The application of surface mount technology (SMT) has become very common, and the proportion of electronic products assembled by SMT has exceeded 90%. With the development of small SMT production equipment, the application range of SMT is further expanded. Aviation, aerospace, instrumentation, machine tools, and other fields are also using SMT to produce a variety of small-volume electronic products or components.
In recent years, in addition to electronic product developers developing new products with SMD devices, maintenance personnel have also begun to repair a large number of electronic products assembled with SMT technology.
At present, the models of chip resistors are not uniform and are set by each chip resistor manufacturer, and the model is particularly long (consisting of more than a dozen English letters and numbers). If the various parameters and specifications of the chip resistors can be correctly proposed when purchasing, then the required resistors can be easily purchased (or ordered).
There are 5 parameters for chip resistors, namely size, resistance, tolerance, temperature coefficient, and packaging.
1. Size series chip resistors generally have 7 sizes, which are represented by two size codes. One size code is an EIA (Electronic Industries Association of America) code represented by a 4-digit number. The first two digits and the last two digits indicate the length and width of the resistor, in inches. The other is a code in millimeters, which is also represented by 4 digits. Different sizes of resistors have different power ratings.
2. Resistance series, the nominal resistance value is determined by series. Each series is divided by the tolerance of the resistance (the smaller the tolerance, the more the resistance is divided), among which the most commonly used is E-24 (the tolerance of the resistance is ±5%).
On the surface of the chip resistor, three digits are used to indicate the resistance value, among which the first and second digits are valid digits, and the third digit indicates the number of zeros followed. When there is a decimal point, it is represented by “R” and occupies one effective digit.
3. Tolerance of chip resistors (carbon film resistors) has 4 levels, namely F level, ±1%; G level, ±2%; J level, ±5%; K level, ±10%.
4. Temperature coefficient The temperature coefficient of the chip resistor has 2 levels, namely w level, ±200ppm/℃; X level, ±100ppm/℃. Only resistors with tolerances of class F use class x, and resistances with tolerances of other classes are generally class w.
5. There are mainly two types of packaging: bulk and strip-shaped rolls.
The working temperature range of SMD resistors is -55–+125℃, and the maximum working voltage is related to the size: 0201 is the lowest, 0402 and 0603 are 50V, 0805 is 150V, and other sizes are 200V.