Chip Scale Packaged (CSP) LEDs (or ‘Direct Mount Chips’ as Nichia refer to then) are the latest incarnation of flip-chip LEDs. CSPs have been used for low and mid-power applications for a few years, mainly for TV backlighting, but are now moving into high-power general lighting applications.
According to the IPC, “to qualify as chip scale, the package must have an area no greater than 1.2 times that of the die and it must be a single-die, direct surface mountable package”. To achieve this LED manufacturers’ remove all superfluous elements of the packaging, including the ceramic submount and the wire bonds. Instead, the LED die is over-moulded, the P and N are contacts metalised to create solderable lands and a coat of phosphor is applied to the emitting surface(s). This reduces the cost of the LED as it removes both materials and manufacturing steps. Industry analysts Yole Développement predict that CSPs will make up 34% of the high-power LED market by 2020.
There is however a significant challenge that is unique to CSPs. Removing the ceramic submount means CSP LEDs are soldered directly onto a module PCB. There is then a direct thermal path from the die junction to the PCB. As CSP LEDs have very small lands (CSPs can measure 1x1mm) this creates an intense point source of heat that the module PCB needs to conduct away to prevent the LED die overheating, potentially causing the LED to fail.
This issue is exacerbated by one of the key selling points of CSPs: the ability to tightly pack LEDs together to create small, very bright, modules (this is particularly true of Nichia’s D.M.C.s as they only project light from the top surface, reducing optical cross talk). This high power density significantly increases the thermal challenge associated with CSPs and makes the job of the module PCB critical to the thermal management of the light engine.
And this is the challenge. Traditional packaged LEDs rely on the submount to spread the heat from the die before it ever reaches the PCB, helping to avoid concentrations of heat. As the heat spreading ability of the ceramic submount has been removed from a CSP it is down to the module PCB to provide this function. But, the traditional remedy of a thick layer of copper on the PCB cannot be used because the tiny dimension of CSP LEDs means they need a wiring trace with very fine tracks and gaps. Thus the challenge is to conduct the concentrated thermal flux through the dielectric layer of the PCB and into the metal board where it can be spread and removed by the heat sink.
The key to understanding this challenge is the dielectric layer of the PCB. All LEDs that require serious cooling use metal clad PCBs (MCPCBs), due to their high thermal performance. Most MCPCBs have a dielectric layer that is made of an epoxy resin mixed with ceramic to create a thermally conductive, but electrically isolating, barrier between the core and the copper tracking. The issue is that there’s only so much ceramic that can be added to the epoxy before the dielectric becomes friable. This fixes a ceiling on how thermally conductive these MCPCBs can be.
Nanotherm’s unique approach to thermal management offers a solution. Nanotherm LC has a dielectric layer created using Nanotherm’s heavily patented Electro-Chemical Oxidation (ECO) process that converts the surface of an aluminium board into a super-thin alumina dielectric layer. This Nanoceramic alumina has a thermal conductivity of 7.2 W/mK which, coupled with being just tens of microns thick, makes for a composite thermal performance of 115 W/mK, much higher than any competitive MCPCB. This means that the intense point heat from the CSPs is conducted efficiently through the dielectric and into the aluminium board, maintaining a safe operating temperature for the LED die and, just as important, uniform temperature distribution over die arrays.
Nanotherm LC’s unique ability to conduct heat so efficiently is driving a new generation of CSP designs, particularly for devices like the replacement bulb market.