SURREY, UK -- The market for PCBs with enhanced thermal and power management will top $3.2 billion by 2020, according to BPA Consulting.

"LED packaging is moving rapidly from single chips to multichip devices due to a combination of improvements in efficacy (luminous output per watt) and opportunities in both new and replacement high brightness (>700 lumens) applications ranging from Edison socket retrofits to automotive headlights," explain Bill Burr and Nick Pearne of BPA, authors of a new report on the technology. "This move is driving changes at the substrate level- the illumination flexibility provided by multichip arrays means that the single level of wiring provided on an aluminum-based board may not be sufficient to address circuit complexity, and the power density created by the multichip array needs a lower impedance thermal path to the reference heatsink."

Figure 1. Relative size of discrete vs. multichip packaging.

Substantial investments are being made in production capacity for aluminum-based boards in anticipation of a massive requirement for LED substrates. Some recent forecasts call for over 1 million sq. m/month additional capacity by 2015, but at the same time the economics of LED retrofits have been driving this market towards other technologies.

The economics of LED retrofits are changing rapidly, as is the lighting market. But not fast enough: while LEDs are presenting the potential for a paradigm shift from single, bright sources to distributed illumination, the majority of the more than 12 billion sockets up for retrofit are rapidly being occupied by alternative technologies such as CFL (cold fluorescent) as legislation mandates phaseout of incandescent bulbs. LEDs are coming up, but need to either drastically decrease the price point or drive a change in the way lighting is designed and used. Both will impact the requirements for mounting, interconnection, and thermal management.

The emergence of multichip LED modules is a consequence of these trends. Grouping bare die onto a substrate removes the thermal resistance represented by discrete packaging, leading to decreased temperature rise from the LED semiconductor junction to the substrate for a given luminous output. At the same time, elimination of individual packages reduces the required footprint area: for example a four-device array of earlier-generation Luxeon Rebels from Philips occupied about 100mm² including thermal fanout giving a power density of some 2W/cm² at around 800 lumens output. In contrast, Cree's MCE occupies 55% less board area, leading to a power density at the thermal pad of about 49 W/cm² when driven to 750 lm. This represents a dramatic increase which requires a complete rethink of the whole packaging approach beginning with the substrate. The challenge is compounded when the four (Red, Green, Blue, White) die need to be individually driven to provide color balancing.