In PCB assembly process, six precautions before using SMT chip resistors

It is well known that in SMT processing, the detection and selection of chip resistors is critical. The chip resistor is the most commonly used electronic component. However, some chip resistors use small encapsulation, making it easy to confuse the chip resistors during usage. The following FASTPCBA will introduce the precautions before using the chip resistor.

1.When the technician uses a multimeter to measure the chip resistance, the power supply in the circuit should be disconnected, and one end of the chip resistor should be disconnected from the circuit to avoid parallel connection with other circuit components, which will affect the accuracy of the measurement value. When measuring resistance, you cannot use both hands to touch both ends of the meter at the same time. This will form a parallel connection between the chip resistor and the body resistance, which will affect the accuracy of the measurement value. To measure a high precision chip resistor, a resistance bridge measurement is required.

2. Before using the resistor, use a multimeter to measure the resistance value, and check unless it’s correct then use. A chip resistor with a text mark, with the mark side facing up when mounting, so that it’s convenient for later inspection.

3.Potentiometers are prone to noise and other faults after usage, and unpackaged potentiometers with switches have a higher probability of occurrence. The contact resistance is unstable mainly due to damage of the resistance film. In less severe conditions, the resistive film can be cleaned with alcohol to remove dirt and toner from friction. In more severe condition, replacing the new potentiometer can be considered.

4.The choice of rated power. The use of high-power chip resistors increases the volume and  cost, which is not conducive to the design of the circuit. The power should not be too small, which will affect the normal use of the circuit. Generally, the value of rated power is twice of the actual power.

5.The choice of the size of the error. The choice of chip resistor is generally ±10% of the circuit diagram value. Where individual accuracy requirements are high, they may be indicated separately.

6.Since a large number of small and ultra-small resistors are used in electronic devices, a soldering iron tip is used for welding, and the power is below 30W . The lead wire cannot be cut too short to avoid heat transfer into the resistor internal during welding, which causing errors.