Here is the exact language from FTC.gov
Assembled in USA Claims
A product that includes foreign components may be called "Assembled in USA" without qualification when its principal assembly takes place in the U.S. and the assembly is substantial. "For the "assembly" claim to be valid, the product’s last "substantial transformation" also should have occurred in the U.S. That’s why a "screwdriver" assembly in the U.S. of foreign components into a final product at the end of the manufacturing process doesn’t usually qualify for the "Assembled in USA" claim.
Example: A lawn mower, composed of all domestic parts except for the cable sheathing, flywheel, wheel rims and air filter (15 to 20 percent foreign content) is assembled in the U.S. An "Assembled in USA" claim is appropriate.
Example: All the major components of a computer, including the motherboard and hard drive, are imported. The computer’s components then are put together in a simple "screwdriver" operation in the U.S., are not substantially transformed under the Customs Standard, and must be marked with a foreign country of origin. An "Assembled in U.S." claim without further qualification is deceptive."