808ROSS,
If you were able to successfully re-flash it then the ISP header must be working properly, so no need to continue testing it.
For lead free solder you should have your iron set to at least 350 degrees C. Depending on the type, you may need to set it more in the range of 370. Keep in mind, the extra hot temperatures are more prone to damage the board or the components so use your flux so the solder flows quickly.
Good luck!
-Brach