I'm pretty sure the cracking is actually the spark going to ground through the coil pack body, for this to happen there'd have to be really high resistance somewhere in the wires or plugs, or your wire routing is not right. To begin with I'd ohm the plug wires and make sure none of them are open, resistance will be in thousands of ohms but probably shouldn't be higher than 5K or so given the length of the wires. I have Bosch wires and they were all at 2500 or so when new.

Did this happen once and then you replaced the wires and plugs and it keeps happening to brand new coil packs?

EDIT: One other thing to check is all the wiring going to the coil pack, these wires are known to break and cause problems, so give that a good visual inspection.

 

All brand new Bosh wires, plugs. yes it happens to brand new coil pack. Brand new alternater and die hard battery.

 

^This

 

Are you using $15 Amazon coils that have uneven bottoms to crack when tightened down? Try also not tightening so much, they really don't have to be that tight. Plastic, you know. People around here love to crack the stat housings too, again, plastic.

Why did you change the coil mount? If not dead flat the out of plane (say one corner high or low) will crack coil when tightened down.

The longterm cracking on these that normally occurs after long periods of time is from temperature.

 

All new Two Ford Motocraft coil packs, Two Autozone Durolast. Got to be a short in the wireing or some thing else. Hopens with in 5 miles on test drive.