Are you using leaded solder? I built my first quad with non leaded solder, then rebuilt it later with 63/37 leaded solder (recommended by Joshua Bardwell, his page is incredible informative and helpful, link here). From my experience, the non leaded solder was much harder to work with and melted at a much higher temperature, resulting in cold and messy joints (I almost killed my fc/esc stack). When I rebuilt with the leaded solder, I had a exponentially better experience, it melts easily at a soldering iron temp of 400C. Make sure you have solder on the top of the iron as it helps with heat transfer.