Working with Thin Film Resistor Components in Your Next PCB
I remember when I bought my first electronics kit as a student. It contained a range of capacitors, resistors, inductors, a few diodes, and some basic integrated circuits (NAND gates, 555 timers, etc.). These resistors were the classic cylindrical resistors with the colored bands, and I remember a time when I had all the bands memorized and could pick out specific resistors by sight.
This is not so simple with thin-film resistors, which do not have a color code. If you are trying to decide which type of resistor to use in your next PCB, there is plenty to consider, ranging from geometry to precision and thermal stability. However, the electrical and thermal behavior of thin-film resistors is well-documented in component datasheets, and it is easy to analyze circuit behavior of your circuits with the simulation tools.
Different Types of Resistors
Resistors can be broadly classified as fixed or variable resistors. As their name implies, variable resistors have resistance that can be changed, either by control of the device’s user or through an external stimulus (electrical, mechanical, thermal, magnetic, etc.). Fixed resistors are intended to provide stable resistance at any voltage or temperature, but this is not always the case. The table below shows a brief comparison of the common types of resistors.
Note that the temperature coefficients for these resistors can be positive or negative (i.e., the resistance can increase or decrease as temperature increases, respectively). The maximum DC and AC voltage values for these components will differ. From a precision and thermal stability standpoint, the quick overview in the above table shows that thin-film resistors are quite stable compared to the other types of resistors.
Thin-Film Resistor Characteristics
Thin film resistors are SMD components, so you won’t find them in a typical electronics starter kit. They have a small solderable pad area which makes them prone to tombstoning if not soldered properly. Most common cylindrical carbon film or metal film resistors are through-hole components, but thin-film resistors can be made from a variety of different materials. Carbon and metal thin-film resistors are available, but higher precision thin-film resistors are normally made from nickel chromium (NiCr) or tantalum nitride (TaN), with NiCr being the far more popular resistive material.
NiCr typically offers the lowest TCR and widest resistance range. In terms of component size and manufacturing, there is only a limited range of feasible sheet resistance values, typically ranging from ~5 to 250 Ω per sq. Despite the limitations in available sheet resistance values with thin-film resistors, they can provide extremely tight tolerances reaching as low as 50 ppm. They also provide low parasitic capacitance and temperature coefficients as low as 1 to 2 ppm.
Any component that offers some advantages and benefits also carries some disadvantages. The manufacturing process for thin-film resistors is more costly than for a comparable thick-film resistor or cylindrical resistor. Thin-film resistors also have lower mass than thick-film resistors, so they tend to have lower power ratings.
Construction and Non-ideal Behavior
Thin-film resistors are fabricated using internal interdigitated electrodes, which wrap across the resistive film inside the resistor. The resistive film is mounted on a ceramic, glass, or silicon substrate, and the entire housing is covered with a protective coating. The ends are metalized, which are soldered to the mounting pads. This structure is shown in the image below.
Thin-film resistor construction [Image source].
The experienced circuit designer should be able to see this Just like inductors and capacitors are not truly ideal devices, the same applies to thick-film and thin-film resistors. These non-ideal behaviors result from parasitic capacitance and inductance in the thin-film resistor package. The simplest non-ideal model for a real resistor is an inductor in series with a parallel RC circuit. A more accurate model is shown below, which takes account of the external capacitance from mounting pads, the external inductance from the leads, and the internal shunt capacitance.
Equivalent circuit model for a real thin-film resistor.
The key to simulating signal behavior with these types of components is to use frequency sweeps and transient analysis. The pairing of various L and C elements may lead to some self-resonant behavior. However, component manufacturers have managed to design these resistors such that their transient response is overdamped, which eliminates this self-resonance. Therefore, they function like low-pass filters with rolloff near ~1 GHz. These components are inherently linear, so you don’t need to worry about a DC sweep.
No matter which type of resistors you want to use in your next design, you’ll have all the tools you need to build a board and simulate its behavior when you use the best PCB design and analysis software. The design and simulation tools in PSpice Simulator for OrCAD and the full suite of analysis tools from Cadence are ideal for designing and analyzing thin-film resistor circuits and components.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.