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Archive for September, 2010

The Relationship Between Hot/Cold Factor And LED Efficacy

30 Sep

Energy saving is one of the key combinations between the selling point for LEDs and technology. Compared to the traditional light bulbs, LEDs can significantly reduce electricity consumption for illumination and improve the efficiency of lighting systems. Although the advantage is significant, there is a negative factor: under the same driving current, the junction temperature increase will lead to the decrease of light output, which both reduces the light output and luminous efficacy at the same time.

To compensate for this phenomenon, designers often employ the method of using low current to drive more LEDs to maintain a reasonable junction temperature. The use of multiple LEDs may consumes extra power and increases system costs. However, LED’s hot/cold factor can reduce the influence and improve the system performance.

What is the cold/hot factor?

This terminology describes junction temperature as the function of light output decreasing, the industry has not defined standards for the cold/hot factor. Lower temperature is always 25 ℃ (room temperature), but higher temperature can be any value within the LED limits. In this paper, we define the hot/cold factor as the light output proportions at 25 ℃ and 100 ℃. Figure 1 shows the relationship between standard luminous flux and heat sink temperature. Heat sink temperature used is equivalent to the LED junction temperature under the condition of a very short pulse test.

Figure 1, the relationship between heat sink temperature and luminous flux

At 25 ℃, the standard luminous flux is 1, at 100 ℃, the standard luminous flux is 0.84, so the cold/hot factor is 0.84. This means that when the heat sink temperature is 100 ℃, LEDs will lose 16% of luminous flux.

The influence of hot/cold factor

At first glance, 16% LED luminous flux reduction may have little influence. However, when considering that a lighting device is composed of many LEDs, this is a serious problem. Comparing a recessed down light with 10 LEDs and a flashlight with 1 LED, the influence of hot/cold factor will emerge.

For an ordinary user, a flashlight reducing 16lm light output does not seriously affect its application. However, 160lm light output reduction will seriously influence the use of a recessed down light, so we need to add one or more LEDs to compensate for the light loss. Thus, the overall power consumption and costs of the recessed down light will increase. ENERGY STAR has very strict requirements for the light efficiency of LED lighting fixtures, and such light output reduction makes it difficult for the LED lighting fixtures to to meet these requirements.

Table 1: The influence of hot/cold factor to 10 LEDs and single LED respectively
Lighting type Numbers of LEDs Hot/cold factor Total light output at 25℃ (lm) Total light output at 100℃ (lm)
Flashlight 1 0.84 100 84
Recessed down light 10 0.84 1,000 840

The improved cold/hot factor

The latest LED technology in the chip extension level, phosphor, mold accessories and other aspects has been developed, hot/cold factor is improved accordingly.

Currently, the hot/cold factor some high-power LEDs in the market is 0.94. This means that when LEDs work at 100 ℃, they will lose 6% of the standard luminous flux. Figure 2 shows the function of light output decreasing under a typical and improved hot/cold factor.

In Figure 2, the improved hot/cold factor makes LED’s brightness improved significantly

Hot/cold factor improvements increase the working temperature range of LEDs, which enables lighting designers to have the opportunity to work at any junction temperature within LED limits.

Performance comparison

In many cases, the product instruction provided by many LED suppliers will give a high light output efficiency. Lighting designer may be premature to conclude that LEDs which have a high light output in the data table will perform better in the real world. But this may be a wrong conclusion, because the all the values in the data table are limited to condition that when the LED junction temperature is 25 ℃. The performance of LEDs in lighting system must be assessed at a higher junction temperature. Once this is done, we can compare according to real conditions and pick out better products.

For instance, we analyze two warm white LEDs(see table 2): LED1 has an improved hot/cold factor, while LED2 has a typical hot/cold factor.

Table 2: Performance of two types of LEDs at the junction temperature of 25℃ and current of 350mA
Light output (lm) at junction temperature of 25℃ Maximum input current (mA) Maximum junction temperature (℃)
LED 1 80 1,000 150
LED 2 84 1,000 150

At 25 ℃ junction temperature and 350mA forward current described in the data table, LED2 performs better than LED1. However, a more realistic comparison would be carried out at a higher junction temperature (see table 3).

