Matching your COB to a heat sink is one of the most difficult parts of designing your own COB grow light. Unless the manufacturer of the heat sink actually specifies the wattage you can get away with running, you’re left to do the calculations yourself. Luckily, there are tons of heat sinks available now that come pre-drilled for your COB or board, and are rated, in plain English, for their power handling ability. You might see smaller pin-fins (120mm or so) that are rated for 50 watts of power, larger pin fins at 140-150mm rated for 75 watts, or even bigger sinks that are 160mm+ and rated for 100W.
That being said, you might be a glutton for punishment and have a burning desire to source your own heat sink, or figure out if the one you’ve got kicking around will work. Fortunately, like most things in the DIY LED world, we can figure this out with a little research and perseverence. Fair warning though, this is pretty dry stuff.
The ultimate goal of a heat sink is to dissipate as much heat from the individual LED junctions (the actual point in each individual diode that electrical current flows through) as possible. Some COBs can have over 100 of these junctions, so quite often, manufacturers prefer to deal with the temperature of the “case”, which is the substrate that all these diodes are mounted to. Keeping each of these junctions under their maximum allowable temperature is critical so it’s very important that the heat sink you select is capable of shedding the heat your light produces.
