It's a well known problem with early active matrix LCD's. You need a lot of luck to find good B&W active matrix screen without fading.
The fading of the corners will get worse the longer the powerbook is powered on.
As far as I know the fading issue is only with B&W active matrix panels: I haven't seen it with color LCD's.
As for the Powerbook 520/540 series, all screens are interchangeable between different models: it's best to swap the interconnect board as well, I'm not sure it's necessary.
A good tip: recap the replacement LCD panel first before install and replace the backligh bulbs when the brightness is low or when the CCFL bulbs are black on both ends.
I replaced the backlight CCFL bulbs in my powerbook 500 series, together swith recapping the LCD panel, the picture quality looked like brand new.
The fading of the corners will get worse the longer the powerbook is powered on.
As far as I know the fading issue is only with B&W active matrix panels: I haven't seen it with color LCD's.
As for the Powerbook 520/540 series, all screens are interchangeable between different models: it's best to swap the interconnect board as well, I'm not sure it's necessary.
A good tip: recap the replacement LCD panel first before install and replace the backligh bulbs when the brightness is low or when the CCFL bulbs are black on both ends.
I replaced the backlight CCFL bulbs in my powerbook 500 series, together swith recapping the LCD panel, the picture quality looked like brand new.