By K.Vijay Kumar Gupta, MD, Kwality Photonics P Ltd .
Friday, January 03 2013: The thermal management design of an LED application is very important to ensure its reliability and optimum performance. The maximum junction temperature of the die inside the package is based on the allowable thermal stress of the package material which can not be exceeded to avoid a catastrophic failure of the device. The thermal resistance is the most important parameter that determines the amount of heat dissipates or travels out of the component. The lower the thermal resistance, the higher ambient temperature that the component can operate. The basic concept of thermal management shows the importance of selecting the right materials from the substrate material, thermal interface to the heat sink, or other cooling methods to ensure the device is operating reliably within the expected ambient temperature range. 
The thermal resistance of the LED is determined by the effective heat removal Design of the LED manufacturer. An efficient thermal engineering of the package can also help in reducing the device dimensions thus greatly improving the future scalability of these devices.The thermal resistance of the package needs to be sufficiently low if the advantages obtained at die level are to be maintained at module level.
Currently, Polyphthalamide (PPA) is the major lead-frame material for packaging low-power chips. The Ceramic and Liquid Crystal Polymer (LCP) are primarily used for high-power packages. While Ceramic has a good melting point, the ceramic-based lead frame, like PPA and LCP lead frames, has to be made with tooling, hence limiting the frame reduction to miniaturisation. micron scale. However, a PPA lead frame, for instance, can be miniaturized to 3mm long by 1.4mm wide at best, holding a 9-mil (0.225 square millimeter) LED chip.
Silicone Large Wafer technology experience
Use of Silicon wafers to grow GaN LEDs could lead to drastic fall in cost of LEDs, once uniformity across 12” area is mastered. Silicon has an excellent heat resistance ( see graph).Si has best suited coefficient of thermal expansion (CTE). Silicon melts at 1,000 C, its CTE or coefficient of thermal expansion is below 4 ppm/C, and its TC or thermal conductivity is in the 150-180 W/m.k
In near future, one can build LED driver ICs also into the WLP LED package by etching circuits on the bottom of the silicon lead frame, thereby shrink the size of LED modules into a miniscule.
Challenges in the packaging of GaN-on-Si devices
Direct growth of GaN-on-Silicon substrate is one of the good option to get rid of Sapphire. Yet this will require the reuse of mainstream technologies such as leadframe and wirebond packaging.
This is because, GaN-on-Si devices for LED applications require Si substrate removal since light is emitted in the same direction as the substrate. It is a technology challenge for substrate transfer to get the GaN film effectively flipped and electrically connected.
Wafer-level packaging
This brings us to option of growing GaN on traditional way, then removing them by LLO ( laser liftoff technology) and then transferring it to Silicone substrate using wafer to wafer processes to derive complete package there in itself. Wafer-level packaging and advanced technologies such as through-Si vias (TSVs) could offer a solution: they could provide effective means of substrate transfer and allow wafer-level integration of some more packaging elements like optics and drivers..
Among the cost of ceramic packaging is the highest whereas WLP is the cheapest, with PPA and LCP in the middle. If a ceramic package costs US$0.25, a PPA would be around US$0.06, an LCP would be around US$0.08 and WLP would be around US$0.03.
LED bonding process
Die Attach Process also known as Die Bond or Die Mount, is the process of attaching the LED chip to the die pad of the leadframe of the package.
There are three main stages: First, the adhesive is dispensed on the die pad. Then the die must be ejected from the wafer tape. A push-up needle pushes upward on the die backside to dislodge the die off the wafer tape. Third, a pick-and-place tool picks the die from the wafer tape and positions it on the adhesive. The key factors are:
1. The amount of the adhesive: Although the junction high is 75~145μm, too much adhesive will cause the p-n junction short.
2. Dimension of push-up needle: should fit the chip with tip radiun as 0.25~0.6 mm only.
3. Pick-and-place of the LED Chip is achieved by either antistatic plastic tool which is made of rubber, though tools made of hard materials like tungsten carbide , ceramic, or steel, are also popular.
Alternate is Eutectic bonding , achieved by low melting alloy like gold-zinc alloy is used to affix LED chip to silicon lead frames. While, epoxy glues used in most LED packages melt at 180 C, Eutectic bonding is much better in thermal resistance and does not absorb the emitted light in the package during high temperatures as epoxy. Here to prevent the high melting temperature from destroying the lead frame construction during the bonding process, the LED chip is first bonded on a heat-resistant board and then the board is adhered to the lead frame ( ref Cree).
Wire Bond Process comprises of using the gold ball bonding as electrical connection. A gold ball is first formed by melting the end of the wire through electronic flame-off (EFO). Then free-air ball is brought into contact with the bond pad on the chip. The bonder applies pressure, heat, and ultrasonic forces to the ball, forming the metallurgical weld between the ball and the bond pad. Then the wire is run to the lead frame, forming a loop between the bond pad and the lead frame. Pressure and ultrasonic forces are applied to the wire to form the second bond. Bonding force of first bond should be fine tune to prevent the stress damage the bond pad and chip.
