To make matters worse, an excess of thermal paste is often applied to the graphics card from the factory. This sends the computer into panic mode, and it refuses to boot. Eventually a connection is no longer made between the graphics card and the board contact points. Over time the heating and subsequent cooldown of the graphics card leads minuscule cracks to develop in the soldered micro-connection between the GPU chipset and the logic board (or MXM board in this case). So where does all this heat originate? Each time the graphics card is taxed (when the computer is running Photoshop, playing Youtube videos, etc.), hot air builds up. This results in much less effective airflow. Furthermore, all aging iMacs have a substantial amount of dust buildup on all components and most importantly the fan blades. In the iMac the fan is not optimally placed to force out the hot air. In the red desktop graphic card shown for comparison, you’ll notice the fan sits atop the GPU chipset and pushes the hot air directly out the opposite end of the card through large vents on top of the input ports.

The fan’s job is to force the hot air out of the computer and away from the graphics card. The fan is situated far away from the cooling fins and is mostly obstructed by the optical drive. You’ll notice in the photo below that the fan sits down and to the right of the graphics card. The copper pipes then transfer heat to the silver cooling fins and out the backside of the iMac through a horizontal cut near the top. In the iMac heat is transferred from the graphics card chipset via copper pipes. Early symptoms are an iMac that fails to wake up from sleep or one that wakes up but only shows a black screen. Due to internal space constraints, many iMacs develop an overheating graphics card issue that results in a black/blue/grey screen while the computer is powered on or a screen that shows vertical stripes or columns. There is a common flaw in the all-in-one iMac design.

Graphic card reflow