Jeff
We have been specifying via plugging from the top side only on some of
our dense boards. This is an additional process step and does add some
cost. About 2/3 of the hole ends up filled with a heat cured polymer.
The bottom side of the filled hole can still be accessed for testing. I
don't think chemical entrapment is a problem with this, but I don't have
any data to prove it. One extra little benefit of this is that you can
put silk screen nomenclature over the top of the plugged vias. We have
done this for reducing shorting and also for reducing secondary reflow
on boards that are wave soldered after surface mount reflow. The plugged
hole reduces heat conduction because the solder from the wave does not
get into the plugged hole very far.

In your case just making the solder mask openings for the vias smaller
on the top side may help. We have some boards where we have the LPI
soldermask covering about half of the pad on top, to reduce shorting.
This works fine and the hole is still open. This would be easy to
implement. OSP would help also.

Nick Wander
Printed Circuit Engineering
Unisys, Roseville, MN  

>----------
>From: 	Jeff Seeger[SMTP:[log in to unmask]]
>Sent: 	Tuesday, February 25, 1997 2:02 PM
>To: 	[log in to unmask]
>Cc: 	[log in to unmask]
>Subject: 	Fab/Assy:  Class3, 1/2 tented vias
>
>Good day, TechNet -
>
>	Can't help needing to post in this time of too much traffic.
>	Perhaps you'll soon see why:
>
>	I've been asked to make a soldermask change on a very dense,
>	double-active, sensor type design.  The assembly is subject
>	to In-Circuit-Test, and the non-test-side clearances are such
>	that the contract manufacturer is requesting that we tent the
>	testpoint vias on the non-test side.  There are troubles with
>	solder shorts.
>
>	The board is HASL, 2x reflow, with hand solder, and an aqueous
>	cleaning process.  The soldermask is LPI.  Volumes are modest.
>	The via geometry is small, such that the current soldermask
>	appears to give a reliable covering.
>
>	The application is life support.
>
>	My fear is that one-sided tenting will allow entrapment of
>	processing chemistry and will not allow adequate cleaning.
>	It would seem to me that switching to OSP or shrinking the
>	via clearance to just above the finished hole size would be
>	a safer alternative for long term reliability.
>
>	Can a sometimes-too-conservative ol' design guy draw out any
>	support or denial opinions?
>
>	Thanks in advance, and regards,
>-- 
> 
>      Jeff Seeger                             Applied CAD Knowledge Inc
>      Chief Technical Officer                      Tyngsboro, MA  01879
>      [log in to unmask]                               508 649 9800
>
>
>
>

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]>   *
* with <subject: subscribe/unsubscribe> and no text in the body.          *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************