Mix brown sugar, butter, corn syrup and salt in mixing bowl and heat in microwave 2 minutes on full power. Take out and stir in 1/2 teaspoon soda. Stir vigorously; mixture will foam. Put popped popcorn in a large paper bag. Pour brown sugar mixture over popped corn and shake. Roll shut and put in microwave 1 1/2 minutes on full power; take and shake put back in microwave for another 1 1/2 minutes; take out and shake and let set for 10 minutes. Break apart and enjoy!