Table 3: Performance of two types of LEDs at the current of 350mA and higher junction temperature
Number of LEDs Junction temperature (℃) Total light output (lm)
LED 1 9 106 662
LED 2 10 119 608

As a result of LED1’s high hot/cold factor, the total light output of 9 pieces of LED1 is 50lm higher than that of 10 pieces of LED2. Although at 25 ℃, the rated luminous flux of LED1 is lower than that of LED2, under the driving of 350mA current, its performance is obviously better than LED2’s. Figure 3 (left) shows that under any forward current driving, LED1’s driving current is 100mA higher than that of LED2’s. Figure 3 (right) shows that when drived by the same driving current, LED1’s efficacy is higher than LED2’s.

Figure 3: Performance comparison between 9 pieces of LED1 and 10 pieces of LED2

Therefore, improving the hot/cold factor can significantly improve the performance of LEDs which operate at higher junction temperature, we can get the same light output with fewer LEDs to reduce power consumption and overall system costs. When selecting LEDs for a specific application, it is important to assess the performance of LEDs under real conditions, rather than depending solely on data sheet.

 

LG Taps LEDs in HDTVs to Improve Image Quality in Thinner Lower-power Designs

28 Sep

INFINIA, an innovative new family of LED LCD HDTVs from LG Electronics that delivers “freedom through infinite possibilities,” highlights the company’s 2010 lineup of LED LCD HDTVs introduced here today at the International Consumer Electronics Show (Booth #8205).

LG INFINIA HDTVs (the LE9500, LE8500 and LE7500 series) combine a slim design and thin bezel with enhanced connectivity and abundant content options. Leading the way to the ultimate home entertainment experience, the 55- and 47-inch class* LE9500 sets will be LG’s first 3D-ready models available in the United States.

INFINIA is the flagship of LG’s 41-model LED LCD HDTV line – six new series of LED LCD HDTVs and five new series of LCD HDTVs. Leading these introductions are two new LED technologies – Full LED Slim and LED Plus – that provide cutting-edge picture quality. The unique backlight structure on its Full LED Slim models (LE9500 and LE8500) allows for the INFINIA line’s ultra-slim depth without sacrificing picture quality. Together, these features provide consumers with infinite possibilities in home entertainment.

“We’re removing barriers to entertainment with very slim LED LCD TVs that couple wireless connectivity with the most access to online content,” said Peter Reiner, senior vice president, marketing, LG Electronics USA. “With seamless connectivity and limitless content, LG INFINIA is resetting the standards for design and entertainment as LED LCD TVs are expected to grow to more than 20 percent of the market this year.”

“Consumers will no longer have to compromise on picture quality in order to enjoy an ultra-slim design. Together this new Full LED Slim technology and our wireless connectivity options allow consumers to ‘live borderless’ with the ultimate in content access and convenient installation,” Reiner added.

LG’s new Full LED Slim technology elevates picture quality with a slim LED structure that supports detailed local dimming of up to 240 addressable segments (on the 55-inch class LE9500), resulting in an HDTV that provides the deeper black levels and uniform picture quality which typically could not be achieved on an ultra-thin set.

The LE9500 series cabinet depth is only .92 inches with a bezel width of only 8.5mm. LG’s LED Plus technology (available on the LE7500 and LE5500 series), also improves picture quality and energy efficiency by adding a basic local dimming capability of up to 16 addressable segments.

The LE9500, LE8500 and LE7500 were all recognized with CES 2010 Innovations Awards, including the “Best of Innovations” distinction in the Online Audio/Video Content category for the LE9500.

Shattering Expectations

Broadening consumer entertainment options, LG’s latest series of HDTVs affords consumers superior picture quality, advanced energy saving options and flexible access to content-on-demand. LG’s LED LCD HDTVs challenge consumers’ current perceptions of home entertainment by illustrating what’s possible with superior display technology.

LG’s Full LED Slim series (models LE9500 and LE8500) for example, join an elite group of LED LCD HDTVs that have achieved THX Display Certification – the industry standard for having the correct gamma, luminance, and color temperature. This certification demonstrates that select series of LG HDTVs can recreate the cinema experience at home, making the picture resemble movie theatre quality. To date, LG is the only manufacturer who has attained this designation for LCD TV in the U.S. market. LG is also the first manufacturer to include the “THX Bright Room” setting on its LED LCD HDTVs. This new feature to the THX certification program optimizes the contrast, gamma and other settings for watching movies in rooms with a lot of ambient light.

