When working together, the packager and silicone provider 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.
The packaging process of LEDs is always evolving. Historically, with surface-mount-type LEDs, silicone gels were used as an encapsulant to fill the void between the diode of an LED and a lens (Figure 2). Lenses were made of various materials, like glass or polycarbonate, and adhered onto the housing of the LED in a separate process made of various materials such as glass or plastic. This design left a gap between the diode and the lens. Consequently, a material was needed to fill this gap and, for several reasons, silicone was used.
One main reason was silicone gel’s low modulus, which protected wire bonds and left them unaltered when the silicone was cured. Also, silicone was used to increase the efficiency of light transmittance from the diode to the lens. Ultimately, this increases the amount of light that reaches out into the environment. By increasing the silicone gel’s RI to be closer to the diode emitting the light, the amount of light and power can be better transported to the lens. Other reasons silicone gels have been utilized to fill the gap between the die and lens over epoxies are their ability to adhere to multiple substrates; optical stability; low modulus; and, again, the ability to alter the RI.
While this conventional method worked, LED packaging has undergone several optimizations.1
The next evolutionary step was to begin using a silicone material for the lens and the encapsulant. Previously, the CTE differences between the silicone encapsulant and the lens made of a different material would cause problems such as voids or bubbles. By switching to a higher durometer silicone elastomer for the lens, the CTE of the lens and the encapsulant were very similar. This reduced the amount of voids due to the silicone’s much larger CTE compared to thermoplastics and more importantly decreased cure time, as the gel fill and silicone lens could be rapidly heat cured together without voids. Increased adhesion of the encapsulant to the lens was another benefit because silicone adheres to silicone well.
This alteration was short lived, however, as manufacturers quickly realized that if silicone can be used in two separate joining parts, why couldn’t they combine them and drastically reduce their work in progress? The need to combine the encapsulant and lens also required a new packaging process to achieve this goal. With processes such as overmolding or using injection or compression molding, packagers combined the encapsulant and lens into one silicone material and one step (Figure 3). The material of choice is one that is compatible with the equipment for compression or liquid injection. This silicone must also protect the diode and transmit the light generated efficiently. Molding processes allow for lower-cost and higher-volume LED manufacturing in a fraction of the time. Also, by eliminating multiple separate components, molded LEDs can be made smaller, allowing for more LEDs to be produced per shot in the same surface area.
We begin to see how important the relationship is between the packager and the silicone material provider. As dynamic as the LED market is, changes in formulations need to become available as packaging procedures and demands evolve. However, silicone formulary changes are not only dictated by processing needs but by performance and demand.
Increasingly, the demand for High Brightness LEDs (HBLEDs) is growing. From back lighting for a laptop computer to illuminating a room, the demand on silicone to transmit this increase in power is rising. To accomplish this, the industry has had to address two separate, but related, issues concerning silicone: a) what phosphor to add and how to add it to produce brighter LED light, especially white light, and b) how to increase the RI of the material housing the diode to allow a greater transmittance of the brightness?
Typically, in white High Brightness LEDs (HBLEDs), it has been found that using a 405nm blue gallium nitride LED covered by a yellowish cerium doped yttrium aluminum garnet (YAG) phosphor coating produces the desired light. This brought about a challenge in itself when considering the most efficient way to introduce the phosphor to the light path of the diode to the environment. HBLED packagers initially added phosphor by mixing it into the silicone gel to get the desired effect. However, many factors — such as phosphor concentrate and silicone temperature or viscosity — affected the efficiency of this process.2 After silicone packagers moved from surface-mount LED processing to more advanced methods, the phosphor added could be introduced into the equation separately, on top of the diode and before the silicone is molded. This effectively allowed for the increased brightness and increased efficiency of phosphor addition for the LED packager.
Next, to address the second issue, silicone manufacturers needed to assist the LED packager and develop a material with a higher RI to transmit this increased brightness. By increasing the refractive index, the material is able to reduce internal reflections improving light extraction. Previously, a dimethyl silicone was used and had heritage in several industries. As described above, altering the R groups attached to the siloxane backbone can alter the silicone polymer in many ways. For example, adding trifluoropropyl groups to the siloxane backbone lowers the refractive index to around 1.38. More important however, by altering the substituent R groups on the backbone of the silicone polymer chain with phenyl groups, a higher RI can be achieved compared to a traditional methyl system. Paramount to achieving a higher RI is the addition of phenyl. Depending on the amount of phenyl added, the RI can range from 1.43 to greater than 1.55 (Figure 4). In addition, adding phenyl provides a secondary benefit of lowering permeability of moisture and other various gases.3
However, LED applications that use silicones with increased RI should also take into account the increased thermal energy associated with increased brightness. While the physical properties of silicone can withstand a wide temperature range, typically -100°C to +200°C, studies have shown that phenyl containing silicones exposed to higher temperatures over time may begin to discolor, ultimately decreasing the transmittance over time.4
Here is where the manufacturer must make a decision. If a need for higher RI silicones is required, proper thermal management must be considered such as thermally conductive interfaces and heat sinks attached to the LED assembly. The more heat generated, the greater the need and size of the heat sink. This need for thermal management and higher phenyl content increases the cost of the LED. Applications that will exist in harsher environments and have a need for a longer operating life would benefit from using a dimethyl silicone such as the headlight of a car. On the other hand, lighting inside the car that is in a controlled environment could benefit from a higher IR, phenyl-containing silicone.
Obviously, the relationship between the LED packager and the material provider is crucial. Both entities are responsible in different ways for ensuring the end product is high quality. Because LEDs are a relatively new lighting source, industry standards are continuously refined as more is understood about LED performance in specific markets.
On the material level, Accelerated Life Testing (ALT) and other accelerated aging tests have set test conditions and industry standards for various applications. While it is important for the silicone material provider to supply initial ALT data, the LED packager has the most control over design, materials and processing. The packager must, ultimately, develop rigorous ALT testing regimes that ensure the chosen materials not only give expected light output over time, but also high yields once in production. A close relationship between the material supplier and LED packager can ensure the best candidate is chosen for the specific design and process, which will accelerate time to market and keep high yields.
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