SAN DIEGO—On a hill in the Scripps area of north San Diego, employees of Nokia spend their time breaking mobile phones.

Really.

They drop the phones, twist them, toss them in water tanks, enclose them in a dust-riddled box, and pound their keys repeatedly.

They’ve got a machine nicknamed Spanky that swats phones off a pedestal. And another one that uses Levi 501 jeans (size 32 by 30) to simulate taking the phones in and out of jeans pockets.

These Nokia workers aren’t happy until they break the phones. That’s because they need to break the phones to figure out why they broke and how to prevent it from happening again.

Nokia’s facility is one of about 10 the company operates in the world but the only one in the United States. About 600 people work there, including those who work in one of the labs designed to test, and fix, Nokia phones.

The labs put the phones through mechanical and environmental rigors, as well as a “failure analysis” lab that tries to figure out what went wrong. Usually the phones are in their early stages of development, using prototypes to test various pieces. Some of the testing involves components that Nokia buys from suppliers, such as glass.

Mike Wellborn, who works in the failure analysis lab, showed a group of reporters in town for the CTIA Wireless I.T. & Entertainment show how he uses microscopes and X-ray machines to look deeply into the innards of a phone. The microscope was able to detect a minute stress fracture in a soldered joint. An X-ray measured the thickness of plating used in the phone cover.

Mike Myers in the mechanical lab used corrosive chemicals to rub the phone exteriors, needle-like pointers to try to scratch the surface, a machine that bends a phone screen until it breaks, as well as the Levis. He joked with reporters that he gets to wear a lot of cut-offs.

The San Diego lab mainly tests Nokia phones under development in the United States, although information is shared with other labs around the world.