Anything that will melt the sauder to the contact points and wire should be enough. You really don't need a higher wattage iron for what you are doing anyways, because you could damage the board or ribbon.

Though if you find that it doesn't stick, try using a knife and scratch the top of the contact. Just barely enough, but it can help if you get a point where it just does not want to stick.

My suggestion is to take the ribbon out of the equation if you can and use wire strait from board to board. The ribbons are hard to come by and in case something happens, you will have the backup.
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