LG’s exclusive Full LED Slim technology includes detailed local dimming capability, but also enables the LE9500 and LE8500 to achieve a slim depth usually limited to conventional edge-lit models. This unique technology makes it possible for these two models to achieve the picture quality worthy of THX Display Certification and helps minimize the front bezel of the TV. This works with the single, edge-to-edge panel of glass to create a design, perfect for any home environment. Boasting a thin bezel of only 8.5mm, the LE9500 brings advanced technology into the home without being obtrusive. Available in 55-and 47-inch class sizes, this series also incorporates TruMotion 480Hz for reduced motion blur during fast moving action sequences.

Connectivity

LG’s full line of LED LCD HDTVs ― series LE9500, LE8500, LE7500, LE5500 and LE5400 (in screen sizes 32-inch class and above) ― boast a connectivity package with a variety of entertainment options, including NetCast Entertainment Access. With NetCast, consumers can access the following content sites for an almost endless array of entertainment options:

Skype: Newly added in 2010, this allows consumers to make free video and voice calls over the Internet to family members and friends (separate camera and other equipment needed).
Netflix: Updated with Netflix 2.0, consumers can stream thousands of movies without a PC.
VUDU: Allows consumers to instantly buy or rent from an extensive library of movies and TV titles, including a catalog of more than 3,000 high-definition movies – with no monthly fees or additional hardware.
YouTube: Offers the ability to instantly stream millions of Web videos directly from the Internet (without a personal computer).
Napster: Now Napster subscribers can enjoy unlimited on-demand streaming music from millions of songs on their NetCast TV.
Yahoo! Widgets: Enables access to various applications called TV Widgets that allow viewers to interact with popular Internet services and online media through applications specifically tailored to the needs of the watcher, such as up-to-the minute Yahoo! News, Weather and Finance, and new widgets, including CBS, Showtime and CNBC.
LG also has incorporated the Digital Living Network Alliance (DLNA) technology across the full line of LED models. DLNA allows consumers to access content stored on other DLNA-certified devices within the home, such as computers, making content options almost limitless.

Providing easy options for connecting to the Internet, in addition to the wired Ethernet jack, all NetCast-enabled sets can integrate into a wireless home network by using a USB wireless broadband adaptor (sold separately). All models with NetCast also support multi-media playback from a connected USB device including photos (JPEG), music (MP3) and video (DivX HD).

For greater convenience and flexibility in setup and installation, all HDTV series with NetCast also offer wireless Full HD 1080p wireless transmission from a “Wireless Media Hub” from up to 98 feet. Connecting source components, such as Blu-ray players, cable or satellite boxes and video games to the media hub enables transmission to a compact receiver adaptor, which attaches to the back of the TV, hidden from view. This eliminates the need for individual components to be connected directly to the TV, making for a clean and easy installation and removal of the unsightly wires (Media Hub and receiver adaptor sold separately as a package).

Controls

LG’s LE9500 incorporates a unique “Magic Wand” remote system that provides an immersive interaction with the set. This “Magic” user interface brings together menus, component controls and even embedded games, which can be accessed using a simple remote that combines minimal buttons and gestures to control the on-screen activity, mirroring a “Wii-like” experience.

Energy Savings

Understanding consumers’ desire for products that reduce their household energy costs, most of LG’s LED and CCFL HDTVs have a variety of energy-saving features, such as Intelligent Sensor, to automatically calibrate and optimize brightness, contrast, white balance and color, based on the ambient light in the room, saving on energy output under most circumstances. Additionally, ISFccc calibration options allow consumers to work with a professional to set “day” and “night” levels for optimal viewing and brightness levels. All of LG’s 2010 LED LCD series also qualify for ENERGY STAR 4.0 certification.

 

Improve Thermal Control to Enhance The Lifetime of White LEDs

28 Sep

LED industry has ever tried to develop large-size LED chips in the past in order to achieve the desired goal of getting full white LED light beam, but actually when the imposed power of white LEDs exceeds 1w, the light beam will be weakened, and the luminous efficiency will reduce by 20 ~ 30%, in other words the white LED is several times bright as conventional LED, if we want the power consumption characteristic of white LED to surpass fluorescent lights, then we must first overcome the following four topics including inhibition of temperature increase, ensuring lifetime, improving light efficiency, and equalization of luminescence properties etc.

