An SMD LED is a semiconductor package that is designed exclusively for automatic population of printed circuit boards using surface mount technology (SMT). SMD is short for "surface mounted device". SMD LEDs are designed to be compatible with existing industrial SMT processes where automated pick and place equipment provides the best placement accuracy and a standard SMT reflow profile can be used to reflow the LED packages on PCBs via solder-capable contacts. They follow the standard nomenclature for SMT packages and are commonly named by their physical footprint. For example, the 2835 package relates to a 2.8 x 3.5 mm footprint, the 3030 package relates to a 3.0 x 3.0mm footprint, and the 5630 package reflects its physical footprint to be 5.6 x 3.0 mm. Other SMD form factors include 3020, 3014, 3528, 4014, 3535, 5050, 5730, etc. This convention has enabled a high level of standardization across the lighting industry, allowing system integrators to easily switch between vendors without any PCB re-design.
The majority of SMD LEDs are supplied in plastic leaded chip carrier (PLCC) packages in which the construction is supported by a molded frame and lead frame. One or more LED dies are mounted on a silver-plated metal lead frame which is surrounded by a plastic cavity. The plastic cavity is formed by highly reflective material that helps increase light extraction by redirecting emissions out of the package. The lead frame serves as the cathode lead for the LED die. The connection to the anode is provided by a wire bond. A phosphor mix fills the cavity to cover the die and bonding wire as well as to provide wavelength down-conversion.
The initial lumen/watt for PLCC packages can look very attractive because the highly reflective leadframe and plastic housing minimize light absorption within the package. However, these components have a poor resistance to chemical contamination and thermal stresses. Discoloration of the plastic housing due to thermal and photo degradation is a dominant color shift and lumen depreciation mechanism in PLCC packages. Oxidation of exposed lead frames and wire bonding breakage due to electromigration or mechanical stresses are another two major failure mechanisms of PLCC packages. These disadvantages may not be a critical concern when they're used in indoor markets, especially commercial and consumer markets. PLCC packages are the usual choice for interior lighting in home, office, retail and hospitality settings. The popularity of PLCC packages is rooted in their high efficacy, low packaging cost, optical design flexibility, a rich ecosystem of compatible components, and advantage in high volume applications where reduced assembly cost is required.
The cost-per-lumen benefit of plastic SMD LEDs eventually attracted the interest of system integrators from the industrial and outdoor sectors which have traditionally been dominated by high power LEDs. SMD packaging using the PLCC architecture was originally intended for mid-power applications where the light sources are not exposed to excessive thermal, electrical and environmental stresses. In industrial and outdoor environments, conventional mid-power LEDs are prone to premature failures because these plastic packages by nature are unable to survive high drive current operation and harsh environments. An alternative to the PLCC packages which is finding its way into high power applications such as high bay lighting and street lighting is the EMC or SMC based Quad Flat No-lead (QFN) SMD packages. EMC stands for epoxy molding compound. SMC stands for silicone molding compound. Compared with mid power LEDs that are packaged in a polyphthalamide (PPA) or polycyclohexylenedimethylene terephthalate (PCT) resin, EMC and SMC compounds have a significantly improved resistance to discoloration at higher temperatures. The QFN design maximizes solder joint reliability and thermal transfer capacity for high flux density applications.
The majority of SMD LEDs are supplied in plastic leaded chip carrier (PLCC) packages in which the construction is supported by a molded frame and lead frame. One or more LED dies are mounted on a silver-plated metal lead frame which is surrounded by a plastic cavity. The plastic cavity is formed by highly reflective material that helps increase light extraction by redirecting emissions out of the package. The lead frame serves as the cathode lead for the LED die. The connection to the anode is provided by a wire bond. A phosphor mix fills the cavity to cover the die and bonding wire as well as to provide wavelength down-conversion.
The initial lumen/watt for PLCC packages can look very attractive because the highly reflective leadframe and plastic housing minimize light absorption within the package. However, these components have a poor resistance to chemical contamination and thermal stresses. Discoloration of the plastic housing due to thermal and photo degradation is a dominant color shift and lumen depreciation mechanism in PLCC packages. Oxidation of exposed lead frames and wire bonding breakage due to electromigration or mechanical stresses are another two major failure mechanisms of PLCC packages. These disadvantages may not be a critical concern when they're used in indoor markets, especially commercial and consumer markets. PLCC packages are the usual choice for interior lighting in home, office, retail and hospitality settings. The popularity of PLCC packages is rooted in their high efficacy, low packaging cost, optical design flexibility, a rich ecosystem of compatible components, and advantage in high volume applications where reduced assembly cost is required.
The cost-per-lumen benefit of plastic SMD LEDs eventually attracted the interest of system integrators from the industrial and outdoor sectors which have traditionally been dominated by high power LEDs. SMD packaging using the PLCC architecture was originally intended for mid-power applications where the light sources are not exposed to excessive thermal, electrical and environmental stresses. In industrial and outdoor environments, conventional mid-power LEDs are prone to premature failures because these plastic packages by nature are unable to survive high drive current operation and harsh environments. An alternative to the PLCC packages which is finding its way into high power applications such as high bay lighting and street lighting is the EMC or SMC based Quad Flat No-lead (QFN) SMD packages. EMC stands for epoxy molding compound. SMC stands for silicone molding compound. Compared with mid power LEDs that are packaged in a polyphthalamide (PPA) or polycyclohexylenedimethylene terephthalate (PCT) resin, EMC and SMC compounds have a significantly improved resistance to discoloration at higher temperatures. The QFN design maximizes solder joint reliability and thermal transfer capacity for high flux density applications.
