3.9ohm SMD Chip SMD Resistors:

How do I choose a chip SMD resistor?

Chip SMD resistors are commonly used for pull-up/pull-down operations to ensure a signal is at valid logic level when external devices are removed, voltage divisions, current limiting, and filtering signals at certain frequencies in high-pass/low-pass/band-pass filters. You can use 0Ω resistors as jumpers to disable a sub-section of a circuit. There are three kinds of Chip SMD Resistors:

1. Thin Film or Metal Film resistors are used in high precision applications like audio, medical, or test equipment. They have lower variation (0.1%-2%), a lower temperature coefficient (5 ppm/K), and are less noisy compared to thick film resistors. However, they are more expensive.
2. Thick Film resistors are the most common type of resistor, and are used for most applications. They have higher variation (1%-5%), a higher temperature coefficient (50 ppm/K), and are noisier than thin film resistors. If there are no specific performance requirements, thick film resistors are usually the preferred choice.
3. There are also Metal Foil resistors which provide high rated power and excellent load life stability along with extremely low Temperature Coefficient of Resistance (TCR).

SMD resistor sizes

The shape and size of chip SMD resistors are standardized, with most manufacturers using the JEDEC standards. The size of chip SMD resistors is indicated by a numerical code, such as 0603. This code contains the length and width of the package. So, the imperial code 0603 indicates a length of 0.060" and a width of 0.030".

Although SMD package codes can be given in either imperial or metric units, the imperial code is used more often to indicate the package size. Confusingly, even when the imperial naming convention is used, the metric dimensions are often used during the design of the printed circuit boards (PCBs). In general, you can assume the code is in imperial units, but the dimensions used are in mm. The SMD resistor size used depends primarily on the required power rating, the minimum feature size of the PCB manufacturing, and the limitations of the pick-and-place equipment.