The detailed solution to isuues related to temperature increase is to lower the package thermal impedance; the detailed methods to maintain the lifetime of LEDs is to improve chip shape, utilize small size chips; the detailed methods to improve the luminous efficiency of LEDs is to improve the chip structure, utilize small size chips; as to the equalization of luminescence, we can achieve by improving LED package method, and all these methods have already been developed in succession.

Solving the thermal control issue of package is the fundamental method

Since increasing power will result that the thermal impedance of package rapidly decrease to 10K/W, the foreign LED industry have developed high temperature resistant white LEDs to improve the above mentioned problems, however, the heat generation of high-power LEDs is actually tens of times higher than low power LEDs, and temperature increase can also sharply decrease the luminous efficiency, even if the package technology allows high heat generation, but the junction temperature of LED chip is likely to exceed the allowable value, the industry has finally realized that the fundamental method is to solve the thermal control issue of package.

As for the lifetime of LEDs, for instance, switching current package materials to silicon and ceramic package materials can improve the lifetime of LEDs by ten times, especially the white LED’s light spectrum includes short wavelength light whose wavelength is below 450nm, traditional epoxy resin package material is easy to be destroyed by short wavelength light, large quantity light output of high-power white LEDs light is easier to accelerate the deterioration of package materials, industry tests showed that if the light has been lit continuously for less than ten thousand hours,  brightness of high-power white LED has been degraded by more than 50%, which totally can not meet the basic requirements of long life for illumination light source.

As for LED luminous efficiency, improving both LED chip structure and package structure, can achieve the same level as low power white LED, which is mainly because that when the current density increases by more than 2 times, it will not be easy to extract light from the large size LED chips , whereas it will cause the luminous efficiency of high power white LEDs is even lower than low-power white LEDs, if we improve the electrode structure of the LED chip, theoretically the light extraction issue can be solved.

Try to reduce thermal resistance and improve thermal control

As for the uniformity of luminescence properties, it is generally believed that as long as we improve the intensity and uniformity of fluorescent material of white LEDs and the manufacture technology of fluorescent material, the above mentioned difficulties can be solved. As noted above, while increasing exerting power, we must try to reduce thermal resistance, improve heat dissipation issue, the specific content is: reducing thermal resistance from LED chip to package, inhibiting thermal resistance from package to the printed circuit board, improving cooling smoothness of LED chips.

In order to reduce the thermal resistance, many foreign LED manufacturers locate LEDs in the surface of the heat sink which is made from copper and ceramic material, and then use metal wire to connect the thermal control system of printed circuit board to the heat sink which uses cooling fan to achieve forced air cooling through welding, according to the German OSRAM Opto Semiconductors Gmb experiment results, the thermal resistance from LEDs with the above mentioned structure to the soldering point can reduce 9K/W, which is only 1/6 as the traditional LED’s, when imposing 2W power to the packaged LEDs, LED chip junction temperature is 18K higher than the temperature of the soldering point, even if the printed circuit board temperature rises to 500 degree Celsius, junction temperature is at most 700 degree Celsius; in contrast, in the past once the thermal resistance reduces, LED chip junction temperature will be affected by the printed circuit board temperature, therefore we must try to reduce the temperature of LED chips, in other words reducing the thermal resistance of LED chip to the the same level as sodering point can effectively reduce the burden of LED chip cooling operation. Conversely, even if the white LED has the structure which can inhibit the thermal resistance, once the heat can not be conducted from the package to the printed circuit board, temperature rise of LEDs will cause the rapid decrease of luminous efficiency, so Panasonic Electric developed the integration technology of printed circuit board and package, the company packaged 1mm square blue LED chip on a ceramic substrate by the way of flip chip, then stuck the ceramic substrate onto the surface of the copper printed circuit board, the thermal resistance of the entire module including printed circuit board is around 15K/W.

LED manufacturers demonstrate their thermal design capability

Since the adhesion between heat sink and printed circuit board directly impacts the heat conduction effects, the printed circuit board design becomes very complicated, under such circumstance, lighting device and LED package manufacturers such as Lumileds in the United States and CITIZEN in Japan, have developed simple cooling technology for high-power LEDs, CITIZEN’s white LED package which started sample shipments in 2004 can emit the heat of heat sink with a thickness of 2-3mm out requiring no special bonding technique, although the thermal resistance of 30K/W from LED chip junction to the heat sink is higher than the OSRAM’s 9K/W, and under normal environment the room temperature will increase the thermal resistance by about 1W, however even under the condition that traditional printed circuit board has no cooling fan to achieve forced air cooling, the white LED module can be lit continuously.

