"To make it light and thin, the design is very complicated. It takes time to learn how to make this new device. Practice makes perfect. Our productivity has been improving day by day", said a Foxconn executive.
Although the iPhone 5 was launched on 21 September, new customers ordering from the Apple website will currently have to wait 3-4 weeks before they receive their new phones.
Foxconn also addressed the 'scuffgate' scandal which saw many customers receiving their brand new iPhone 5 with scratch marks on the metal casing.
The source said the firm had recently implemented a new quality check procedure to reduce the chance of damages and noted that the handset 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.
Foxconn has faced various disruptions in recent months, with accusations surrounding working conditions in its Chinese factories.
Yesterday it admitted to using interns as young as 14 on its production line, violating China's legal working age of 16.
The company's second-largest plant, in Zhengzhou, which makes the iPhone5, was this month rattled by a dispute between quality control staff and line workers, and last month thousands of workers at the Taiyuan factory rioted after an argument between a worker and a security guard.