I have been following this link, somewhat, but what Dave says below is
the most important point so far, the avoidance of soldermask defined
pads and ground planes on the top layer (solder layer).
Soldermask-defined pads are somewhat problematic during manufacture, but
more importantly they can significantly reduce the long term reliability
of the solder joint. All of the other issues pale in the face of
fractured SJs due to soldermask compression during cyclic operation.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Schaefer
Sent: Thursday, September 17, 2009 8:54 AM
To: [log in to unmask]
Subject: Re: [TN] Ground Pour Under BGA

Toby,

For HDI designs (via in pad) I pour ground underneath BGAs and ensure
that
any connections to the BGA ball are thermals. This eliminates soldermask
defined land areas which are problematic during manufacture.

For most non-HDI designs your observation is correct - there's not
likely a
lot of pour under the BGA anyways, so not much point in having it there,
and
I'll normally block out the area if pouring on the surface. I do
normally
employ a stackup where the adjacent layer (t-1, b-1) is a plane in these
situations. The one reason I would consider keeping the pour under the
BGA
is if it is large enough (and required) to provide some thermal benefit.

Hth,
Dave

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