Why are there swirls or zig-zag lines on the PCB samples?

I go through this every time I have PCBs I need to populate with SMD parts, and this has become more an issue as pin spacings have gotten tighter, and my hands have become less steady with age. How do you hold SMD part in place while soldering?

I have received the final prototypes from manufacturer and all is good after our checking. And we want to keep it well. Do you have any tips about that?

Solder paste should be stored in a refrigerator and it has an expiration date. But what exactly will go wrong when it is stored at a higher temperature or past its shelf life?