At Apple's (NASDAQ:AAPL) annual developers conference in June, the company released a pair of new premium tablets. The first was an iPad Pro with a 10.5-inch display, and the second was an iPad Pro with a 12.9-inch display, illustrating its commitment to the iPad lineup.

What I found interesting was that these tablets didn't arrive a year after their predecessors did. The 10.5-inch iPad Pro, which succeeded the 9.7-inch iPad Pro, launched roughly 15 months later.

It took Apple even longer to update the larger iPad Pro. The initial 12.9-inch version launched in November 2015, meaning the time between the launch of the older model and the current one was 19 months.

Though I'd love to see Apple launch new iPad Pro models in June or July of next year, that doesn't look likely, thanks mostly to OLED challenges.

An Apple iPad Pro on the left, and an Apple iPhone on the right.

Image source: Apple.

Can Apple build big OLEDs?

Apple recently launched a trio of new iPhones, the highest-end of which is known as the iPhone X (pronounced "iPhone ten"). This phone uses a new generation of displays based on organic light-emitting diode (OLED) technology that Apple markets as a "super retina display."

The OLED display is reportedly difficult for Apple to build, with the company apparently only recently having solved some of the seemingly painful problems in manufacturing it.

For the next iPad Pro, Apple is widely believed to be planning to use OLED displays, moving away from the liquid crystal display technology it has traditionally endowed the iPad with. That brings some good news and bad news from a time-to-market perspective. The good news is that Apple's iPad Pro shipment volumes are minuscule compared with its iPhone shipment volumes. During fiscal 2016, Apple shipped more than 211 million iPhones but only about 46 million iPads -- and a lot of iPads shipped are of the lower-cost, non-Pro variety. The iPad was about 11% of revenue in the most recently reported quarter.

The bad news is that iPad displays are substantially larger than their iPhone cousins, so they can be much harder to manufacture and more expensive to build.

But Apple has to do it

One of the marquee features of the latest iPad Pro tablets is the ProMotion display. This is Apple marketing-speak for displays that can refresh their contents at a rate of 120 times per second -- twice as many times as previous iPad displays could, as well as all current iPhones.

Higher refresh rates dramatically improve the user experience by making everything smoother, but such displays are more difficult to produce and can consume more power. Apple reportedly tested a ProMotion display on the iPhone X, but for reasons unknown to those outside Apple and its display manufacturing partners, Apple didn't include it in the device. (I have offered up some educated guesses here.) 

Since Apple must include a ProMotion display, or some enhanced version of the technology, on all future iPad Pro tablets, it follows that Apple needs to figure out how to build a large OLED display with ProMotion technology at a cost and in volumes appropriate for the next iPad Pro.

This isn't an impossible feat, but it'll be challenging, which is why I wouldn't be at all surprised if the next-generation iPad Pro tablets launch sometime after the next iPhone does -- more than a year after the current iPad Pro tablets hit the market.

Ashraf Eassa has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Apple. The Motley Fool has a disclosure policy.