Knowledge Types of LED Packages for Lighting Applications

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An LED package is a fully functional LED light source that produces optical radiation through electroluminescence and, in many instances, photoluminescence within a semiconductor device. LED packages are packaged LED dies (chips) with all elements available to facilitate integration of the light sources with its operating environment. Since LED chips are bare semiconductor dies, the package into which one or more LED chips are assembled should provide electrical connection, environmental protection, thermal conduction, mechanical stability, and wavelength conversion. There are a multitude of solutions regarding the package design and packaging materials employed. Currently on the market four types of LED packages are used as the light sources for lighting applications.

High-power packages attach the LED chip(s) to a metallized ceramic substrate which provides a high efficiency thermal conduction path to draw heat away from the LED junction. The fundamental packaging philosophy of high power LED packages is to maximize thermal performance of the packages while eliminating the use of packaging materials that are prone to thermal degradation. High-power packages deliver a large amount of light in a small package with minimal lumen depreciation over the rated life. These packages are currently mainly used for applications where high flux density and high package reliability are needed. These applications include street lighting, high bay lighting, high power floodlighting and spotlighting.

Mid-power packages are low-cost, plastic-molded lead frame or plastic leaded chip carrier (PLCC) packages that are widely used in interior lighting products such as light bulbs, tubes, troffers, ceiling lights, pendant lights, cove lights, and under cabinet lights. These packages follow the standard nomenclature for SMT packages such as 2835, 5630, 3535, 4014, etc. In a typical mid-power package, the LED chip is mounted on a silver (Ag)-coated metal lead frame surrounded by a plastic cavity. This design gives mid-power packages a high initial efficacy due to the use of high reflectivity materials. The high probability of discoloration of the plastic resin and oxidation of exposed lead frames makes PLCC packages an unqualified candidate for applications that require the light source to have good lumen maintenance and color stability in harsh and high ambient temperature environments. An alternative to the PLCC package is the EMC (epoxy molding compound) plastic package which has an improved resistance to discoloration at higher temperatures and thus is finding its way into high power LED packaging.

Chip-on-board (COB) packages mount a large array of small LED dies onto a metal-core printed circuit board (MCPCB) or a ceramic substrate. COB packages provide a large light emitting surface (LES) with uniform light distribution, making it particularly suitable for applications requiring a very large optical assembly to achieve a directional light output and high beam quality. With an efficient thermal path, the lumen maintenance and color stability of COB LEDs are comparable to high-power packages as long as the operating temperature is tightly controlled.

Chip scale packages (CSPs) strip off superfluous packaging elements such as wire bonding, lead frames, substrates and plastic housings to provide a compact, low-cost alternative to the high-power and mid-power packages. These package-free LEDs are flip-chip assemblies that reduce both the length of thermal path and the thermal resistance along the thermal path. Less is more. The CSP platform makes a lot sense in terms of cost reduction and performance improvement of LED packages.
 
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