Electronic – Should I apply soldermask over SMT thermal pads

heatsinkmosfetreflowsurface-mount

On page 8 and 9 of the following datasheet, there's a description of thermal pads for this MOSFET: https://www.vishay.com/docs/63302/si2342ds.pdf

What I am wondering is: I will design the footprint on the copper layer to have the shape of the footprint on page 8, with the "copper spreading". This increases the heat transfer, so better cooling.

Should I also leave holes as big as that in the soldermask? Or can I cover up the thermal pads so that the footprint with the soldermask will look like the one on page 9 (standard SOT-23). This way during reflow, the parts will stay nicely in place.

Best Answer

As I read the datasheet, it is expected that there will be soldermask over the thermal pad.

This paragraph from the datasheet seems to be saying that you should only have holes in the solder mask for the pins of the transistor:

Even if additional planar copper area is used, there should be no problems in the soldering process. The actual solder connections are defined by the solder mask openings. By combining the basic footprint with the copper plane on the drainpins, the solder mask generation occurs automatically.

So, your idea of having solder mask over the whole thing with holes just for the transistor pins is correct.

Related Topic