Thick film resistor failure seldom occur due to a failure of the resistive element. More often, the root cause is external environmental, electrical or mechanical stress.
Designers must trade off between cost and reliability. A cheaper thick film resistor device may be appropriate, but it’s important to consider the potential implications. Low cost generally means some compromises in the resistor design and manufacturing process.
Where failures do occur, they usually show as an open rather than a short circuit. Seven of the most common causes of thick film resistor failure are:
- Mechanical stress.
- Environmental effects.
- Thermal issues.
- Constant overload.
- Electrical pulse events.
- ESD.
- Ageing.
Damage due to thermal and mechanical stresses may not be immediately obvious. A degradation of resistor performance can cause errors that are difficult to trace.
Mechanical Causes of Thick Film Resistor Failure
Thick film resistors can suffer mechanical damage during the system manufacturing process. Potential problems include mishandling, inappropriate device mounting and device coating.
Mishandling can cause substrate chipping or microcracking. Inappropriate mounting can cause solder joint stress via ongoing compression or extension of the resistor lead. Coatings can introduce compressive forces, which can lead to longer-term reliability issues.
Mechanical stress, such as vibration, thermal shock and thermal cycling, can all cause microcracks in the resistive film or substrate. This may not cause the resistor to fail immediately but can make the device more prone to failure over the medium to long term.
If vibration or shock disrupts the particle-to-particle contact in the resistor film, it can cause a permanent change in resistance value.
Environmental Factors
Environmental conditions can lead to material degradation, performance drift or outright resistor failure. Environmental factors to consider include:
- UV and radiation exposure
- Humidity, moisture and salt spray
- Contaminants and chemical elements
- Oxidation
- Pressure and altitude
Radiation exposure can cause the breakdown of both the ceramic substrate and the resistive film, weakening the thick film resistor’s structural integrity and making it more susceptible to other forms of environmental degradation.
High-energy radiation can also ionise the materials in the resistor, changing its electrical properties and causing shifts in resistance values. Over time, the combined effects of UV and radiation can lead to a cumulative drift in resistance values. However, radiation-hardened materials and ceramics can help improve performance.
Exposure to moisture leads to oxidation of the metal elements within the resistive film. This degradation process accelerates when combined with UV light exposure.
Moisture absorption by the resistive film can significantly alter its electrical properties, especially when contaminants like dirt and salts are present, as they retain moisture and increase absorption into the resistor material.
The presence of dust and oils on the surface can also create a conductive layer that enables leakage currents. Moisture can form conductive paths across the resistor’s surface.
Pressure and altitude variations can impact the mechanical performance of thick film resistors, as these environmental factors can induce mechanical stress on the resistor materials.
High-pressure environments can cause compression that affects the resistor layers, potentially weakening the bonding between the resistor material and its substrate, increasing the risk of delamination or cracking. At high altitudes, thick film resistors face additional challenges, such as more extreme temperature variations and reduced air pressure, which diminishes cooling efficiency and exposes them to increased levels of UV radiation and atmospheric ions.
Considering harsh environments during the resistor design phase can help reduce their potential impact. Utilising materials with superior thermal and mechanical stability, as well as protective coatings, can improve the resistor’s resilience.
It is also important to consider the interface between the resistor device and the system board, as improper treatment of the system board surface can cause short circuits or changes in resistance value, and high-temperature environments can facilitate metal migration between the resistor terminals.
Thermal Issues Impacting Resistor Failure
Resistor selection requires careful consideration of the operating environment and thermal management. In thick film resistors, mechanical failures are often aggravated by temperature-related stress.
When current flows through a resistor, it generates heat. This causes different materials within the component to expand at varying rates. These thermal stresses can disrupt particle-to-particle contact within the resistive film, resulting in permanent changes to resistance values.
Over time, prolonged exposure to high temperatures leads to material degradation. Thermal cycling can cause fatigue, resulting in micro-cracks. Over time these cracks can lead to delamination, resistance drift or complete thick film resistor failure.
In thick film resistors, power ratings and thermal performance are closely linked. The relationship is influenced by resistor dimensions, substrate material, thermal conductivity and ambient temperature.
Exceeding power ratings generates excessive heat, causing thermal stress. This can trigger thermal runaway, where increasing resistance due to heat creates additional heat generation.
The Temperature Coefficient of Resistance (TCR) indicates how sensitive the resistive element is to temperature changes. In thick film resistors, TCR typically ranges around ± 100 ppm/°C or better.
