内容编辑过去LED业者为了获得充分的白光LED光束，曾经开发大尺寸LED芯片试图藉此方式达到预期目标。不过，实际上白光LED的施加电力持续超过1W以上时光束反而会下降，发光效率相对降低20~30%。换句话说，白光LED的亮度如果要比传统LED大数倍，消耗电力特性超越荧光灯的话，就必需克服下列四大课题：抑制温升、确保使用寿命、改善发光效率，以及发光特性均等化。
温升问题的解决方法是降低封装的热阻抗；维持LED的使用寿命的方法是改善芯片外形、采用小型芯片；改善LED的发光效率的方法是改善芯片结构、采用小型芯片；至于发光特性均匀化的方法是改善LED的封装方法，这些方法已经陆续被开发中。
解决封装的散热问题才是根本方法
由于增加电力反而会造成封装的热阻抗急剧降至10K/W以下，因此国外业者曾经开发耐高温白光LED，试图藉此改善上述问题。然而，实际上大功率LED的发热量比小功率LED高数十倍以上，而且温升还会使发光效率大幅下跌。即使封装技术允许高热量，不过LED芯片的接合温度却有可能超过容许值，最后业者终于领悟到解决封装的散热问题才是根本方法。
有关LED的使用寿命，例如改用硅质封装材料与陶瓷封装材料，能使LED的使用寿命提高一位数，尤其是白光LED的发光频谱含有波长低于450nm短波长光线，传统环氧树脂封装材料极易被短波长光线破坏，高功率白光LED的大光量更加速封装材料的劣化，根据业者测试结果显示，连续点灯不到一万小时，高功率白光LED的亮度已经降低一半以上，根本无法满足照明光源长寿命的基本要求。
有关LED的发光效率，改善芯片结构与封装结构，都可以达到与低功率白光LED相同水平。主要原因是电流密度提高2倍以上时，不但不容易从大型芯片取出光线，结果反而会造成发光效率不如低功率白光LED的窘境。如果改善芯片的电极构造，理论上就可以解决上述取光问题。
设法减少热阻抗、改善散热问题
有关发光特性均匀性，一般认为只要改善白光LED的荧光体材料浓度均匀性与荧光体的制作技术，应该可以克服上述困扰。如上所述提高施加电力的同时，必需设法减少热阻抗、改善散热问题。具体内容分别是：降低芯片到封装的热阻抗、抑制封装至印刷电路基板的热阻抗、提高芯片的散热顺畅性。
为了降低热阻抗，许多国外LED厂商将LED芯片设置在铜与陶瓷材料制成的散热器(heatsink)表面，接着再用焊接方式将印刷电路板的散热用导线连接到利用冷却风扇强制空冷的散热器上。根据德国OSRAMOptoSemiconductorsGmb实验结果证实，上述结构的LED芯片到焊接点的热阻抗可以降低9K/W，大约是传统LED的1/6左右，封装后的LED施加2W的电力时，LED芯片的接合温度比焊接点高18K，即使印刷电路板温度上升到50℃，接合温度顶多只有70℃左右；相比之下以往热阻抗一旦降低的话，LED芯片的接合温度就会受到印刷电路板温度的影响。因此，必需设法降低LED芯片的温度，换句话说，降低LED芯片到焊接点的热阻抗，可以有效减轻LED芯片降温作用的负担。反过来说即使白光LED具备抑制热阻抗的结构，如果热量无法从封装传导到印刷电路板的话，LED温度上升的结果仍然会使发光效率急剧下跌。因此，松下电工开发印刷电路板与封装一体化技术，该公司将1mm正方的蓝光LED以flipchip方式封装在陶瓷基板上，接着再将陶瓷基板粘贴在铜质印刷电路板表面，根据松下报导包含印刷电路板在内模块整体的热阻抗大约是15K/W左右。
各业者展现散热设计功力
由于散热器与印刷电路板之间的致密性直接左右热传导效果，因此印刷电路板的设计变得非常复杂。有鉴于此美国Lumileds与日本CITIZEN等照明设备、LED封装厂商，相继开发高功率LED用简易散热技术，CITIZEN在2004年开始开始制造白光LED样品封装，不需要特殊接合技术也能够将厚约2~3mm散热器的热量直接排放到外部，根据该CITIZEN报道虽然LED芯片的接合点到散热器的30K/W热阻抗比OSRAM的9K/W大，而且在一般环境下室温会使热阻抗增加1W左右，即使是传统印刷电路板无冷却风扇强制空冷状态下，该白光LED模块也可以连续点灯使用。