The high power LED chips that Lumileds began sample shipments in 2005 allow a junction temperature as high as +1850 degree celsius, which is 600 degree Celsius higher than the same level products of other companies, when using traditional RF4 printed circuit board to package, ambient temperature range within 400 degree Celsius we can input the current equivalent to 1.5W power. So Lumileds and CITIZEN utilized the method of improving allowable junction temperature, while OSRAM located LED chips in the heat sink surface, and reached 9K/W ultra-low thermal impedance record, the record is 40% less than the thermal resistance of same level products developed in the past, it is worth mentioning that when packaging the LED module, they use the method of flip chip which is the same as traditional method, however when bonding LED module and heat sink, they choose the surface which is nearest to light-emitting layer of LED chips as the joint face, which can conduct and emit the heat of light-emitting layer in the shortest distance.

In 2003, Toshiba Lighting ever built low thermal resistance white LEDs with a luminous efficacy of 60lm/W on the surface of 400mm square aluminum alloy, there were no special cooling components such as cooling fan, they tried to make LED module with a light beam of 300lm, due to rich trial manufacturing experience of Toshiba Lighting, the company indicated that as a result of advances in simulation analysis technology, since 2006 the white LEDs with luminous efficacy of more than 60lm/W, can easily use light fittings, framework to improve thermal conductivity, or use cooling fan to achieve forced air and design the thermal control of lighting devices, the module structure without special heat dissipation technology can also use white LEDs.

Change package materials to inhibit material deterioration and the decreasing speed of light transmittance

As for the longevity of LEDs, LED manufacturers now utilize the method of changing package materials, meanwhile distribute fluorescent material within the package materials, especially silicon package material can more effectively inhibit material deterioration and decrease speed of light transmittance than the traditional blue and near-UV LED chip package material-epoxy resin material. Since the absorption percentage of epoxy resin to light with wavelength of 400 ~ 450nm light is as high as 45%, which of silicon package material is as low as 1%, the time for epoxy resin to degrade LED brightness by 50% is less than ten thousand hours, while silicon material can extend the time to around forty thousand hours, which is almost the same as the designed lifetime of lighting devices, which means there is need to replace white LEDs during the using period of lighting devices. However, silicon resin material is a kind of high elastic soft materials, when processing we must use the production technology which will not scratch the surface of the silicon resin, in addition during processing silicon resin is easy to attach dust, so in the future technology of improving the surface property must be developed.

Although silicon package material can ensure forty thousand hours lifetime of LEDs, the lighting device manufacturers in the industry have different views, the main argument is that the lifetime of traditional incandescent and fluorescent lamps is defined as “the brightness degrading by less than 30%”, the time for LEDs to degrade brightness by 50% is around forty thousand hours, if it is converted into the time to degrade brightness by 30%, it will be only about twenty thousand hours. There are two measures currently to extend the service life of components, inhibiting the overall temperature rise of white LEDs and stopping using the resin package method.

It is believed that if the above two life extension measures can be fully implemented, the time for LEDs to degrade brightness by 30% can achieve forty thousand hours. We can utilize the method of cooling LED package printed circuit board to inhibit white LED temperature rise, which is mainly because that package resin under high temperature and strong light will quickly deteriorate, when the temperature decreases by 100 degree Celsius, the lifetime of LEDs will extend to 2 times as original life. Stopping using resin package  can completely eliminate deterioration factor, because the light generated by LEDs will be reflected within the packaging resin, if we use resin material reflector which can change the light travelling direction of LED chip side, the reflector will absorb light, so the quantity of light extraction will rapidly decrease, which is also the reason that LED manufacturers are more willing to utilize ceramic and metal package materials.

 

LED Component Assembly Benefits from SMT Processes

24 Sep

Automated SMT assembly of LED components, enabled by high-quality equipment such as nozzles and feeders, is able to effectively increase manufacturing throughput and minimize defects, explains Zachery Shook.

