The accuracy of low resistance sampling resistors (usually at the milliohm or even micro ohm level) is crucial for the entire measurement system. The installation method directly affects the accuracy, stability, and long-term reliability of the measurement. Here are several key installation methods and their core considerations, following the principles of minimizing additional impedance, optimizing heat dissipation, and reducing stress:
Kelvin connection (four wire connection)
Principle: This is the gold standard and core installation requirement for measuring low resistance resistors. It uses two independent pairs of wires/traces:
Current terminal: A pair of thicker wires/traces used to carry the main current through the resistor.
Voltage detection terminal: Another pair of wires/traces is specifically used to measure the voltage drop across the resistor. The current flowing through this pair of wires is extremely small (close to zero).
Key installation points:
Physical isolation: In PCB design, voltage detection pads must be directly and tightly connected to the metal electrodes at both ends of the resistor (ideally the voltage sampling points inside the resistor), and must be physically isolated from the pads of the current path. Usually achieved through special pad shapes (such as split pads) or slots.
Wire separation: Voltage detection wires should be as short and wide as possible (to reduce their own impedance impact) and kept away from high current wires to avoid induced noise. It is best to trace wires inside the PCB or use ground plane shielding.
Connectors/Terminals: If external connectors or terminal blocks are used, it is necessary to ensure that the current terminals and voltage detection terminals are completely independent and isolated.
Purpose: To completely eliminate the influence of current lead resistance, contact resistance, and solder joint resistance on voltage measurement, and only measure the true voltage drop at both ends of the resistor body.
Optimization of welding process
Solder selection: Use a sufficient amount of high-quality solder (such as SnAgCu lead-free solder), but avoid excessive amounts that may cause the solder to climb onto the resistor or form solder beads. Some ultra high precision applications even consider low resistance special solder (such as solder with higher silver content).
Welding temperature and time: Strictly control the welding temperature curve (reflow soldering or wave soldering) to avoid overheating. Excessive temperature or prolonged welding time may:
Changing the microstructure of resistance alloys leads to permanent drift in resistance values.
Damaging the connection points or substrate inside the resistor.
Intensify the growth of intermetallic compounds between the solder alloy and the resistance electrode, increasing the contact resistance.
Solder joint quality: Ensure that the solder joints are full, smooth, and well wetted, without virtual soldering, cold soldering, voids, or cracks. Poor solder joints can introduce significant and unstable additional resistance.
Integrated heat dissipation design
PCB copper foil area: Use large-area copper plating below and around the resistor. This not only helps to reduce wiring resistance (which is particularly important for current paths), but more importantly, provides a huge heat dissipation area to quickly dissipate the heat generated by the resistor. The thicker the copper foil (such as 2oz, 3oz), the better the heat dissipation effect.
Heat dissipation through holes: densely drill heat dissipation through holes in the copper plated area below the resistor to transfer heat to the inner copper foil or back copper foil layer of the PCB, further expanding the heat dissipation area. The via needs to be filled with tin to enhance thermal conductivity.
Air circulation: In the system layout, try to place the sampling resistor in a location with good air circulation and avoid being close to other high-temperature components. If necessary, forced air cooling can be considered.
Thermal conductive materials: For extreme power applications, thermal conductive silicone grease can be applied between the top of the resistor (if allowed) and the heat sink, or thermal conductive pads can be installed. Even specialized power resistors with heat dissipation substrates (such as TO247 packaged resistors) can be used and directly fixed on the heat sink.
Minimize mechanical stress
Avoid bending stress: During installation and welding, avoid applying bending, twisting, or impact force to the resistor body or pins. These stresses may alter the characteristics of the resistive material or cause internal connection microcracks, leading to resistance drift.
Thermal expansion matching: Consider the differences in thermal expansion coefficients of resistance materials, PCB substrate materials, and solder. Excessive differences can generate shear stress during temperature cycling, leading to fatigue cracking of solder joints or internal damage to resistors. Choosing a resistor type with good CTE matching (such as thick film resistors, which are usually more compatible with FR4 PCB than some precision alloy foil resistors) can help alleviate this problem.
Fixed: For large or high power resistors, additional mechanical fixation (such as brackets or buckles) may be required in addition to welding, but the fixation method cannot introduce additional stress to the resistor body.
Connection method selection
PCB surface mount: currently the most mainstream method. SMT resistors (such as 2512, 3720 and other large-sized packages) are easy to automate production, have good thermal performance (through PCB heat dissipation), and are easy to achieve precise Kelvin connection design. It is the preferred option.
PCB through-hole insertion: suitable for certain specific high-power or situations that require additional mechanical strength. Pins can provide a certain heat dissipation path. But it is still necessary to ensure the implementation of Kelvin connection (four wire connection).
Screw fixed terminals: commonly used in high-power splitters. The resistor itself has screw holes or copper bars, which are directly fixed to the busbar or terminal through bolts.
Key point: It is necessary to ensure that the contact surface is flat, clean, and free of oxidation. Use appropriate washers and tighten bolts with the specified torque to ensure even and sufficient contact pressure without causing deformation. Contact resistance is the main source of error in such installations and requires special attention. Usually, a four wire connection is also required.
Summary of core principles:
Mandatory use of Kelvin connection: This is the absolute basis for precise measurement of low resistance values, and any installation method must be designed around this principle.
Minimize parasitic resistance: Reduce additional resistance introduced by leads, contact points, and solder joints by optimizing welding, selecting appropriate connection methods, ensuring clean contact surfaces, and pressure.
Strictly control thermal effects: optimize heat dissipation design to prevent resistance changes (temperature coefficient) and long-term aging caused by self heating of resistors, while also controlling welding heat input.
Eliminate mechanical stress: Avoid unstable or damaged resistance caused by stress during installation and operation.
Matching process and requirements: Select the most suitable resistor packaging type (SMT/THT/screw) and corresponding installation details based on power level, accuracy requirements, space limitations, and production conditions.
Following these installation principles and methods is necessary to ensure that low resistance sampling resistors can achieve their expected high precision and reliability in practical applications.