iPhone 5c
Pre-Orders for Apple’s iPhone 5c are about to begin, while the premium iPhone 5s is still not up for the grab online. Apple

At a time when retailers are taking online pre-orders for Apple’s (NASDAQ:AAPL) new iPhone 5c, with tech enthusiasts around the world waiting to see if the handset can garner consumer interest as expected, the company’s flagship iPhone 5s is still not up for pre-orders, suggesting that customers will have to wait until Sept. 20 to get the device from stores or to purchase it online.

Major U.S. carriers such as Verizon and Sprint said, on Thursday, that the iPhone 5c would be available for pre-orders at 12:00 midnight PST (2 a.m. Central Time and 3 a.m. EDT) on Friday, while AT&T is expected to begin iPhone 5c pre-orders early in the morning on the east coast.

However, none of the carriers have provided any information about iPhone 5s pre-orders, and according to some recent reports, supply constraints could be the reason holding Apple back from offering iPhone 5s pre-orders.

According to a report from DigiTimes in August, production delays of the iPhone 5s’ Touch ID fingerprint sensors were expected to affect initial shipments of the smartphone. The report said that Apple was expected to ship 10 million units of the iPhone 5s in the third quarter of 2013, but low yield rates of the fingerprint sensor could hamper the phone’s release, and the company could end up shipping only three million to four million units of the smartphone during this period.

DigiTimes reported a similar buzz in July as well, saying that volume production of the LCD driver chips and the fingerprint sensor chips of the iPhone 5s were scheduled to begin by end of June or early July, but issues related to yield rates might have delayed commercial production of both chips, eventually affecting the initial shipments of the iPhone 5s.

According to a report from iDownloadBlog, another reason why Apple is not offering the iPhone 5s pre-orders for now could be that the company wants to encourage would-be buyers to line up outside retail stores on the release date of the iPhone 5s next week, and grab headlines as it always does with new iPhone launches.

At the same time, the report observed that big pre-order numbers also have the potential to get Apple enough media attention. Last year, the company claimed that more than two million people had pre-ordered its iPhone 5 in the first 24 hours after the handset was up online, helping Apple’s stock cross the $700 mark.

And, while all of this is speculation, there also is the possibility that Apple may want the iPhone 5c to retain the attention of customers in the first week after the unveiling of both the models.