SMD and COB UV LED Products – A Comparative Overview

Light-emitting diodes are available in various form factors to enable different types of applications. The most commonly available UV LED products are offered as Surface Mount Devices or SMDs. As the industry standard, SMDs are offered in a range of dimensions and specifications, from single chip SMDs to high power SMDs wherein multiple chips are encapsulated by a single lens. Currently in the UV LED industry, square-shaped packages are most common with a length/width varying between 3mm to 6mm.

Figure 1A: An SMD where the LED chip is mounted on ceramic substrate.

Figure 1B: An SMD where the LED chip is mounted on a copper substrate.

Since the packaging technologies used to manufacture SMDs may differ significantly, the range of their performance can be very broad. While many UV LED SMDs on the market today are created from ceramic packaging material to reduce costs, others utilize aluminum or copper-based materials to further boost performance, reliability, and heat dissipation abilities. In addition, availability of different types of optics such as a domed lens, a flat panel lens, with varying grades of UV transmission, viewing angles, and absorption, bring about a variety of products to choose from. Because of the many existing packaging techniques, SMD prices vary greatly and so it becomes important to evaluate each model while keeping in mind the price, the optical output, and the performance of the product.

SMD products are usually mass-produced and readily available, making them convenient for mounting and testing on PCBs. These ready-to-go packages can allow system designers to have flexibility in design as SMDs can be easily configured to the desired form depending on the application. Furthermore, replacement of failed components is much easier in the case of individual SMDs. This versatility of SMDs allows system engineers a lot of flexibility in LED design with minimal adjustments to existing overall system design. This is one of the reasons why SMDs are popular at the prototype stage.

Figure 2: An array of SMDs mounted on a metal core printed circuit board (MCPCB).

One of the more recent developments in LED packaging has been Chip-On-Board (COB) LED modules, as an alternative to SMDs. With reduced thermal resistance and increased optical advantages, COB solutions are made by directly bonding a single or multiple chips onto a custom size, thermally efficient PCB with a unique circuit layout.

The main difference between the two packaging methods is that in COBs, the chip is bonded directly onto the PCB by the LED manufacturer, eliminating the need for soldering on the customer’s end, as in the case with SMDs. This allows for easier installation into custom systems, greater chip density potential, and decreased thermal resistance.

Figure 3A: An SMD mounted on a metal core printed circuit board (MCPCB).

igure 3B: A COB module with an integrated metal core printed circuit board (MCPCB).

Without the need to purchase a separate PCB or the need to solder the package onto a substrate, COBs are ideal for “plug and play” applications. This minimizes the extra steps of designing and configuring an additional PCB, as the COB solution is delivered as an integrated solution by the manufacturer. There are various forms COB solutions can take, including ring shapes, large rectangular arrays, narrow light bars, etc. which can be adopted for virtually any application. Simulations can help identify the optimal optical design based on measures such as irradiance/dosage, area of illumination, system design, and size/power restrictions.

Moreover, COBs can encompass a much greater packing density in terms of the LED chips, which facilitates system designers to develop dense, uniform arrangements of flux output, directing the radiant flux more efficiently. Furthermore, with custom optics options, there is the potential of various viewing angles or molded optics for the COB. These characteristics are especially important for applications such as curing and disinfection because uniformity is paramount to creating crisp images or for ensuring every part of the surface is equally sanitized.

Figure 4A: A high density, bare die COB module in which the LED chips are bonded 0.2 mm apart for high flux density and uniformity.

Figure 4B: A high density, lensed COB module in which an array of LED chips is encapsulated by a custom optic.

COBs exhibit lower thermal resistance than SMDs, and this effect is even more pronounced for arrays with more LED chips. Thermal resistance can loosely be defined as the total module heat retention. It is an aggregate value of the thermal resistance of the chip, board, and heatsink, indicating the rate at which the UV LED junction temperature rises with the optical output. In the long run, a lower junction temperature improves the lifetime of the chip and prevents thermal decay. With the elimination of the SMD layer, COBs allow for a direct thermal path from the chip to the board and are valued for high power UV applications that have immense cooling requirements.