Gel or Elastomer
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 surfacemount 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.