Foxconn says the shortage in iPhone 5 supplies is due to the difficulty in assembling its complicated design, reports the WSJ.

“The iPhone 5 is the most difficult device that Foxconn has ever assembled. To make it light and thin, the design is very complicated,” said an official at the company who declined to be named. “It takes time to learn how to make this new device. Practice makes perfect. Our productivity has been improving day by day.”

The site also learned that steps have been taken to prevent phones from being shipped out with scratches.

The executive said Hon Hai has taken steps to improve its productivity and address scratches on the metal casings of some new iPhones that were shipped. Hon Hai has recently implemented a new quality check procedure to reduce the chance of damages. But he noted the iPhone 5 uses a new coating material that makes it more susceptible to scratching.

“It’s always hard to satisfy both aesthetic needs and practical needs,” said the executive.