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August 2009

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Subject:
From:
Bob Willis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Bob Willis <[log in to unmask]>
Date:
Thu, 27 Aug 2009 16:26:48 +0100
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Always important to provide the complete story

If you try the oil test on a sample with and without via plug that should
show you.

If you search the archives you will find the procedure and video links I
have posted in the passed on normal plated through holes. The images in the
database show small vias but not via in pad. The image of a BGA shows via in
pad

Although solder mask does absorb moisture and solvent it very unusual to
outgas for a long period of time.

Many thanks
 

Bob Willis
2 Fourth Ave, Chelmsford, Essex, CM1 4HA England
Tel: (44) 1245 351502
Fax: (44) 1245 496123
Mobile: 07860 775858
www.ASKbobwillis.com
www.SolderingStandards.com 


New Package on Package Workshops 24th November
www.ASKbobwillis.com/PoPWorkshops.pdf 
PCB Design for Manufacture & Assembly Workshop 15th October
www.ASKbobwillis.com 
PCB Inspection & Quality Control Workshop 3rd November
www.ASKbobwillis.com/faworkshops.pdf
Book Bob's "Step by Step Failure Analysis Workshop" 4th November
www.ASKbobwillis.com/faworkshops.pdf

-----Original Message-----
From: Chris Mahanna [mailto:[log in to unmask]] 
Sent: 27 August 2009 15:47
To: Bob Willis; TechNet E-Mail Forum
Subject: RE: [TN] Via-in-pad blowing bubbles

Bob et al,
Sorry we don't have blinds outgassing.  If we see some I'll send a sample to
you. Obviously I forgot to mention the via type.  They are through vias with
single sided solder mask plug.  The plug is very deep (>75%), and maybe a
separate mask process.  Different mask?  I doubt that the board is
outgassing through the via because we've got ~1.8 mil of uniform plating.
How about incomplete plug cure?  I think that I may have some evidence of
that.

Chris

-----Original Message-----
From: Bob Willis [mailto:[log in to unmask]] 
Sent: Thursday, August 27, 2009 7:23 AM
To: 'TechNet E-Mail Forum'; Chris Mahanna
Subject: RE: [TN] Via-in-pad blowing bubbles

If you have via in a pad and its solder coated or reflowed with paste what
you describe can happen. If there is still volatile material either in the
hole, a void below the level of the solder or thin copper plating outgassing
through the copper plating can happen. Not however on gold over nickel
boards. The eruption and outgassing will be random depending on what is
there, speed of heating and how long the solder remains molten.

Just like any gassing from a plated through hole you can see a nice ball, a
balloon blow out or a sunken joint and this can be all associated with
outgassing. The classic problem with through vias is when engineers try to
fill small vias on side one reflow, then spend hours cleaning balloons off
side two before printing again.

If you simply find a sample board with the problem you describe and place it
under a microscope, fill the view with a group of pads in question. Take a
micro soldering iron and touch sample pads and wash what happens. You can
use the old oil outgassing test I have used for years to see the outgassing
from the vias on a bare board Unfortunately I don't have a via example to
show you but if you look at http://defectsdatabase.npl.co.uk and do a search
for via outgassing you will see some a video and gif animation and images.

If someone has some good examples of blind vias that are outgassing, send me
a board and I will test it and share the video on TechNet. I have tested
vias in the is way before for customers but never videoed the results.

Many thanks

Many thanks
 
Bob Willis
2 Fourth Ave, Chelmsford, Essex, CM1 4HA England
Tel: (44) 1245 351502
Fax: (44) 1245 496123
Mobile: 07860 775858
www.ASKbobwillis.com
www.SolderingStandards.com 


New Package on Package Workshops 24th November
www.ASKbobwillis.com/PoPWorkshops.pdf
PCB Design for Manufacture & Assembly Workshop 15th October
www.ASKbobwillis.com PCB Inspection & Quality Control Workshop 3rd November
www.ASKbobwillis.com/faworkshops.pdf
Book Bob's "Step by Step Failure Analysis Workshop" 4th November
www.ASKbobwillis.com/faworkshops.pdf
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: 27 August 2009 02:34
To: [log in to unmask]
Subject: [TN] Via-in-pad blowing bubbles

Hello netters-

Have we a term for vias-in-pad blowing solder ball bubbles to the point of
bridging?
Regardless, is there a unique cure for the problem?
I wonder because- how does one get just enough pressure to blow the large
bubble, but not blow-out?   Bad luck?
Or is it not that unusual, and just usually gets screened before it gets to
me?

Chris


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