Thermal management is one of the most important aspects of a good quality grow light. The lifetime of an LED is simply dependent on its running temperature.
The hotter the LED runs the less time it will last. If you do not dissipate the heat away from the LED effectively by using a good quality and well weighted heat sink then you could lose 1 or 2 years from the operation of your light.
That's up to16,000 hours of your 50,000 hour life span.
But wait that's not all. . . .
Light / Spectrum Decay, if your LED are not cooled sufficiently you could lose between 10-15% of your total output of umol over a 2 year period. Via decay of the actual phosphorus coating from prolonged over heating the spectrum will deteriorate at a rapid rate, coupled with the loss of umol and 16,000 hours life span its not looking good for your cheap LED unit from the far east..
There is a rise in cheap aluminum profiles being used as heat sinks, extremely light and not fit for purpose, designed to reduce the cost of shipping from the far east, these aluminum profiles do not have the required weight to dissipate heat correctly. They also do not use thermal conductors to assist the removal of heat from the PCB, ideally either thermal paste or tape is a good option, this encourages the heat to escape the PCB. If your LED is creating more heat at the source than your PCB and heat sink (profile) is capable of dissipating, then there will be a back up of heat building up in the LED which will cause Light and Spectrum decay along with a large loss of life span.
Image below : Here is an example of a poor heatsink, very low dissipation rating, too small and light to be effective. Designed for cheaper shipping from China. Will cause loss of spectrum, output and longevity. This extrusion weighs less than 0.80kg and is expected to dissipate 100w of power
A poor heatsink, very low dissipation rating, too small and light to be effective.
Image below : Here is an image of an excellent HEATSINK with high dissipation rates, our very own extrusion used on our F BAR system, as you can see it dwarfs the Chinese variant both in size and weight, 3.3kg in total dissipating 120w of power.
The hotter LED get the bigger the decay curve. To run LED efficiently and reach the run time of 50,000 hours, manufacturers should be aiming for a junction temperature of around 65c or less, this will protect the LED from rapid decay as described above and give them longevity to the end of their 50,000 hour life span.
Using your dissipation rate of your aluminum PCB and the dissipation rate of the HEATSINK you can calculate how large a HEATSINK is required. Initially the PCB then needs to have a dissipation rate of 1.0w.MK or more to move the heat from the PCB into the HEATSINK, any additional help you can give by including thermal paste, lube or tape will be of benefit. Now the rest is up to the heatsink, it needs to be able to dissipate the heat it is receiving quick enough to allow the flow of heat through the metal.
Think of it as a tap slowly running into a sink with an over flow into a bucket. Depending on the rate of flow and the size of the sink and also overflow bucket will be the deciding factor on if you flood your kitchen. However, once your heatsink backs up with too much heat, the heat will start creeping backwards into the PCB and then into the LED.
See illustration below for a good example of thermal management
, the heat sinks on those F bars are beasts need one of those in my life 
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