Apple has delayed the release of a white iPhone 4 a second time, blaming the delay once again on manufacturing challenges.

The company announced the delay Friday, while also releasing details of its free iPhone 4 case program. Apple chief executive Steve Jobs last week promised a free case, or bumper, to prevent drop calls when the user's hand comes in contact with a certain part of the phone's exterior antenna. Apple is also offering refund to people who previously purchased the $29 wrapper.

Apple has yet to ship anything but the black model of its latest iPhone. In starting pre-orders for the phone in mid-June, the company notified customers that the white phone wasn't available.

On June 23, the company blamed manufacturing challenges as the reason for delaying the release of the white iPhone 4 until the second half of this month. Friday's announcement said manufacturing problems still plagued the model's release.

"White models of Apple's new iPhone 4 have continued to be more challenging to manufacture than we originally expected, and as a result they will not be available until later this year," the company said in a statement. "The availability of the more popular iPhone 4 black models is not affected."

Not having a white model doesn't appear to have hurt sales. Apple sold more than 1.7 million iPhone 4s three days after its June 24 launch, making the phone the company's more successful product launch.

The much bigger problem has been iPhone 4s exterior antenna. To ease customer anger, the company is offering a free case to people who buy an iPhone before September 30.

Under the program, iPhone 4 buyers must first download a program app from Apple's App Store, launch the app on the phone and sign in using their iTunes Store account or Apple ID. People can then select a bumper.

To be eligible for the giveaway, people who bought the iPhone 4 before July 23 must apply no later than Aug. 22. Otherwise, people have 30 days from the time of purchase to order a case.