First thing u need to do is clean the motherboard using isopropyl alcohol. Use a small brush to clean all the parts. Only after that should u consider any other option and one more thing, solder melts at 183 degree centigrade and the non lead solder melts at 217 degree centigrade. Also to the reflow be successful u need to use liquid flux. But clean first. Write back on what u did. good luck.