Thermal design: Get the heat out of the electronics




If you have high-powered LEDS, or a power supply, or are trying to control larger motors, you have to get a lot of heat out of your circuit boards. The classic way to dissipate heat is to bolt your power transistor to an aluminum heat sink. That is a slow, messy, and expensive proposition, especially if you need thermal grease between the transistor and heat sink (Figure 1).

Thermal design Get the heat out of the electronics

Figure 1
 This 5V regulator can supply more power when bolted to an aluminum heatsink using thermal grease to better transfer the heat.Another problem with bolted-up heatsinks is that the assembly operation is less controlled. Circuit board fabrication and assembly are tightly-controlled repeatable processes. When you have people hand-assembling things there is a greater chance for errors. This is especially true if the people doing the work are on the other side of the world, speak a different language, and are functionally illiterate.
It’s always a good idea to make thermal management inherent in your PCB design. Experience has shown you can get about 2W of heat out of a 3×5” copper area on a conventional FR4 PCB.
Wayne Yamaguchi learned some great thermal design tips for his kits to convert a Maglite to an LED flashlight (Figure 2). He thought the hard part would be the switching regulator design. It turns out the mechanical engineering and thermal design was the hardest thing.
Figure 2 Wayne Yamaguchi found that Linear Tech had made the circuit design for his LED Maglite kit easy, it was the thermal design that took the most work. Click to enlarge.

Wayne figured out how to use vias and copper pours to get the heat of the LED transferred to the aluminum body of the Maglite. Getting the heat out is why LED flashlights are not made of plastic. An LED is more efficient than an incandescent bulb, but the incandescent bulb radiates its waste heat out as IR (infrared) light, along with the viable light. The heat made by an LED stays in the LED.

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The first place you look to dissipate more heat is in the PCB copper. You may want to experiment with thicker copper. Normal PCBs have 1 oz copper, or 1 oz plated up to 2 oz copper. That is measured in ounces per square foot. To covert to the actual thickness in mils (thousandths of an inch), you multiply by 1.37. It’s easy to order PCBs with 6oz/ft2 copper foil. Then you go to what PCB fabricators call “heavy copper,” where the weight can get up to 20oz/ft2, that is, 27.4 mils thick. If you need even thicker, you can shop around for a PCB fab house that does “extreme copper.” This is anything above 20 oz/ft2 thickness.

Any airflow over a heatsink or copper planes will greatly improve heat flow. If you uncover the solder mask of the plane, it improves heat transfer to ambient. Do not finish these large bare copper areas with HAL (hot-air-leveling). The solder can pile up and while it will improve heat dissipation, it will make big droopy lumps of solder. It is better to nickel plate or use gold immersion finishes. Both will protect the copper from corrosion and the gold finish looks great.




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