Recent growth in LED technology and solid-state lighting has provided the electronics manufacturing industry with viable solutions for its addition into today’s electronic devices. LEDs have become an alternative light source to conventional incandescent and fluorescent bulbs. The electronics manufacturing industry sees the greatest benefits from the small size and lower power consumption of today’s LEDs.

There also is the recent trend to use “green technology” in consumer and commercial electronics. LED components offer high brightness and power efficiency, as well as lower carbon emissions than traditional lighting technologies. This aspect has made LED lighting popular with government organizations, which are now installing LED lighting in public places and government offices worldwide.

When used for illumination purposes, LEDs are more cost-effective than traditional lighting sources. Thus, the global LED component market is witnessing an increasing demand. As a result, companies in the surface-mount technology (SMT) industry are expanding their manufacturing capabilities to meet this demand. High brightness LED components currently are experiencing high growth as the backlighting application in TVs is shifting from traditional CCFL technology to LEDs. The range of new and potential applications for LEDs in electronics is practically endless.

In order for electronics manufacturers to get these LEDs into their products, they must use one of two methods: hand assembly or automated assembly. Hand assembly is where skilled technicians carefully place individual LED components onto circuit boards using specialized tooling. This is a long, tedious process that can slow the production rates of electronic devices and tie up major labor resources.

On the other hand, automated assembly uses the company’s existing SMT equipment to quickly and efficiently place thousands of LED components every hour. Most SMT equipment is capable of placing multiple components on a circuit board at one time, increasing the overall production rates while decreasing labor costs. Obviously, the goal of a high-volume electronics manufacturing company would be to transition assembly into automated production.

Pick and place

The principle of using vacuum pressure and precision nozzles to enable component placement is repeated in every type of SMT equipment. There are five distinct stages of the pick-and-place process:

* Picking: components are withdrawn from a feeder or tray by a vacuum nozzle
* Holding: components are steadied for rapid movement while the machine detects proper alignment
* Transport: components are transferred from the picking location to the PCB for assembly
* Placement: components are lowered to their specific location on the circuit board
* Release: components are released by the nozzle, which returns to the picking area to restart       the process

No SMT equipment can place components accurately or run efficiently without quality nozzles and feeders, which are at the core of the pick-and-place process. If the machine is either unable to pick parts consistently or hold on to the components during the transport from feeder to PCB, defects will result. An increase in defects means a decrease in production, costing the company more money over a short period of time. Proper feeder and nozzle selection is critical, especially with the current market growth and technological advancements in SMT equipment.

Nozzles are the first and last thing to touch all components placed, and they move tens of thousands of these parts every hour. With components sizes reaching microscopic proportions, nozzle manufacturers must strive to maintain precision tolerances and exact dimensions in their designs. These nozzles are required to hold the part during transport to the board while the machine is moving and/or rotating at high speeds. providers must use this technology to get LED components into their customer’s products.

EMS providers

Debron Industrial Electronics, Inc. is a leading electronics manufacturing service (EMS) provider specializing in high-technology electronic assemblies, printed circuit board assemblies, electronic wiring, cable assemblies and box build. Mark Hoch, SMTA Certified Process Engineer for Debron, said “We are a contract manufacturer that caters to several customers specializing in cutting-edge LED technologies. They rely on our expertise to develop, document, implement and sustain their manufacturing processes.” When one of its customers needed a product that required the placement of LED components in its design, Debron decided to move forward with the automated assembly process.

Since making the transition, Debron has helped several of its customers to fully automate the production of products that previously had been assembled by hand. The company was able to do this by creating custom pick-and-place trays for LEDs that were available only in bulk for hand assembly. Debron also has been working with tooling companies such as Count On Tools, Inc. to develop custom pick-and-place nozzles that enable LEDs to be picked, vision centered and placed with high-speed, automated SMT assembly equipment. Streamlining the automated placement process has allowed Debron to free needed manpower to use in other areas of the assembly process.

As with any new project, there are some challenges associated with the placement of LED components in the SMT production environment. It is the goal of the EMS provider to overcome each of these obstacles to cut production costs and provide quality product to its customers and their end users. Some of the major challenges that EMS companies face when trying to place LED components in SMT production include:

1. Component handling in the feeder

During the picking process, LED components are withdrawn from the feeder by a vacuum nozzle. SMT technicians must ensure that the LED components are correctly positioned in the feeder pocket to guarantee that pick-up is achieved and that the LED is properly handled during the transport stages. “Slop in the pocket” may require nozzle centering during the picking process while excessively fast advancements of the feeder may skew the part in the pocket, preventing component pick-up.

