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[on:]

On page 6 of the data sheet
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00043711.pdf

...It shows the heat slugs on the bottom of this VNH2SP30 chip. Do they have to be soldered to the board?  I can SMD solder the chip's pins, but not under there.  I'm assuming if they are supposed to be soldered, and you don't, it would just limit the current rating, and otherwise be fine?  The chip is rated for 30 amps, but I'll just be drawing 6 amps in my project.
Thanks.

[Reply #1 on:]

yes they are

if its just sitting on a pad there is going to be little air gaps, and air is a good insulator thus reducing its effectiveness, since your not hitting the chip very hard it might not matter as much, best way to tell is to put it in a worst case situation and take its temperature (and adjust for die to body temp, its in the datasheet) ideally when the chip hits its a temp where it just wont get any hotter.

you can solder that pad with some paste and hot air, thats how the machines do it

if theres a pad on the other side of the board with via's you can flux up the chip side real good and heat from the oppsite side shoving solder into a via, it will wick a little

or you can drill a small hole where the slug would be and solder it that way

that is if its needed

[Reply #2 on:]

Figure 38 shows varying degrees of heat-sinking using the PCB copper - ideally you'd solder the pads but for just heat thermal heatsink compound could be used I think.   The amount of heatsinking needed depends on the power dissipation, fig 40 shows the thermal resistance in deg C/W for various areas of PCB copper - so you should be able to calculate the temp rise etc.

And be aware the heat slugs are internally connected to various different voltages signals so must not be shorted together!