I've been changing my own batteries for awhile now, and I noticed when I change them there is a clear coat on the solder joint where the batteries are at. Sometimes it's this yellow or brown. I normally clean this off before de-soldering my batteries because it jus doesn't look right when it's yellowed. But I also noticed when I solder new batteries or holders in my solder points do not have this clear coat. Does anyone know what this coating is? And if I am changing my own batteries do I really need to start putting this on my joints? I'm using standard rosin core solder. Haven't had any issues with my hundreds of battery changes yet.