Product stability
LEDs, which will be used in outdoor full-color signs, must use robust packaging materials. For example, epoxy encapsulation is very sensitive to near-UV or UV radiation. Moreover, the blue emissions from the LED chip discolor the epoxy over time, decreasing the light output.
A much better material that can be used is silicone encapsulation material. Silicone exhibits superb stability towards heat, near-UV and UV radiation as a result of the use of a stronger siloxane bond.
This enables long life performance of a multicolor or high-brightness LED with little intensity degradation. Additionally, silicone can be formulated with different hardness properties. For example, silicone used in LEDs for full color signs should be formulated elastic to absorb potential thermal stress in the application. This relaxation also helps to prolong the lifetime expectancy of the LED.
A lead frame dissipates a majority of the heat from the LED chip to the thermal pads on the PCB. Therefore, a highly thermally conductive material should be used as the base material used in the lead frame.
Ideally, the lead frame is made from copper, which has the best thermal conductivity factor (min. 350W/m*K) among any currently available metals. Additional silver plating makes the soldering easier and inhibits the growth of tin whiskers in the later product.
Colors and color mixing
In a full color display, it is important to be able to display colors with high purity or saturation. This can only be achieved if the basic red, green and blue colors are highly saturated.
For example, a very saturated blue (dominate wavelength ~465nm) enables very good color mixing with green or red. This will then create highly distinguished mixing colors with excellent saturation.
Of course, the same approach also applies for red (dominate wavelength ~621nm) and green (dominate wavelength ~528nm). To achieve a D65 white color point, the ratio between red, green and blue is estimated to be approximately 3.5:8:1 for wavelengths of 621nm, 528nm and 470nm.
Different wavelengths will determine a different ratio. This means, if you have 200mcd blue light, you need 700mcd red and 1600mcd green lights to generate a neutral white color from RGB multicolor LEDs.
The LED manufacturer can help the display manufacturer by preselecting the proper chips upfront of the LEDs' assembly. Depending on the installation and size of the display, it may be quite important to choose the right internal dice arrangement as well. There are two types of a rmarket: a triangular or "star" arrangement of the chips .(Figure 3 below ) and an in-line or linear chip arrangement.
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| Figure 3: Besides horizontal viewing, the "Star" chip arrangement is also suitable for vertical viewing as well. |
The "Star" chips positioning is especially favorable for centered screens with a 4:3 or 16:9 ratio as this arrangement enables excellent light mixing and high color uniformity from a wide range of viewing angles. People watching or passing the display with an angle of 120 will perceive the screen to be very uniform. Besides horizontal viewing, the "Star" chip arrangement is also suitable for vertical viewing as well .
In-line chip positioning is recommended for screens such as U-TVs, which can be found in soccer stadium across Europe, that exceed the ratio of a conventional screen. Because these displays are very wide horizontally but relatively small in the vertical direction, they are being viewed from directions that can easily exceed 120. The linear placement of LEDs helps to ensure a uniform perception of the display and good color mixing from wider viewing angles (Figure 4, below).
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| Figure 4: The linear placement of LEDs helps to ensure a uniform perception of the display and good color mixing from wider viewing angles. |
Soldering
It is recommended that designers of large displays follow the JEDEC guidelines to assemble a display made of surface mount (SMT) LED components. Display manufactures that used through-hole LEDs before switching to SMT had to be especially careful because SMT components are sensitive to moisture.
The moisture sensitivity level (MSL) indicates how long the LEDs can be exposed to a factory-controlled environment prior to soldering without the risk of moisture intrusion. Moisture inside the LED prior soldering will vaporize immediately during the reflow soldering.
The result of vaporization sometimes can cause the chip to lift or the bond stitch from the lead frame leading to a broken mechanical or electrical connection. This effect is called "delamination" and can lead to field failures of an LED display.
Andreas Pohl is a Field Application Engineer at Avago Technologies.
