TECHNET Archives

June 2005

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
"Stadem, Richard" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard
Date:
Tue, 14 Jun 2005 07:59:34 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (265 lines)
Yes. The 42 mils of solder mask between the via hole and the pad should
be enough, provided the mask adheres fairly well. I have run boards like
this without any problems. In fact, I think it may prove beneficial in
preventing venting into the solder paste if the paste is slightly
misregistered, reducing voiding and other issues. I assume there is some
clearance of the solder mask around the ball pad (non-soldermask
defined).
However, I am just curious about this design. In the dogbone patterns
that I see, the via pad is usually smaller than the solder pad, not the
other way around. I understand this is a smaller diameter ball pad to
accomodate a fairly small pitch and probably a high pin count, but
usually a design of this type uses via-in-pad. However, I don't like
via-in-pad, as it often induces voiding in the solder ball, usually
because of improper cleaning of the vias after laser ablation.
I once had to deal with via-in-pad done so poorly that you could see all
kinds of crud in the via hole of the pwb with a 40x microscope, the vias
were not plated all the way down, and due to the subsequent outgassing
into the solder joint, nearly every ball either had a large void, and
some balls literally exploded, leaving no ball but many irregularly
shaped surrounding balls that had absorbed most of the solder from the
ball that exploded. However, if done correctly, via-in-pad certainly
makes life easier in a design of this type. The vias should be
fill-plated up to the surface or nearly to the surface to prevent the
voiding and venting.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ofer Cohen
Sent: Tuesday, June 14, 2005 2:07 AM
To: [log in to unmask]
Subject: Re: [TN] Immersion Silver - Via Plugging

Richard,
Thank you for the explanation. I have a question: Assuming a 1mm-pitch
BGA, with 17 mils pads and via hole of 12 mils and pad of 24 mils: Would
you consider implementing tented vias (mask defined) and opening the
solder mask to diameter of hole+2 mils, yielding a solder barrier of 42
mils, as sufficient dam?

Regards
Ofer Cohen
Manager
Quality Assurance, Reliability and Production Technologies Seabridge
Ltd. - A Siemens Company Siemens COM FN A SB 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
Sent: Monday, June 13, 2005 9:11 PM
To: [log in to unmask]
Subject: Re: [TN] Immersion Silver - Via Plugging

Open vias, in this case I mean unplugged vias, not electrically open,
can cause problems when used inside of the BGA pad pattern. The design
of a dogbone pattern where the BGA ball pad and its smaller via are
connected together with only a small amount of soldermask (a solder dam)
overlapping the small trace connecting them (the center of the dogbone)
but leaving the via itself open is frowned upon. 
The reason for this is that the small mask dam does not adhere as well
as a cap of solder mask over the via, thus allowing the solder to wet
and flow under the dam into the via. Vias are notorious in the BGA
reflow/rework world as proficient solder thieves. Quite often I have
seen X-rays of production hardware with either a smaller-than-normal
solder ball or no solder ball, but the via next to it is plugged with
solder, whereas the rest are open. That particular BGA solder sphere
drained into the via. In many cases the solder sphere can be seen in the
X-ray, but it is actually on the opposite side of the board, and
anchored to the via.
This problem is compounded 50x or better if the BGA needs to be removed
and replaced (reworked) for any reason, including the open solder ball
from the initial reflow process I just described. Now, the solder mask
is going to be heated up a second time to remove the BGA, and then
someone is going to have to remove the old solder from the pads on the
board. This invariably causes more of this type of soldermask dam
damage. This means that the solder mask must be very carefully repaired
using a two-part soldermask epoxy before a new part can be put on and
reflowed. You do not want to do this on a 1024 solder ball pattern! 
Plugging the via with soldermask during fabrication and leaving only the
pad available for solder typically eliminates this problem. The
soldermask cap usually adheres much better than the small dam, probably
because there is more surface to adhere to, and it is somewhat anchored
into the via hole.
As someone here pointed out, the plug on one end only is problematic for
trapping chemicals left over from the etching process. My philosophy on
that is that there should not be any "residual chemicals" left behind.
However, if there are, at least they cannot interfere with the BGA
solder joint formation as easily, due to the cap. This does not fix the
problem of them continuing to eat away at the via hole wall, especially
on pwb's with immersion tin or immersion silver finishes.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kaye Clayo
Sent: Monday, June 13, 2005 11:34 AM
To: [log in to unmask]
Subject: Re: [TN] Immersion Silver - Via Plugging

