Attention should be paid to the selection of chip resistors

The application of Surface Mount Technology (SMT) has become very common, and the proportion of electronic products assembled by SMT has exceeded 90%. Our country should begin to enter SMT technology since 80s. With the development of small-scale SMT production equipment, the application scope of SMT is further expanded. In the field of aviation, aerospace, instruments and meters, machine tools, SMT is also being used to produce all kinds of small quantities of electronic products or components.
In recent years, besides the development of new products by SMT devices, the maintenance personnel have begun to maintain a large number of electronic products assembled by SMT technology.
At present, the SMD resistor model is not uniform, set by the manufacturers themselves, and the model is particularly long (consisting of more than a dozen letters and numbers). If you can correctly put forward various parameters and specifications of chip resistance, it will be convenient to purchase (or order) the required resistance.
SMD resistors have 5 parameters, namely size, resistance, tolerance, temperature coefficient and packaging.
The 1. Series SMD resistor series generally come in 7 sizes and are represented in two sizes. A size code is the EIA (American Electronic Industry Association) code, represented by 4 digit numbers. The first two bits and the last two bits represent the length and width of the resistor, in inches. The other is metric code, which is also represented by 4 digit numbers, in millimeters. Different sizes of resistors, the power rating is also different. Table 1 lists the codes and power ratings for these 7 resistance sizes.
2. resistance series, nominal resistance is determined by series. Each series is divided by the tolerance of the resistance (the smaller the tolerance is, the more the resistance is divided), among which the most commonly used is the E-24 (the tolerance of the resistance value is + 5%), as shown in table 2.
Three bit digits are used to represent the resistance on the patch resistor surface, in which the first and second bits are valid numbers, and the third bit numbers indicate the number of zeros after the connection. When there is a decimal point, it is represented by "R" and occupies one significant digit. The nominal resistance code representation is shown in table 3.
3. tolerance chip resistance (carbon film resistor) tolerance is 4, namely F class, + l%; G class, + 2%; J class, + 5%; K class, + 10%.
4. temperature coefficient chip resistance temperature coefficient has 2 levels, that is, w class, + 200ppm/ degrees; X class, plus or minus lOOppm/ degrees centigrade. Only x tolerance is allowed for class F, and the resistance at other levels is generally W.
5. packing mainly consists of two types: bulk and strip coil.
The working temperature range of chip resistor is -55--+125 DEG C, and the maximum working voltage is related to size: 0201 minimum, 0402 and 0603 for 50V, 0805 for 150V, and other sizes for 200V.