Skinny Divining Rod Solder, tells where iron tip is

[petemoore]

  and makes mess board.
  something strongly pulls the core toward the hot tip, then up it, and the copper pad has a solder repellant effect, it's like the tip and solder and pad are reactive magneticically [which makes no sense?].
  The cure for it IME is use a less skinny core of solder to add solder with, using this thin stuff again is no fun at all, works like bad spirits trying to get into an Ouiga board.

[Ronsonic]

Flux. It's all about the flux. The iron tip is relatively immune to oxidation compared to the copper pad. Copper gets hot, oxidizes and repels solder. Iron (literally, iron) needs less flux to remain solderphilic (yes, I made that up).

Thin solder has obvious potential problems in the flux department. Only the better solders have good flux, well distributed - I've had good results with both Alpha and Kester, nongood results with others. Also, any pulling or stretching of the solder, which is much more likely with the fine stuff will tend to squeeze solder out of the narrowed section of solder. It is not uncommon to find a bunch of bad solder near the beginning of a spool of fine solder.

Hope some of that helps.

Ron