just about cost, although that is clearly a factor, but also about capability. To surface mount bare LED die requires semiconductor cleanroom
facilities, whereas CSP LEDs can go down a standard PCB assembly
line. The flexibility opens up the benefits of CSP LEDs to the whole of
the Level 2 manufacturing base and provides a cost-effective alternative to standard packaged LEDs.
One interesting benefit of CSPs being so small is the ability to
place them very close together to create extremely power-dense
modules. The downside is such close groupings, combined with high-power LEDs and the lack of a heat-spreading ceramic layer, generate
a lot of heat — anathema to electronic devices. The thermal challenge that had been mitigated by the LED packaging manufacturers
with ceramic submounts now fell squarely into the lap of the Level
2 integrators. The only option open to Level 2 integrators was to
mount the CSP LEDs onto a PCB with better thermal performance.
A new thermal approach is required
The LED industry uses thermally efficient metal-clad PCBs
(MCPCBs) for most modules and arrays ( http://bit.ly/2fsNHNr).
MCPCBs are typically constructed from a sheet of aluminum (
occasionally copper) usually around 1. 5 mm thick with a thin sheet of
copper, usually 30 µm or so, glued on with a dielectric epoxy. This
epoxy is filled with particles of a thermally conductive material such
as AlN to increase its thermal performance without impacting the
electrically isolating properties of the material.
At best this approach to manufacturing MCPCBs results in a thermal conductivity of around 100 W/mK — usually considerably less.
While this performance is perfectly acceptable for most LED modules, the challenge with CSPs is exacerbated by the way they conduct heat. CSPs are a point heat source — their small size and high
temperature mean they quickly saturate any substrate that doesn’t
have the requisite thermal conductivity, leaving the LED vulnerable to overheating.
The price of failing to deliver adequate thermal conductivity and
having the LED chip overheat is a reduced lifetime, reduced reliability,
poor light quality, and ultimately catastrophic failure. Clearly, there’s a
requirement for a board-level thermal management solution that can
deal with the thermal profile of CSPs without adding too much cost,
while maintaining manufacturability and the ability to use standard
PCB assembly lines.
There is an alternative to the standard process of creating an
MCPCB using a patented electro-chemical oxidation (ECO) process. For example, Cambridge Nanotherm converts the surface of
aluminum into a layer of thermally conductive, but electrically isolating, ceramic. As this layer of Al2O3 has high electrical isolation,
it only needs to be tens of microns thick to offer sufficient dielectric strength to meet most requirements. This combination of an
exceptionally thin dielectric with a high thermal conductivity gives
nanoceramic an exceptionally high thermal conductivity. With a
thin layer of epoxy to attach the copper circuit layer, Nanotherm LC
comes in with a thermal conductivity of 115 W/mK, ideal for power-dense CSP LED modules and arrays.
While CSP LEDs are continuing to gain traction in the industry,
their uptake will necessarily be limited by thermal factors unless
the industry adopts a new type of thermal management. Given the
enormous opportunity for differentiation and cost saving through
CSP LEDs, there’s no doubt the industry will respond with appropriately innovative solutions. And in this regard nanoceramics are
leading the charge.
FIG. 3. CSP LEDs are being deployed in outdoor lighting
applications such as street lights, among many other uses.