While the burn-in issue is reportedly invisible to users, the panels do not meet Apple's specifications and thus the high rejection rate has resulted in low panel yields for the tablet. As noted in a report earlier this week analyzing the new iPad Air's use of IGZO technology, Sharp has experienced difficulties ramping up production of its IGZO panels in the past, with it taking until now for Apple to bring the technology to its products.
The burn-in problem was caused by the drastic reduction of the pixel size. The resolution of the 7.9-inch iPad mini with Retina display is 2048×1536, about four times clearer than the existing 1024×768 products. LG Display used the amorphous silicon (a-Si) for the substrate, whereas Sharp used IGZO. The pixel of the smartphone display is smaller than that of the iPad mini Retina panel, but as the Low Temperature Polycrystalline Silicon (LTPS) technology is used, it can be produced stably. The pixel of the iPad mini Retina is the smallest among those products using a-Si. Sharp failed to solve the chronic problem of IGZO, i.e. uniformity, and its yield went down.The report also elaborates on how Apple may be negotiating with Samsung to produce displays for the device starting next year, turning back to its rival in part due to Samsung's experience solving a similar problem with third-generation iPad's move to a Retina display.
Apple is negotiating with Samsung Display for the supply of displays starting next year. Apple discussed cooperation with Samsung Display when it was planning on the iPad mini Retina. AUO, which was the iPad mini display supplier, was excluded from the retina version because of its yield problem. Apple was planning to receive 15 million panels from LG Display, Sharp and Samsung Display at the end of this year. However, as the negotiation with Samsung Display fell through, only LG Display and Sharp made it to the final list of primary suppliers.If the issue of screen burn-in persists, the report also states that Apple may move to LTPS technology such as that seen in the Kindle Fire HDX, as it is less prone to the problem. However, while LTPS is commonly used on displays for smaller devices such as the iPhone, Apple is unlikely to mass produce the displays for larger screens in part due to scalability issues that simply won't support the tens of millions of tablets that the company is producing each year.
Apple CEO Tim Cook stated last week during the company's fourth quarter earnings call that it was "unclear whether we will have enough for the quarter or not," as the company has only promised that the tablet will arrive by the the end of November. As noted by Apple during the introduction of the second-generation iPad mini, a Retina display has been one of the most requested features for the smaller iPad since its release last year.
Update: MacRumors spoke with Ray Soneira of DisplayMate Technologies, who noted that LG is almost certainly not using a-Si for its Retina iPad mini display panels due to significant power issues with trying to drive a display of that pixel density using the technology. The company has also been shipping its own IGZO display panels for some time now, and Soneira pointed out that "it would be very inefficient to engineer the iPad mini to ship using two significantly different display technologies."
Soneira also clarified that "burn-in" is the incorrect term for what would be happening with these LCDs. It would be a short-term image retention issue perhaps similar to what some early Retina MacBook Pro users experienced, although it appears in this case that the effect is invisible to users.