QUOTE(brandogg @ Aug 16 2008, 08:12 PM)
I really don't think 0020 is a bridged solder joint, if it was, it would pretty much impossible to fix without reballing the chip that was causing the area. Also, the system on it's own won't ever get anywhere nearly hot enough to reflow the solder to begin with. The only way you could get a bridged joint would be from a bad reflow, or it was "born that way", though I'm sure each system is power cycled and tested before it's placed in the box and shipped.
I had a few 360s that make me come to this result.
One of the 360s I am talking about is one that I bought cheaply on ebay, the GPU was flatly pressed on the mainboard and was shorting out completely, it had 0020.
Another one I bought on Ebay had 0102, so I did the X-Clamp replacement and got rid of it, however it showed 0020 afterwards(0102 overrides 0020 since the number is higher), so I loosened the screws a bit because it sometimes happens when you over tighten them...
I couldnt get rid of the 0020 so I checked the mainboard and found 2 of the bottom RAM out of which the solder leaked by the look from the side.
They were completely flat as well...
And I could manage to get the error a few times when I was searching for a 3.3V source for the 9V mod, when you draw power from the regulator that powers the GPU and RAM you get it.
Missing capacitors under the GPU can also cause it.
The fact that solder balls are completely shorting out sounds pretty weird imo as well the only thing I could think off is that they move around/ crack when you tighten the screws too much and make them short out like that.
It also doesnt make much sense that you can often get rid of it by reflowing the GPU/RAM.
Someone posted a quite interesting thread that somehow didnt catch too many eyes but the guy had a quite nice theory that would actually explain this.
He was talking about a phenomenon called "Tin whiskers" it is like that solder can kind of get little antennas that can grow up to a length of 1mm per year and shorten out other stuff if they get in contact with them.
If you regard that the solderballs under the GPU are like a lot smaller than 1mm this actually makes a lot of sense since if this is the case they can short stuff out within less than a year.
When you reflow the chip/ take it under pressure/ overheat it you break these little antennas and the solderballs are disconnected and get back its original form and are no longer shorting out.
Here is some more information on this phenomenon http://en.wikipedia.org/wiki/Tin_whiskers
It sounds pretty far fetched to me as well but it is an addition to the X-Clamp theory that explains this certain issue very well imo.
On the wikipedia article it also says that "mechanically induced stresses" and "thermally induced stresses" support the growth of these tin whiskers which obviously is the case for the solderballs under the main chips of the 360
And it is mainly lead free solder that is effected by it which is also the case here.