IVEC Insights

SMD Resistors in Thick-Film and Thin-Film Technology – Structure, Series, Differences

SMD resistors, manufactured using thick-film or thin-film technology, are likely among the most common components on a PCB. Both differ significantly in manufacturing, structure, and electrical parameters, with further distinct differences existing within each technology. With few exceptions, all featured KOA series are tested according to AECQ-200.

Thick-Film Resistor Structure

For both thick-film and thin-film resistors, aluminum oxide (Al2O3) is used as the ceramic substrate material. In thick-film technology, the actual resistive material consists of a ruthenium oxide (RuO2)-based paste, which is applied to the ceramic by screen printing and then fired. The “thickness” of the RuO2 paste is in the range of 10-14µm. More precisely, the thick-film paste is a mixture of RuO2 (conductive) and small glass particles (non-conductive). Different paste mixtures are used depending on the resistor series and resistance value.

To achieve the desired, precise resistance value, the resistive layer is adjusted with a laser, a process known as trimming. Depending on the resistor type, various incision patterns are possible. For example, the L-cut is frequently used.

Naturally, an SMD resistor also requires terminals. In most cases, these consist of three layers (silver and NiCr, nickel barrier layer, tin layer), with the inner top termination (AgPd) forming the transition to the resistive layer. On the surface, the resistive layer is covered by an inner glass layer and an epoxy resin layer.

KOA Thick-Film Series

The various KOA series exhibit partly very different (electrical) properties.

  • The standard thick-film series RK73 alone is available in variants RK73B (Tol. 5%, TC 200ppm/K), RK73H (Tol. 1%, TC 100ppm/K), RK73G (Tol. 0.5%, TC 50ppm/K), and as a jumper RK73Z in sizes 008004 to 2512 inch.
  • The SG73 series is equipped with higher rated power and, above all, pulse resistance. Depending on the type of pulse and precision requirements, the variants SG73, SG73P, SG73S, and SG73G are offered.
  • The HV73 series is designed for higher continuous voltages. Due to an additional internal insulation against overvoltages, an operating voltage of 800V can be applied, for example, in size 1206. For size 2512, up to 3000VDC are possible. Accordingly, the HV73 is also a high-ohm resistor, with values up to 100MΩ being offered.
  • For higher continuous loads, the Wide Terminal Series WK73R is particularly suitable. As the name suggests, the terminals are located on the long sides. This enables, among other things, improved heat dissipation into the PCB. Thus, a rated power of 1.5W is possible for size 0612, and even up to 3W for size 1225.
  • Also noteworthy are KOA’s low-ohm/shunt series in thick-film technology, particularly the SR73, UR73V, and WK73S series. UR73V and WK73S achieve minimum values of 10mΩ. The UR73V2B in size 1206 can handle up to 1W, while the WK73S3A in 1225 can handle up to 4W.
  • Finally, a new thick-film series with special properties in terms of precision should be described. The RS73 allows initial tolerances of ±0.1% and a TC of ±25ppm/K, values that were previously reserved only for thin-film resistors. The long-term stability of the RS73 is also exceptionally good, and the RS73 has better ESD compatibility than a thin-film component with comparable parameters.

Thin-Film Resistor Structure

Thin-film resistors are also called metal film resistors. Thin-film technology is based on a significantly more complex manufacturing process under cleanroom conditions. A very thin nickel-chromium layer is applied to the ceramic substrate using the sputtering process. The layer thickness ranges between 10 and 100nm. The final structure of the resistive element is then created by a photolithographic process. Laser trimming is then used in the designated areas to achieve the desired resistance value (Figure 3).

 

 

For thin-film resistors, the terminals also consist of three layers (CrNiCu and NiCr, nickel barrier layer, tin layer), with the upper termination overlapping the resistive layer. Here, too, the resistive layer is covered by epoxy resin layers.

KOA Thin-Film Series

KOA’s thin-film series are not as numerous and varied in structure as KOA’s thick-film series, but each series has a high number of variants, due to the broader spectrum of initial tolerances and temperature coefficients.

KOA’s high-runner in the thin-film sector is the RN73 series. This is available in the variants RN73R and RN73H. The lowest initial tolerance possible is ±0.05%, and the tightest TC is ±5ppm/K. As is typical for thin-film, RN73R/H also feature high long-term stability over several thousand hours. The RN73H differs from the RN73R by an additional internal SiO2 protective layer against moisture penetration on the resistive material, making the RN73H suitable for demanding automotive applications (e.g., engine compartment).

If a thin-film resistor with higher power is required, the wide-terminal type WN73H can be used. Here, too, the terminals are located on the long sides of the resistor for better heat dissipation, and the internal structure, like the RN73H, is protected with an SiO2 protective layer. The two sizes 0306 and 0612 are available. For example, the rated power of the WN73H2B in 0612 is specified as 1W – more than double that of the comparable RN73H2B in 1206 with standard terminals.

Furthermore, KOA also offers special series in thin-film technology based on silicon, such as a high-precision voltage divider in an SOT-23 package, series name RTY. Also outstanding is the high-voltage voltage divider network of the HVD series. Here, precise voltage division of max. 1000V and a division ratio of 10:1 up to 1000:1 is possible.

Differences Between Thick-Film and Thin-Film Resistors

Outstanding properties of thin-film resistors are certainly precision and long-term stability. Even though KOA is venturing into areas with the new thick-film series RS73 that were previously reserved only for thin-film, for very small tolerances and tight TCs, as well as very low resistance drift over many thousands of hours, thin-film technology is still indispensable.

When it comes to transmitting signals as unaffected as possible, the metallic thin-film element shows better values. Thick-film resistors are at a disadvantage regarding current noise, as the granular structure of the resistive paste affects the signal quality. The higher the resistance value, the higher the proportion of glass particles in the paste, and the stronger the noise.

With regard to ESD and pulse resistance, as well as rated power (for comparable size), the thick-film series have an advantage over thin-film. Here, the very robust thick-film paste pays off. As can be seen from the multitude of series, thick-film resistors are applicable in a wide variety of applications and are generally a comparably more cost-effective solution.