ikor_leds-csp-y-flip-chip

As we have already commented in other articles in our blog during the last few years, the world of lighting is experiencing an almost overwhelming revolution. In a very short time we have gone from living happily with lamps with light bulbs to converting all lighting to use LED (Light Emitting Diode) technology.

The more traditional LED initially addressed low power requirements focusing on signalling applications, but leaving the middle to high power segment to incandescent lighting.

With the appearance of medium and high power LEDs, the non-LED lamps finally lost their dominance completely, giving way to LEDs that were cheaper (€/lm), smaller in size (lm/mm2) and with higher efficiency (lm/W).

But obviously these types of LED are not a panacea. They are very sensitive to temperature and this critical factor very much conditions their useful life.

The useful life of a LED is defined as lasting until the moment when the luminosity falls to 70% of its nominal value. This degradation is not linear and is accentuated as time passes. As a representative example, a temperature increase of 25°C reduces the useful life from 50,000 hours to 16,000 hours, which is a reduction of 68%.

If we bear in mind that the medium and high power LEDs are polarised at high currents (in the order of one ampere) and also that around 75% of the energy generated in a LED is converted to heat, it is clear that without adequate heat management its durability will be seriously compromised. The following graph shows the impact of this temperature factor:

ikor_leds

This considers the thermal resistance of the classic LED encapsulations is high,  the conclusion is: a major impact on useful life.

In order for luminaire designers to be able to estimate a LEDs useful life, LED manufacturers generally follow the LM80 standard. The contents of this standard make it possible to estimate the useful life of the LED in the design conditions of a luminaire. Although it is the key to design, it is unusual for LED manufacturers to make the recommendations of this standard public.

ikor_lm80-test-report

 

However, in an increasingly competitive market where ever cheaper solutions are sought, year by year, LED manufacturers analysing the manufacturing costs of LEDs concluded that the key area to work on was the encapsulation.

As published in the DoE Solid-State Lighting Research  the encapsulation of LEDs accounts for almost 50% of the final cost, which is why a reduction in this would directly affect manufacturing costs.

And as if these problems were not enough, the fact that the traditional LED has connection lines (bounding) to the anode and cathode represents a weak point when this is exposed to mechanical stress and ageing (rupture by heating).

In addition, in an increasingly polluted environment, the sulfides that are normally present react with the reflective silver of the LEDs, turning this darker and changing the colour of the LED ( Cause study of LED)

case-of-study-leds_ikor

The LED manufacturers, aware of all these drawbacks, began working on innovative ideas that would allow them to minimise the problems of traditional LEDs. From the different alternatives that they looked at, the winning solution was the FLIP CHIP, also known as CSP (Chip Scale Led).

Sources:

EETimes: Rudolf Hechfellner, Philips Lumileds Lighting

Samsung: Case Study of Discoloration

Office of Energy Efficiency & Renewable Energy 

 

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