Without the plug, the solder from the sphere can wet down into the
adjacent via to the backside of the pwb during reflow.
On a dogbone pattern, with one side the pad and the other side the
connecting via, the small soldermask dam that goes over the thinnest
part of the dogbone pattern is not enough to stop the solder wicking.
The solder will wick right under that flimsy little dam, and the via
will rob the solder. This is called solder thieving. The soldermask plug
prevents that from happening. If you ever need to rework that BGA, the
site prep operation nearly always makes the little soldermask dam worse,
and after you replace the part you get even more solder thieving, and
the entire rework/X-ray/test/rework cycle spins on down into a black
hole called scrap. One almost never sees BGAs with daisychained dogbones
without the soldermask plug, and that is a good thing, as Martha says.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Black, Paul
Sent: Wednesday, November 30, 2005 7:50 AM
To: [log in to unmask]
Subject: [TN] Plugging vias necessary?

Hi Everyone,

I have a board with a 357 pin PBGA. Each pad has a short etch leading to
an adjacent via. Currently, the fabrication drawing calls for each via
to be plugged. The assembly process for this board does not contain a
wave solder step and the test engineer assures me that plugging the vias
is not necessary for vacuum during incircuit testing, so is there some
other reason for plugging these vias? They are plugged with soldermask,
and I feel like we are unnecessarily introducing potential problems into
the process.

Thanks in advance for your opinions.

Paul Black
Manufacturing Engineer
Kronos
[log in to unmask]

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