I used the cheapest solder I could find until I got instructions from an electronics teacher using good quality Kester solder. The difference is huge. I had a cheap solder with Chinese writing and tried Kester and the ease of use was very noticeable. This material melts more easily, flows where it needs to go, and doesn't form awkward spots with tails. I thought I'm doing something wrong and end up with ugly solder joints. Turns out crappy solder makes crappy connections. Kester solder (in the right proportion) provides excellent solder joints.