Are there any thermal vias on the thermal pad? They have a small wicking effect thus allowing you a little leeway on how much thermal pad paste you use. Without them, the solder's got nowhere to go but the pin pads.
What is your reflow method? Are you using a paste stencil and an oven, or wetted pads and hot air for example?
For development, you could just not apply paste to (or avoid wetting) the thermal pad as it's already tied to Vss.
FWIW, for reworking these, I use hot air and wet the pads, I don't use paste or stencils. How much solder you use to wet the thermal pad is probably an experience thing, but the short answer is not too much and not too little.
When applying hot air, the device should be gently nudged with tweezers under the microscope in a way that you can see it returning itself to the correct position without you forcing it. Then, drag solder the exposed castellations of the QFN with a 0.2mm bit.
post edited by Howard Long - 2019/11/13 06:32:09