Kerry,
Pure tin balls?!  I have not heard of that before.  Pure tin melts considerably higher than SAC alloys.  Hmm, 206C you say, 14 layer board?  It is quite possible that your temperature/time is not enough to get a significant amount of diffusional mixing of the solder paste and the balls. But then you mention intermittents at the corners as well as the center.  

Two other possibilities are warping of the part (but would expect that to give you intermittents in one or the other area, not both) or there is a severe problem with the construction of the component itself and there is internal damage to the component.

You don't say, but I presume the board itself solders well and there are no other soldering problems on the board that would lead you to suspect the solder paste?
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Kerry McMullen
Sent: May 23, 2005 1:10 PM
To: [log in to unmask]
Subject: [TN] BGA Intermittents


Hi Technetters,
I have a 1089 BGA with Sn balls.
Paste is 63/37 SnPb
Board is 14 layer,  .098 thick Immersion Ag.
Ran thermocouples.  Inner balls are seeing 206 degrees C.

What could be causing intermittents at the center and corners of the BGA?

Thanks,

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Kerry



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