Pad and via design issues

1. Why does the tin on the QFN pads seem insufficient? 

2. I noticed some bubbles or tiny solder balls under the BGA pads, and the bottom looks covered with solder — is that normal?

This is not a normal phenomenon. Usually, such a situation occurs because there are through-holes in the pads or the through-holes are not sealed properly, causing solder leakage during reflow soldering. As a result, gases or residual fluxes get trapped beneath the solder joints, forming bubbles or solder balls.

We suggest: 

Use the process of resin filling holes + copper cap sealing or solder mask covering (tented vias). At the same time, check the amount of solder paste printing and the reflow soldering temperature curve to ensure a stable heating and exhaust process.