A common LED chip mounting technology used in the market today is “flip chip” mounting because the active layer (electrical side) is facing down (flipped) while the transparent sapphire substrate is facing up. The chip is attached directly to the substrate (e.g., ceramic). The flip-chip approach offers some advantages over earlier SMT (surface-mount technology) approach including better durability and light performance and lower thermal resistance. Thus, flip-chip technology has been widely adopted by many LED and lighting manufacturers.
But for high-luminance directional lighting (HLDL) applications, flip chips are not ideal. Instead, vertical chip technology offers superior performance for directional lighting. An important difference between flip chip and vertical chip design is the light emitting plane (the p-n junction, shown as a red line in Figure 1, below). The whole emitting surface of a flip chip includes both the top and side walls of the chip (the phosphor layer shown in yellow in Figure 1). A vertical chip’s only emitting surface is on the top of the chip, thus it has a narrower light emitting angle with more light on axis and higher on-axis intensity.
Figure 1 –Illustrating the flip chip vs. vertical chip LED structures, and the relatively narrower angle of light spread from the vertical chip design.
Light output from the sides of a flip chip reduces the performance of the LED for HLDL purposes. It also can degrade light color quality especially for white-light devices, as it tends to create a yellow ring. The Luminus line of vertical LEDs uses a unique white phosphor technology, which delivers high color uniformity for color-over-angle and keeps the emitting area as small as the chip itself. These factors further support the high lumen density needed for HLDL.
The vertical die has lower thermal resistance and the vertical structure transfers heat more efficiently for better dissipation. Many standard flip-chip LEDs cannot support a high current density due to the limitations of horizontal current flow, but the vertical chip can tolerate a higher current intensity—a higher driving current. This means it can deliver higher flux per LED and higher lumen density, or “chip luminance” (the nits of the chip).
The smaller emitting area and narrower beam angle require smaller optics. The absence of side walls also results in better color-over-angle uniformity and on-axis intensity. All these characteristics are ideal for HLDL devices. A vertical chip design can also reduce the number of LED chips needed in a device due to its high flux per LED and the uniform beam color and brightness. Additionally, Luminus has implemented advanced packaging and chip technology to further optimize this product line for directional lighting, including a flat window package, multiple junctions per chip, and a round emitter.
A single vertical chip with multiple p-n junctions offers optical advantages including minimizing or eliminating dark lines in the beam spot and making it easier to collimate the LED light into very tight beam angles (Figure 2, below).Having multiple junctions per chip eliminates the extra wires required when using separate chips. The multi-junction chip allows the device to be run at a higher voltage, which makes it easier to match to a wider range of power supplies. The multi-junction chips give designers the option to run at low voltage or in series at higher voltages, making them a versatile option for a wide range of applications. For example, the Luminus SST-70X WxS chip is available in two versions with maximum drive current of either 5.25 A (6V) or 2.625 A (12V).
Figure 2 – Comparing the beam characteristics of the Luminus SST-70X multi-junction LED (left) with the beam from a typical 4-chip LED (right)
Combining round and vertical design features has an additive effect on performance (the benefits of round emitter design are discussed in the Help Center article “What LED Optical Characteristics Are Best for Directional Lighting?”.
Table 1 below compares vertical chip performance using a round emitter vs. a flip chip with square emitter for several optical configurations. Using the same optics with the same flux of 200 lumens, the vertical chip/round emitter (SST-12-WxH) provides a narrower beam angle, 57% higher on-axis intensity (cd), a higher K Factor (cd/lm), and 19 meters longer throw.
Table 1 – Comparison of square flip chip (3535) vs vertical round emitter chip (SST-12-WxH).
To summarize, the benefits of vertical chip technology for directional lighting are:
- Higher optical efficiency
- Higher on-axis intensity
- Narrower beam angle
- Smaller optics
- Fewer LEDs needed
- Uniform beam color and brightness
Learn more about how to optimize lighting device design and components to achieve the best HLDL performance in the white paper High-Luminance LEDs for Directional Lighting Applications.
Guidance on selecting the right LED for your HLDL application can be found in the Help Center article, “How Do I Select the Right LED for my Directional Lighting Product?” .
* Chip on board (COB) technology uses wire bonding: LED chips are electrically connected to a substrate with wires, in contrast to flip-chips where the electrical connections are bonded directly to the substrate.
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