Fixed SMT resistors are especially ideal where board real estate is at a premium
Surface-mount resistors sit on top of PCBs, where they are soldered onto mating landing pads. Their small size is ideal for space-constrained boards and they are used in mass circuit-board production. Available in either thick- or thin-film options, thin-film surface-mount resistors offer improved precision and thick-film surface-mount resistors are typically less expensive to use. In either case, a resistive metal alloy film is placed between a ceramic base and glass/epoxy coating and connected to terminating conductive edges.
Available in such standard sizes as 0805, 0603 and 0402, a surface-mount resistor’s claim to fame is that SMT technology minimizes stray inductance and capacitance in wire leads that can hamper circuit function. They are particularly appropriate for high-speed, high-frequency and ultra-compact designs, and they have fewer unwanted RF signal effects. Surface-mount resistors can be used in flat-pack and chip-carrier packages.
In addition to size advantages, SMT packing density is high so that components can be mounted on both sides of a board when necessary. Additionally, there can be a greater number of connections per component. Surface-mount resistors are typically suitable for high-volume production, enabling a lower unit assembly cost that is not available with through-hole technology.
While the capital investment necessary for SMT equipment and production is higher than its through-hole counterpart, set up time for producing each board is less and fewer holes are necessary on the board resulting in simpler and faster assembly. And they perform better when shake and vibration conditions exist. Finally, because of the small package and lead inductance, surface-mount resistors offer better EMC compatibility.
The 4-2176055-8 from TE Connectivity belongs to the RL73 Series. This thick-film chip resistor for current sensing features a special metal glaze resistive element and nickel barrier layer under the solder that prolongs terminal life. The RL73 Series satisfies the demand for a low-ohmic shunt resistor that acts as a current sensor.
Features include up to 2 watts at 70° C, eight available chip sizes ranging from 0201 to 2512, and clearly-marked values on the resistors. Applications include audio, communications, automotive, low-voltage power supplies and power management applications.
The RK73G1ETTP1301F fixed single-surface mount resistor by KOA Speer Electronics, is a high precision resistor with a T.C.R of ±50ppm/°C and tolerance of ±0.5 percent or ±1 percent This thick-film resistor is AEC-Q200 qualified, and products with lead-free terminations meet EU RoHS. They are often used in environmental applications.
Figure 1: Dimensions and construction of the RK73G by KOA Speer Electronics. (Source: KOA Speer Electronics)
Yageo’s SR0805JR-0710RL surge chip resistor is a fixed single-surface mount resistor that is RoHS-compliant and halogen-free. Often used in telecommunications and power supplies, the SR0805JR-0710RL features high component and equipment reliability, surge withstanding voltage and pulse withstanding voltage superiority, and the reduction of environmentally hazardous waste.
The aforementioned SMT resistors are all thick-film examples. In comparison, a thin-film resistor, the NPCT50-50W-50ROJ, is available from Aeroflex. With this high-power chip, available in both 50 and 100 ohms, when they are mounted on an appropriate heat sink, they offer high-power dissipation in terminations and are balancing resistors in Wilkenson power divider networks. They have a power rating of 5 W and a tolerance of 5 percent. This resistor is also EU-RoHS complaint.
Surface-mount technology use has grown to a point where it has surpassed many of the through-hole technology advances and, while the latter is used more often in text and prototype applications, SMT is increasingly used in prototype situations today.