I have not met a designer yet who doesn't worry about thermal issues in their LED packaging. Everyone has a thermal interface material solution, but not everyone has opted for metal interfaces – just yet.
I wondered if one reason for this might be simply a lack of understanding of the metal thermal interface materials or their design possibilities. From my experience, designers of high power LEDs often choose metal thermal interface materials which are stiff, and resistant to oxidation. The solder is supplied as preform discs or rectangles which the manufacturers use to solder their chip to a Heat Sink Slug. To minimize the thermal resistance of their LED package (and maximize the thermal power that can be dissipated), the preforms used in this application are very thin. They can be manufactured as thin as 0.0005" in thickness. At this point, the resistance of the heat sink which the chip is soldered to has a higher resistance than the metal thermal interface material, making the interface resistance a negligible concern if it has been soldered adequately.
One reason that the high temperature preforms are the material of choice and not other alloys in other forms such as solder paste is that the preforms can be soldered without using a flux. For assistance with choosing the best preform for your LED application, please e-mail me!
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