Out of the Wood BY MIKE WOOD How do LEDs work? Chip-on-board OVER THE LAST YEAR OR so, I’ve written a series of articles in this journal on the mechanics and physics of LEDs and how they operate. This issue, I want to continue that theme and take a look at a method of mounting and using LEDs which is gaining popularity in entertainment lighting products. You’ll see it referred to as chip-on-board or COB. What does that mean, and what advantages does it give to the product designer? to the semiconductor. What was a 2 mm x 2 mm die that’s 1 mm thick becomes a 10 mm x 20 mm package that’s 5 mm thick, perhaps 250x the volume. This potential for extreme reduction in size is one of the fi rst reason COB devices were used. With a COB device, the semiconductor die has no packaging and instead is mounted directly onto the circuit board or substrate. can see in the photograph that provides protection. The end result is tiny compared to a packaged chip. Let’s take a look at how LEDs are packaged and how they could also take advantage of chip-on-board techniques. To recap from an earlier article, the most familiar and oldest LED package is shown in Figure 2 . Bond wire LED die The fi rst thing to say is that COB is not actually a new technique. It dates back many years both in LED usage and with semiconductors as a whole. Chip-on-board is a method of packaging and using semiconductors and has nothing to do with the design and manufacture of the semiconductor die or LED itself. We are most familiar with seeing semiconductor dies assembled into a plastic or ceramic housing to create a component. These packages form the familiar computer chips we see on every circuit board and are supplied with standardized leads or connections that make them easy to assemble onto boards. This packaging also provides protection to the semiconductor die. One downside of the chip-style packaging is that it also adds a lot of bulk WINTER 2013 “ “ The closer spacing of the dies improves color homogenization and helps get rid of multi-colored shadows. Anode Cathode (Large to provide Heat Sink) Heat flow Figure 1 – Chip-on-board processor in a calculator Figure 1 shows an example of a COB chip in a pocket calculator. The circular black blob to the right in the photograph is the processor that provides all the calculating power. It’s a complex digital semiconductor that, if packaged conventionally, might be 3 cm square just to accommodate all the connections. Instead, the silicon semiconductor die is adhered directly to the green circuit board, and the connections are made through fi ne gold wires that are bonded from the die straight to points on the circuit board with no intermediate connectors. The entire assembly is then encased in the blob of black epoxy that you Substrate (Circuit board) Solder Through Hole Packaging Figure 2 This is a through hole package where the LED die is mounted to one of the lead wires (which also provides the heat sink), and the entire assembly is surrounded by a transparent domed package. The two lead wires pass through holes on a circuit board where they are soldered to copper traces on the board. Figure 2 is roughly to scale, and you can see how large the package is 22 WINTER 2013