It is widely known that the light emitting diode (LED) R&D and commercialization community is striving to produce higher power, bright-white-light LEDs. From both a temperature and UV-resistance standpoint, silicone is rapidly becoming the material of choice for the next-generation chip encapsulant. But as LED manufacturers look to employ silicones in their manufacturing processes, they must be aware of potential problems. Cure inhibition can lead to unacceptable variations in the manufacturing process and thus, the finished product. This article takes a brief look at the benefits of using silicones in high-brightness LED (HBLED) applications, with a focus on potential production floor issues and their solutions.

Material Choices

Figure 1: The effect of UV light on epoxy can be seen in this transmission versus wavelength graph.
Since the introduction of the high brightness LED, manufacturers have used silicone as a packaging encapsulant as high light flux and associated heat prove too much for traditional epoxies. An example of the move to silicone as an encapsulant is the Luxeon lamp, which was introduced by Lumileds (San Jose, CA) several years ago. Lumileds data confirms silicone encapsulants provide a longer optical transmission life than epoxy encapsulants, making silicone a mainstay for encapsulation of both HBLEDs and low-power LEDs.

Figure 2: The effect of UV light on light transmission using curable acrylate is better than epoxy.
Manufacturers of blue LEDs with wavelengths near 405 nm and other LEDs that emit deeper into the UV (365-399 nm) spectrum have concerns regarding the long-term effect of near-UV radiation on an encapsulant’s light transmission. Recent studies show that silicones perform better than acrylates, which perform better than epoxies. The UV VIS spectra shown in Figures 1 through 3 demonstrate the findings.

As companies transition to silicone encapsulants, they must take care because the cure mechanism of many silicone encapsulants can be permanently inhibited or poisoned. Cure inhibition occurs when adjacent substrates, monolayers, or gases slow down or deactivate a crosslinking reaction. Platinum-catalyzed silicones, which can be encapsulating gels or thermosets, generally are two-part systems with each part containing different functional components. These two-component systems can be formulated in various ratios with the most common being 10:1 and 1:1. Generally, the Part A component contains vinyl-functional silicones and the platinum catalyst, whereas Part B contains a vinyl-functional polymer, hydride-functional (Si-H) crosslinker, and cure inhibitor. Cure inhibitors are additives used to adjust the cure rate of the system and are different from the poisons discussed in this article.

The cure chemistry involves the direct addition of the Si-H functional cross-linker to the vinyl-functional polymer, forming an ethylene-bridge crosslink. The vulcanization of addition-cured silicone elastomers can be heat-accelerated. Depending on the specific product, addition-cured elastomers can be fully cured at temperatures and times ranging from 10 minutes at 116 °C to two minutes at 150 °C. Cure conditions vary with product mass.

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