Hi Gary,
 
if you look at a solder cream reflowing under a microscope you'll see the flux flow out and the solder particles collapse, before solder fusion takes place. The exact point of this occurrence can be shifted depending on the reflow profile and the organic system used.
 
Work undertaken indicates that voids in joints may be a result of wetting issues rather than out-gassing issues. Certainly producing a solder joint over a tin lead HAL finish is easier than with gold than with OSP. One assumes that the tin lead system is more compatible with the solder cream than the gold, which requires dissolution into the molten solder. I'm also assuming that the OSP is hardest to wet out because of a higher level of oxide present the with HAL or gold finishes. Unfortunately it's fairly difficult to determine the oxidation under an OSP finish due to variation in process and finish, also any handling or sample prep will potentially create change.
 
Best regards,
 
Andrew Hoggan
BBA Associates Ltd
www.bba-associates.com
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Erickson, Gary
Sent: 09 August 2001 21:11
To: [log in to unmask]
Subject: Re: [TN] VIAS in pads...

            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]]
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

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