Hi,
I do not think that this is a solder mask burning, but flux which is burning. The soldering wire contains, beside SnAg and so on the so-called flux (few percent, depending of wire type). The flux help the solder to "wet" by cleaning your solder pad. This is a normal way to clean flux after soldering.
I assume, according to the photo, that it is iron soldering.
Please give us some info :
- What is the type of flux that is in your solder wire (R0L0, REH1...) ? It should be indicated on your solder wire.
- did you add extra flux (with flux pen, or brush...) ?
But, it looks like :
- there is too much flux !
- the temperature is probably too high !
According to the type of flux, a particuliar solvent needs to be use to remove flux after soldering.
But you can find general purpose flux remover. I use this one : farnell reference 3916613
regards
RG