Today I learned a cool thing about soldering.
Solder is usually 60% Tin, 40% Lead, and this is referred to as 60-40.
When the solder joint cools, the different metals will solidify at different times, and there is a short time when one metal is solid while the other is liquid, called the "plastic phase". Unless the solder joint is kept absolutely still you risk creating artifacts in the joint such as voids and cracks that will decrease the quality of the joint considerably.
The solution is to switch up the numbers. You can find solder with the perfect balance of 63% Tin to 37% Lead which completely removes this problem.
So now you know what solder to get next time!