One can substitute both DieBond & WireBond steps with one step Flip Chip Bonding technique in large chips of high power..
Encapsulation Materials
The lifetime of LED is not only due to the chip, but also the encapsulation materials. The silicone resin provides many advantages which positively influence the lifetime. Using silicone resin would be very helpful for luminous maintenance.
ESD Protection
Electrostatic discharge (ESD) may damage chips. To prevent chips from ESD damages, Equipments must be properly grounded & Use wrist band or anti-electrostatic glove when handling the chips.
In solid-state white lighting technology, phosphors are applied to the LED chip in such a way that the photons from the blue gallium nitride LED pass through the phosphor, which converts and mixes the blue light into the green-yellow-orange range of light. When combined evenly with the blue, the green-yellow-orange light yields white light. The notion of multiple colors creating white may seem counterintuitive. While, in reflective pigments, mixing blue and yellow yields green. With emissive light, however, mixing such complementary colors yields white.
In LED manufacturing processes, normal variations in the brightness and exact color LED die and variations in the phosphor coating processes during die packaging lead to variation in the brightness and the “whiteness” of manufactured PCLEDs. During final testing, these LEDs are sorted into different intensity and color bins. Within one intensity bin, some PCLEDs will be a bluer white, others a yellower white, and so on. LED manufacturers need to find applications for each of these bins to keep manufacturing costs under control.
CRI: Another limitation of many PCLEDs stems from how colored objects appear when illuminated by the type of white light they produce. A white light source’s ability to accurately reveal colors depends on the number and intensity of the colors contained in the light coming from that source. The red or green objects aren’t as vivid when illuminated by PCLED white light made from a mixture of blue and yellow light. New phosphors that can convert blue LED light to other wavelengths besides yellow are now being combined with YAG:Ce to improve the color rendering of blue InGaN PCLEDs.
Another approach to making PCLEDs is to use UV InGaN LEDs and a blend of phosphors that convert the UV into blue, green and red emission which combine to appear white. This approach improves color rendering and can reduce manufacturing variability (range of whiteness) of the light made by the PCLED. Packaging UV LEDs presents more challenges for some of the packaging materials, including lower reflectivity of metal surfaces and mold compounds which reduces brightness and photo-degradation of epoxies and other plastic package parts which reduces LED lifetime. As with research on phosphors for use with LEDs, research on packaging materials better suited for use with UV InGaN LEDs is getting a lot of attention.
Combining phosphors with monochromatic LEDs is not the most energy efficient way to make white LEDs. Phosphors convert higher energy LED light into lower energy, longer wavelength light and the energy difference is lost as heat in the phosphor. The energy difference between absorbed and emitted light is called the Stokes shift. As expected, the Stokes shift is larger (more energy lost as heat) when UV LEDs are combined with phosphor blends than when blue LEDs are combined with YAG:Ce, but since UV InGaN LEDs radiate more optical power than blue InGaN LEDs, both methods make about the same amount of visible light with a given amount of electrical power (Lumens per watt or Lm/W)
Driving White LEDs is another entrepreneurial opportunity for todays engineering students.
As LEDs have a very steep Current vs Voltage curve, a small change in voltage (say 0.2V) will change current by more than 100mA which could lead to the LED burnout. Also the LED Voltage drop Vf has negative temperature coefficient which leads to lower Vf as the LED gets warmer during ON condition. Hence, it is imperative to hold the current constant. A simple resistor in series with the LED, can absorb the changes in voltage effectively, when the Supply Voltage is far higher than the Vf. But this wastes lot of power in the resistor as heat.
A far more efficient way is to use SMPS also called as LED Driver with current sensing feedback in Buck or Boost modes. Here upto 95% power efficiency can be achieved. Modern Drivers come with power Factor Corrections & EMI/EMC compatibility. Dimmability with triac dimmers as well as Input –Output isolation is also preferred by consumer markets & government regulations.
| Kwality Photonics P Ltd |
 Kwality Photonics P Ltd, are PIONEER MANUFACTURER of LEDs in India. With Lamps & incandescent Bulbs since 1966 & LED Manufacturing since 1987. Awarded with INDIA’s TOP LED BRAND by EFY readers in 2012, Kwality is most TRUSTED vendor for thousands of electronic industries over last 25 years for High Power LEDs Medium & Low Power LEDs for Lighting & Signages, SMD Automotive LEDs, LED Segment Displays, Dotmatrix,LED Light Bars, Bargraphs, & Indicator LEDs with a range depth of of 500 types of LEDs. All LEDs are RoHs Compliant & made at the ISO9001-2008 certified Plant. All systems run on SAP ERP and customers response time is less than one hour generally.The author K.Vijay Kumar Gupta BSc( Mumbai), BE ( IISc 1977) is Past President ELCINA, ELIAP & Member BIS23/24, ELCOMA-BEE Standards, Advisor -DEITY (Opto & Materials) R&D Review Committees. He has 35 years experience in LED technology field at IISC, BARC, TIFR and CEL/NPL/DST & Kwality group of industries. |
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