Apple's iPhone 5 has been suffering from supply shortages since its launch in September, and the device's manufacturer, Foxconn, blames the complicated design for its slow production.

"The iPhone 5 is the most difficult device that Foxconn has ever assembled. To make it light and thin, the design is very complicated," an unnamed official at the company told The Wall Street Journal. "It takes time to learn how to make this new device. Practice makes perfect. Our productivity has been improving day by day."

New quality control measures are thought to have been put in place at Foxconn factories following complaints that the iPhone 5s were arriving at customers' houses with scratches and dents in them.

The Foxconn official told WSJ that the iPhone 5 uses a new coating material that is easier to scratch. "It's always hard to satisfy both aesthetic needs and practical needs," the official said.

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