A while back last week or so I posted a thread about " How many of you do your own computer maintenance? great responces everyone, anyways some one asked What is reballing the graphics chip ,Last night I attempted to reball a problem 2009 Imac 21.5" there are a ton of videos on Youtube on the subject, and although the methodology and execution may differ the in fo is the same and the process, remains . Here's how I approach it: back ground , My Vetrinarian gave me a 21.5" Imac, problem was the graphics chip goes all intermittent and locks up the unit. I have tried reballing it 3 times to see If it will fix the problem, No it didnt work. so I installed a fan control app that allows me to monitor and control the fan speeds for CPU, HD, and DVD plus it accesses the Graphics Processor temperature sensor, with some tweaking I was able to reduce the crashing feature by ramping the fans before the GPU went critical. This is a good thing, so Last Night I dissmantled the Imac to try one more time to reball the graphic chip to reduce the high temperature I was getting. CPU fan was set to 2600 RPM CPU temp 75 F HD fan was set to 3700 RPM HD temp 75-80 F DVD fan was set to 3200 RPM DVD temp 83 F GPU 135 F ( too hot) all other perameters are well under 100 F so I dis mantled the Computer and removed the GPU ( ATI Radion HD 2400 with 128 MB video ram) when I do anything inside a computer I always wear a static discharge band digital electronics is really susseptable to static electricity After cleaning all of the old Thermal paste off of the Video ram chips, Graphics chips, and heat sinks with 99 % alcohol and Q-tips and really cleaning them well, I wrap the Graphics Card in multiple layers of tin foil with a window cut out to expose the actual chip this protects the rest of the board from excessive heat. I place a metal plate over top of the wrapped GC again exposing the chip I turn on my heat gun to 250 F for a few minutes to heat the chip then raise it to 310 F to Reball any broken solder joints with in the chip, there alr tons of contrary opinions on this, but this works for me I test this to ensure the heat is enough to melt solder by melting a small piece of solder using just the heat from the heat gun , I blast the chip from 5-10 minutes tops then let it cool for same undisteerbed before unwrapping it. In this case I gave the card a small blast of heat to ensure there were no broken solder joints on the card , this is not something i would normally do ,but the symptoms indicate a possible issue here. the rest is total re-assemblly of the computer and Viola it works this did not successfully repair the issues with this computer but with the fan app I was able to prevent the graphics chip from failure , it still over heats to 135 F ,and may require a new Graphic Chip or coolng array, My other machine that had the dust bunnies from my first post the GPU runs at 87 F under load and the temperature has dropped several degrees after the cleaning. I did this for those who may be interested , check You tube for more or different approaches , DO NOT ATTEMPT THIS IF YOU HAVE NO EXPIRIENCE WITH THIS TYPE OF REPAIR, I ASSUME ZERO LIABILITY FOR THIS TO GO BAD DUE TO LACK Of KNOWLEDGE OR INEXPIRIENCE.