MOSFET Heat sink

[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

[Lakshmi]

May I know the part Number of MOSFET?

[federico.massimi]

Sure, the IRFZ44
but my question is generic, for any component that generates heat. I wanted to know the solder mask has a role in the heat dissipation in the air.

[Lakshmi]

It's good practice to keep but you must always check the data sheet whether those thermal pad should be connected to GND or left unconnected.
It looks like IRFZ44 is Through hole component Why don't you try keeping this component vertical and save your board space just a suggestion.

[federico.massimi]

IRFZ44 is the series, the specific part is IRFZ44VZSPBF cha package D2PAK. But I have the same problem even with LDOs, the question does not concern the component, I try to explain myself better:
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?

[robertferanec]

the question is whether this area should be covered or not with the solder mask?

- I normally leave it covered by mask. However I need to say, usually I design different kind of boars (e.g. often I have multiple GND planes and heatsink available)

I wanted to know the solder mask has a role in the heat dissipation in the air.

- I have never experiment with this, but I would say, solder mask will influence heat dissipation

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

- if you do gold finish, I believe it should not oxidate

If the heatsink area is GND, I would probably not worry about making it big and unmasked. If it was not GND - then I would be more careful - making it big could cause problems with EMC/EMI and in that case it may be better to consider placing heatsink on the plastic package of the chip.

We often use thermal adhesive tape + simple and cheap heatsink (it is cheap and quick solution). Looks like this: