Even though its Friday, I thought another comment is needed:

Ted Edwards is correct regarding the aspect ratio of the via holes and the 
need for technology to assist plating in the hole.  The portion of his reply 
that requires more discussion is the issue of "plating" via holes shut.  In 
most cases a reputable board fabricator does not plate small holes to closure. 
 Rather the holes are open through the entire wet processing cycle and are 
closed during the reflow or HAL process (depending on your specification).  
Both HAL and IR reflow fluxes are generally based on an inorganic acid (HCl) 
and could retain corrosive by-products if the holes were plugged.  While I 
haven't done the studies, I suspect that the holes will be plugged with solid 
solder (no pockets of liquid).  If you are dealing with a board fabricator 
that has invested in the equipment to bump or ultrasonically agitate the 
boards during plating, that fabricator may have the studies available to 
demonstrate that there are no corrosive residues in the holes or should be 
willing to perform the testing to demonstrate that there are no reliability 
problems.  If this is a problem for the fabricator, you might want to examine 
the other fabrication services available in the marketplace.

Don Vischulis
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_________________________

Edwards, Ted A (AZ75) wrote:
> 
> I do not see on your list the first question that should be asked, why the
> small hole size in the first place?  On the 0.093 your aspect ratio( hole to
> thickness) is over 7and could be 9 depending on what size it is drilled at
> to get the finished 0.013.  Unless you have a supplier with bump or
> ultrasonics on the plating tank and electroless line you are creating a
> challenge for the solution to get through the holes and probably have a dog
> bone plating.  They pass electrical test because they probably do have an
> electrical connection at that time.  When you assembly and thermal shock the
> board at soldering the poor little barrel can not take the z asis expansion
> and lets go.  Don't blame the fabricator for this go talk to design.
> When they are plated shut one always wonders what chemistry is along for the
> ride and will it cause premature failure later.
>  ( Opinions are those of the author and not of his employer)
>  ---------
> From: [log in to unmask]
> To: [log in to unmask]
> Cc: [log in to unmask]
> Subject: Open vias problem
> Date: Friday, May 17, 1996 8:32AM
> 
> Hello,
> 
> We assemble circuit boards in a high product mix/low volume environment.
> I'm uncomfortable about some PWB defects that were detected by our ATE
> test equipment recently (i.e. testing of fully assembled/soldered circuit
> board assemblies).  "Open" vias were detected in a few instances.  In other
> words, there wasn't continuity between the top side and bottom side
> pads for the via.
> 
> For one particular part number we had 3 boards with an open via on each
> out of 94 boards total.  The previous month we had 1 board out of 114
> exhibit this problem.  This board is a .093" thick 4-layer SMOBC surface
> mount board.
> 
> Another part number we experienced this problem with had an open via on
> each of 3 out of 210 boards total.  This board is a .062" thick 6-layer
> SMOBC surface mount board.
> 
> In both cases, the vias are specified to have a finished diameter of
> 0.013".  However, in both cases we've allowed them a minimum diameter
> of 0 per their request (i.e. we won't reject any for vias plated shut).
> 
> Both of these part numbers are supposed to be 100% electrically tested
> by the PWB supplier with a clamshell tester.  Therefore, ideally, I
> would've expected zero instances of this type of problem.
> 
> QUESTIONS...
> 
> Is there anything wrong with allowing vias to plate shut?
> 
> Which PWB fabrication process(es) would cause an occasional open via?
> 
> Isn't it fair for me to expect their electrical test to detect these?
> 
> Should I be concerned about the via integrity in every board supplied
> by this PWB fabricator, regardless of whether it passed our ATE test
> or not?  (i.e. is this a sign of something far worse)
> 
> How would you proceed in this situation?
> 
> Please e-mail responses to [log in to unmask]  Thanks - any assistance
> is appreciated.
> 
> Regards,
> 
> Mark Lettang