The key for adding cheese to soup so that it melts just right has to do with regulating the temperature, but with cream cheese or sour cream, you also need to mind the risk of curdling...
No matter how you add the dairy, avoid other mistakes that will cause it to curdle. Add it in towards the end of cooking, and don't allow the soup to reach a boiling point...
[
Collection
]
[
|
...
]