I'm new to PCBs and I'm working on a project in which I'm dropping ~22.8V across a resistor. I'm using 0.5W resistors (they'll be dissipating about 0.43W with my application). In my prototypes the resistors get fairly warm, and I'm wondering if a heat sink wouldn't be a bad idea. Additionally I could end up with up to 5 of these resistors and if several are on at once, it concerns me that they'd be able to overheat each other. What's the best way to sink the heat off of these guys? Would a something like this be appropriate?
Electronic – Sinking heat from resistors
heatsinkpcbresistors
Related Solutions
LM317 in TO-92 only does 100mA, wheras the TO-3 and TO-220 are both 1.5A - so it depends on how much current you need!
"Best for attaching a heat sink" - hard to say. It's easy to attach a TO-220 to a heatsink, just a hole and thermal compound (or one of the clip on ones). You can also just attach them to the PCB for a bit of extra cooling.
T0-3 can have a lower thermal resistance to the heatsink, but generally are more awkward to mount.
TO-92 isn't really possible to attach to a heatsink.
According to the note just below the thermal data table from that datasheet:
For packages with exposed thermal pads, such as QFN, PowerPAD™, or PowerFLEX™, \$\theta_{JP}\$ is defined as the thermal resistance between the die junction and the bottom of the exposed pad.
The way I interpret that:
So if you are attaching the heatsink in the normal way, on the back, against the pad, use the junction to pad thermal resistance.
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Best Answer
Remember the power rating of resistors is a maximum rating. 0.43W for a 0.5W resistor is a bit close to the edge.
Now, SMD resistors conduct heat well into their pads, and if the pads are connected to a large copper area (like a ground or power plane) then this will suck the heat away and spread it, using the board as a heat sink. In this case, a 0.5W resistor used at 0.43W will be warm, but not close to its maximum temperature.
Thus, if one side of your resistors is connected to a ground/power plane, things will get easier.
Several resistors of lower rating in parallel spread the heat better, too, since you can connect them all to the copper plane. You can also wire them in series, but then only one will be connected to the copper plane.
Another strategy if you use multilayer and have a ground plane on layer 2 is to use wide pads. Since the prepreg between layers 1 and 2 is thin, heat will tend to flow through it and into the ground plane where it is spread around the board, which then acts as a heat sink.
Vertical boards get a lot better convection cooling than horizontal ones.