All:

            Just to push this a little further (I'm interested in this
response), does a finish of HASL present less issues with wetting
            and outgassing of the solder paste volatiles to the uVias, say
versus a finish of OSP or ENIG ??

            If what you say is true, then will the wetting of the uVia walls
during HASL eliminate or reduce the tendency of
            entrapment of gases (or air) during BGA assembly ??

            Regards,

            Gary Erickson

-----Original Message-----
From: Tim Jensen [mailto:[log in to unmask]]
Sent: Thursday, August 09, 2001 2:15 PM
To: [log in to unmask]
Subject: Re: [TN] VIAS in pads...


Gary brings up a very good point regarding various via-in-pad technologies.

The microvia technology has proven to be much more difficult than the vias
that pass completely through the board.  My experience is related to
soldering and have worked with various customers using both technologies, I
can provide the following insight on each:

Through hole via-in-pad: The biggest problem that I have run into with this
technology has been related to solder paste volume.  Since the solder will
wet down into the via, it is necessary to print a large deposit of paste.
In addition, solder paste is half flux(by volume) and the resultant solder
joint volume will be half of the paste deposit (another reason to print more
paste).  I have had a number of situations where people have taped off or
masked the bottom side of the via to keep the paste from flowing out the
bottom side.  This typically creates some of the same problems seen with the
microvia technology(see below), so I would avoid sealing the bottom side of
the via.  The opening at the bottom of the via provides an addition path for
the volatile components of the solder paste to escape the molten solder.

Microvia-in-pad: Voiding is the primary problem that I have seen here and
typically people are seeing a very large void right in the middle of the
solder joint.  In the process, you print solder paste over a small opening.
Air becomes entrapped underneath the solder paste printed and many people
theorize that it is this entrapped air that is causing the void.  However,
our work has shown that this is probably not the case.  It is more likely
the entrapped volatiles from the solder paste(although I often say that "the
paste is never the problem," in this case it seems that the paste could be
the problem).  As the flux is cleaning the surface to promote wetting, it
produces volatiles through this fluxing reaction.  The more difficult a
surface is to wet to, the more volatiles that are produced from the flux.
Since the microvia openings are quite small, it is possible that the plating
of the via walls isn't that great.  In addition, as the constituents of the
flux go from a liquid to a vapor, their volume can increase up to 100x.

I am no chemist, so I hope that my comments are clear.  Again, this is just
what we have seen from a solder paste supplier's perspective.  This is a
very interesting topic to me and any additional comments would be great!

As this is my first posting to TechNet, I would like to add one comment to a
particular member of the Ontario, Canada contingent(Since I don't want to
mention names, we will call him Bev).  You thought it would never happen,
but the central NY engineer is finally among your ranks!!!

Regards,

Tim Jensen
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Erickson, Gary
Sent: Thursday, August 09, 2001 9:34 AM
To: [log in to unmask]
Subject: Re: [TN] VIAS in pads...



Ken:
Are you talking about Microvia ( single or multi-tier ) or Thru hole "via in
pad" ??

GaryE

-----Original Message-----
From: Ken Patel [ mailto:[log in to unmask] <mailto:[log in to unmask]> ]
Sent: Wednesday, August 08, 2001 2:53 PM
To: [log in to unmask]
Subject: [TN] VIAS in pads...


Assemblers,
How robust is the VIA in the pad technology now? Can someone share the
process of handling board that has vias in pads?

Most of our boards have VIAS in the pads and there is no way out.

re,
Ken Patel

----------------------------------------------------------------------------
-----
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET
Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site ( http://www.ipc.org/html/forum.htm
<http://www.ipc.org/html/forum.htm> ) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
----------------------------------------------------------------------------
-----