PTB >> HBLEDs: Tackle Your Silicone Packaging Problems

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 UVresistance 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

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.

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 crosslinker 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.

In an encapsulating gel or thermoset composition, the material’s cured mechanical consistency is particularly sensitive to the final crosslink density. In other words, materials with low-crosslink density (like silicone gels) are greatly impacted by inhibition. Weak cure inhibition on a substrate’s surface causes the gel to appear cured in bulk, but “wet” at the substrate interface. Modest cure inhibition results in a lower final cured durometer (i.e. softer) than expected, while severe cure inhibition can lead to complete cure elimination (the mixed gel’s form exhibits little or no perceptible viscosity increase after the nominal cure time).

Avoid Cure Inhibition

It is imperative to analyze adhesives, plastics, and elastomers for cure inhibition prior to selection for use near or in contact with a silicone compound. This analysis should include materials used in any transfer containers, dispensing hoses, or utensils that come in contact with the silicone components. The following list gives a partial inventory of suspect materials:
Adhesives
• Amine-cured epoxies
• Amide-cured epoxies
• UV-cured epoxies
• Elastomers
• Peroxide-cured silicones
• Organotin-cured silicones
• Certain grades of addition-cured
silicones
• Chlorinated elastomers
• Polyvinylchloride (PVC), plasticized
• Certain grades of THV fluoropolymers
• Neoprene
• Buna N (nitrile)
• Natural rubber
• Latex
Plastics
• Some chlorinated plastics
Other Materials
• Certain sulfur-containing compounds,
especially thiols, sulfides, sulfites, and
thioureas
• Certain tin-containing compounds,
especially tin salts, and tin soaps
• Some mold-release lubricants or other
agents
• High-pH organic materials
• Wood, leather, and clay
• Some solder flux
• Paper tapes and masking tapes
• Certain grades of vinyl tapes
• Cellophane tapes

Testing candidate materials that may contact a curing optical gel for compatibility is the most effective way to eliminate “poisons.” It is important to test each material’s particular manufacturer’s grade. For example, some vinyl electrical tapes show no cure inhibition at all, whereas others may exhibit inhibition. Placing fragments of the suspect material into uncured silicone can reveal whether or not an adverse reaction has occurred (after the silicone has undergone the proper cure schedule).

The use of silicones in HBLED encapsulating applications can provide device developers with significant benefits. Understanding the proper use of silicone systems, and, more specifically, the elimination of cure poisons, can ensure the smooth transition from prototype to finished product.

This article was written by Bill Riegler, product director-engineering materials, and Stephen Bruner, marketing director, at NuSil Technology LLC. For more information, contact Mr. Riegler at billr@nusil.com or visit http://info.ims.ca/5785-201