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Do LEDs need pure metal thermal interface materials?

Simply stated, YES.

In effort to stay on top of industry technology I spend as much time as I can afford attempting to learn about the current thermal solutions being used on various devices. LEDs have long been controversial regarding their requirement (or non-requirement as the case may be) for high efficiency thermal solutions. Although I have spoken to many LED designers in the past who were adamant about and enforced the use of metal thermal interface materials on their LEDs, after sitting through the recent Photonics West trade show, I have to admit now that these engineers are no longer the minority. LED packages are now commonly including Aluminum or Copper heat sink slugs which transfer heat directly from the chip to a printed circuit board. Taking this a step further, LED design engineers are now even considering the use of active cooling which could reduce their thermal resistances by another factor of ten. None of this makes sense though if the design skimps on the die bonding material. Low power LEDs regularly use silver-filled epoxies, but most high power devices are now using solders due to their low thermal resistances.