We have been seeing more and more of these later range iMacs with LCD blemishes, so we thought it was about time we cleared the air (so to speak), as there is a lot of uncertainty on a consumer level as to how these panels are built.
In 2012 Apple completely overhauled their iMac ranges. They slimmed it down, removed the optical drive, and in 2014 they released their first Retina display model. (5K in 27″ and 4K in 21.5″)
These were the first model that didn’t have a separate glass panel over the top of the LCD panel, which was great from a repairers point of view, as our engineers no longer needed to clean the LCD and both sides of a glass panel to then rebuild a machine in a work zone that was not hermetically sealed!
Now, the troubling bit…
These are often referred to as a ‘bonded’ display. In reality, that is not the case. The LCD panel itself is bonded to the glass, but the main issue for dust ingress in displays is not between the glass and LCD surface, But rather, between the LCD and the backlight sheets. Tiny specs of dust show up as either dark patches, marks and occasionally are mistaken for dead pixels. As seen in the photo below, all that seals the edges besides what is seen is a single strip of tape. (not the definition of ‘bonded’ I’d use)
Brown patches are a common occurrence on older panels (2007-2011 iMacs), as are minor patches and streaks to the lower corners. Apple even did an extended replacement programme for the LCD is the 2009 21.5″ iMac for brown patches in the upper left corner (right over the internal power supply).
But that brings us to the 21.5″ and 27″ later slim line iMacs (2012-2019). These have far less severe symptoms to the earlier ranges, but do sometimes suffer from darker ‘patches’ or ‘streaks’ in the lower 2 corners. The only explanation we can come up with is the tape in the corners of the LCD is weakened over time and a small amount of dust gets through from the air circulation generated by the internal fan. Its not a hardware fault, but is just dust under the surface that cannot be removed (without a clean room environment).
We often sell machines with these blemishes, but the extent of the ‘patching’ varies from one machine to the next. For that reason, we’ve provided a few photos showing a few different examples of the fault, but the severity will always differ.
As an additional reference, Apple look to have faced a class action lawsuit regarding this issue…
thebookyard.wordpress.com 
I have a late-2013 27″ iMac with an insect and a spec of dust between the lcd and backlight. A teardown for getting between those to clean doesn’t seem to be available.
I have seen a YouTube clip showing a suction cup being used to vibrate the LCD in the hopes of dislodging such debris. It’s worth a try, I suppose, though I’m not sure how effective that will be given the bonded glass in front of the LCD.
Hi Geoff,
Did you ever attempt this solution? If so, let us know how it went! Typically anything between the layers of these bonded LCDs is a nightmare to resolve and opening the layers usually makes the image messier than before with dust etc. now getting inside once the seal is broken. Would be interesting though to hear if this had any joy as a solution.