SMD LEDs are rectangular or square. As with other SMD components, the edge lengths are included in the type designation. The specifications are given in one tenth of a millimeter.
For example, if the type designation of an SMD LED is 3528, the edge lengths are 3.5 mm x 2.8 mm. The smallest SMD LEDs are 0.4 mm x 0.2 mm, far below 1 mm, while the largest and most powerful SMD LEDs can be 5.6 mm x 5.0 mm or even larger. To ensure that SMD LEDs are not installed incorrectly in production, they have an external marking of the negative electrode. This can be a green color bar, a dot or a notch.
SMD LEDs have good heat dissipation because the package also serves as a heat sink for the LED chip. The good heat dissipation means that the chip temperature drops and the light emitting diode can be operated with a higher current, which in turn causes a higher brightness. Another LED design that features even better heat dissipation is the CoB LED, which stands for chip on board (CoB). In this design, the LED chip has direct heat dissipation to the PCB via a thermal paste. This further improves the efficiency compared to the SMD LED.