Surface-mount Technology - Identification

Identification

Resistors

SMD resistors usually are marked with their resistance values using three digits, two significant digits and a multiplier digit. These are quite often white lettering on a black background, but other coloured backgrounds and lettering can be used.

The black or coloured coating is usually only on one face of the device, the sides and other face simply being the uncoated, usually white ceramic substrate. The coated surface, with the resistive element beneath is normally positioned face up when the device is soldered to the board although they can rarely be seen mounted with the uncoated underside face up, whereby the resistance value code is not visible.

For 1% precision SMD resistors, the EIA-96 code is used, as three digits would otherwise not convey enough information. This code consists of two digits and a letter: the digits denote the value's position in the E96 sequence, while the letter indicates the multiplier.

Typical examples of resistance codes

102 = 10 00 = 1000 Ω = 1 kΩ

0R2 = 0.2 Ω

684 = 68 0000 = 680 000 Ω = 680 kΩ

68X = 499 × 0.1 = 49.9Ω

There is an online tool to translate codes to resistance values on the Hobby-Hour website.

Capacitors

Non electrolytic capacitors are usually unmarked and the only reliable method of determining their value is removal from the circuit and subsequent measurement with a capacitance meter or impedance bridge. The materials used to fabricate the capacitors, such as Nickel Tantalate, possess different colours and these can give an approximate idea of the capacitance of the component.

Light grey body colour indicates a capacitance which is generally less than 100pF.

Medium Grey colour indicates a capacitance anywhere from 10pF to 10nF.

Light brown colour indicates a capacitance in a range from 1nF to 100nF.

Medium brown colour indicates a capacitance in a range from 10nF to 1μF.

Dark brown colour indicates a capacitance from 100nF to 10μF.

Dark grey colour indicates a capacitance in the μF range, generally 0.5 to 50μF, or the device may be an inductor and the dark grey is the colour of the ferrite bead. (An inductor will measure a low resistance to a multimeter on the resistance range whereas a capacitor, out of the circuit, will measure a near infinite resistance.)

Generally the larger the physical size, the larger the capacitance or voltage rating will be when all other ratings are held constant. For example, a 100nF 50v capacitor may come in the same package as a 10nF 500V device. If both appear on the same board, the two can be told apart by their usage (context). The device rated 500V will typically be designed into a high voltage or high energy circuit, whereas the 50V rated device would be found in the small signal part of the circuit.

SMD (non electrolytic) capacitors, which are usually monolithic ceramic capacitors, exhibit the same body colour on all four faces not covered by the end caps.

SMD electrolytic capacitors, usually tantalum capacitors, and film capacitors are marked like resistors, with two significant figures and a multiplier in units of pico Farads or pF, (10−12 Farad.)

Examples

104 = 100nF = 100 000pF

226 = 22μF = 22 000 000pF

The electrolytic capacitors are usually encapsulated in black or beige epoxy resin with flat metal connecting strips bent underneath. Some film or tantalum electrolytic types are unmarked and possess red, orange or blue body colours with complete end caps, not metal strips.

Inductors

Due to the small dimensions of SMDs, SMT inductors are limited to values of less than about 1mH. Smaller inductances with moderately high current ratings are usually of the ferrite bead type. They are simply a metal conductor looped through a ferrite bead and almost the same as their through-hole versions but possess SMD end caps rather than leads. They appear dark grey and are magnetic, unlike capacitors with a similar dark grey appearance. These ferrite bead type are limited to small values in the nH (nano Henry), range and are often used as power supply rail decouplers or in high frequency parts of a circuit. Larger inductors and transformers may of course be through-hole mounted on the same board.

SMT inductors with larger inductance values often have turns of wire or flat strap around the body or embedded in clear epoxy, allowing the wire or strap to be seen. Sometimes a ferrite core is present also. These higher inductance types are often limited to small current ratings, although some of the flat strap types can handle a few amps.

As with capacitors, component values and identifiers are not usually marked on the component itself; if not documented or printed on the PCB, measurement, usually removed from the circuit, is the only way of determining them.

Discrete semiconductors

Discrete semiconductors, such as transistors, diodes and F.E.T.s are often marked with a cryptic two- or three-symbol code in which the same code marked on different packages or on devices made by different manufacturers can translate to different devices.

Many of these codes, used because the devices are too small to be marked with more traditional numbers used on through-hole equivalent devices, correlate to more familiar traditional part numbers when a correlation list is consulted.

GM4PMK in the United Kingdom has prepared a correlation list, and a similar .pdf list is also available, although these lists are not complete.

Integrated circuits

Generally, integrated circuit packages are large enough to be imprinted with the complete part number which includes the manufacturer's specific prefix, or a significant segment of the part number and the manufacturer's name or logo.

Examples of manufacturers' specific prefixes:

• Philips HEF4066 or Motorola MC14066. (a 4066 Quad Analog Switch.)
• Fujitsu Electric FA5502. (a 5502M Boost Architecture Power factor correction controller.)