When working together, the packager and silicone pro vider should take into account several details. One major consideration is the methodology used when packaging an LED, such as surface mount LED, compression molding or injection molding. Besides that, the end use of the LED and the longevity of the LED must be addressed. As we often find, producing an LED is a game of compromise – durability versus reliability. We will explore some of the basics of silicone to get a better understanding of the material. Also, we will delve into a few questions to be asked when packaging an LED, such as what type of material to use or what RI should be targeted. In all cases, the necessity of a relationship between the silicone provider and the LED manufacturer will become clearer.
Understanding the basic structure of the silicone polymer is the first step in deciding what silicone to choose for a particular LED application. First, we have the basic structure of the silicone polymer (Figure 1). Silicone polymers are chains (backbones) comprised of repeating Si-O units, termed siloxane, with organic groups occupying any of the remaining bonding sites (R) on the silicon atom not already occupied by oxygen atoms. Because of this combination, these polymers are often referred to as polyorganosiloxanes.
During the polymerization process, there are two main factors that are controlled: the substituent groups and the degree of polymerization. Although many combinations are possible, the main pendant groups (R) attached to the silicon atom of the polymer backbone can be methyl, phenyl or trifluoropropyl. Generally, a material is referred to as a “polymer” if a molecule contains only one type of organic group, while it is called a “copolymer” if a molecule contains a combination of substituent groups. Altering these substituent groups is how one is able to control many factors about the polymer important to the LED packager such as permeability or Refractive Index (RI).
Furthermore, all silicone polymers can be synthesized to a desired degree of polymerization. The degree of polymerization dictates the average molecular weight, which in turn governs the viscosity. A silicone polymer may possess a viscosity close to that of water (20 cP) or be so large as to be a solid (millions of cP). This aspect plays a crucial role in how the silicone will be used for the LED packager. Whether one needs gels of varying durometers or elastomers of varying durometers, altering the degree of polymerization can help optimize a formulation for a specific application.