Heatsink soldermask on PCB

[federico.massimi]

Hello,
I'm designing a pcb but I have a question: on my pcb there is a free zone used as a heat sink for some MOSFETs, the question is whether this area should be covered or not with the solder mask? For clarity I attach a screenshot, solution A or solution B?
the surface finish is HASL LF

In the case of solution B, could I have trouble leaving that big exposed PAD, such as oxidation or something like that?

thanks in advance, Federico

 

[barry]

If you intend to solder the MOSFET tab to that area (it's not really clear from your post), you sure don't want solder mask there. As for your second question, didn't you just say that there's a HASL finish? If oxidation is a problem with that, then it would be a problem EVERYWHERE on your board. (It's not.) Unless you're operating in some atmosphere that would require conformal coating or some other protection.

 

[federico.massimi]

No, I don't have to solder the mosfet tab, this is just a free space on the board. Reading the datasheet and the application notes I estimated that for the power dissipation on my board I need a free area of about 1.2 square Inches around the MOSFETs. The question is simply, should this area be covered or not covered by the solder mask?

Said in other words, the solder mask significantly limits the thermal dissipation in the air or is negligible for thermal purposes?

 

[marce]

Have a read of this...
https://www.ti.com/lit/an/snva183b/snva183b.pdf

Done both ways...

 

[barry]

The solder mask contribution is negligible. If it would make that much of a difference in your application, then you need more copper area.

 

[marce]

If we fit a heatsink we remove the solder resist even when using thermal pads, but generally just using the PCB we leave the solder mask on...