Resistors are available in a large number of different package styles and sizes. The most commonly used today are the rectangular surface mount (SMD) resistors, but also the good old axial resistor is still used extensively in through-hole designs. This page provides information on the dimensions of SMD, axial, and MELF packages. It also provides some recommended land patterns for SMD components for solder attach to PCBs.
The shape and size of surface mount resistors are standardized, with most manufacturers using the JEDEC standards. The size of 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".
The SMD package code can be given in either imperial or metric units. In general, 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. The following table lists the dimensions and specifications of commonly used surface mount packages.
|Code||Length (l)||Width (w)||Height (h)||Power|
When designing with surface mount components, the right solder pad size and land pattern should be used. The following table shows recommended dimensions of the land pattern for common surface mount packages. The table lists the dimensions for reflow soldering. For wave soldering, smaller pads are used.
|Code||Pad length (a)||Pad width (b)||Gap (c)|
The size of axial resistors is not as standardized as the SMD resistors, and different manufacturers often use slightly different dimensions. Furthermore, the size of an axial resistor depends on the power rating and the type of resistor such as carbon composition, wirewound, carbon film, or metal film. The following drawing and table give an indication of the dimensions of common carbon film and metal film axial resistors. Whenever the exact size needs to be known, always check the manufacturer datasheet of the component.
|Power rating||Body length (l)||Body diameter (d)||Lead length (a)||Lead diameter (da)|
|1/8 (0.125)||3.0 ± 0.3||1.8 ± 0.3||28 ± 3||0.45 ± 0.05|
|1/4 (0.25)||6.5 ± 0.5||2.5 ± 0.3||28 ± 3||0.6 ± 0.05|
|1/2 (0.5)||8.5 ± 0.5||3.2 ± 0.3||28 ± 3||0.6 ± 0.05|
|1||11 ± 1||5 ± 0.5||28 ± 3||0.8 ± 0.05|
Metal electrode leadless face (MELF) is another type of surface mount resistor package.. The main advantage of using MELF instead of standard SMD packages is the lower thermal coefficient and better stability. The temperature coefficient of resistance (TCR) of thin film MELF resistors is often between 25-50 ppm/K while standard thick film SMD resistors often have a TCR of > 200 ppm/K. The lower TCR of MELF resistors is due to their cylindrical construction. This cylindrical construction also gives the package distinct disadvantages, mainly when the components have to be placed using pick and place machines. Because of their round shape a special suction cup and more vacuum is required.
There are three common MELF package sizes: MicroMELF, MiniMELF and MELF. The following table lists the characteristics of these types.
|MicroMELF||MMU||0102||2.2||1.1||0.2 - 0.3|
|MiniMELF||MMA||0204||3.6||1.4||0.25 - 0.4|
|MELF||MMB||0207||5.8||2.2||0.4 - 1.0|