Which vehicle/engine? If this is the 5.4 in the F150, you most likely have rust buildup in one of the rear two spark plug boots (the two in the very back closest to the cab). The coil packs themselves are expensive from Ford ($100), but you can replace the boots for about $3 each from your local parts house.
I have had my dealings with Ford "Quality Care", and what I found is that A) The quality is seriously lacking, and B) There is no care whatsoever. My issue was that the boot on the number 8 spark plug got rust inside causing my engine to miss. My truck was less than 50 miles out of warranty. I pleaded with both my local dealer and with Ford Quality Care (via telephone) and they would not admit to having a quality problem. This despite having literally thousands of hits via a Google search for this problem AND having a Ford dealership mechanic tell me POINT BLANK that this was a major issue with these trucks/engines. I ended up having to pay the full amount for the repair as well as the required "fee" for the mechanic plugging my truck up to their super computer (it must be super if they expect us to pay over $100 just to plug the damned thing up and turn it on).
Just like the engine miss that I experienced, this spark plug issue will generate too many search results to count. It IS a quality issue, but Ford won't admit it ... nor will they do anything to help you. A class action lawsuit is the answer, but getting it together takes money for attorneys and most intelligent attorneys will tell you that Ford has a much larger legal budget than they do.
I would sign on to a lawsuit against Ford in a heartbeat, but I am not so sure that it will ever come to fuition. Sadly Ford will win this one by simply denying the truth about their substandard products.