2. Component handling on the nozzle and proper nozzle selection

Some LEDs, such as those from Cree, require special handling operations to prevent damage to the optical lens. They must avoid placing mechanical stress on the LED lens by not touching the optical surface during the component picking or placement processes. This eliminates the possibility of degraded performance from the LED after the circuit board is assembled. Proper nozzle selection also is important for the transport and placement processes. Not only does the SMT nozzle have to pick the component, it also must move it to the board and accurately place it. Most LED suppliers work directly with nozzle and tooling manufacturers, like Count On Tools, to develop nozzle designs that meet their individual process requirements.

3. LED sensitivity

The fragile optical surface is not the only issue with the use of LEDs in SMT production. Early LED designs were very temperature-sensitive, forcing assembly using unconventional methods, such as hand assembly. LEDs often were bonded to heat dissipative substrates using conductive epoxies or low-temperature solders. This required special assembly processes that lengthened the manufacturing process, increasing product build costs.

4. Scaling up to high-volume production

As LEDs become more robust, assembly via means of more conventional assembly processes such as automated SMT equipment allows EMS companies like Debron to focus on other challenges such as repeatable part picking and vision centering, as well as effectively increasing throughput and minimizing defects. Repeatable performance is the major challenge with scaling up to high-volume production. EMS providers must strive to maintain a high level of performance to keep their production on track to meet customer demands. This requires fine tuning the assembly process.

As with any problems in a production environment, there is always a solution. By capitalizing on its current knowledge of SMT production and partnerships with quality suppliers, Debron was able to overcome most of the challenges associated with this process. Debron developed custom trays for the LED components to allow for more accurate picking and transport processes while eliminating issues with component handling in the feeder. It also worked with its equipment manufacturers and custom-tooling manufacturers such as Count On Tools, Inc. to develop custom SMT pick-and-place nozzles that increase LED/nozzle compatibility, allowing for greater placement accuracy and increased throughput.

Due to the partnership with Count On Tools, Inc., Debron was able to fine tune its automated assembly process and scale up to high-volume production of LEDs. Using the custom nozzle that it purchased from Count On Tools, Inc., Debron was able to reduce LED fallout to 2.3 percent. Defect rates dropped significantly and first pass yields increased steadily up to 99.4 percent.

Summary

In conclusion, the latest LED technology opens up wide areas for new applications, new technical possibilities and reduced costs in both the SMT and electronics manufacturing industries. Today, many companies are crossing traditional business boundaries and streaming into the LED lighting market. This, in turn, has created a large demand for LED use in general markets and not the traditional niche applications. By partnering with component manufacturers and nozzle/tooling suppliers, EMS companies like Debron can guarantee success by lighting the way for customers seeking LEDs in their SMT production.

About the Author
Zachery Shook is the Marketing Director of Count On Tools, Inc.

 

SMD LED PCB Design

23 Sep

SMD LED is a new type of surface-mount semiconductor light emitting device with the advantages of small size, large beam angle, excellent light uniformity, high reliability and so on, light color can be various colors including white, so it is widely used in variety of electronic products. PCB is one of the main materials to manufacture SMD LEDs. The development of each new SMD LED product starts from designing drawings of PCB, when designing the PCB drawings, the front and back drawings of the PCB, the assembly drawing and the finished product drawing of SMD LED shall all be given, then give the completed PCB drawing to professional LED PCB manufacturers to produce, the design quality will have a direct impact on product quality and the implementation of manufacturing process. Therefore, designing a perfect SMD LED PCB is not an easy task, many factors which affect the design shall be taken into account. Therefore, this article will discuss SMD LED PCB design from the following aspects.

1. Choose the right SMD LED PCB structure

SMD LED PCB can be sorted into two types according to their structure: via hole type structure and chamfer hole type structure; according to the wafer number that single SMD LED chip uses, SMD LED PCB can be sorted into: single wafer type, double wafer type and three wafer type. The difference of via hole type structure PCB and chamfer hole type structure PCB is: the former need to be cut in both directions when cutting, single finished electrode is semi arc-shaped; the latter only need to be cut in one direction when cutting. Choosing the design structure of PCB and the number of wafers for SMD LEDs depend on the market and users’ requirements. If the user does not have special requirements, we generally choose chamfer hole type structure to design PCB. The substrate of PCB is BT board.