Hot TCR measures resistance increase with temperature rise, while Cold TCR becomes relevant only in extremely cold conditions or during cold starts. Lower TCR values are preferred for precision applications to minimise resistance drift.
Effective thermal management involves careful material selection, thoughtful system board design and component layout. When insufficient, forced air cooling, heatsinking or cooling in oil or deionised water may be necessary.
Resistor manufacturing defects like pinholes in the resistor track or non-uniform patterns from abrasion can create hot spots. These can lead to unexpected failures or performance changes. The solution is thicker films and wider tracks, but these modifications may affect other electrical characteristics.
Derating curves help determine safe operating power levels in various thermal environments. At higher temperatures, resistors are often operated below their maximum rated power to prevent thermal overload. The resistor’s power rating, operational environment and space constraints all influence performance.
Overload and Pulse Driven Resistor Failures
Thick film resistors fail under pulse conditions because they are unable to dissipate the heat generated by the pulse electrical energy.
The system’s ambient temperature can impact a resistor’s pulse handling capability, as higher temperatures reduce the margin for additional thermal load. Environmental factors, like humidity or contaminants, can alter the resistor’s thermal properties and contribute to localised failures under pulse conditions.
Understanding pulse energy is crucial to determine if a resistor can handle a particular pulse without failure. Excessive pulse energy can create thermal stress gradients that cause micro-cracking, film separation or material degradation.
It is important to understand the amplitude and duration of the pulse and pulse repetition (if any). Pulse duration, frequency and shape influence thermal and mechanical stress on the resistor. For instance, single high-energy pulses can cause immediate damage, while repetitive low-energy pulses may lead to gradual degradation.
Pulse voltage, power, energy ratings and derating curves are often quoted in resistor datasheets. These can help with resistor selection for pulse-heavy applications. Operating resistors below their maximum rated pulse conditions improves reliability.
The key element in determining 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. Increasing the surface area facilitates more heat dissipation.
Appropriate design and manufacture of the resistor can reduce the impact of pulse events. Resistor film thickness, substrate material and pattern layout all contribute to a thick film resistor’s pulse performance.
The final factor to consider is an adjustment for the final resistance value. During resistor trimming, weak spots (hot spots) can be created. Over time these can cause failure in resistors subjected to high-energy pulse conditions.
ESD Events & Resistor Failure
The most common cause of 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. Energy concentration in a small area of the resistor’s active element generates heat which can lead to irreversible damage.
ESD damage has three main categories: parametric failure, catastrophic damage and latent damage. When subjected to ESD thick film resistors almost always experience negative resistance changes.
The damage caused depends on the thick film resistor’s ability to dissipate energy. Resistor ESD sensitivity is therefore directly related to the dimensions of the resistor. 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. Hence, the choice of thick film paste influences the ESD performance of the resistor device.
Changes to the track profile – Widening the track profile will improve resistor ESD performance. 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 has a direct impact on the power resistor ESD sensitivity.
The Impact of Ageing
Resistor ageing, caused by environmental exposure, mechanical issues and electrical overstress, impacts resistor performance. Over time, this ageing process can lead to a drift in resistance value and mechanical damage. In extreme cases, ageing can result in complete resistor failure.
Several factors can accelerate these degradation processes. Higher operating temperatures speed up both chemical and physical deterioration, while the combination of frequent voltage or power surges with high humidity can increase failure rates.
Additionally, environmental factors play an important role. Prolonged exposure can degrade the ceramic substrate. This can lead to cracking and delamination under thermal and mechanical stresses.
The mechanisms behind ageing are primarily material-based. The resistive layer can degrade through oxidation, while metal migrations within the component can cause decreases in resistance or short circuits.
However, the impact of these factors is significantly less than that of temperature, electrical stress, and mechanical issues. If these stress factors are removed, the impact of ageing is minimal, with thick film resistors able to operate without degradation for ten years or more.
To combat ageing effects, two main strategies are employed. First, appropriate materials and protective coatings mitigate degradation. Second, operating the resistor below its maximum rated values helps reduce overall stress on the component, extending its operational life.
Resistors may be the lowest-cost and simplest components in a system, but failure can be just as damaging as a failure of any other component. When specifying a thick film resistor it is, therefore, important to understand potential failure modes and how they may be addressed.
Specialist thick film resistor manufacturers often build safety margins into the design and manufacturing process to minimise the risk of resistor failure. This minimises the risk of drifts in performance.