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Q: When a datasheet calls out an area of copper for heat dissipation, how do I interpret that requirement for my board?
Cherie Litson: Look for the watts, or calculate it yourself—remember Ohm’s law and Watt’s Law? What kind of heat do you need to get rid of? How much copper do you have on your board? Then go to the online calculators (Saturn PCB Toolkit comes to mind) and crunch the numbers. If you still have questions, call the component manufacturer and talk with the electrical engineers there.
Carl Schattke: The first thing I want to know is how many watts the part is going to need to dissipate. Then, how am I going to be able to do that? A copper land area is not the only way to get heat away from a part. What’s the environment? We can get heat away with convection and conduction, or some combination of that. If you do not have thermal modeling tools available, the lowest risk is going to be following the guidelines on the data sheet. Usually, you need less area than what is called out on a datasheet, but you may have a better method of dissipating the heat than they do. You may have a large ground plane, or some structure that acts as a conductor away from the part, or some active cooling components that draw heat away.
Lee Ritchey: Deciding on how much copper area is needed for heat dissipation requires a good thermal modeling tool. Any other approach is simply guessing.
Chris Young: I suggest reading the following application report from Texas Instruments: AN-1520A Guide to Board Layout for Best Thermal Resistance for Exposed Packages. This report details an approach to making low thermal resistance connections to copper regions within a PCB. A datasheet will typically give you a copper area and a copper weight. Rarely will datasheets give details about the environment where the test/characterization measurements were conducted. This leaves you in a bind because there is not enough data present to work your way back to any specific and “safe” requirements for your board. I have found that many IC manufacturers will do whatever is possible to show the best possible performance of the part in the datasheet. I have also found that this “whatever is possible” is not a reasonable approach to manufacturable designs. The most reasonable course of action is to build a prototype and take measurements in the environment(s) you expect your design to be operating in.
Cherie Litson is the founder of Liston1 Consulting and an instructor with EPTAC and Everett Community College.
Carl Schattke is a lead PCB design engineer with a North American automobile manufacturer, and a PCB design instructor.
Lee Ritchey is a PCB design instructor and founder and president of the high-speed consulting firm Speeding Edge.
Chris Young is the owner/lead engineer of Young Engineering Services and a columnist with Milaero007.
This article originally appeared in the July 2021 issue of Design007 Magazine.