Why would open (assuming you mean unplugged) immersion silver vias
increase assembly problems and cost more money?
Should immersion silver vias be plugged as a standard all the time?

Kaye Clayo
Keithley Instruments, Inc.

At 11:39 AM 6/11/2005, you wrote:
>We have been manufacturing boards with immersion silver for 8 years, 
>six of those we have supplied plugged vias.
>The process works best BEFORE the immersion silver process.  It should 
>not be done after immersion silver because of the possibility of 
>contaminating the silver.
>There NEVER should be anything in the via holes that could etch the 
>hole walls.  They must have a very marginal via plugging process.  Best

>pick another fabricator until they improve their via plugging.  Open 
>vias could increase assembly problems and cost you more money.
>
>Happy Holden
>Westwood Associates
>
>
>
>
>Kanaiyalal Patel <[log in to unmask]> Sent by: TechNet 
><[log in to unmask]>
>06/10/2005 05:23 PM
>Please respond to
>TechNet E-Mail Forum <[log in to unmask]>; Please respond to Kanaiyalal 
>Patel <[log in to unmask]>
>
>
>To
>[log in to unmask]
>cc
>
>Subject
>[TN] Immersion Silver - Via Plugging
>
>
>
>
>
>
>Our supplier is recommending against plugging via holes the immersion 
>silver boards and here is their explanation. Need some input into how 
>you guys are managing.
>
>"We can not plug before silver because chemicals used during the silver

>process may become trapped. Over time these chemicals can eat away at 
>the hole wall copper creating voids.
>2. Because the boards have passed through rout and are in piece form 
>when silver is applied the board can not be plugged in panel form as 
>are the gold boards.
>3. Immersion silver is soft and easily damaged. It is our last 
>manufacturing process and we avoid handling it as much as possible 
>after being coated, this particularly applies for the plug process."
>
>Appreciate your feedback.
>re,
>Hemant
>
>
>---------------------------------------------------
>Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 
>or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET 
>Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
>posts: send e-mail to [log in to unmask]: SET Technet Digest Search the 
>archives of previous posts at: http://listserv.ipc.org/archives Please 
>visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for

>additional information, or contact Keach Sasamori at [log in to unmask] or 
>847-615-7100 ext.2815
>-----------------------------------------------------
>
>_______________________________________________________________________
>______ Scanned by IBM Email Security Management Services powered by 
>MessageLabs. For more information please visit http://www.ers.ibm.com 
>_______________________________________________________________________
>______
















CONFIDENTIALITY NOTE: This message is intended only for the use of the
individual or entity to which it is addressed and may contain
information that is privileged, confidential, and exempt from disclosure
under applicable law. If the reader of this notice is not the intended
recipient, you are hereby notified that any dissemination, distribution
or copying of this communication is strictly prohibited. If you have
received this communication in error, please notify the sender
immediately.



________________________________________________________________________
_____
Scanned by IBM Email Security Management Services powered by
MessageLabs. For more information please visit http://www.ers.ibm.com
________________________________________________________________________
_____

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 or
(re-start) delivery of Technet send e-mail to
[log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing
per day of all the posts: send e-mail to
[log in to unmask]: SET Technet Digest
Search the archives of previous posts at:
http://listserv.ipc.org/archives
Please visit IPC web site
http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100
ext.2815
-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 or
(re-start) delivery of Technet send e-mail to [log in to unmask]: SET
Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
archives of previous posts at: http://listserv.ipc.org/archives Please
visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for
additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
-----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
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 or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------

ATOM RSS1 RSS2