.
The fundamental principle of a HALT process is to subject the product under test
conditions to gradually increase environmental stress until latent failures occur. The
most commonly used stresses are thermal and broadband vibration. These stresses are
used individually and in combination.
If a product has inbuilt latent flaws from “birth”, the environment in which it is used
will cause the product to fail sooner or later. A meticulously planned and implemented
HALT process will reveal these latent sources of failure in a fairly short time and
hence allow costs arising from guarantee claims and complaints to be kept to a
minimum.
Experience and data from the HALT test are used as a basis for implementation of
Highly Accelerated Stress Screening (HASS), which is a very short test, often taking
just a few minutes, and any flaws in the product may be found during this additional
stressing.
Conclusion
Thick film resistors are used in very large numbers and are inexpensive, but they must be selected
and used with care on account of their small dimensions, component density and failure
mechanisms. The environment in which resistors are used has a major effect on the function and
reliability of the products in which they are included, and verification of whether the end product
meets the specified requirements is important. HALT and HASS tests are effective tools for
validating the robustness of a product in the environment in which it is expected to operate
flawlessly over a long period of time.
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