Lumileds于2005年开始制造的高功率LED芯片，接合容许温度更高达+185℃，比其它公司同级产品高60℃，利用传统RF4印刷电路板封装时，周围环境温度40℃范围内可以输入相当于1.5W电力的电流(大约是400mA)。所以Lumileds与CITIZEN是采取提高接合点容许温度，德国OSRAM公司则是将LED芯片设置在散热器表面，达到9K/W超低热阻抗记录，该记录比OSRAM过去开发同级产品的热阻抗减少40%.值得一提的是该LED模块封装时，采用与传统方法相同的flipchip方式，不过LED模块与散热器接合时，则选择最接近LED芯片发光层作为接合面，藉此使发光层的热量能够以最短距离传导排放。
2003年东芝Lighting曾经在400mm正方的铝合金表面，铺设发光效率为60lm/W低热阻抗白光LED，无冷却风扇等特殊散热组件前提下，试制光束为300lm的LED模块。由于东芝Lighting拥有丰富的试制经验，因此该公司表示由于模拟分析技术的进步，2006年之后超过60lm/W的白光LED,都可以轻松利用灯具、框体提高热传导性，或是利用冷却风扇强制空冷方式设计照明设备的散热，不需要特殊散热技术的模块结构也能够使用白光LED。
变更封装材料抑制材质劣化与光线穿透率降低的速度
有关LED的长寿化，目前LED厂商采取的对策是变更封装材料，同时将荧光材料分散在封装材料内，尤其是硅质封装材料比传统蓝光、近紫外光LED芯片上方环氧树脂封装材料，可以更有效抑制材质劣化与光线穿透率降低的速度。由于环氧树脂吸收波长为400~450nm的光线的百分比高达45%,硅质封装材料则低于1%,辉度减半的时间环氧树脂不到一万小时，硅质封装材料可以延长到四万小时左右，几乎与照明设备的设计寿命相同，这意味着照明设备使用期间不需更换白光LED。不过硅质树脂属于高弹性柔软材料，加工时必需使用不会刮伤硅质树脂表面的制作技术，此外加工时硅质树脂极易附着粉屑，因此未来必需开发可以改善表面特性的技术。
虽然硅质封装材料可以确保LED四万小时的使用寿命，然而照明设备业者却出现不同的看法，主要争论是传统白炽灯与荧光灯的使用寿命，被定义成“亮度降至30%以下”。亮度减半时间为四万小时的LED，若换算成亮度降至30%以下的话，大约只剩二万小时左右。目前有两种延长组件使用寿命的对策，分别是，抑制白光LED整体的温升，和停止使用树脂封装方式。
一般认为如果彻底执行以上两项延寿对策，可以达到亮度30%时四万小时的要求。抑制白光LED温升可以采用冷却LED封装印刷电路板的方法，主要原因是封装树脂高温状态下，加上强光照射会快速劣化，依照阿雷纽斯法则温度降低10℃寿命会延长2倍。停止使用树脂封装可以彻底消灭劣化因素，因为LED产生的光线在封装树脂内反射，如果使用可以改变芯片侧面光线行进方向的树脂材质反射板，则反射板会吸收光线，使光线的取出量急剧锐减。这也是LED厂商一致采用陶瓷系与金属系封装材料主要原因。
In the past, in order to obtain sufficient white LED beam, LED manufacturers have developed large-sized LED chips in an attempt to achieve the desired goal. However, in fact, when the applied power of white LED lasts more than 1W, the light beam will decline, and the luminous efficiency will be reduced by 20-30%. In other words, if a white LED is several times brighter than a traditional LED and consumes more power than a fluorescent, it must overcome four major challenges: limiting temperature rise, ensuring service life, improving luminous efficiency, and equalizing luminous properties.
