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November 2005

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Subject:
From:
Eddie Rocha <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Eddie Rocha <[log in to unmask]>
Date:
Wed, 30 Nov 2005 08:58:20 -0700
Content-Type:
text/plain
Parts/Attachments:
text/plain (188 lines)
A via plug isn't necessary to avoid the solder wicking
problem that is being described. Simply coating the vias in
the BGA area with LPI soldermask could accomplish this
without the additional processes required for "via
plugging".


Thank you,
Eddie Rocha
South Bay Circuits


Hi Paul,
I am fully agreed with Richard. The problem is even worse
if your finish
is ENIG (adhesion problem of the solder mask to the gold).

Regards,

Dr. Gil Zilber
Technologist, Elta system Ltd.
Tel. +972-8-8577240
Mobile +972-54-4983222
http://www.iai.co.il/site/en/iai.asp?pi=17887



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem,
Richard
Sent: Wednesday, November 30, 2005 3:59 PM
To: [log in to unmask]
Subject: Re: [TN] Plugging vias necessary?

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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