An XO on its last legs?

[John Watlington]

On Dec 4, 2010, at 4:01 PM, Tiago Marques wrote:

> On Tue, Nov 30, 2010 at 2:11 AM, John Watlington <wad at laptop.org> wrote:
> 
> The symptoms you describe (power light and Mic light coming on, but nothing else)
> indicates that the main processor isn't executing any code.   This is usually caused
> by one of two problems: corruption of the SPI ROM containing the boot loader or
> a cracked solder ball underneath the processor.   I doubt it is the former, as once it
> died it wouldn't restart.
> 
> You can test for the latter by pressing down on the processor (biggest chip on the
> board) and seeing if it improves the situation.   Unfortunately, it is impossible to fix
> this in the field.   It requires reflowing the board again.
> 
> 
> Not exactly, although the fix isn't 100%, he can try to heat it up with a heat gun at some electronics repair shop. It worked with Xbox 360s, which are plagged by the same issue, even though it sometimes isn't a definitive fix.

The heat gun would have to heat the solder balls under the chip back
up to around 700 - 750 F.

> Then again, can you even successively reflow it with the epoxy around the chip?

Yes --- there are gaps in the glue allowing gas under the BGA to expand.

> Out of curiosity though, is this a problem that plagues more XOs or has the solder's chemistry been worked out?

The laws enforcing a reduction of harmful substances (RoHS) force us to use
a solder which contains no lead.   Not only does this solder require a higher temp. to reflow,
it can form tin whiskers which short, and most importantly, is more brittle than regular solder.
Unlike the military, space, and watch industries (exempted from RoHS), we are prevented
from fixing this by "working out the solder's chemistry" (i.e. adding lead back to the alloy).

The problem is cracking caused by mechanical forces acting on the motherboard. 
We did move back to gluing all sides of the processor down, instead of just the corners,
trying to improve the situation.

wad