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RE: BGA Assembly
I have been told that it is a very bad idea to put vias in the pads for
BGAs. As discussed, the solder is drawn into the hole and there is not
enough to provide a connection.
I think the "limiting feet" are also a bad idea. I had several
boards manufactured with BGAs and they use normal amounts of paste and
flux and let the BGA "float" on the molten solder. The BGAs all
came out perfectly (out of 28).
We had another house attempt to assemble our prototypes by hand. They
first tried using a *lot* of paste. This literally floated the chip off
the pads. Then they tried using a small amount of glue. This had the same
problem Dimiter found. The chip can't settle on the pads.
So use normal amounts of paste and flux and let the chip float on the
pads. These things are very easy to assemble in production. You just have
to have faith that they will work and not try to force the issue.
At 01:27 PM 11/24/00, you wrote:
At one time Agilent (then HP) had
a high density interconnect method for probing with their logic analyzers
that required via-in-pad.
As I recall, the relevant application note recommended
applying solder paste and reflowing the board twice.
On the first pass one applied paste only to the pads that
incorporated vias. The amount of solder paste was sufficient to
fill each via. Then one reflowed the bare board, with no
components. After that, using a different stencil, one applied
normal amounts of solder paste for the components, placed the components,
and reflowed again.
I never saw discussion of this by third parties, nor do I
know what kind of yields one might experience using this approach.
From: Dimiter Popoff
Sent: Friday, 24 November, 2000 9:25 AM
Subject: Re: BGA Assembly
>I was specifically recommended not to put vias or holes
under the ball, by
>someone with past experience of solder being drawn
through the hole and away
>from the BGA. I guess this isn't what you wanted to hear
I guess you just saved a board from an unsuccessful
Overall, though, this is no problem, because I don't expect
device to go into automatic production quantities soon, and
I seem to manage it quite successfully. Moreover, from what
seen so far the solder drawn through the holes is not much
never makes a drop beneath the board, just fills the hole),
there is a big pad at the opposite side to be wetted; and
there is no problem, the connections are quite intact.
have hard times imagining what should happen to make the
be drawn away so there is no more a connection between the
and the board (I guess we all know how do solder drops of
size behave), but as I said earlier, I'll take easily
risks at a later stage :-) (trying if the BGA will float
this without limiting feet, that is).
All this is valid for holes drilled with 0.3mm or less; this
about 0.2mm after plating. The holes had anyway to be that
in order to allow 3 traces between each pair of BGA pads;
turn, made routing possible on only two signal
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