2. Choosing direction for chamfer hole

If we choose to use the chamfer hole type structure to design PCB, we must consider which direction to choose for the chamfer hole. Under normal circumstances the chamfer hole is designed along the direction of PCB board width, which can minimize the PCB deformation after molding.

3. PCB dimesion selection

Selection of each new SMD LED PCB dimension must take following factors into consideration: ① the specified designed product number on each PCB. ② Whether the extent of PCB deformation after molding is in the acceptable range.

When it does not affect the workmanship, the number of products on each PCB shall be designed as much as possible, which will help reduce the cost of single product. Meanwhile because the gel will shrink after molding, PCB is easy to deform, the number of each group SMD LED can not be too much, but the the number of groups can be designed more when designing PCB. This will not only meet number requirement of SMD LED on single PCB, but also decrease the PCB deformation after molding and gel shrinking. Serious PCB deformation will cause that PCB can not be cut and the gel is easy to be stripped from PCB after cutting.

The decision of PCB thickness is based on the entire thickness requirement of SMD LED that the client uses. PCB thickness can not be too thick, too thick may cause failure to weld wire after die bonding; PCB thickness can also not be too thin, too thin may cause gel shrink and serious PCB deformation after molding.

We can take 0603 type ordinary SMD LED products with thickness of 0.6mm as an example. If we choose the PCB with thickness of 0.3mm, the thickness of gel can only be 0.3mm, and then choose the wafer with thickness of 0.28mm to die bond, the entire thickness has reached 0.58mm, then the wire welding can not be operated. If we choose the PCB with thickness of 0.1mm, the thickness of gel will be 0.5mm, since the gel is thick, the gel will obviously shrink after molding, while the PCB is thin, which will cause serious deformation of PCB. Therefore, when designing the thickness of PCB, we must choose a suitable thickness, which on the one hand can make the PCB suitable for SMD LEDs with different thickness, on the other hand will not cause serious deformation of PCB after molding.

4. PCB circuit design requirements

1). Die bonding area: the size design of die bonding area is determined by the size of the wafer. Under the condition that the wafer can be fixed safely, the die bonding area shall be designed as small as possible. The adhisive property between gel and PCB will be better and the gel is not easy to be stripped from PCB, meanwhile we shall consider to design the die bonding area in the middle of single SMD LED PCB.

2). Wire welding area: the size of wire welding area shall basically be greater than the bottom size of the magnetic mouth.

3). Distance from die bonding area to wire welding area: Distance from die bonding area to wire welding area shall be determined by the wire arc, if the distance is far, the pull of the wire arc is not enough, if the distance is near, the metal wire may contact the wafer when welding wire.

4). The electrode width: electrode width is generally 0.2mm.

5). The diameter of the circuit wire: We also shall consider the diameter of circuit wire which connects the electrode and the die bonding area. Use of circuit wire with small diameter can increase the adhesive force between the substrate and the gel.

6). Via hole diameter: If we use PCB with via hole design, the minimum via hole diameter is generally Φ0.2mm.

7). Chamfer hole diameter: If we use PCB with chamfer hole design, the minimum width of chamfer hole is generally 1.0mm.

8). Width of cutting line: since the cutting blade has a certain thickness when cutting, PCB will be worn after cutting, when designing cutting line width, we should consider the thickness of the cutting blade, and compensate on the PCB, Otherwise, after cutting the width of the finished product will be narrow.

In addtion, we should also consider the diameter of positioning holes and other issues. Generally the product number design of a PCB within the circuit range shall be even number.

5. The quality requirements for the PCB substrate

When designing PCB, we shall make the following technical description on PCB:

1). Sufficient accuracy is required: the degree of uneveness of PCB thickness shall <± 0.03mm, the tolerance of positioning holes to circuit board shall <± 0.05mm.

2). the thickness and quality of gold plating layer must ensure the pull test of the gold wire > 8g after die bonding.

3). The surface of finished products of PCB shall contains no dirt, the adhisive property between it and the gel shall be strong.

 
 
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