I am working on the design of dual 5V/3.3V power supply based on TI's TMZ12003EXT buck converter module. It's an older TI part, apparently intended for "rugged/military applications," but the only integrated-inductor module that I can find currently in stock from U.S. distributors or TI itself.
In the course of working on my design, I tried TI's WEBENCH Power Designer tool, just to see what it would produce. The schematic it produced is unremarkable and matches up with what's in the datasheet and what I was planning.
However, the suggested PCB layout is weird. It appears to show large regions of bare copper on the front and back of the board. Here's what the tool generated for a suggested PCB layout:
(Note: I'm assuming the yellow regions are bare copper, since the green is clearly solder mask.)
My question is: do you really need large regions of bare copper with these types of modules? I understand heat dissipation is important. But having large regions of bare copper on the PCB front and back seems really risky to me in terms of the potential for short circuits, corrosion, etc. Is it normal for these types of modules to require bare copper planes, as opposed to "normal" copper pour regions that are covered in a thin layer of solder mask?
Also, won't bare copper make it hard to reflow solder this module? The paste will melt and spread out all over the bare copper, will it not? I don't see anything to stop it in this image.
(Is it possible that the orange area is just to emphasize the copper pour and is actually intended to be covered by solder resist, not exposed to world bare?)