The solution to the temperature rise problem is to reduce the thermal impedance of the package. The way to maintain the service life of LED is to improve chip appearance and adopt small chip. The way to improve the luminous efficiency of LED is to improve chip structure and adopt small chip. As for the homogenization of luminous characteristics, methods to improve LED packaging methods have been developed.
To solve the heat dissipation problem is the fundamental method
Due to the increased power will cause the package thermal impedance to drop below 10K/W, so foreign manufacturers have developed high-temperature white LED, trying to improve the above problem. However, in fact, the calorific value of high-power LED is more than ten times higher than that of low-power LED, and the temperature rise will make the luminous efficiency drop sharply. Even if the packaging technology allows high heat, but LED chip joint temperature may exceed the allowable value, finally the industry finally realized that the solution to the problem of heat dissipation packaging is the fundamental way.
About the service life of the LED, for example, to switch to the siliceous materials and ceramic packaging materials, can increase the service life of the LED a digit, especially the white LED the luminous wavelength spectrum contains less than 450 nm wavelength light, traditional epoxy encapsulating materials are easily destroyed by short wavelength light, high power white LED for big quantity of light more accelerated degradation of encapsulation material, according to industry test, according to the results of less than ten thousand hours of continuous light, high power white LED brightness has more than halved, cannot satisfy the basic requirement of lighting light source, long life.
About LED luminous efficiency, improve chip structure and packaging structure, can achieve the same level as low-power white LED. The main reason is that when the current density is increased by more than 2 times, it is not easy to remove light from the large chip. As a result, the luminous efficiency is not as good as that of low-power white LED. If the electrode structure of the chip is improved, the above light harvesting problem can be solved theoretically.
Try to reduce thermal impedance and improve heat dissipation
Concerning the uniformity of luminescence characteristics, it is generally believed that as long as the uniformity of fluorescence material concentration of white LED and the manufacturing technology of fluorescence material are improved, the above problems can be overcome. As mentioned above, while increasing the applied power, ways must be found to reduce the thermal impedance and improve the heat dissipation problem. The specific contents are: reducing the thermal impedance from chip to package, inhibiting the thermal impedance from package to printed circuit substrate, and improving the heat dissipation smoothness of chip.
In order to reduce the thermal impedance, many foreign LED manufacturers set the LED chips on the surface of copper and ceramic heatsink, and then connected the heat dissipation of printed circuit board with wires to the heatsink using cooling fan to force air cooling. According to the German OSRAMOptoSemiconductorsGmb experimental results confirmed that the structure of the LED chip to the weld thermal impedance can reduce 9 k/W, is around 1/6 that in conventional leds, encapsulation after applying the 2 W LED power, LED chip bonding temperature is higher than weld 18 k, even printed circuit board temperature rise to 50 ℃, joint temperature at most only around 70 ℃; In contrast, once the thermal impedance is reduced, the junction temperature of the LED chip will be affected by the temperature of the printed circuit board. Therefore, it is necessary to try to reduce the temperature of the LED chip. In other words, reducing the thermal impedance of the LED chip to the welding point can effectively reduce the cooling burden of the LED chip. On the other hand, even if a white LED has a structure that inhibits thermal impedance, if the heat cannot be transferred from the package to the printed circuit board, the result of rising LED temperature will still cause a sharp decline in luminous efficiency. As a result, panasonic developed integrated PCB and packaging technology. The company encapsulated a 1mm square blue LED with flipchip on the ceramic substrate, and then pasted the ceramic substrate on the surface of the copper PCB. According to panasonic, the thermal impedance of the whole module including the PCB was about 15K/W.
Each industry shows the heat dissipation design ability
Because the density between the radiator and the PCB directly affects the heat conduction effect, the design of the PCB becomes very complicated. For this reason the Lumileds and Japanese CITIZEN, lighting, LED packaging manufacturers such as successively developed the high power LED with simple cooling technology, CITIZEN in 2004 began to start making samples of white LED encapsulation, do not need special joint technology can also about 2 ~ 3 mm thick radiator heat emissions directly to the external, according to the CITIZEN reports while the LED chip junction to 30 k/W heat radiator of impedance is bigger than the 9 k/W OSRAM, and in general environment at room temperature can increase the thermal impedance around 1 W, The white LED module can be used continuously for lighting even under the condition of forced air cooling without cooling fan of traditional printed circuit board.
Lumileds was started in 2005, high power LED chip joint allow higher up to + 185 ℃ temperature, is higher than other companies products at 60 ℃, using traditional RF4 PCB encapsulation, 40 ℃ ambient temperature range can input electric current is equivalent to 1.5 W (about 400 ma). So Lumileds and CITIZEN is to improve the junction allowable temperature, Germany OSRAM company is a set of LED chips in the surface of radiator, to 9 k/W low thermal impedance record, the record than OSRAM the past development of products at the same level of thermal impedance is reduced by 40%. It is worth mentioning the LED module encapsulation, using the same way of flipchip with traditional methods, but the LED module and radiator joint, choose the most close to the LED chip light-emitting layer as the joint surface, to make the light emitting layer heat to be discharged in the most short distance transmission.
In 2003, Toshiba Lighting once installed a 400mm square aluminum alloy surface with a low thermal impedance white LED with a luminous efficiency of 60lm/W, no cooling fan and other special cooling components, and trial-produced an LED module with a beam of 300lm. Because Toshiba Lighting has rich manufacture experience, the company said due to the evolution of simulation technology, in 2006 after more than 60 lm/W white LED, can easily use the lamps and lanterns, the box body to improve thermal conductivity, or using the way of cooling fan forced air cooling design Lighting equipment heat dissipation, no special cooling technology can also use white LED module structure.
Changing the packaging material limits the rate of deterioration and light penetration
As for the longevity of LED, currently LED manufacturers take measures to change packaging materials and disperse fluorescent materials in packaging materials. Especially, compared with the epoxy resin packaging materials above traditional blue light and near-ultraviolet LED chips, silicon packaging materials can more effectively inhibit the deterioration of materials and the speed of light penetration rate reduction. Due to epoxy resin absorption wavelength of 400 ~ 450 nm light is as high as 45%, the percentage of siliceous encapsulating materials is below 1%, luminance in half the time of the epoxy resin is less than ten thousand hours, siliceous encapsulating materials can extend to forty thousand hours, almost the same as the design life of lighting equipment, this means that during the lighting equipment use does not need to replace the white LED. However, silicone resin is a kind of flexible material with high elasticity. During processing, it is necessary to use the manufacturing technology that will not scratch the surface of silicone resin. In addition, when processing, silicone resin is easy to adhere to powder, so it is necessary to develop technology that can improve the surface characteristics in the future.
Although silicon encapsulation materials can ensure that LED 40,000 hours of service life, lighting equipment industry has a different view, the main argument is the service life of traditional incandescent lamps and fluorescent lamps, defined as "brightness down to 30%". If the brightness of the LED is reduced to less than 30% with a time of 40 thousand hours, there are only about 20 thousand hours left. Currently, there are two ways to extend the service life of components, one is to suppress the overall temperature rise of white LED, and the other is to stop using resin packaging.
It is generally believed that if the above two life-extending countermeasures are thoroughly implemented, the brightness can reach 30% when 40,000 hours of requirements. Inhibition of white LED temperature can adopt the method of cooling the LED packaging printed circuit board, the main reason is that encapsulating resin, under high temperature and strong light irradiation degradation rapidly, according to the ray new algorithms reduce 10 ℃ temperature 2 times longer life. Stop using resin packaging can completely eliminate the deterioration factor, because the light generated by the LED is reflected in the packaging resin, if you can change the direction of light on the side of the chip resin material reflector board, the reflector board will absorb light, so that the amount of light taken out sharply reduced. This is also the main reason for LED manufacturers to adopt ceramic and metal packaging materials.
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