Thick film resistor failures seldom occur due to a failure of the resistive element. More often, the route cause is external environmental, electrical or mechanical stresses.
A wide range of thick film resistor devices is available to a variety of quality standards. The designer needs to understand the trade-off between cost and reliability. A cheaper thick film resistor device may be appropriate for the application, but low cost generally means some compromises have been made in the resistor design and manufacturing process.
Failures can be classified as degradation of resistor performance or complete failure. Where failures do occur, they usually show as an open rather than a short circuit.
Six of the most common causes of resistor failure are:
- Mechanical stress.
- Environmental effects.
- Thermal issues.
- Constant overload.
- Electrical pulse events.
The designer must always be aware of thermal and mechanical stresses and their impact on the resistor. Damage to a resistor device may not be immediately obvious. This can cause errors in system performance that are difficult to trace.
Mechanical Causes Of Thick Film Resistor Failure
Thick film resistors can suffer mechanical damage during the system manufacturing process. This may not cause the resistor to fail immediately but can make the device more prone to failure when in service. Potential problems include:
Inappropriate mounting of a device can cause ongoing compression or extension of the resistor. This increases its susceptibility to one or more of the stresses discussed below.
Microcracking due to mechanical stress can cause subtle changes in resistance that show themselves as system performance errors. Tracing the error back to a resistor that shows no external signs of damage can be difficult.
When in service resistor damage can be caused by vibration or shock. The particle to particle contact in the resistor film can be disrupted by mechanical stress causing a permanent change in resistance value.
Although the makeup of the resistor film material does make thick film technology more susceptible to damage than other resistor technologies, careful resistor design and an appropriate selection of materials can minimise issues caused by mechanical stress.
Environmental factors to consider include ambient temperature, moisture and chemical elements. It is important to consider both the resistor device and its interface to the system board.
Considering environmental factors during the resistor design phase reduces their potential impact. Thick film resistor devices can be protected from moisture and chemical elements by applying a suitable coating at the end of the manufacturing process.
The system board interface can often be the point of failure. A tin-coated lead can be directly soldered to the system board, whereas a direct gold/tin interface should be avoided. Tin is comparatively cheap but can (depending on the environment) tarnish with age and compromise the connection.
A variety of lead coating materials are available. These include gold, platinum silver and platinum palladium. Palladium alloys can be a better option than gold in some applications as there is no need to remove gold from the lead before soldering. Platinum silver is often used on surface mount pads as the material provides acceptable performance at a relatively low cost.
Failure to treat the surface of the system board correctly can cause metal migration between the terminals of the resistor in high-temperature environments. This can cause a short circuit or a change in resistance value.
Thermal Issues Impacting On Resistor Failure
When choosing a resistor, the thermal operating environment and thermal management are important considerations. Mechanical failure modes of thick film resistors are often propagated by high temperatures.
When current passes through a resistor it generates heat and causes a differential thermal expansion of the different materials used to manufacture the resistor component. The particle to particle contact in the resistor film can be disrupted by thermal stresses, causing a permanent change in the resistance value.
The Temperature Coefficient of Resistance (TCR) defines the resistive element’s sensitivity to temperature change. TCR is a known factor and, depending upon materials used, is typically ± 100 ppm / °C or better in thick film resistors.
Hot TCR and cold TCR are usually specified. Hot TCR quantifies the increase in resistance with an increase in temperature. Cold TCR is the opposite and is only relevant in extremely cold operating environments or cold start situations.
It is important to understand the heat dissipation properties of the resistor. Forced air cooling and/or heatsinking are used to cool thick film resistor components. Cooling in oil or deionised water is another option.
Errors in the thick resistor manufacturing process can result in pinholes in the resistor track. Abrading of the resistor track to set the resistor value can cause a non-uniform resistor track pattern. Both of these issues can cause hot spots that may result in unexpected failure of the resistor film or changes in performance. One solution is to design products with as long a resistor path as possible within a given area.
Overload Pulse Conditions
Thick film resistors fail under pulse conditions because they are unable to dissipate the heat generated in the resistor device by the electrical energy of the pulse.
Significant single transient (pulse) events or multiple voltage pulses can degrade thick film resistor performance or cause resistor failure. Appropriate design and manufacture of the resistor may reduce the impact of pulse events but cannot ensure the device will survive in all circumstances.
It is important to understand the amplitude and duration of the pulse and pulse repetition (if any). The key element in determining a thick film pulse resistor performance is the mass of a resistor element. This is proportional to its thickness multiplied by its surface area. A larger surface area results in a higher film mass. The increased surface area also allows for more heat dissipation.
The final factor to consider is an adjustment for the final resistance value. Depending on the method used for resistor trimming weak spots (hot spots) can be created. Over time these can cause failure in resistors subjected to high energy pulse conditions.
The most common cause of power resistor ESD damage is a direct transfer of an electric charge from a human body or a charged material to the thick film resistor device. ESD damage has three main categories, parametric failure, catastrophic damage and latent damage.
Thick film resistors almost always experience negative resistance changes when subjected to ESD. The damage caused depends on the thick film resistors ability to dissipate energy.
Resistor ESD sensitivity is therefore directly related to the dimensions of the resistor. Energy concentration in a small area of the resistors active element generates heat which can lead to irreversible damage. The smaller the resistor, the less area is available to spread the energy delivered by the ESD event.
The best method to protect a thick film resistor is to establish systems and processes to reduce the probability an ESD event can occur. However, often it is not possible to eliminate all ESD risks. Design and manufacturing options include:
Improvements to the thick film paste. – The amount of ESD damage is directly related to the conductive mechanism of the resistive material, which is in turn related to the composition of that material. The choice of thick film paste can, therefore, influence the ESD performance of the resistor device.
Changes to the track profile.– Modifications (widening) to the track profile will improve the ESD performance of the device. Hence, for known ESD threats, an element of ESD survivability can be designed in.
Firing temperatures and profile. – Firing is one of the most critical elements of the thick film process. Optimising the process can have a direct effect on the power resistor ESD sensitivity.
A resistor may be the lowest cost and most basic component in a system, but failure can be just as damaging as a failure of any other component. It is, therefore, important to understand potential failure modes and how they may be addressed.
A design and manufacturing partnership with an experienced specialist resistor manufacturer can ensure all application issues are covered. To minimise the risk of resistor failure, a specialist manufacturer will often build safety margins into the design and manufacturing process. This minimises the risk of drifts in performance or resistor failure.