I've never previously designed a board for mass production (as opposed to boards I hand-solder myself with a soldering iron and rare application of a hot-air station), but it's looking like I'm going to need to quite soon. Most things I can figure out myself, but I'm having a hard time finding information on how to design footprints for QFN packages (particularly whether the recommendations for very large QFNs also apply to small QFNs like those used by the LT8336 and LT8618C, both of which are used on my board).
Many of the resources I can find say that using paste over the entire thermal pad is too much solder, and will raise the part off the board enough to prevent the pins from reflowing properly. But is this true of all QFN devices, or would this not matter as much for the tiny ones where the thermal pad isn't that much bigger than the pins?
Also, what's the best way to place the thermal vias? I don't want to put too many thermal vias that all the solder wicks away instead of forming a bond between the pad and the device, but I don't have the budget to use plugged vias. LT doesn't provide any recommendations on this, unfortunately.
They also don't say anything about whether I should be using solder-mask defined or non-solder-mask defined pads, which I know can be important for fine-pitch SMD components, but I don't understand why. Is that important in this case, or is that not until you get to the ultra-fine pitch BGAs that you need to worry about that?
In short: PCBs and PCB assembly are expensive, how do I make sure it works right first time with QFNs? I have enough worries about whether the design itself will work adequately, I'd like to not have to worry about whether the parts will even solder properly.