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Video walls using multicolor LEDs are in demand, especially if the LED can be mounted using surfacemount technology. This technical article deals with the latest RGB LED package approaches for fullcolor indoor and outdoor displays, as well as technical considerations, which designers should take into account when designing these large full color displays.
There are different packages available in the market that can be used to build up a video wall using a surface mount approach.
For instance, PLCC-2 LEDs can be used to build up a mono-chromatic display or full-color display while one pixel is made from multiple LEDs. On the other hand, due to the size of a PLCC-2 LED, the pixel pitch of such a screen can be quite large. This might be acceptable in screens with a certain distance to the viewer, at least 10-15m.
In situations where the viewer is closer to the screen, like at a shopping mall, a finer resolution would be more favorable. To achieve smaller pixels, 3- in-1 RGB packages can be found in the market as well. The most popular 3-in-1 RGB packages are based on either a PLCC-4 or PLCC- 6 platform.
Avago released a new 3-in-1 RGB PLCC-4 LED (ASMT-QTB0). This PLCC-4 LED has highly reflective white package material, which provides a high extraction of light. Another feature required for a high contrast ratio is the black surface on the upper side of the LED.
|Figure 1: The pixel pitch of the aforementioned PLCC-4 can go down to 8mm for high resolution video screens.|
The black surface needs to be non-reflective to absorb as much light as possible shining onto the screen from various sources, e.g. spot lights or the setting sun. The pixel pitch of the aforementioned PLCC-4 can go down to 8mm for high resolution video screens (Figure 1 above ).
A completely black PLCC-4 leads to an even better increase of the contrast ratio. The visual performance and appearance of indoor full color screens can be improved tremendously using a LED with a completely black package.
Avago's ASMT-QTC0, for example, provides a very small pixel pitch down to 8mm. This makes the screen appear like an LCD display, but it can be designed much bigger compared to what can be done with current LCD technology.
Another popular package platform for full color video screens is a PLCC-6 LED which has 6 leads with an individual anode and cathode for each chip, like Avago's ASMT-YTB0. A high reflective white package enables good color mixing and high brightness performance. Because of the high brightness performance, it can be used in outdoor full-color screens as well.
|Figure 2: For better product performance, the recommended pixel pitch is 10mm.|
The black surface helps to achieve a high contrast ratio in indoor and outdoor video walls. Additionally, the small package design enables a small pixel pitch to 8mm. For better product performance, the recommended pixel pitch is 10mm (Figure 2, above ).Thermal management and LED component behavior
Higher brightness is considered to be a good tool to boost the visual performance of full-color displays. The ASMT-QTB0 and ASMT-YTB0, for example, have a typical total brightness of 2.1cd @ 20mA for each chip; the fully black ASMT-QTC0 comes with typical total 925mcd @ 20mA for each chip.
The LED chips are made of semi-conductive materials, called aluminium gallium indium phosphide (AlInGaP) and indium gallium nitride (InGaN). As in physics or electronics, semiconductors tend to be quite sensitive to heat. In the case of AlInGaP and InGaN chips, this means heat leads to a significant decrease in brightness.
For example, at 125 degree Centigrade , the light output of a green InGaN chip decreases by up to 40 percent compared to its performance at 25 degree Centigrade . Thus, good heat dissipation from the LED chip to the supporting PCB or heat sink is even more important than pure brightness at 25 degree Centigrade junction temperature.
The thermal resistance between the junction of the chip and the pin of the package is about 280K/W in a conventional PLCC-2 and about 140K/W for a mono-color PLCC-4 LED. These values appear to be quite high, especially in a 3-in-1 LEDs with three chips heating up each other.
As a result of this consideration, the LED package of a multicolor LED has to be designed to the lowest possible thermal resistance. The PLCC-4 ASMT-QTB0 or ASMT-QTC0 has been designed to this requirement with a typical thermal resistance junction to pin, R?-JP of 140 degree Centigrade /W.
RGB display panels are not only being run at night, but in the afternoon or early evening to display a wide range of televised events such as soccer matches or rock concerts. In the afternoon, the LEDs and the sunlight combine to add heat to the display.
Good resolution and visibility can be ensured by either increasing the contrast ratio with low-reflective potting material or by increasing the forward current. With the first solution being costly, driving the LED at a higher current seems to be an alternative.
Thus, a key benefit of low thermal resistance is to drive the LED at a higher forward current in warm ambient conditions. For example, the ASMT-QTB0 can be driven at the maximum current of 50mA for the red and 30mA for green and blue at 70 degree Centigrade ambient temperature.
PLCC-4 LEDs can only be driven at 25mA (red) and 13mA (green and blue) at the same ambient conditions. If the ambient temperature around the LED increases to 85 degree Centigrade, red can still be driven at 35mA, green and blue at 22mA.
By comparison, conventional RGB PLCC-4 LED's can only be driven at 15mA or 8mA at such high ambient temperatures. This means that with a low thermal resistance value of the LED, the brightness of the display will still be very high in warm environments. Additionally, this will also ensure a good readability of the screen in sunlight or when a spotlight pointing towards the display.
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.
|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 ).
|Figure 4: The linear placement of LEDs helps to ensure a uniform perception of the display and good color mixing from wider viewing angles.|
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 .