boric acid + isopropyl alcohol
For safety reasons, I would stick with ethanol or isopropanol, as they are less flammable than methanol (your using hot soldering equipment) and much less toxic, even though methanol is a better solvent for $\ce{H3BO3}$. If it weren't for the expense, due to heavy taxation of ethanol, it would be preferable to isopropanol. From the table below, you'll need almost twice as much isopropanol (though 1-propanol is the solvent listed, it is an isomer of the more common 2-propanol, AKA isopropanol).


  Solubility of $\ce{H3BO3}$ in various alcohols at 25 C, percent by weight:
  
  Methanol:     22.66% 
  
  Ethanol:     11.96% 
  
  1-Propanol:    7.34%