In the near future, one can build LED driver ICs into the WLP LED package by etching circuits on the bottom of the silicon lead frame, thereby shrinking the size of LED modules to a miniscule level.
LEDs or LED packages (often wrongly referred to as LED chips in India) are the component form of LED semiconductors, made suitable for rough handling and soldering on to PCBs.
Starting with LED semiconductor chips, the Group III-V compounds have their electron energy band gap falling in the ‘visible light’ region. The indium gallium nitride (InGaN) multi-layer epitaxial layered on diodes emit light the most efficiently in 450nm (blue) by way of a process called electroluminescence [EL]. Part of this monochromatic light is converted to yellow and red colour by applying a suitable phosphor coating of the right thickness in the epoxy lens and the resultant light is seen as ‘white light’. This type of white LED is also called a PCLED.
Unlike with incandescent bulbs, where only 3 per cent of the total power is emitted as light, in the case of LEDs, which are 12 times more efficient, nearly 35 per cent of the power used is emitted as light. The balance 65 per cent of power is manifested as heat within the LED and this has to be conducted out to the atmosphere smoothly, to let the LED continue to work at its best efficiency.
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 PPA and LCP lead frames have to be made with tooling, hence limiting the scope for miniaturisation with regard to frame reduction. However, a PPA lead frame, for instance, can be miniaturised to 3mm x 1.4mm, at best, holding a 9-mil (0.225mm²) LED chip.
The use of silicon wafers to grow GaN LEDs could lead to a drastic fall in the cost of LEDs, once uniformity across a 30 cm area is mastered. Silicon has an excellent heat resistance. It has the best suited coefficient of thermal expansion (CTE). Silicon melts at 1,000º 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 range.
In the 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 shrinking the size of LED modules to miniscule levels.
Challenges in the packaging of GaN-on-Si devices
Direct growth of the GaN-on-silicon substrate is one of the good options to get rid of sapphire. Yet, this will require the reuse of mainstream technologies such as lead frame and wire bond 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 to get the GaN film effectively flipped and electrically connected.
Wafer-level packaging
This brings us to the option of growing GaN the traditional way, then removing it by LLO (laser lift off technology) before transferring it to a silicon substrate using wafer-to-wafer processes to derive a complete package. Wafer-level packaging and advanced technologies such as through-Si vias (TSVs) could offer a solution—they could provide an effective means of substrate transfer and allow wafer-level integration of some more packaging elements like optics and drivers.
Regarding the cost, 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 about US$ 0.06, an LCP around US$ 0.08 and WLP US$ 0.03, approximately.
The LED bonding process
The die-attached process
Also known as die bonding or die mounting, this is the process of attaching the LED chip to the die pad of the lead frame of the package. There are three main stages to the process. 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 back side of the die to dislodge the die from 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 to short.
2. Dimensions of push-up needle: This should fit the chip with the radium tip being 0.25mm ~ 0.6mm only.
3. Picking and placing the LED chip is achieved by either an anti-static plastic tool which is made of rubber, though tools made of hard materials like tungsten carbide, ceramic or steel, are also popular.
The alternate is Eutectic bonding, achieved by low melting an alloy like gold-zinc to affix the LED chip to the silicon lead frames. While epoxy glues used in most LED packages melt at 180ºC, Eutectic bonding is much better in thermal resistance and does not absorb the emitted light in the package during high temperatures. Here, to prevent the high melting temperature from destroying the lead frame’s 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.
The wire bond process
The wire bond process involves using the gold ball bonding as the electrical connection. A gold ball is first formed by melting the end of the wire through the electronic flame-off (EFO) process. Then a 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. The bonding force of the first bond should be fine tuned to prevent the stress from damaging the bond pad and chip.
One can substitute both the die bonding and wire bonding steps with the one-step Flip Chip Bonding technique in large chips of high power.
The encapsulation process
The encapsulation of bonded LED chips not only provides protection from the ambient conditions but also helps shape the out beam, be it lambertian, focused, batwing, etc. The lifetime of an LED is not only due to the chip, but also the encapsulation materials and phosphor used. The silicon resins are expensive but far superior to epoxies.
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 colours creating white may seem counter-intuitive. While, in reflective pigments, mixing blue and yellow yields green, with emissive light, however, mixing such complementary colours yields white.
LED testing
In LED manufacturing processes, normal variations in the brightness and exact colour of the 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 colour 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.
Here are a few issues of importance regarding the application of LEDs.
ESD protection: All InGaN chips are prone to electrostatic discharge (ESD) damage. The equipment must be properly grounded. Wrist bands or anti-electrostatic gloves must be used when handling the chips. The non InGaN chips are robust and have no ESD problems.
CRI: Another limitation of many PCLEDs stems from how coloured objects appear when illuminated by the type of white light they produce. A white light source’s ability to accurately reveal colours depends on the number and intensity of the colours 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 colour rendering of blue InGaN PCLEDs.
Another approach is to use UV emitting LEDs and a blend of phosphors that convert the UV into blue, green and red emissions which combine to appear white. This approach improves colour rendering and can reduce manufacturing variability (the 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 mould compounds, which reduce brightness and photo-degradation of epoxies and other plastic package parts, which in turn reduce the LED’s lifetime. Just as there is ongoing research on phosphors for use with LEDs, packaging materials better suited for use with UV InGaN LEDs are getting a lot of attention.
The author of this article is the MD of Kwality Photonics Pvt Ltd.
