Thick film resistor failures seldom occur due to a failure of the resistive element. More often they are caused by external environmental, electrical or mechanical stresses.
Failures may be classed as a degradation of resistor performance or complete failure. Where failures do occur they usually occur 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.
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
A thick film resistor 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.
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 listed below.
The resistor could also be damaged while in service by vibration or shock. Mechanical stress can cause microcracking of the resistor material. Or, in severe cases, substrate and resistor failure.
Subtle changes in resistance value caused by microcracking can be a significant problem. They show themselves as system performance errors. Tracing the error back to a resistor that shows no external signs of damage can be difficult.
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.
If environmental factors are considered during the resistor design phase their potential impact can be reduced. The resistor device can be protected from moisture and chemical elements by applying a suitable coating at the end of the manufacturing process.
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. Care must be taken to ensure protective coatings are not damaged during system manufacture.
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 the need to remove gold from the lead before soldering is removed. Platinum silver is often used on surface mount pads as the material provides acceptable performance at a relatively low cost.
Thermal Issues Impacting On Resistor Failure
When choosing a resistor the thermal operating environment and heat management are important considerations. Mechanical failure modes of thick film resistors are often propagated by high temperature.
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. This, in turn, induces relative mechanical changes (stresses) in the resistor.
It is important to understand the heat dissipation properties of the resistor. A low power resistors primary heat dissipation mechanism is via conduction through its component leads or connections. A high power resistor tends to dissipate heat via radiation.
Forced air cooling and/or heatsinking can be used to cool the resistor component. Cooling in oil or deionised water is another option.
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.
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
The amplitude and duration of the pulse must be understood. Pulse repetition (if any) is also a factor.
The key element in determining a thick film resistor performance under pulse conditions is the mass of a resistor element. This is proportional to its thickness multiplied by its surface area.
The geometry of a resistor is also a factor. 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 the adjustment for 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 either a human body or a charged material to the thick film resistor device. ESD damage can be divided into 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 power resistors ability to dissipate energy. Power 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. Energy concentration in a small area of the resistors active element generates heat which can lead to irreversible damage.
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. If so, working with a specialist thick film resistor manufacturer can minimise the risk of thick film resistor failure.
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.
Wherever possible, action should be taken to reduce the possibility of an ESD event that could cause a thick film resistor failure or (worse still) to continue to function but with degraded performance. Unfortunately, in some applications, it is impossible to remove the potential for an ESD event entirely.
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 minimise the risk of resistor failure.