Figure 3. LED Dispensing Applications
Thermal interface material (TIM) dispensing is important for heat dissipation although all LED package types are applicable. The material is applied between the LED package and the LED lamp substrate to transfer the heat from the package to the lamp and ultimately to the fixture. Many of the TIM materials are silicone.

Reflection material dispensing is done around the LED chip to prevent light from coming out of the side wall of the LED. Titanium oxide (TiO2), which is a high viscosity material, is commonly used. The dispenser needs to have excellent location and placement accuracy because the dispenser has to negotiate LED chips as small as

Adhesive dispensing is required for phosphor plate attachment. In this package, adhesive is dispensed on the top of the LED chip and a phosphor plate is then die bonded to that. The material dispensed is usually silicone. A consistent weight is important to avoid fluid bleeding or lessening the adhesion.

Silicone casting is used to cover an LED chip with an exposed phosphor layer. In other words, the phosphor surface needs to be protected after phosphor plate attachment. Pure silicone is dispensed on the surface.

Remote phosphor is the latest package technology and has better light emission extraction than the others because of its unique structure. A space is created between the LED chip and phosphor to avoid yellow light and heat generated from the phosphor being absorbed by the chip. The space between the chip and phosphor is filled with silicone by pure silicone dispensing. In a flip chip package, reflection material is dispensed for the same reason. The space gap is sometimes defined by the amount of silicone dispensed, therefore in silicone dispensing, consistency is a key requirement.

As you can see from the package descriptions above, silicone dispensing is quite popular in LED packaging. However, dispensing silicone is quite challenging. The silicone is stringy and leaves a long tail that has to be cut (Figure 2). If it is not cut cleanly and quickly, a long trail of fluid lays down next to the part instead of going into the cavity. The amount of silicone dispensed is precisely measured because the fluid volume plays a key role in the way the LED performs. If all the fluid doesn’t go into the cavity or place where it’s intended, then the amount of fluid won’t be accurate and performance will suffer. Not only is it messy, but in a tightly packed assembly that tail overlaps into the next package, altering the volume in that LED. For every LED manufactured to be the same, a consistent amount of fluid has to be dispensed.

Z-retraction is the key movement used to cut the silicone tail, but there is a distinct difference between the z-retraction of needle and jetting valves. A needle requires a long retraction in the z-axis. Jetting, on the other hand, needs a short one. Jetting has a unique technology that uses a valve with an active nozzle to easily make the cut. It requires only a tenth of the retraction that a needle does. The retraction is actually very time consuming and often accounts for a considerable percent of dispensing time. Therefore, using a jet vs. a needle not only helps solve the tail problem, but it significantly contributes to improvement in units per hour.

LEDs are becoming more commonplace in all aspects of our life so manufacturers will need to find the most cost effective way to produce them in large quantities while optimizing their color, performance, and product life. There are many manufacturing challenges because their packages are small, they are tightly packed with components, silicone is not easy to work with, and they need to be manufactured with precision, accuracy, and consistency. Fluid dispensing plays an important part in many aspects of LED manufacturing. Because fluid dispensing has been an important component of microelectronic packaging and manufacturing for many years, the equipment, processes, and capabilities are already available and proven. Fluid dispensing equipment manufacturers will continue to innovate and adapt their equipment and processes to play a vital role in the manufacture of LEDS.

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