A method of manufacturing a semiconductor device by attaching a flip chip die to an organic substrate using solder comprises applying no-clean flux to the flip chip die or the organic substrate; heating the flip chip die and the organic substrate to bond the flip chip die to the organic substrate, and cooling the flip chip die and the organic substrate. The step of heating the flux includes controlling oxygen and moisture content of an atmosphere surrounding the flux, preheating to a temperature of about 145° C. to about 165° C., soaking at a temperature of about 145° C. to about 165° C. for about four to about six minutes, and reflowing above the solder's melting point.