Electronics > PCB/EDA/CAD

TO252 thermal vias

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luky315:
I have a TO252 LDO which gets a bit warm and needs aditional cooling (layout attached). When the Tab is GND I would place a lot of thermal vias to spread out the heat into the ground plane(s). But this LDO has Vout (3.3V) on the Tab and most of the VCC-Plane is reserved for another (more critical) supply voltage. I can't connect the heat source directly to a big copper area.
Are there any design considerations for this kind of problem?

tszaboo:
First thing you should do is: put a fill or solid region to overwrite the poligonconnect design rule. Then, if you can, make a poligon on the other side of the PCB, and connect it to the vias. Also, make sure, the vias are not connected with relief. Remove solder mask on if possible, but of course keep some around the footprint. Extend the copper on top as mucha s possible. You might want to increase the density of the vias. Also, I'm not sure about the diameter you are using, but AFAIK 0.3mm-0.4mm is the best. Somewhere I read that more than two rows of via has very small effect.
If nothing helps, there are SMD heatsinks, which you can solder "around" the component. If you bring the ground plane close (~0.2mm)to the VCC that also helps. How much are you dissipating? How big is the pour you have?

luky315:
There is a connected Polygon on the Bottom layer. The Via diameter is 0.5mm and there are no thermals on them (I don't know a good reason for thermal reliefs on vias. No Idea why it's the Altium default setting...) I need the thermal relief otherwise the production guys will kill me. Of course it would be great not to use thermal reliefs and to put thermal vias directly under the component, but than you can't solder it with a normal reflow process.

lorth:

--- Quote from: luky315 on January 28, 2014, 02:03:35 pm ---(I don't know a good reason for thermal reliefs on vias. No Idea why it's the Altium default setting...)

--- End quote ---

It helps when soldering components to large cooper planes/traces, you need less heat (and less time with the soldering tip on the pad). Also, it avoids tombstoning when mass producing.

luky315:
I use the thermal reliefs on the pad (and a symmetrical connection) to avoid tombstoning. Thermal reliefs on "Ground"-Vias add unwanted inductance.

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