LEDs are the light of choice for growing numbers of applications due to their shrinking size, as well as declining cost and longer life. But LEDs pose a host of issues to the PCB layout designer including thermal management challenges.
Within thermal management, there are a few other key issues to ponder. Those involve PCB material, thermal vias versus copper plate, and junction temperature. As far as PCB material, there is the traditional, cost-effective, widely available FR4 versus a metal core (MC) PCB. The biggest issue with FR4 is poor thermal conductivity. On the other hand, a MCPCB, as shown in Fig. 1, is a great thermal conductor, however, it costs 20 to 30 percent more than FR4.
For a more comprehensive look at LED PCBs and their design, check out our article in Modern Printed Circuits Magazine.
Meanwhile, here are a few tips and hints that’ll give you a brighter outlook on LED PCBs and some design considerations.
- A metal core PCB or MCPCB is used for LEDs instead of FR4 material because it effectively spreads the heat without encountering issues that otherwise FR4 does.
- Design an LED system with a controlled temperature rise for a given power dissipation for greater reliability. For this, use a controlling temperature resistor and high thermal conductivity heat spreader material.
- An MCPCB and natural graphite, which is a heat spreader, are used for power LEDs.
- Assure that junction temperature is properly measured. If not, it won’t be effectively dissipated.
- Also, if thermal management isn’t properly performed, it’ll degrade color at an LED’s output.
- Via in pad is a good tool to use to better manage LED heat.
When designing via in pad technology in an LED board, the following steps are important for the PCB designer to follow. Via hole size should be small, around 6-8 mils. Vias should be placed on a 25-mil grid. They should be electrically connected to one of the copper planes on the internal layers and preferably on the bottom layer of the board, as well. Fab notes should properly identify the via types that will be filled and plated during manufacturing.